In this
treatment of the Cobar belt, its extent, general nature and pattern of
mineralization, nature of hostrocks and wallrock alteration are first
introduced, along with a listing of the more important references.
The individual ore deposits and prospects are then grouped and
described. Genetic theories, and
further generalization and speculation regarding mineralization, such as
zonation, are discussed subsequent to the deposit descriptions.
The Cobar belt is the major metalliferous strip of the Cobar sheet.
It is considered to be geologically bounded along much of its eastern
side by a fault network interpreted as a syn-depositional extensional system
which saw later reactivation during deformation, as part of a linked thrust
system (Glen 1988). This gives a
ready conceptual boundary, virtually the Devonian/basement contact, for the
eastern side of the belt. The
western side is more empirical, being simply a line drawn initially as that
beyond which significant prospects were unknown prior to gold discovery at
McKinnons Tank. Numerous
individuals have contributed in greater or lesser amount to the current
understanding and documentation of Cobar belt geology, mineral deposits,
exploration and mining. The many
contributors include the following: Adams and Schmidt (1980), Andrews (1919,
1923), Besley (1966), Binns (1983), Binns and Appleyard (1983, 1986), Bouffler
(1981), Brill (1988, 1989, 1991), Brooke (1964, 1975, 1976), Brunker
(1969-1970), Bryan (1974b), Byrnes (1975, 1978), Cannings (1988), Carne (1899,
1906-1908), Chapman (1989), Chesney (1889), Conolly (1943, 1946, 1950), Davis
(1980), de Roo (1987, 1989), Doe et al. (1990), Gilligan (1980), Gilligan and
Suppel (1978), Glen (1978-1980, 1982, 1984-1989, 1991), Glen and Hutton
(1983), Glen and Pogson (1981), Glen et al. (1983, 1985, 1986, 1991), Gow
(1965), Gray (1918, 1942), Hinman (1989-1991), Hinman and Scott (1989), Iten
(1952), Iten and Carter (1951), Jaquet (1895-1896), Joklik (1950), Kappelle
(1970), Kelso (1982), Kirk (1983-1984), Lawrie (1991), Leahey (1990), Lloyd
(1936-1939, 1943, 1950), Longman and Meares (1972), Marshall (1991), Marshall
and Sangameshwar (1982), Marshall et al. (1981, 1983), McClatchie (1984),
McLeod (1973), Mulholland (1941, 1957, 1959), Mulholland and Rayner (1952,
1953), O'Connor (1980), Phillips (1989), Plibersek (1982), Pogson and Felton
(1977-1978), Pogson and Hilyard (1981), Pogson et al. (1976), Rayner (1955,
1959, 1961, 1969), Rayner and Lloyd (1952), Robertson (1968), Robertson
(1974), Russell and Lewis (1965), Sangameshwar and Marshall (1980), Sangster
(1979), Schmidt (1981, 1986), Schmidt (1980, 1983), Scott (1987), Scott and
Phillips (1990), Scott and Taylor (1987), Seccombe (1990-1991), Seccombe and
Brill (1989), Shields (1986), Sullivan (1947, 1950, 1951), Suppel (1979,
1984), Suppel and Lewis (1989), Suppel and Stevens (1972, 1973), Taylor
(1914), Taylor (1982-1983), Taylor et al. (1984), Thomson (1951, 1953),
Warburton (1977), Wood (1980), Worotnicki (1977), Worotnicki and Alexander
(1977). Besides these, much change
many other writers have made mention in passing of the geology along the Cobar
belt or the nature of its metalliferous deposits.
In addition to the contributions of the above individuals, a large
volume of prospect generation and evaluation reportage will be found under
company titles. The numerous
companies which have made single and conjoint (joint venture) contributions to
exploration and understanding of the Cobar belt include Abminco NL (1978),
Amax Australia (Operations) Pty Ltd (1983), Amax Exploration (Aust) Inc
(1971), Amax Exploration (Aust) Inc and Australian Selection Pty Ltd (1971),
BHP Minerals Ltd (1983), BP Australia Ltd (1985-1986), Buka Minerals NL (1976,
1983), Clifford McElroy & Associates Pty Ltd and Mt Hope Minerals NL
(1971), Cobar Mines Pty Ltd (1960-1962, 1965 1966, 1968-1973, 1975, 1977-1979,
1981, 1983-1986, 1988), Cobar Mines Pty Ltd and CRA Exploration Pty Ltd
(1988-1990), Cobar South Pty Ltd (1980, 1982, 1984), CRA Exploration Pty Ltd
(1981-1984, 1986-1990), CRA Exploration Pty Ltd and Samedan Oil Corporation of
Australia (1977), Electrolytic Zinc Co of A/asia Ltd (1974-1975, 1977-1984,
1986, 1989), Electrolytic Zinc Co of A/asia Ltd and Dampier Mining Co Ltd
(1976-1978), Electrolytic Zinc Co of A/asia Ltd and Geoterrex Ltd (1975), Esso
Exploration and Production Australia Inc (1976), Enterprise Exploration Co Pty
Ltd (1958), Getty Oil Development Co. Ltd (1982-1984), International Mining
Corporation NL and Esso Exploration and Production Australia Inc (1976), Jones
Mining Ltd (1984), Jones Mining Ltd et al. (1985, 1989, 1990), Jones Mining NL
and Aquitaine Australia Minerals Pty Ltd (1983), Jones Mining NL and Metals
Exploration Ltd (1988), Magnum Gold NL (1990), McIntyre Mines (Aust) Pty Ltd
(1967), Mines Exploration Pty Ltd (1975, 1977), Mines Exploration Pty Ltd and
Cobar Mines Pty Ltd (1971), Mt Hope Minerals NL (1970-1973), Mt Hope Minerals
NL and International Mining Corporation NL (1975), Newmont Holdings Pty Ltd
(1981, 1984), Newmont Pty Ltd (1979-1982, Norgold Ltd (1989), Pasminco Pty Ltd
and C.R.A. Exploration Pty Ltd (1990), Peak Gold Mines Pty Ltd (1990),
Penarroya (Australia) Pty Ltd (1978), Penarroya (Australia) Pty Ltd and
Preussag Australia Pty Ltd (1981), Preussag Australia Pty Ltd (1978), Swiss
Aluminium Mining Aust Pty Ltd (1974), Swiss Aluminium Mining Aust Pty Ltd and
Le Nickel (Aust) Exploration Pty Ltd (1973-1974), and The Zinc Corporation Ltd
(1949-1950, 1952).
The Cobar belt is currently postulated to extend NNW on the Louth sheet
via the Kerrigundi area, and SSE on the Nymagee sheet.
The southwards continuation of the belt through Nymagee and Mount Hope
has long been accepted but the view as to its northwards course have changed
since 1970. Now thought to trend
NNW via Elura and Kerrigundi, the favoured view formerly was to extend it
north of Cobar towards Gunderbooka gold field via Mount Drysdale.
Although historically best known as an important copper producer, the
field centred on Cobar has also been a major producer of gold, silver, lead
and zinc. Within the Cobar sheet
area, between Elura and Queen Bee mines, the total metal content of the belt
down to an extractable depth limit is estimated as 1.1 Mt Cu, 1.6 Mt Pb, 2.6
Mt Zn, 5000 t Ag, 90 t Au. Summary
production figures for different periods are given in Russell and Lewis (1965)
and Kappelle (1970). Some inferred
1990 conservative reserve figures, and production figures for the major
centres are as follows:
| Deposit
|
Reserve
(=R) |
Production
(=P) |
Copper
(
%) |
Gold
(g/t)
|
|
Elura
|
22.3 Mt
|
|
|
|
|
CSA
|
5.0 Mt
|
|
|
|
|
Great Cobar
|
3.5 Mt
|
3.9 Mt
|
2.3%(P),2.0%(R*)
|
1.4(P),0.2(R*)
|
|
New Cobar
|
2.4 Mt
|
1.0 Mt
|
1.0%(P),<0.9%(R)
|
7.0(P),<6.8(R)
|
|
Chesney
|
5.0 Mt
|
0.7
Mt
|
1.7%(P),2.2%(R)
|
2.5(P),0.2(R)
|
|
Gladstone
|
2.2 Mt
|
0.03 Mt
|
6.3%(P),2.5%(R)
|
?(P),0.5
|
|
Young
Australian
|
?
|
0.01 Mt
|
4.2%(P),?(R)
|
4.3(P),?(R)
|
|
New
Occidental
|
1.2 Mt
|
2.1 Mt
|
very low
|
9.6(P),6.5(R)
|
|
The Peak
|
3.9 Mt
|
0.03
Mt
|
?(P),0.8%(R)
|
20.0(P),7.1(R)
|
|
Queen Bee
|
1.2 Mt
|
0.04 Mt
|
7.9%(P),2.4%(R)
|
very
low
|
(*) - A less
conservative estimate of Great Cobar reserve grade is 2.8% Cu, 2g/t Au.
The Cobar region has produced a
large output of metals since production commenced in 1870.
Estimated totals
(updated to mid 1990s) are:
2.5 million ounces of gold
0.6 million tonnes of copper
1.5 million kilograms of silver
0.6 million tonnes of lead
1.2 million tonnes of zinc
In addition, an exploration
program in the five years following the compilation of these notes, by Peak
Gold Mines, identified further resources of 1 million ounces of gold.
Much
about the later major developments at the Peak is not included in these notes
and was still confidential at the time (and any updating of these late 80s-90s
compiled notes may be placed in separate files. Also lacking are
production figures for Elura and CSA. A start was made on
compiling Elura's production but it was found that there were multiple sets of
figures - those recorded by company head office, annual figures published in
commodity reviews by the BMR and 'mine figures' (possibly the most accurate)
by the metallurgical staff (believed to backwardly adjust to great metals
content accuracy following the returns of smelter result figures months after
intial reports of production (based on 'grab' sampling of dispatched
concentrates?) had been reported. The full collection/resolution
of all the figures could not be completed prior to the end of the Cobar
metallogenic project.
Geology
The Cobar Belt, largely a high strain zone, occurs between two broad
structural regions. To the west
lies a wide expanse of progressively less deformed Devonian deposits of the
Darling Depression, comprising marine deposits of the Darling and Baka Basins
and the subsequent fluviatile Mulga Downs Group which is part of a wider
blanket. West of Cobar these less
deformed Devonian strata extend for 300 km or more, to beyond Wilcannia.
To the east of the Cobar belt there is a broad basement-dominated area
extending at least 120 km to Nyngan. The
area to the east contains some Devonian sequences of Cobar Supergroup age, in
the Mineral Hill Synclinorial Zone. However,
these tend to be richer in coarse clastics, volcanics and carbonates than for
the region west of Cobar. Thus the
Cobar belt, the region's zone of strongest mineralization, coincides with the
major structural hinge zone apparent from Devonian rocks.
Whether the line of the Cobar belt has similar significance with
respect to the Ordovician or older basement rocks is unknown as drilling west
of Cobar generally has not penetrated below the Darling Basin sequence.
It is clear that the Cobar mineral belt coincides with a major
structural change, probably of considerable crustal penetration.
The nature of detachment and other tectonic processes possibly
connected with the Cobar belt, and hence the structural setting of
mineralization, has been speculated upon at length by Glen (19xx-19xx) and
Scheibner (19xx-19xx). Multiple
reactivition of older structures features in some of the tectonic models
advances.
The eastern edge of the Cobar belt, as noted above, appears to be over
a major tectonic line. It had
mid-Palaeozoic expression as a structural and palaeogeographic hinge zone.
Roughly speaking, the eastern edge of the Cobar belt is the eastern
faulted margin of the deeper water turbiditic deposits of the Cobar Basin.
Co-eval shelf deposits are known immediately to the east, across fault
zones, in a number of places. The
precise correlations across fault zones, from shelf sequence in the east to
deeper water sequence in the west, remain imprecise.
In some areas, such as at the Mallee Tank mines, conglomeratic facies
and carbonate facies occur more or less in proximity.
However, much faulting is present and the conglomeratic facies
(dominant to the west, lower Nurri Group) is not expected to intertongue with
the carbonate facies (dominant to the east, Kopyje Group).
Precise correlations across the shelf edge along the eastern Cobar belt
margin must await more detailed stratigraphic studies.
The regional biostratigraphic framework remains in need of elaboration.
It is hampered by the infrequency and poor preservation of fossil
material. Biostratigraphic
zonation problems have existed and no biomeres are recognized.
Biostratigraphy is more advanced for the Cobar Supergroup than for
other rocks in the sheet area, and in time a more accurate tracing of
sedimentary units across the palaeogeographic hinge zone of the eastern Cobar
belt margin will be possible. The
trend of the shelf edge become progressively better known as additional
drilling intersects carbonates along the eastern edge of the belt north of
Cobar. Compared to their deeper
water equivalents, the shelf sediments are also currently suspected to be in
places richer in carbonaceous material (including bituminous residues?).
The shelf edge will probably be traced over a long distance.
From south of the Rookery it passes east of Elura into the southeastern
corner of the Louth 1:250,000 sheet area.
Fossil preservation in the shelf edge zone is locally excellent by
regional standards, and it is suspected that this zone may have been a
relatively stable and less deformed one even prior to Cobar Basin formation.
As a generalization, the eastern side of the Cobar belt has been the
more stable, and strata from further west may be locally overthrust to the
east. The tectonic movements
involved, such as those along the Rookery fault zone and the Myrt syncline and
fault zone, have been the subject of many changes in interpretation (e.g. Glen
19xx-19xx).
The deposits
The Cobar belt in the Cobar sheet area contains over 60 deposits and
prospects hosted in slates and deformed turbidites of the Early Devonian Cobar
Supergroup. The majority of Cobar
belt metal deposits have strong structural controls apparent.
These controls, and general genetic theories are discussed at the end
of the Cobar belt chapter. Some
regional structural elements, especially cleavage attitude, show considerable
uniformity over large area. The
direction 330oT is a very common trend direction, usually reflecting cleavage.
The regional strike from the CSA to the Occidental mine averages 350o.
Bedding dips are steep in proximity of the mines, e.g. 80oW.
Folding is generally broad, except in some shear zones, and strong
axial plane cleavage strike 340o and dips typically 80oE, generally cutting
across the bedding. Wherever
clearly exposed (e.g CSA mine), the ore bodies are seen to follow cleavage and
not bedding. Major departures from
these generalised trends of bedding and cleavage occur north of CSA mine.
Within the CSA-Occidental segment of the Cobar belt there also occur
deviations from the average trends. Various
explanations of these deviations have been given in terms of interference
cross-folding; buckles, warps and
fault jogs generated under shear couple forces;
and other mechanisms. The
deviations from the broader northnorthwesterly trends have commonly been
regarded as important factors in ore localization.
Some deposits occur within broad weakly mineralized zones associated
with faulting. Silicification and
dissemination of sulphides or magnetite may occur in broad zones.
Most major deposits are composite ore systems comprising multiple
discrete orebodies. Widespread
primary ore minerals include chalcopyrite, pyrrhotite, sphalerite, galena,
pyrite, gold, silver and magnetite in major amounts.
Bismuth minerals may be common locally.
Minor minerals, including arsenopyrite, tetrahedrite and cobaltite,
occur in small amount. The
orebodies typically occur as steeply plunging lenses, in zones of deformation
with associated wallrock alteration. The
term "Cobar type" has been used often but as the Cobar belt deposits
vary significantly among themselves, it has little distinctive meaning.
Common to most, but not all, of the major ore systems in the Cobar belt
would be the presence of plunging lenses enriched in sulphides and quartz, and
encased by sheared fine-grained sediments.
As such general features are found worldwide, there is little necessity
to discern them as "Cobar type".
Perhaps more useful would be to name a series of more specific deposit
types based on detailed consideration of individual ore systems or even
individual orebodies. This is
considered beyond the scope of the present study.
For many of the larger deposits relationships are sufficiently well
documented to show that orebodies lie aligned to cleavage or shears, and
oblique to the nearest observable bedding.
Because many of the deposits are in silicified and mineralized shear
zones which retain little or no trace of bedding, the relationships of
orebodies to bedding are often inferred over short distances rather than
directly observed. Likewise, the
broader fold structures elsewhere defined by bedding may become indistinct or
totally obliterated within the shear zones hosting mineralization.
An exception is at Elura where the fold pattern closely associated with
the mineralization is not greatly obscured.
The Elura orebodies are domally surrounded by their hostrocks (CSA
Siltstone). In this context the
strongly cross-cutting relationship of the massive sulphide periphery with
surrounding strata lacks the ready option of multiple interpretation which
linear shear zones may provide elsewhere along the Cobar belt.
It has long been recognized that occurrences of mineralization along
the Cobar belt are structurally controlled, discordant to bedding, and highly
elongate down dip. In plan view
mineralization may appear to broadly follow stratification.
However, in vertical transverse sections of the ore systems the broad
overall conformity of ore is with cleavage not bedding.
The frequent pattern along the eastern edge of the cobar belt is for
ore bodies and cleavage to dip easterly where bedding dips westerly.
This is quite clear on an ore system scale although close to individual
ore bodies and shear zones the bedding may steepen or be overturned, so that
the disparity between cleavage and bedding becomes less pronounced.
Although cleavage may be more or less planar and dip to the east in
unmineralised areas, it is by no means as consistent in the vicinity of ore.
Close to ore there are innumerable observations of undulating and
dislocated cleavage. Likewise, the
bedding is generally oblierated in mineralised zones.
Early investigations deduced the mineralized shear zones of the Cobar
belt near Cobar to be typically overthrust from the east.
Deformation of veins and cleavage indicates near vertical, east side up
movement within the shears. In
plan view there are many minor sinistral and dextral displacements apparent.
Form of mineralisation and
orebodies
The
mineralization is generally in the form of disseminations or numerous
veinlets, and less often, or seldom, in the form of massive ore (although
"massive sulphides" are often mentioned and Elura is much more of
this type than other mines).
The
Cobar
belt ore systems may extend thousands of metres vertically and in plan section
they vary from very elongate narrow to almost equant.
The CSA ore system is of the latter type, being only slightly longer
along strike than it is broad. Individual
orebodies may be up to 120 m wide (e.g. Elura) but are mostly in the 6-20 m
range. Their strike lengths mostly
fall between 30 m and 200 m. Over
much of the length of the Cobar belt the orebodies dip vertically to steeply
east. Increased frequency of vein
quartz may characterise even the economically blank portions of mineralized
zones. Enhanced quartz-carbonate
veining and alteration may occur between ore lenses, sometimes following shear
zones. Quartz veining with minor
sulphides is believed to commonly indicate the upper extension, or strike
extensions, of orebodies. Indeed,
quartz veins commonly occur in swarms around mineralisation.
Some veins are parallel to cleavage or bedding, but most occur at low
angles to these features. A few,
usually ptygmatic, lie in markedly discordent other orientations.
this suggests a long history of vein formation throughout the life of
shear zones, with earlier veins being rotated to higher angles as shearing
progressed. Coarsely fibrous vein
quartz, of probable syntectonic growth, is a prominent feature of the Cobar
belt along much of its length. In
some areas (e.g. New Cobar - New Occidental area) quartz with weak but
distinct colloform banding has also been noted, both in drill core and from
mine dumps.
Orebodies along the Cobar belt mostly show steep northerly plunge,
although plunges may be southerly in The Peak area.
The cause of the general northerly plunge shown by the orebodies is
uncertain. The deviation from
vertical is too great to merely invoke a simple regional tilting in
post-Palaeozoic times. A
combination of east block up and the much evidenced dextral movement along
structures at the eastern edge of the Cobar belt would perhaps suffice.
Such explanation could not be extended as far west as Elura and,
indeed, the Elura orebodies are non-plunging vertical pipes.
Various other explanations of northerly plunge can be given in terms of
structural intersections, as noted in the discussion herein at the end of
Cobar belt deposit descriptions.
Zones of disseminated sulphides, and less often magnetite, are known at
many places along the Cobar belt. Disseminated
pyrrhotite zones are of uncertain origin and may not be connected with base or
precious metal mineralization. It
is considered that the pyrhotite disseminations might pertain to a phase of
basin deformation pre-dating economic mineralization.
Brecciation is prominent in the Cobar belt.
The largest breccia body known is within The Peak ore system and
reaches of 300m in width. Similar
siliceous breccia as at The Peak also occurs at New Cobar, where it is up to
50m thick, and at other sites. The range of breccia features includes
"floating" angular clasts in silicified breccia bodies, rare
close-fitted clasts possibly indicative of hydrothermal fracturing,
oligoclastic sedimentary rock breccias of uncertain slumping versus tectonic
origin and tectonically-rounded clasts in shear zones (e.g "pebble"
shear zones, CSA ore system). Masses of brecciated sedimentary rocks related
to deep faults or shears occur in many metalliferous mining areas (e.g.
Thomson, 1965). There is often no simple relationship between breccias and
mineralization. However, fault activity, and/or fluid transfer along faults,
could be involved in both brecciation (hydrofacturing) and metals transport.
Cobar
belt mineralization varies a great deal, from low sulphide siliceous gold ores
to massive sulphide ores. Detail
on types of mineralization, ore types and ore minerals is contained in many
studies. These include Rayner
(1969) and in more recent years many studies on ores and hostrocks as they
became exposed in the workings of the new CSA and Elura mines:
Adams & Schmidt (1980), Besley (1966), Binns & Appleyard
(1986), Bouffler (1981), Brill (1988, 1989, 1991), Cannings (1988), Chapman
(1989), Cobar Mines Pty Ltd (1965, 1972, 1979, 1981), Davis (1980), de Roo
(1987, 1989), Doe et al. (1990), Gow (1965), Kappelle (1970), Lawrie (1991),
Leahey (1990), Marshall (1991), Marshall et al. (1981), McLeod (1973),
Robertson (1968), Robertson (1974), Russell and Lewis (1965), Sangameshwar and
Marshall (1980), Schmidt (1980, 1983), Scott (1987), Scott and Phillips
(1990), Scott and Taylor (1987), Seccombe (1990), Taylor (1982-1983), Taylor
et al. (1984). Certain of the
studies which consider microscopic ore details have placed quite different
interpretations on textural features (patterns of intergrowth, exsolutions,
twinning etc.) in terms of paragenesis, temperature of formation,
metamorphism, etc.
The chief primary ore minerals are chalcopyrite, pyrite, pyrrhotite,
and magnetite, with variable sphalerite, galena, and native silver-gold.
Minor minerals include arsenopyrite, tetrahedrite, cubanite, bornite,
and cobaltite. Significant
bismuth, in the form of native bismuth, bismuthinite, guanajuatite,
galenobismutite, or tetradymite is occasionally present.
Traces of stannite, valerite and other ore minerals have been recorded.
The proportions of these minerals vary considerably and the belt
contains large ore systems which are variously rich in gold (e.g. New
Occidental) or copper (e.g. Great Cobar, CSA) or silver-lead-zinc (Elura).
Some of the ore systems contain a variety of ore types, including both
massive sulphides and disseminated sulphides in silicified or quartz veined
intervals. Sphalerite commonly
occurs in banded, drawn out or wispy patches aligned with cleavage, whilst
chalcopyrite occurs mainly in massive banded form or in veins.
The most sphalerite-rich ores may be of highly deformed, almost
foliated appearance. Some have
interpreted such sphalerite as severely deformed because of early deposition
whereas others have taken an opposite view that the sphalerite was a late
addition favouring the most strongly sheared and last-active zones during its
introduction. Disseminated
sulphide aureoles and depletion haloes commonly occur about orebodies, often
in rocks which are chloritized or silicified.
Thomson (1953) and others have broadly divided Cobar belt major ore
types into three categories, being the most siliceous and the most
sulphide-rich extremes, and an intermediate category.
In generalized terms these types may be characterized as follows:
* Siliceous ore consists of disseminated gold and chalcopyrite, with
associated pyrrhotite, pyrite, and magnetite.
The silicified host rock, commonly thought to be silicified slate, is
in its purest form locally known as "elvan" (a misnomer).
This highly siliceous rock is somewhat chert-like, with a duller more
cryptocrystalline appearance than vein quartz.
Subordinate galena, sphalerite, arsenopyrite, and bismuth minerals may
occur, but sulphide abundance is not closely associated with elvan.
the observation that cleavage and black chloritic shears appear to wrap
around the margins of elvan zones suggests that elvan formed before the
principla introduction of sulphides. Siliceous
ore is well known along the Eastern Line at Cobar (New Occidental-Chesney-New
Cobar line of mineralization), at CSA mine, and at Mt Drysdale.
* Siliceous-pyritic ore, consisting of chalcopyrite and pyrite in a
siliceous gangue, with subordinate arsenopyrite and marcasite, but little gold
or magnetite. Pyrrhotite is minor.
The Queen Bee, Gladstone, and CSA ore systems contain some ore of this
type.
*
Massive sulphide ore, consisting of various proportions of
chalcopyrite, pyrrhotite, magnetite, pyrite, galena, sphalerite, and
marcasite. Virtually all massive
developments of chalcopyrite along the Cobar belt carry a significant
proportion of intimately intergrown pyrrhotite.
Sphalerite (marmatite), generally less abundant than the pyrrhotite, is
usually also present in massive chalcopyrite ore.
Minor associated minerals include "ekmannite" (stilpnomelane),
arsenopyrite, cubanite, tetrahedrite, covellite, galeno-bismutite and native
bismuth. Exsolved cubanite is
found in the chalcopyrite of this ore type, which Rayner (1969) used as
evidence of high temperature of formation (+450oC) for the iron-rich copper
ores. Good examples of this ore
type occur at Great Cobar (pyrrhotite-chalcopyrite-magnetite), Dapville, and
CSA (copper-zinc lodes). The
grainsize of chalcopyrite-rich massive ore is generally less than a
millimetre, with occasional larger grains.
Mineral banding in the more massive ores is discontinuous and is
usually parallel to ore boundaries. It
lacks any typical sedimentary structures.
It is generally envisaged as formed by post-emplacement shearing and
small-scale remobilisation. An
outright marine sedimentation origin for the banding is rejected but
influences during the stages of basinal dewatering cannot be as readily
evaluated or discounted. For
example rhythmic crystallisation from a metal rich colloid has been suggested
by J. Elliston (unpublished basinal dewatering study for CRA Exploration,
1989).
Following the discovery of Elura, with its overall more massive ore
types, a special case exists there of threefold division of ores: - into
massive (pyrrhotitic), massive (pyritic) and siliceous.
The threefold ore classification at Elura has some similarity of intent
to that following Thomson (1953), as above, but the subdivision categories
used at Elura are not directly comparable with those applied elsewhere along
the Cobar belt. The Cobar belt
literature also contains varying usage of "massive ore".
The term "massive" is now generally restricted to dense
sulphide ore as at Elura. Earlier
references to so-called "massive" ore, as at Great Cobar, often
meant lenses of metasomatized cherty slate.
The latter "massive" ores broke in a blocky manner and were
very often densely transected by a multiplicity of quartz and sulphide
veining. To a smaller extent,
intervals of dense pure sulphides also occur.
During the Great Cobar period, the major ore types were variously
blended for smelting, in ways controlled by changing metallurgical practices
and metal prices. A typical blend
was about three parts of Great Cobar massive sulphide pyrrhotitic copper ore
to one part of siliceous gold or gold-copper ore.
There has been no smelting in the region since the Great Cobar period,
subsequent to which the metals have been exported from Cobar in the form of
separated sulphide concentrates. The
concentrates and fine-ground sulphide ores require care in handling because of
their chemical reactivity. Many
Cobar belt orebodies are rich in pyrrhotite which is particularly prone to
oxidation heating and cementing. Mined
ore and bulk tailings with as little as 15% pyrrhotite, has been observed to
heat to above 200oC (Paton 1952, Bean 1976).
Mining and processing cycles are planned to avoid any unnecessary
accumulations which might be prone to self-heating.
Patterns of metals
distribution variation in mineralised areas
Various
writers over time have suggested that certain "zonations" of metals
can be observed.
The
compositional distribution of mineralization in the Cobar belt, it has been
suggested, can be further
generalized as follows: Gold to
the east, lead-zinc and sulphur increasing to the west, copper strongest in
between.
Such
a generalisation is by no means a
rigorous observation but it has been suggested as applicable both to the whole
belt and to single deposits. For example,
only
minor concentrations of galena and sphalerite occur in the Great Cobar, New
Cobar and Peak deposits - whereas further
west in the belt there are important discrete Pb-Zn ore lenses appearing such at the CSA
mine;
and at Elura it is Pb-Zn which dominates. The
more copper-rich deposits occupy an intermediate position.
The pattern of Zn/Pb/Cu/Au ratios increasing westwards overall within
the belt is related to the stratigraphic-structural positions of the deposits.
The siliceous gold deposits are hosted in the interval from basal Great
Cobar Slate down to basement. The
most copper-rich deposits occur higher within the Great Cobar Slate, and Pb-Zn
increases greatly in the westerly deposits hosted within the CSA Siltstone.
A general compositional factor which Rayner (1969) strongly stressed is
that the Cobar belt copper ores are iron rich.
Chalcopyrite ore is associated with pyrrhotite, cubanite, magnetite,
stilpnomelane and pyrite. There is
abundance of iron, in excess of sulphur. In
this respect the Cobar belt mineralization differs distinctly from that of the
Canbelego and Girilambone belts.
Wide zones
of alteration and depletion typically surround Cobar belt ore systems or
individual orebodies. The Cobar
Supergroup host rocks consist essentially of a quartz + muscovite + chlorite +
albite + calcite + ankerite assemblage. In
the vicinity of some orebodies marked alteration of this assemblage is
present. The alteration recognized
within and surrounding orebodies includes silicification and the development
of chlorite, talc, carbonates, sericite, biotite and kaolin (e.g. Rayner 1962,
Bouffler 1981, Schmidt 1980). For
the strongly shear-related deposits the most widespread form of alteration is
chloritization and silicification, with the latter often favouring the more
arenaceous hostrocks. Chlorite is
often a dominant mineral. Intensely
chloritized intervals have been encountered in many ore zones, sometimes with
associated subordinate talc or kaolinite zones.
Kaolinite zones, although widespread, are usually thin and very minor
compared to the extent of associated chloritic zones.
Moreover, there is difficulty in determining Palaeozoic kaolinite from
more recent near-surface alteration. Biotite
is not common and at CSA mine may indicate local high temperature zones in the
country rocks. A little biotite is
also recorded in wall rock at the Chesney mine.
Talc is common and at the CSA mine has at times given rise to
metallurgical difficulties. Sericitization
has often been mentioned but is inadequately documented and may best be
disregarded unless confirmed. Extensive
depletion haloes have been recognized in otherwise unaltered rocks surrounding
Cobar belt orebodies (e.g. Robertson and Taylor 1987).
Many elements may be depleted (e.g. Na, K, Sr).
A few may show increase rather than depletion (e.g. Mg, Fe).
The major components of the alteration haloes, such as chlorite and
introduced silica, are envisaged as pre-deformational by some workers but the
alteration zones are likely of multiphase development (viz. Hinman's 1989-1991
studies at The Peak). The gangue
minerals are the same as those involved in alteration.
Quartz, chlorite, calcite and siderite are dominant.
Stilpnomelane, a complex iron silicate, is a characteristic and
widespread associate of ore in the Cobar belt.
Both stilpnomelane and massive black chlorite occur in little-deformed
cross-cutting veins at some of the major deposits, and the black chlorite may
also be common in late stage sphalerite-galena mineralization.
The Cobar belt orebodies, especially at the Great Cobar mine, have
often been stated to have shown consistent fall of values with depth.
This was generally attributed during the Great Cobar period to the
effect of diminishing enrichment with increasing distance from the rich
supergene zone where the processes of enrichment were the most concentrated.
Below the supergene zone, enrichment was held to extend some distances
into the primary ores, attributed to copper sulphide deposition in Mesozoic or
later times. Some workers have
suggested a greater dominance of chalcopyrite just below the supergene zone
than at lower mine levels. Generally,
mine records are now inadequate to definitely support or refute such a
conclusion. The balance of
evidence from mine grade records, and the formed opinions of earlier
geologists, are in support of the phenomenon of a secondary "yellow
sulphide" enrichment zone, particularly at Great cobar mine.
In the supposed zone with secondary enriched chalcopyrite, much higher
copper grades were mined than is generally the case for the primary sulphide
ores. Cobar belt primary copper
ore in typically not very rich, e.g. 1.5-2.5% Cu, and the richer ores of the
early mining years were secondary ores. Nonetheless,
rich primary copper ore is not unknown in the Cobar belt.
For example, some drillhole intersections at CSA (QTS zone) have
assayed up to 20% Cu over mineable widths (Scott and Phillips, 1990).
Caution needs to be exercised in interpreting the record of a company's
annual production grades as a simple reflection of the deposit's changing
character with increased depth of working.
Various changing metallurgical and commercial factors generated
selective mining practices, group treatment needs, and other complications.
Moreover, in discerning enrichment effects a basic assumption is that
the primary ore compositional variation in the down-plunge direction is
relatively insignificant. Studies
from both the CSA and Elura mines reveal that this may not be so.
Some of the smaller CSA ore lenses have been found to change strongly
down plunge, and at Elura it is thought that some minor metals concentration
at ore pipe tips could be a primary feature.
Forms of mining
Cobar belt mining has been almost entirely hardrock mining, with little
alluvial mining and prospecting. Although
the mines in the Cobar belt from Great Cobar south to the Peak mines, and at
Mt Drysdale, have been collectively the largest hardrock source of gold in New
South Wales, there is almost negligible record of alluvial gold being worked
along the belt. Some alluvial-colluvial
gold was recovered from a short gully immediately flanking the siliceous ore
system at Mt Drysdale, and there are unconfirmed reports that a little
alluvial gold and tin(?) was recovered by early prospectors in the Tinderra
area.
Details of mining are in some of the references and mining engineering
is not treated in these notes in any detail. The older mines where
shaft mines with inter-level stoping; sometimes developing open cuts to remove
the upper levels entirely. Elura was a late discovery and from the
outset had a different more modern design of spiral access decline for trucks
and vertically elongate stopes. It had no open cut but a catastrophic
collapse in the mine, that occurred without warning, produced a surface
depression.
A late
'scavenging' stage form of 'mining' or extraction of copper at some of the
dirstrict mines has been precipitation of copper from acidic or acidified mine
waters pumped to surface vats containing scrap iron.
The designation of deposit
groups
Geographic
and stratigraphic grouping
During
the 1970s in particular a sygenetic 'understanding' was imposed over the
Cobar and Nymagee 1:250,000 sheet are mineral deposits and attempts were made
to organise the mines into groups on that basis. This has
been given up on and more purely geographic grouping returned to.
Ten mineral areas (groups of mineral deposits and prospects) are
described herein along the length of the Cobar belt on the Cobar 1:250,000
sheet (figure xx). Some of these
groupings are of long standing and may be closely associated with a single
dominant well known stratigraphic or structural feature (e.g. New Cobar-New
Occidental line of deposits). More
often, however, the groupings have less internal uniformity.
They are in part simply of geographic convenience for purpose of
description, and some might contain deposits of diverse style and host rock
stratigraphic level. The ten
mineral deposit areas are listed below from north to south.
1.
Warrego-Elura Area
2.
Budumbah-Mount Drysdale Area
3.
Benowa-Kendi Area
4.
CSA-Spotted Leopard Area
5.
Mopone-Bluebell Area
6.
Great Cobar-Gladstone Area (The "Western Line" at Cobar)
7.
New Cobar-New Occidental Area (The "Eastern Line" at Cobar)
8.
The Peak Area
9.
Coronation-Rookery Area
10. Victoria Tank-Nymagee
Area
Of these ten areas, those between CSA and Rookery (Nos 4-9 above) have
generally been the more closely examined.
However, strong exploration continues in all the areas, and has
increased in the vicinity of Elura mine. The
old Mount Drysdale mines also attract intermittent strong attention.
The long-known mineralized tract between CSA and Rookery includes what
is loosely termed the Cobar belt "central area".
The latter comprises the Great Cobar - Gladstone and New Cobar - New
Occidental lines of mineralization (Nos. 6 & 7 above).
The Peak area has usually been treated separately from the central area
although there is a degree of structural continuity which links these areas.
However, strong exploration continues in all the areas, and has
increased in the vicinity of Elura mine. The
Mount Drysdale mines also attract continued attention.
Very detailed geological mapping between C.S.A. and Queen Bee mines was
carried out by Enterprise Exploration Co. Pty Ltd, largely in 1947-1949, at a
scale of 1:4,800. The mapping by
Enterprise Exploration is particularly useful.
It embodies a considerable amount of observation from extensive
costeaning which has since been backfilled and is no longer available.
Much of the Enterprise Exploration data has been transferred to more
modern scales by Cobar Mines Pty Ltd or CRA Exploration Pty Ltd.
Early mapping delineated multiple fractures, often envisaged as
post-attenuation failures, along the submeridional folds.
Overthrusting from the east and sinistral wrenching products arranged
en echelon were favoured structural interpretations in detailed work.
The deposits of the Cobar belt between CSA and Queen Bee were concluded
by Rayner (1969) and others to occupy a series of en enchelon overlapping
shear zones with north-plunging wrench dilations.
Rayner (1969) envisaged maximum attenuation on the limbs of folds in
the central area. Overlapping
shears were considered to collectively define major tecto-lineaments along a
belt of Nurri Group strata characterised by south-pitching folds.
The western limbs of the anticlinal folds were seen as particularly
prone to attenuation and overthrusting from the east.
This model was derived mainly from studies of the central area.
It remains of interest in a general sense but in detail is superseded
by the many subsequent structural studies by D. Glen and others.
The stratigraphic setting of the major deposits along the Cobar belt
shows some generalized northwards younging.
The mineralization at Elura possibly extends into the youngest host
rocks known. Both Elura and CSA
are within the CSA Siltstone and determination of their relative setting might
depend upon future correlation of marker tuff bands.
From CSA south through Cobar, it has been stated often that the
mineralization is confined to three deformed zones, being within the C.S.A.
Siltstone, the Great Cobar Slate, and at or near the contact of Great Cobar
Slate with the Chesney Formation. The
Great Cobar host rocks are usually placed in about the middle of the Great
Cobar Slate (contra placement in C.S.A. Siltstone, Glen et al. 1985, p. 64).
The Eastern Line deposits (including New Cobar, Chesney and New
Occidental) mostly lie in the Great Cobar Slate close to its faulted contact
with Chesney Formation. For the
southern end of the Eastern Line, a greater proportion of the lesser orebodies
would appear to lie within the Chesney Formation.
This trend, for more mineralization to be hosted in the Chesney
Formation, could continue southwards, However,
for the Peak and Queen Bee mine areas further south, stratigraphic
interpretations have been more controversial.
Some regard the mineralization there as hosted mainly in Great Cobar
Slate whereas others have interpreted the host rocks as sheared Chesney
Formation, or else as a transitional sequence between typical Chesney
Formation and Great Cobar Slate. Unfortunately,
no marker beds or fossiliferous strata exist which would enable the
stratigraphic subdivision at Cobar to be confidently extended south along the
Cobar belt, although this has been variously attempted.
Iten (1952) suggested a southwards facies change to coarser sediments
within the Great Cobar Slate. For
The Peak ore system in particular, not only have some workers regarded the
host rocks as mainly Chesney Formation but the involvement of still older
(pre-Chesney Formation) brecciated material has been postulated in the
writings of Hinman (1989-1991). Hinman regarded these oldest host rocks as originally
volcanic, which was at the time he suggested this quite anomalous-seeming for
deposits of the Cobar field.
When considering the generalization that depositional setting youngs to
the north, it may be recalled that the immediate structural context of some of
the ore systems is anticlinal or domal. Elura
and The Peak are in such flexural cores and Great Cobar could also be on the
faulted crest of a local anticline. Thus
host rock age might well vary with depth in a single ore system.
At The Peak the host may change from Great Cobar Slate in the upper
part (where previously mined) downwards to Chesney Formation (unmined), and at
still greater depth the proportion of infaulted or fracture-emplaced basement
host rocks could be expected to increase.
1.
WARREGO-ELURA AREA
The
northerly segment of the Cobar belt wherein these deposits lie is not well
known geologically but it does have features reminiscent of the belt further
south, from CSA through Cobar. Drilling
within the Warrego-Elura segment of the Cobar belt has confirmed northward
continuation of the general tendency for cleavage, veining and mineralization
to dip steeply east. The area has
a few small prospecting pits which appear to predate modern times but there is
scant record of metalliferous search prior to Elura's discovery by modern
methods. The Warrego-Elura area
contains the following two metalliferous sites:
No.
Deposit name
Commodities
1 Warrego
prospect
Zn,Cu,Pb
2 Elura mine
Zn,Pb,Ag (Au,Cu,Cd)
Vertical pipe-like
orebodies occur at Elura. The
mineralization at Elura is structurally controlled and localized in a NNW
trending series of tight domes attributed to later refolding of the NNW
trending folds. Alteration
includes feldspar destruction and carbonate enrichment.
Weak base metal anomalism occurs along the trend of the CSA Siltstone,
extending north-northwest of Elura mine with the chief anomalies situated
between 6 km and 13 km from the mine. The
bedrock geochemical anomalies detected throughout the area are overall very
weak, although values range up to 870 ppm Cu, 770 ppm Pb, 290 ppm Zn and 60 ppm
Sb. Weak but distinctly anomalous
values offer moderate encouragement from Elura mine north through Warrego
prospect into the southeastern corner of Louth 1:250,000 sheet area.
Elura is the most northerly significant deposit of base metals known in
the Cobar belt and its discovery fundamentally influenced thinking on the
trend of mineralization to the north of Cobar.
Prior to Elura's discovery the Cobar belt mineralization had been
widely thought to trend due north towards Gunderbooka.
Subsequent to Elura the likely major trend of expected base metal
mineralization was reassessed as more westerly; more towards Louth and not via
the Billagoe Ranges as was the formerly favoured interpretation.
Elura is a concentrically zoned pipe-like deposit localized by a
north-northwest trending anticline and chain of domal feature (Schmidt 1980,
1983; de Roo 1989, Lawrie 1990). Orebodies
occur in domal culminations along this trend.
The localization of orebodies by folding may appear anomalous for the
Cobar belt, where other deposits are commonly localized by faults.
Anticlinal folding is also a feature of the ore environment at The
Peak, along with shear zones. Likewise,
the major anticline at Elura could be associated with a fault.
It might lie above a blind thrust.
For the latter possibility, Glen (1991) suggests that deposition of
sulphides could relate to reduced permeability above the tip line of a buried
fracture.
In the Elura area the rocks are at lower greenschist grade and
deformation is relatively mild. There
is suggestion that various aspects of sedimentation and ore deposition could
prove clearer and better preserved around Elura than further south in the
Cobar belt. Probable ashfall beds,
of untested usefulness as stratigraphic markers, occur in the mine sequence
and there are also present fossiliferous horizons (I. Kelso, pers. comm. 1991)
which might correlate with a limestone shelf edge (Kopyje Shelf) to the east,
from whence fragmental faunal remains were likely transported.
This is a similar inference for the transition across the Cobar Basin
margin as to the south of Cobar but the strata recording it give some promise
of being less deformed at Elura. Study
of the shelf to basin transition at the latitude of Elura is still in its
early stages, but the contact between CSA Siltstone and Kopyje Group to the
east of Elura mine has already been a focus of exploration.
The contact zone is much fractured and chloritized.
Appreciable Pb/As anomalism occurs along it in auger samples but as no
primary mineralization has been intersected in drilling to date, this
geochemical anomalism has been attributed to scavenging and secondary
enrichment along the contact. The
enrichment appears to cut out below 40m (Electrolytic Zinc Co., 1986c).
The general inferred vicinity of the Kopyje Group faulted western edge
has been prospected from near Elura mine to beyond Warrego prospect,
principally by Electrolytic Zinc Co A'asia Ltd (EZ).
Quartz veined areas return weak metal values, rarely reaching 300 ppm
Pb. One prominent tract of
intensive quartz veining, termed "Big Vein" by EZ geologists, is on
a small rise 6 km north of Elura mine. Quartz
exposure occurs along a length of 100 m and is up to 2 m wide.
The line of quartz outcrop appears to be sinusoidal and is perhaps of
related origin to the quartz-filled "buckle" segments of faulted
contacts close to the eastern edge of the Cobar belt near Cobar.
Warrego
Prospect
The Warrego prospect comprises a geochemical anomaly in the CSA
Siltstone. It lies within a zone
containing other weak anomalies extending north-northwest of Elura mine
subparallel to the Chesney Formation upper contact.
The zone yields values of up to 1700 ppm Pb marginal to quartz veins.
The host sequence is CSA Siltstone with possibly some Great Cobar
Slate. Siderite presence and
possible alkali depletion are suggestive of wallrock alteration.
Drilling at the Warrego prospect intersected a 5-10m wide zone of weak
Zn-Cu-Pb-As mineralization, associated with narrow quartz veins.
The best intersection was 2m of 0.14% Zn, with a maximum of 0.37% Zn +
Cu (Electrolytic Zinc Co. A'asia Ltd, 1986b).
Elura Mine (later re-named Endeavor)
The
Elura (renamed Endeavor) mine is currently owned by Sydney-based mineral
resource company CBH Resources Limited (CBH - Cobar Broken Hill).
It is located 47 km north of Cobar. It was first brought into production
in 1983, as a relatively recent 'new discovery' at blind orebodies found by
geophysical and geochemical exploration. It was developed at a cost of
$270 million. The mine has been operated by CBH since 2003, after
general failure of the large company Pasminco which formerly operated
it. Along its large underground spiral decline roadways the mine
operates haulage units for ore and waste transport of a massive 920
horsepower.
Sulphide
concentrates are sent to smelters in Port Pirie in South Australia and
overseas, predominately to Japan.
Endeavor
has a total capacity of 1,200,00 tonnes of ore per annum with capabilities to
produce 140,000 tonnes zinc concentrates per annum, and 60,000 tonnes of lead
concentrates per annum containing 70,000 tonnes zinc, and 40,000 tonnes lead
and 31,000 kilograms of silver.
The Elura name ("Pleasant Place", aboriginal) was given to a
magnetic anomaly over the deposit first detected in 1972, prior to realization
of its significance. The Elura Zn-Pb-Ag
deposit occurs in CSA Siltstone, 41km north-northwest of Cobar.
Prospect status was reached in 1973 when Electrolytic Zinc Co. of
Australasia Ltd was carrying out a program of aeromagnetic anomaly evaluation.
The 1973 auger drilling at Elura anomaly revealed the presence of
highly anomalous lead values (ultimately the lead anomaly there was found to
extend for 1,200m along strike. Diamond drilling penetrated the
deposit in February 1974. The
geophysics of the deposit, and other aspects of its detection and initial
development are treated in the Electrolytic Zinc Co.'s progress reports (e.g.
1974; 1975a,e; 1977a; 1878f,h; 1979a,d; 1980a), and in Emerson (1980).
Further aspects of Elura may be found described in Adams and Schmidt
(1980), Cannings (1988), Chapman (1989), Davis (1980), de Roo (1987, 1989),
Lawrie (1990, 1991), Leahey (1990), Schmidt (1980, 1983), Scott (1987), Scott
and Taylor (1987), Seccombe (1990), Taylor (1982-1983), Taylor et al. (1984),
and Wood (1980).
The Elura deposit barely touches surface and rapidly expands with depth
as a large discrete vertical plug or pipe-like mass of massive sulphides.
The top of the main plug is separated into two apothyses, one of which
reaches surfaces but has no significant expression.
Drilling to 500m defined an orebody of 27 Mt and a number of smaller
deep vertical orebodies were later discovered.
At least seven orebodies comprise the Elura ore system, located in the
hinge area of a doubly plunging anticline.
The associated Pb anomaly measures 100-300m x 1200m.
The Elura sulphide ores are the most massive encountered to date in the
Cobar belt and the deposit constitutes a world class base metal resource.
The deposit size had been earlier estimated to be as much as 31 Mt of
ore grading 5.8% Pb, 8.4% Zn and 130 g/t Ag.
A 1980 estimate was 27 Mt at 5.6% Pb, 8.3% Zn and 146 g/t Ag (Adams and
Schmidt 1980). Following the
mining out of the supergene zone, ore reserve grade figures were revised
downwards, particularly in silver, to 5.5% Pb, 8.4% Zn and 87g/t Ag.
Metal averages from numerous chip and core samples of ore up till 1990
are 5.2% Pb, 7.7% Zn, 142g/t Ag, 0.6% As, 0.2% Cu.
To date (1990), over 7 Mt has been mined and total ore remaining is
re-estimated as 22.3 Mt, with the deposit open at depth. Proved and probable
reserves total 14.8 Mt. Production
grade has averaged 5.9% Pb, 8.7% Zn, 157g/t Ag.
Full scale exploitation of the silver rich supergene zone commenced in
1986. Silver declines with depth,
averaging around 3300 g/t in the thin supergene cap, 110 g/t in the main
primary ore mass, and 65 g/t in the deeper northern pods.
The 1985/1986 ore raised at Elura averaged 205g/t Ag.
Most of silver leaving Elura is in the lead sulphide concentrates,
which have contained two to three times the amount of silver produced from the
supergene ore. The first lead
concentrate from Elura (15,553t in March quarter 1983) was rich in silver
(1642.1 g/t Ag). Later separate
processing of supergene ore produced a markedly richer silver concentrate
(e.g. 5306t assaying 16.7 kg/t Ag in 1985-1986.
Exploration
of Elura commenced in 1972. Initial
geochemical surveys at the Elura magnetic anomaly in 1973 revealed lead
anomalism without associated Cu-Zn anomalism.
A lead anomaly with dimensions of 100-300 m x 1 200 m was later
outlined. The anomaly was first
diamond drilled in 1974. The first
hole entered gossam at 102m and sulphide ore at 132.5m.
One of the orebodies just reaches surface and minor excavation revealed
an inconspicuous leached gossan with anomalous lead and arsenic.
Some gossan samples contain several per cent arsenic.
The gossan has the highly stable lead aluminium arsenate-sulphate
mineral hidalgoite at the surface, and beudantite at greater depth.
An exploration shaft was commenced in 1976, and the first 1000t of ore
was extracted in 1978 for metallurgical testing. Full mine development was
carried out for EZ Industries Ltd in 1980-1982, and was commenced within a
$160 million project budget, including minor changes to EZ's Risdon plant to
better handle the Elura zinc concentrate.
During the construction phase the workforce at Elura peaked at over 600
employees. Eventually over $200M
was expended on Elura and associated works.
Mining cycle and stope design aimed for below 4% ore sterilization and
eventual mining recovery of 88% total ore.
Final commissioning cost was about $190 (Barrett, 1983) and further
associated downstream facility improvements raised the total to around $200M.
Government infrastructure expenditure was about $35M.
The railway spur line to CSA mine was extended 30 km to Elura.
Concentrates are railed to Newcastle, where new port facilities were
constructed on Kooragang Island to handle them.
The first lead-silver and zinc concentrates were produced in 1983, and
the mine has since supported a workforce of up to 350 employees.
Zinc concentrates are sent to EZ's Tasmanian smelter at Risdon, near
Hobart. Lead concentrates go to
both domestic and overseas markets. Metal
recoveries to the end of June 1989 are conservatively estimated as 457 528t
Zn, 252 500t Pb and 720 482 kg Ag, from 7.11 Mt ore mined.
Minor commodities in the concentrates are antimony, cadmium, copper,
gold and sulphur. The total
contents of the concentrates produced to June 1990, as estimated from assay
values but unlikely to be fully extracted, are:
585 148t Zn, 345 715t Pb, 983 189 kg Ag, 2 346 kg Au, 7 690t Cu, 2 093t
Cd, 1 115t Sb and in excess of 400 000t S.
In 1991, as the industrial economies experienced recession, fall in
base metal prices and demand moved Pasminco Ltd towards cutting production at
Elura. The work force was reduced
to less than one-third, a reduction of over 260 employees.
The Elura
deposit comprises a set of at least seven crudely elliptical pipe-like
vertical lenses, extending over a length of 700m.
Visible carbonate and sulphide alteration define a 100-150m wide
subvertical envelope that is at least 1.5km in strike length, and over 700m in
vertical extent. The main portion
of the deposit consists of the two large partly coalesced near-vertical
cigar-shaped pipes, with maximum diameters of 100-120m (figure ESS).
Typical diameter is 80m. Where
these join at depth, below about 290m, the large resultant ore mass is roughly
elliptical in plan and measures 210m by 120m.
It is elongated roughly north-south and with the other mineralization
pipes is aligned along the axis of a north-northwesterly trending anticline.
Towards the surface the two main pipes separate and extend upwards as
tapering apophyses. The northern
orebody pinches out 50m or more below surface.
The southern orebody also tapers out but just reaches the surface.
The extremely weak surface expression had escaped attention from early
prospectors. Sharp plunge
reversals occur along the trend of the host anticline, sheathing the orebody
pipes in tight domes. All
orebodies may be located in the hinge or core zones of doubly plunging
anticlines which represent culminations along the persistent NNW-trending fold
axis (de Roo 1989a). However,
detailed structural interpretation in the close vicinity of the orebodies is
not straight-forward.
Elura has been drilled to 800 m and at this depth the southern of the
two main orebodies is still continuing. The
northern orebody, re-separates downwards from the coalesced mass below 485m
and pinches out around 610m. A
vein mineralization zone, enveloping five smaller vertical pipe-like massive
sulphide orebodies, the northern lodes, extends north from at or near the base
of the northern orebody. The
northern mineralization terminates 300-400m below surface (Fig. ESS).
These smaller (max. 200m x 20m) near-vertical pods or lenses extend
over a distance of 500m from the main ore mass, and may have a similar
concentric zoning. Alteration
increases in intensity with depth, although visible haloes die out rapidly
above the orebodies.
As first conceived, the configuration of the Elura deposit seemed
markedly different from others in the Cobar belt.
Whereas other deposits tend to be systems of multiple planar shear
lodes containing north-plunging more linear concentrations or rich shoots,
Elura as early known did not fit this pattern.
The Elura ore system was initially thought to form just a single linear
trend. This would contrast with
other major Cobar belt sites (e.g. CSA, Great Cobar, Chesney, New Occidental,
Peak) where there is more than one line of mineralization present.
First known at Elura were the two vertical cigar-shaped orebodies,
concentrically zoned and in turn at the centre of apparent upwarping.
Later on, the delineation of the northern lenses showed Elura to have
the typical linear shear zone characteristic of other Cobar belt ore systems.
Still more recently the possible top of another mineralization pod was
discovered below the crusher. This
deep "crusher pod" would seem to have affinity with the northern
lenses. It lies south of them and
is colinear with them (Pasminco pers. comm.).
This then suggests that the Elura ore system may now be viewed as two
very close-lying en echelon lines of mineralization.
The two main orebodies comprise the western line, whilst the crusher
pod followed by the northern lenses comprise the eastern line.
Such an interpretation further increased the similarity of Elura to
other Cobar belt ore systems. It
is also of interest that for Elura, CSA, and to a lesser extent at The Peak
(Big Lode), there has been a similar discovery pattern followed.
This has been that the most obvious mineralization, outcropping or
nearly so, occurs at the western side of the ore system and deep drilling has
discovered other lines of deeper mineralization to the east.
The Elura deposit occupies the core of an anticlinal structure which
trends about 345oT and might overlie a blind thrust.
Along it are the pipelike orebodies, situated in the cores of tight
domal culminations (doubly plunging anticlines).
These domes with mineralized cores are the shallowest and southernmost
of a series of similar deformations along the NNW striking anticline.
This host anticline is possibly in turn situated within an overall
south-plunging synclinorial structure. The
peripheries of the domal deflections which host the orebodies contain smaller
fold structures which plunge steeply along axes radial to the ore pipes.
The folds close to the orebodies are tighter than the parallel folds
and large open structures which generally occur in the area.
Underground exposures reveal solid sulphide ore sharply demarcated
against the deformed and truncated CSA Siltstone hostrocks (turbidite strata
of mudstone, siltstone and minor thin fine-grained sandstone beds).
The overall shape of the orebodies is grossly discordant, and Schmidt
(1980) estimated that the mineralization transgresses at least 500m of
stratigraphic sequence. A major
north-northeast trending shear abuts to the south-southeast and may truncate
and/or displace the ore system.
The orebodies may have formed syntectonically in doubly-plunging fold
hinges within the subvertical NW-trending D2 shear zone.
If so, later deformation has partitioned around the orebodies, which
became mantled by silicified envelopes. Similarity
between orebody outlines and the flanking bedding trends was early noted.
This, the local frictional obliteration of bed bottom load features
near the ore, and other observations, combine to suggest that massive sulphide
mineralization was in existence prior to the close of deformation and
significantly affected the structural movements in the nearby host strata.
Between individual sulphide pods veining and alteration is developed.
Alteration may be detectable up to 100m from the ore.
NNE-trending D4 shear zones cause minor offset of the pre-existing
mineralised structure, with replacement of the pre-existing alteration
mineralogy by a chlorite-dominated assemblage.
The
two main orebodies have Ag-rich caps. These
are characterised by development of a secondary sulphide assemblage and
localised metal redistribution with secondary concentration of Pb, and locally
Ag, Cu, Au, As, Sb, Hg, Sn, W and Ba within true supergene layers at the base
of oxidation (Schmidt, 1989). Mature
porous gossan with numerous cavities is developed down to an uneven base at
80-100m depth. There is a 1-2m
supergene oxidate zone overlying a 2-4m thick massive sulphide zone enriched
in lead and zinc. In places a thin
black `sooty' layer, which is a supergene copper enrichment (chalcocite,
digenite, covellite), sharply overlies relatively soft (?leached) sulphides.
Leaching of the sulphides continues down to 120m, and marcasite formed
from pyrrhotite is present down to 150m. The
supergene zone in places is markedly enriched in lead (10-55%), barium (1-2%),
silver (3000-6000 ppm) and antimony (9000 ppm).
Lead is readily observed as cerussite, beudantite and minor anglesite
in the basal 2m of the oxidate zone and as copious mimetite dispersed at
higher levels. Gold, tin, tungsten
and bismuth are also enriched, and mercury is notewothy.
Some of the native silver contains up to 20% mercury (Scott 1987).
Copper is strongly enriched in the lower part of the supergene sulphide
interval, where the basal `sooty' layer contains up to 50% Cu.
The ore weathering processes at Elura and the secondary minerals,
including some species of little note in the Cobar belt (e.g. cassiterite,
tripuhyite, blixite, mendipite and lauriontite), have been considered by Scott
(1987), Scott & Taylor (1987), and Slansky (1988).
The secondary zone's high content of silver, present as chlorargyrite
and native silver, was evaluated in 1986, and a reserve of 50,000 t of silver
ore estimated. The supergene ore
was estimated from drilling to average 13.5% Pb, 3290 g/t Ag, 9.9 g/t Au and
1.4% Cu. The 1988/1989 output of
14,570 t from it averaged 14.7% Pb, 3330 g/t Ag, 9.8 g/t Au and 1.01% Cu.
An approximate 5m thick section was mined in panels, taking the top of
the supergene sulphide enrichment zone and the silver-rich basal part of the
oxidate zone. This rich thin
interval, progressively extracted and replaced by tailings/cement concrete
pumped through holes drilled from the surface, had been practically exhausted
by 1990. Once level extraction is
completed, a trial drive over the top of the mined area could recover any
upward irregularities of the supergene zone as might exist but any further
supergene ore quantity is not expected to be great.
Because silver is also enriched at the tops of the deeper northern ore
pipes or pods, the silver enrichment of the main orebodies in the supergene
zone may well reflect primary concentrations further enhanced by supergene
processes.
Following
exhaustion of the supergene ore, average silver grade is lowered, and
decreases with depth. Some 10.4Mt
of ore in the two main orebodies is estimated to comprise 6.0% Pb, 8.7% Zn,
95g/t Ag; and 4.5Mt in the northern lodes as 5.5% Pb, 8.5% Zn, 60g/t Ag.
In the primary zone the margins of the orebodies may be quite sharp.
Discordant sulphide vein networks and outlying concordant sulphide
bands may occur.
The
concordant or stratabound sulphide beds, which may be selective replacements,
often extend no more than a few metres from the main ore mass.
However, the peripheral zone in which veins and stratabound
concentrations can occur in the altered wallrocks extends as much as 200
beyond the distinct edge of the massive ore.
In this zone elongate masses of massive sulphides may be bed-like and
as much as 2m long. They have been
interpreted either as disrupted syngenetic beds or selective replacements.
These locally concordant sulphide developments, of disrupted
appearance, also supported an early view of the orebodies as piercement
structures formed from remobilized sedimentary sulphides (Archibald 1984, de
Roo 1989a).
The two
main pipe-like orebodies have crudely concentric internal zonation, with
massive sulphide cores. A more
planar (elongate ellipsoidal) zone of low grade stockwork to trace
mineralization surrounds and extends between orebodies at depth as a possible
"feeder zone". This
continues north via the northern lodes. A
1-10m wide halo, continuing below the northern orebody as the "feeder
zone", contains low grade stockwork sulphide veins.
This open stockwork consists of variably oriented, 0.5 to 5 cm wide
sphalerite-galena veins with a density of 20 to 50 veins per meter at an
average grade of 10% combined lead-zinc.
Within the main orebodies there is strong concentric zonation, more
pronounced than anything known elsewhere in the Cobar belt.
Although the zoning may be complex and intergradational in detail
(Schmidt 1980), a simplified concept of three concentric ore zones with fairly
distinct boundaries suffices here. The
boundaries are in effect gradational over distances of 1-5 m.
The three major ore zones, from core outwards, are termed pyrrhotite
ore, massive ore and siliceous ore. The
"pyrrhotite ore" consists of pyrrhotitic massive sulphide forming
the orebody cores. It contains
70-80% sulphides in a gangue dominated by siderite.
Monoclinic and hexagonal pyrrhotite, pyrite, sphalerite and galena are
the principal sulphides. Chalcopyrite
is minor. The surrounding
"pyritic ore", occupying the intermediate zone, is pyritic massive
sulphide low in pyrrhotite, with a quartz-siderite gangue.
The pyritic ore contains 70-95% sulphides, dominantly pyrite with some
sphalerite and galena. The
sphalerite and galena define a faint banding.
This ore grades out into the outer zone of "siliceous ore",
which has a similar ore mineral assemblage but is distinguished by the
presence of abundant macroscopic and microscopic siliceous siltstone fragments
regarded as hostrock remnants. The
siliceous ore zone contains 20-50% silica and the sulphides vary from
disseminations to patches of massive ore.
The siliceous ore zone is relatively thin (2-30 m).
It is considered that primary metal zoning patterns in Ag, Pb, Zn, As
and Cu are preserved in the main orebodies.
With depth there is an increase in Zn, Pb and Cu contents, and a
decrease in Ag and As. This trend
is present in all seven orebodies, and is provisionally interpreted as a
function of physico-chemical gradients which probably controlled ore
deposition (Lawrie 1991). Thin
zones rich in Ag and Au are present in the caps of the northern lodes.
As these zones lie several hundred metres below the level of supergene
development they are interpreted as primary features.
Mineralogy and composition of Elura ores and gangue is described and
discussed in Schmidt (1980). The
major minerals of the deposit in approximate decreasing order of abundance are
pyrite, sphalerite, siderite, pyrrhotite, quartz and galena.
These comprise 95% of the deposit.
Other minerals include arsenopyrite, chalcopyrite, tetrahedrite,
tennantite, enargite, barite, feldspars and phyllosilicates.
The primary ores are fine-grained with complex textural relationships,
especially the siliceous ore, and are difficult to process.
Such ore mostly does not lend itself to ready separation and requires
very fine grinding. Thermal
reactivity of concentrates is considered by Jorgensen and Moyle (1984).
Being fine-grained and pyrrhotitic the ore is prone to oxidise quickly
when broken, aggravating any problems of re-cementation, spontaneous heating
or explosion danger. As a result
drilling and blasting are restricted to not exceed loading rates.
Firings are restricted to 12,000t compared to the 30,000t average for
similar mines elsewhere (1990 figures). Base
metal contents are highest in the pyrrhotitic cores of the orebodies, and a
Ag-As-Sb-Au assemblage is noteworthy in the outer siliceous zone.
Secondary zone minerals are discussed in Taylor et al. (1984) and
Chapman (1989).
As with
other deposits of the Cobar belt, a syngenetic exhalative origin has been
considered for Elura (e.g. Gilligan & Suppel 1978, Marshall &
Sangameshwar 1982), usually with some manner of remobilization.
Two main alternative origins have been considered for the deposit:
emplacement of pre-concentrated metals in a rheid state (physical
remobilization), or by replacement of host rock and filling of progressive syn-deformational
dilations. An epigenetic
replacement origin has been favoured by Schmidt (1980,1982,1983), Glen et al.
(1985), Glen (1987), de Roo (1987, 1989), and Seccombe (1990).
Adams and Schmidt (1980) and Archibald (1983,1984) discuss the
contrasting interpretations. Schmidt
(1980) proposed syndeformational replacement of host rock in a zone of
enhanced fluid flow, in an anticlinal core zone during the Tabberabberan
Orogeny.
Subsequent to earlier theories of syngenesis and physical
remobilization, Elura has been usually regarded as epigenetic (syntectonic)
with strong structural control. De
Roo (1989) and Lawrie both suggested that Elura formed as a syntectonic
orebody by a combination of replacement and emplacement in dilatant sites.
Microstructural evidence indicates that the orebodies formed after
lithification, and during the first upright deformation event.
De Roo (1987) concluded that the deposit was formed by metamorphic
fluids, with syndeformational replacement in a tight zone of vertical dilation
characterized by steep plunge reversals. Fluid
inclusion and sulphur isotope data are consistent with syntectonic origin (Sun
1983, Seccombe 1990). In response
to vertical extension the ore contains narrow subhorizontal fractures (max.
2mm) filled by later quartz, siderite and sulphides (the dilation layering of
Schmidt 1980). Microscopy
indicates that the `crack-seal' mechanism of rock deformation proposed by
Ramsay (1980) was a major process in the formation of the dilation layering
(de Roo 1989b, Seccombe 1990).
The subhorizontal dilation layering overprints an earlier syntectonic
ore fabric which is characterised by well developed near-vertical
mineralogical layering parallel to cleavage traces.
Within the orebodies subhorizontal discontinuities are dominant over
subvertical ones. Cleavage traces
mostly trend northerly (NE-NW) but an overprinting E-trending foliation is
apparent in the orebodies near their peripheries (de Roo 1989a).
Thus two generally pervasive preferred orientations exist in the
deposit: the sub-vertical gross mineralogical banding (best seen in the
massive sulphide), and the sub-horizontal predominantly gangue-filled fracture
filling interpreted as dilation layering.
Vertical dilation is discernable down to the scale of pressure shadows
about pyrite or horizontal cracks across siderite spheroids (Schmidt 1980, de
Roo 1987). Vertical lineation
within the ore is defined by streaks, up to 10 cm long, of coarse-grained
(2mm) sulphides. Galena is
enriched in these lineation spindles. Studies
of the sulphides indicate the effects of at least two overprinting deformation
events.
2. BUDUMBAH-MOUNT DRYSDALE AREA
The former gold mining centres of Mt. Drysdale and Billagoe, existed in
this area. Billagoe won early
infamy as an abortive silver field. In
the general Billagoe Range area traces of gold were found as early as 1866.
Impressive prospects for silver and gold began to be reported from
Billagoe from 1880 onwards, with increasingly spectacular assay grades quoted.
A small mining settlement was established there by 1886, the year
following impressive silver finds at Broken Hill.
The excitement over Billagoe was largely fuelled by speculation during
the 1880s silver boom. Spectacular
assay reports flowed from Billagoe, and funds were invested, but little
production resulted. The silver
boom had well-nigh run its course by early 1889 (The Argus, 13 March 1889).
The exploration focus then switched to gold and the centre of activity
subsequently shifted to nearby Mt Drysdale where a village of over 100 miners
came into existence in the 1890s. The
rise and peak of Mt. Drysdale as a centre of gold mining activity, principally
in the period 1892-1897, coincided with a general gold boom elsewhere, e.g.
Queensland (Mt. Morgan, etc.). The
Mt. Drysdale settlement did not prove permanent, however, and ceased to exist
after the closure of the principal mines.
Its peak population is uncertain.
The group of mineral deposits in this area occur in a possible large
cross-structural zone, apparent as a major northeasterly deflection or spur in
the general northnorthwest trend of the Cobar belt.
The concept of a northeasterly trending major cross-structure or fault
zone meeting the Cobar belt here from the west was principally developed in
the 1970s by company geologists but is also suggested in a 1961 compilation by
Rayner (1969). The local structure
is believed to be quite complex (Electrolytic Zinc Co. A'asia Ltd, 1982a).
From Elura mine area with its NNW trending structures, the trends swing
SE-E-NE to join the Myrt Fault and Syncline which are axial to the NE trending
spur. The Myrt Syncline follows a
meridional trend from Cobar for over 20 km north, and then veers north-
northeasterly. A suspected "Myrt
zone" continues into the Bourke 1:250,000 sheet area (Byrnes 19xx).
Structural trends and zones of mineralization have long been thought to
swing northeast approaching from the south but these trends remain poorly
mapped.
In general, a zone of deflections has for some years been thought to
extend northeasterly, passing west of Mt Drysdale, until meeting the Tinderra
granite. However, the structural
interpretation between Mt. Drysdale and Elura has been recently in a state of
change. The Coorari prospect is
regarded as lying possibly in Kopyje Group and just outside the deflected
trend of the Cobar belt. It is
thus not dealt with in this section, but other weak mineralization traces near
Coorari, could lie within the Nurri Group or in higher Cobar belt strata.
A likely deep source magnetic trend runs northeast from the Coorari
prospect. On its southern side
drilling of a thumb print anomaly (GT1) revealed weak disseminated
pyrite-pyrrhotite mineralization with occasional chalcopyrite in quartz veined
and chloritic host rocks (CRA Exploration Pty Ltd 1988a).
The best drill hole traces at GT1 were 1230 ppm Cu over 8.6m; 1610 ppm
Zn with 1020 ppm Pb over 1.1m. Other
weak prospects in the area are recorded by Getty Oil Development Co. (1982b).
The known major mineralization is hosted within the basal Cobar
Supergroup but sparse auriferous quartz veins and trace sulphide
mineralization is also recorded from the basement rocks of the area.
Quartz-feldspar-tourmaline veins occur in possible basement rocks 3 km
west of the Mt Drysdale mines, and drilling at Mt Drysdale itself has
encountered sparse sulphides in Girilambone Group rocks.
West of Mount Drysdale gossans occur in the area of the inferred
Chesney Formation - Great Cobar Slate contact (Earth Resources Aust., GS
1972/340), and at Bundumbah prospect mineralization is within the CSA
Siltstone. The deposits within the
Budumbah-Mt Drysdale area are:
No.
Deposit Name
Commodities
6 Unnamed
Pb
7 Budumbah prospect
Zn, Cu (Pb, ?Au)
11
Telephone line prospect
Zn, Pb, Cu
(Ag)
12
Billagoe mine
Au,
Ag
13
New Eldorado mine
Au
14
Mount Drysdale Mine
Au (Ag)
The most
important part of the Budumbah-Mt Drysdale area is the Mount Drysdale-Billagoe
Gold Field. Only the three major
mine names for this field are listed above as the records for other sites in
this field have not as yet been adequately identified on the ground.
This was a major gold field within the Cobar belt, with an estimated
production of 0.8-0.9t Au and significant silver from some 25,000-30,000t of
ore. At least three auriferous
silicified lenses are known in the gold field.
The largest is the Mt Drysdale deposit which was a major gold producer.
The second is at Billagoe which has small rich shoots but relatively
little size extent. The third is a
minor small lens with only trace metal values, about 1 km southwest of the Mt
Drysdale deposit. The form of
mineralization determined at Billagoe and Mt Drysdale is that of steeply
plunging rich shoots within a zone of silicification.
Budumbah
Prospect
Old
workings exist at the Bundumbah prospect and may be the shafts which Chesney
(1889) referred to as having recorded fair gold prospects west of Billagoe.
Drilling indicates a wide zone of disseminated pyrite high in the CSA
Siltstone. Elongation is
northeasterly and as outlined by geophysics the zone appears in part to be
transgressive of the contact between the Amphitheatre Group and the Great
Cobar Slate (Schmidt and Milovanovic, 1982).
For much of its length, however, the zone appears to be conformable.
It averages about 500 m in width and has been traced 8 km.
Pyrite content is 4% or less but there is little evidence of any
accompanying metals. Subparallel
to the Budumbah pyritic zone a 3 km long magnetite zone (Coorari anomaly)
occurs on the northwestern side of the Myrt Syncline.
The disseminated magnetite lies within the top of the Chesney
Formation. Magnetite comprises
1-2% of the rock over an interval of at least 45m, and hosts very minor
fine-grained pyrite and chalcopyrite. These
sulphides occur intergrown with magnetite or in quartz-chlorite veinlets.
As at the Budumbah dissemination zone, no significant drilling
intersection has been made (Milovanovic, 1980).
Billagoe
Mine
Payable gold was discovered at Billagoe
(a.k.a. Mount Billygoe, Tindarey Run) in 1879 by Alexander Rankin, following reported
earlier prospecting of trace gold in the Billagoe ranges as early as 1866 (Jaquet
1895). Numerous small
"reefs" had been prospected by 1882.
The principal discovery, the Billagoe mine, was developed initially for
gold (1880-1883), and only later for both gold and silver (1887-1901) by Mount
Billagoe Gold and Silver Mining Co. and other earlier parties.
Rich gold ore was reported in 1882 but the strong silver content of the
lode went without note until silver was assayed for in 1887 during a general
silver boom in eastern Australia. The
Mount Billagoe Gold and Silver Mining Company formed in that year.
In consequence of rich assay values announced, shares soon increased as
much as eighty-fold in transaction price and a township to be known as "Billagoe"
was surveyed. Something of a local
silver exploration boom then followed at Billagoe, and to a lesser extent
elsewhere in the region. The
emphasis on silver at this time was likely prompted by the discovery and
mining of rich silver lodes elsewhere in the 1880s (Broken Hill, Zeehan etc).
The silver boom peaked in 1888. During
the 1880s, Broken Hill ("Silver City") was producing one-third of
the world's silver. By 1889 there
were over fifty mineral tenements held throughout the Billagoe and Little
Billagoe ranges. A compilation of
these, showing the main Billagoe mine and the supposed line of lode, is given
in Chesney (1889). Some of the stone
taken from the Billagoe mine was very rich (e.g. silver 199 oz, gold 74 oz per
ton in specimens). The rich gold was reported as very fine, never
visible to the eye. In the 1890s,
after the discovery of the Mt Drysdale deposit which is also in the Billagoe/Billygoe
ranges, the Billagoe mine area became known as Old Billagoe prior to the
separate naming of the `new' Mount Billagoe workings as Mt Drysdale (in an
1889 description of the 'Billygoe' mine, in The Argus, 21 March 1889, page 10,
there is mention of new prospecting at Mount Margaret but it is not known what
that refers to).
The yield
from Billagoe was probably small in comparison with Mt
Drysdale. Production is estimated
at 10-15 kg Au and 30-50 kg Ag. Most
major activity had ceased by 1891 although the Billagoe company was
revitalised in 1899 and reported
breaking out a further 400-500t of ore, after which it ceased work (the ore
presumably being of insufficient yield).
Minor tributor work was done in later years and some further minor rich ore
recovery was reported in 1901.
Two
narrow lenticular rich shoots of gold ore in silicified slate, sandstone and
conglomerate, have been worked from shafts as deep as 61m at Billagoe.
The shoots are 0.2-1m wide and up to 9m in length.
The average width of siliceous lode extracted for crushing was 30 cm.
Reported assays are 31-612 g/t Au and 0.6-9 g/t Ag (with exceptional samples
to 1967 g/t Au and 28.9 kg/t Ag). Such
outstanding assays attracted much attention and from the small centre
established at Billagoe a great deal of prospecting was done throughout the
area. This expanded activity in
the area resulted in
the discovery of the Mt Drysdale mineralization.
Several trial parcels of Billagoe ore sent away to
Ballarat, Adelaide, Sydney and elsewhere gave rich recovery grades of up to
364 g/t Au and 3.7 kg/t Ag.
The Billagoe mine lode is
apparently no longer exposed or accessible.
No material of comparable assay values is found at surface or on the
dumps; and the workings have not been drilled.
Ore minerals are thought to have included native gold, pyrite,
arsenopyrite, cerargyrite, pyrargyrite, embolite, iodyrite and native silver.
Shaft dump material is not remarkable, other than that the conglomerate
mullock occasionally contains interstitial sulphides carrying significant
metal traces. Grab sample values
from this material range up to 268 ppm Cu, 3141 ppm Pb, 80 ppm Zn, 175 ppm Sb,
9 ppm Au and 172 ppm Ag. As at Mt
Drysdale, the Billagoe mineralization is recorded as being in silicified
country rock. There is no reported
association of the gold with quartz veining and it appears to be of similar
type as at Mt Drysdale.
Mt Drysdale-Eldorado Mines
(Drysdale
Gold Field)
A
second exploration boom occurred over the Billagoe ranges in 1892-1894, at
which time the hill carrying the new gold discovery became known as Mt
Drysdale after the discoverer of the principal deposit (Mr David Drysdale).
The area was also referred to as the Drysdale gold field.
Over 350 mining tenements had been surveyed in the Mt Drysdale area by
1895, exclusive of older tenements dating from the earlier Billagoe rush which
peaked in 1888. Peak prospecting
activity in this second phase was probably in 1894, with over 105 miners and
prospectors then at work. Old
workings may be found throughout the line of hills of which Mt Drysdale is
part, but probably most of them encountered very little or no gold.
Many of the outlying claims were abandoned by 1896-1897.
The significant production from the Drysdale gold field was almost
entirely limited to a group of several claims clustered upon a single
lensoidal siliceous ore system opened in 1892-1893 at Mt Drysdale (formerly an
unnamed peak in the Billagoe or Billygoe Hills).
Two principal companies developed mines side by side at the site:
Mt. Drysdale Gold Mining Company and New Eldorado Gold Mining Company.
The Mt Drysdale ore system, worked in the Mt Drysdale, Eldorado, and
adjacent leases appears to be a single silicification lens containing
pipe-like shoots of richer ore. The
area has a complex tenement history, with variation in the names, extent, and
controlling interests of the mines. References
to the Eldorado, El Dorado and New Eldorado mines are roughly synonymous.
The mines have been worked with extremely variable results, the ore
being at times poor and at other times sensationally rich.
Records are too incomplete to be certain of the ore system geometry but
there is believed to be a broad pattern of similarity to other ore systems
along the Cobar belt to the south. The
trend of mineralization is sub-parallel to bedding in horizontal section and
possibly cuts across it vertically, with bedding or foliation dipping steeply
west and ore steeply east. Drilling
has yet to fully clarify this. The
silicification also plunges steeply north.
The smaller rich shoots were apparently in left echelon arrangement.
The three richest near surface shoots were largely worked out by 1896.
Of these three shoots the northern two were in the Mt Drysdale mine
property and the southern one in the Eldorado mine.
The intervening Multum in Parvo mine occupied a relatively barren strip
across the siliceous outcrop. The
Mt Drysdale silicification lens lies close above the Devonian/basement contact
(Figure MDSS). This contact has
been regarded as an unconformity in the core of an anticline but the structure
could be significantly faulted. The
silicification extends about 250m along strike and has a mid-length
constriction. The flattened
pipelike lens is within a weak mineralization halo which continues for some
greater distance along strike. Associated
phenomena include an anomalous IP zone 1.2 km long and 300m wide.
Drilling suggests that this is a zone of disseminated pyrite and
pyrrhotite, locally with trace gold. Sulphide
dissemination may be mostly confined to the Devonian rocks but traces of
pyrrhotite, sphalerite, chalcopyrite and galena also occur below the
Devonian/basement contact (Getty Oil Development Co. Ltd, 1981).
The structures thought to localize the Drysdale gold field deposits are
little understood but may include a blind thrust within the Chesney Formation
(Glen 19xx).
Old workings extend along strike in both directions from the main
deposit. The New Eldorado South,
Jumping Frog, South Drysdale Amalgamated Tunnel Co., Parkers Spur Tunnel,
Little Drysdale and others lay to the south.
The Hard to Find, Morning Star, Silver King and others lay to the
north, with prospecting shafts and pits continuing along strike to the
northeast for up to 1.4 km. Most
of these workings appear to have had little or no production but some did
record gold and silver values up to 4 g/t Au and 7 g/t Ag (e.g. Rankin &
Party's mine). Very low but
definite values, including visible gold, are recorded from the workings as far
south as the Little Drysdale claim. No
consolidated plan of the old workings is now available.
The Mt
Drysdale mine area, like much of the hilly terrain in the Billagoe district,
was secured under mining leases by 1888-1890.
During 1892-1893 some 9.3 kg of colluvial and alluvial little-abraded
gold was recovered on the western slope of what was later named Mt Drysdale.
The alluvial claim (Robert McPherson's) extended along a gully
watercourse flowing directly downslope of the site of the later established
Mt. Drysdale No. 1 shaft. Much of
the gold was in bedrock crevices, with the largest particle recorded being 28
g. Working up-slope, prospector
David Drysdale came upon the primary deposit as a rich small leader carrying
plentiful gold. Development
followed immediately, with all the principal mines along the large
silicification lens being commenced in 1893.
The early holdings along this deposit (New Eldorado, Multum in Parvo,
Mt Drysdale, Hard to Find etc.) were variously amalgamated as mining
progressed. Three of the earliest
claims were amalgamated in 1893 to form the Mount Drysdale Amalgamated Gold
Mining Company. Next south was the
Eldorado claim, which was resurveyed in 1894 as the New Eldorado.
A narrow (4 m) strip left across strike between these two properties,
presumably the result of a surveying discrepancy, was taken up as the Multum
in Parvo claim. Of the principal
properties, the Mt Drysdale and the Eldorado (a.k.a. El Dorado, New Eldorado),
the Mt Drysdale appears to have been the richer or the better equipped.
It is mentioned as the area's most important mine in contemporary
accounts (e.g. Mines Department Annual report for 1899).
The Mount Drysdale Company later effectively absorbed the Eldorado
property, sometime in or before 1906, probably at first on tribute.
Later the two mines were worked conjointly, if not under single
control. Below a depth of 90m the
Eldorado property was worked mostly from the Mt Drysdale shaft.
The joint mining operation at the Mt Drysdale deposit employed 23
persons in 1910, shortly before operation ceased in early 1911.
Between 1911 and 1935 various parties prospected in the vicinity and
raised small parcels of ore, totalling about 600t.
This included work at the Lone Hand, Rankin & Party's tunnel,
Joseph Harvey's mine (M.T. 5), Bodkin's Jumping Frog mine (G.L. 70, within
former South Drysdale Amalgamated Tunnel Company area south of New Eldorado)
(GS 1984/070), Barton's mine, Just-in-Time mine, Mt. Drysdale West mine,
Drysdale Proprietary mine, etc.; for some of which the precise identities or
locations are now uncertain. The
last hardrock mining lease at Mt. Drysdale was cancelled in 1978.
Mineralogical notes indicate a number of silver minerals from
"Mount Drysdale", namely cerargyrite, pyrargyrite, iodyrite(?), and
iodobromite(?). These were
probably obtained at the main workings but confirmation is lacking.
One suspects that at times the term "Mount Drysdale" may have
been used to embrace the Billagoe mine as well in collection records.
The Mt. Drysdale workings extend to at least 183 m depth.
They include two narrow deep open cuts, with an intervening deep very
narrow stope along the mid-length constriction of the auriferous lens.
The northern open cut measures 50 m x 10 m by about 25 m depth.
The southern one (New Eldorado) is 38 m x 15 m and about 40 m depth.
Below the base of oxidation (36-50 m) there is considerable
disseminated sulphide, locally up to 10%.
The sulphides are dominantly pyrite, with lesser arsenopyrite and
traces of base metal sulphides. In
drill cores from holes passing through the east-dipping (85o) basement
contact, the underlying Girilambone Group rocks also contain sulphides.
These are pyrite and pyrrhotite, associated with quartz veining.
The sulphide content within the silicified Devonian beds appears to
show an inverse correlation with the intensity of silicification.
The
extension of the silicification lens as far as the Hard to Find Shaft is not
readily confirmed from available records, but the silicification lens is quite
apparent over the Eldorado-Mt Drysdale length of workings.
The silicification consists largely of chert or "elvan".
As Andrews (1913) noted, the "Indicator" rock type of Mount
Drysdale resembles the elvan of the Big Lode at New Occidental mine.
It has been suggested that a lenticular body of fine-grained sediment
within the Mt Drysdale Conglomerate has been silicified, the silicification
also extending less strongly into the surrounding conglomerate for a short
distance. A quite different
alternative suggested by McClatchie (1984) is that a volcanic pipe or fumarole
feeder existed here in the Devonian but there is little evidence for such an
interpretation. The
"elvan" preserves little textural evidence of its parentage.
The silicified rock in places shows microscopic fine stellate
tourmaline growths, carbonate replacements of a mica mineral, and scattered
occasional quartz sand grains remnants from the replaced rock.
The conglomeratic facies surrounding the high silica lens comprises
alternating intervals of pebbly sandstone and conglomerate with deformed
elongate cobbles up to 35 cm long in a chloritic sandy matrix.
Brecciated zones are prominent (Amax drill hole DDH B1).
The
richest gold leaders, such as ones being followed in 1893-1896 and again in
1899, carried visible gold (including "lumps" showing in the drive
face) and varied from a mere thread to 0.4m width.
They produced rich ore of grades ranging from 185 to 482g/t Au (over
0.6m), and occasionally yielded very rich spot assays (e.g. 918 g/t Au;
exceptionally to 14.2 kg/t Au, 7.39 kg/t Ag).
Where of a distinctive appearance (e.g. black pyritous ore) the
material giving phenomenal assay values was seen to be in masses up to 15 cm
across. Rich "bunches of
silver glance", carrying gold, were found between 60m and 120m depths.
The first 127t of ore averaged 318 g/t Au.
In the Mt Drysdale mine the main shoot was about 15 cm wide near
surface and widened to 1.5m by 38m depth.
The shoot was almost vertical down to 24m and thereafter assumed a
steep northerly plunge to 49m. There
it appeared to terminate although reports as late as 1900 do refer to rich
stone as deep as 99m. Development
at the northern end of the northern open cut also appears to have followed an
orebody plunging north, and produced small amounts of very rich ore (240-300
g/t Au). The northerly plunge of
the gold shoots is roughly parallel to regional lineation.
A study of pebble elongations in the conglomerate gave an overall pitch
of 80oN (360oT) (Glen 1985). In
the Mt Drysdale mine another rich shoot, 10-45 cm thick, was encountered at 84
m depth and was followed to 140m depth.
In the early years, when bagged ore was sent to Sydney, South
Australian and Queensland for treatment, grades below 25 g/t Au did not pay to
work. By 1896-1897 on-site
treatment facilities has been established and the lower grade ore could
subsequently be extracted in glory hole development.
Relatively low grade widths as great as 12m within the silicification
lens were being extracted by 1897. Details
are uncertain but the mass of the cherty siliceous rock appears to have
yielded above 7 g/t Au around the middle of the lenses, declining outwards to
2-5 g/t Au near the periphery. The
tailings at Mt Drysdale were retreated between 1938 and 1948, by Blackman,
Armstrong and party for a further return of 37 kg Au.
During the same period minor silver production by J. Harvey was
recorded.
Drilling
results suggest that the Mt Drysdale silicification pipe approaches the
unconformity with depth. The pipe
is broadly conformable with the host sequence in plan, unlike the small poorly
mineralized silicification lens in the conglomerate 1 km to the southwest
which is clearly discordant. However,
the orientation of the Eldorado open cut lode is 026oT/85oE, whereas nearby
foliation is 037oT/85oW (Roberts, 1985). Rankin
and Party's mine obtained trace gold which followed the foliation direction
(ca. 035oT). Visible gold is also
recorded to lie along joints, (e.g. 360oT) further south in the Jumping Frog
mine (GL 70, Par. Moquilambo). The
record of gold along joints, and the phenomenal precious metal grades obtained
near surface in the Billagoe and Mt Drysda1e mines, is suggestive of supergene
enrichment. The bulk of the very
rich stone appeared to give out by 49m depth in Mt Drysdale mine, and by 37m
depth at Billagoe mine. However
some further rich ore (with veinlets of almost pure gold) was later won below
83m depth in the Mt Drysdale mine. This
deeper ore shoot had a southerly plunge, in contrast to the three rich shoots
followed at higher levels.
3. BENOWA-KENDI AREA
The
deposits in the Benowa-Kendi area are:
No.
Deposit Name
Commodities
8
Benowa prospect
Zn, Pb,
Cu
9 Rough Lizzie prospect
Zn,
Pb
10 Hill shaft
Cu,
Pb
46 Clarke prospect
Au (Zn, Fe)
47 Cougar ironstones
Fe
(Zn)
48 Unnamed
Zn, Cu, Pb
49 Kendi prospect
Zn, Pb, Cu
50 Leslie mine
Cu
(Au, Pb, Zn)
In
the Cobar belt to the north of Cobar, mineralization within the Great Cobar
Slate is generally weaker than that within the CSA Siltstone.
Weak mineralization occurs within Great Cobar Slate at the northern end
of the Benowa-Kendi area (Rough Lizzie and Hill shaft prospects).
These prospects probably lie close to the faulted contact with the
Chesney Formation, or partially within it (as does the Leslie mine to the
south). The northern
mineralization is associated with geochemically anomalous trends.
Utah (Gardner and Colliver, 1975) found that these trends reflect a
"quartz reef alteration zone (QRAZ)".
This zone is up to 55m wide, strikes parallel to cleavage, and contains
individual quartz reef outcrops 1-3m wide.
Many percussion holes have been drilled along this zone, following up
geochemical values from surface sampling and bedrock drilling which range up
to 2500 ppm Cu, 4800 ppm Pb, 4900 ppm Zn, 6750 ppm As.
One drillhole intersected 18m of 1700 ppm Cu.
Gold values are below 2 ppm and mostly negligible.
Benowa Prospect
The Benowa
prospect host strata lie high in the Great Cobar Slate, near the transition to
CSA Siltstone. Drilling has
demonstrated several zones of quartz veining with associated weak
mineralization, within a broader zone of disseminated pyrite.
Values are up to 0.58% Zn, 1050 ppm Cu and 1300 ppm Pb.
The best intersection is 2.6m of 0.5% Zn.
Silicification and chloritization is associated with the mineralization
and largely obliterates all sedimentary features.
The quartz veins are mostly parallel to cleavage.
Kendi Prospect
The Kendi
prospect area has mineralized quartz veins, possibly concentrated along
shearing. The veined and
silicified intervals (4-6m thick) carry low base metal values (e.g. 800 ppm
Cu, 670 ppm Pb, 0.7% Zn). The ore
minerals occur mostly within quartz veins but stringers of
pyrrhotite/sphalerite also occur. Values
range up to 3.4% Cu, 6.4% Pb, 40% Zn, 270 ppm Bi, 105 ppm Cd, and 120 ppm Ag.
The best intersection is 4.1m of 3.1% Cu and represents sugergene
enrichment. The mineralization
hostrocks are situated low in the CSA Siltstone, and some may be in the Great
Cobar Slate.
The system
of weak mineralization at Kendi prospect is dimensionally comparable to that
of the CSA mine, with zones to the east comparable to the QTS and D orebodies
at CSA. It can be considered that
Kendi prospect is the top of a more deeply buried CSA-like system, or
alternatively represents much less intense mineralization regardless of the
level of exposure.
Leslie Mine
The Mines Department Annual Report for 1911 (p.93) makes passing
mention of the Leslie mine and then states: "Some 250 tons of ore were
raised...". This may be in
error as there is no other indication of ore being obtained in the Leslie
mine. Later reports refer to it as
still being in the prospecting stage. The
mine is located at a 15m wide zone of gossanous quartz veins containing copper
values up to 1300 ppm. The veins
are within the Chesney Formation, close to the faulted contact with the Great
Cobar Slate. Sparsely scattered
pyrite subhedra (1-2.5 mm) occur in the hostrocks, mainly along cleavage
planes. Occasional sulphide
aggregates reach at least 3 mm diameter, and splashes of ore are recorded from
the mine workings. The sulphides
are mainly pyrite and pyrrhotite, locally accompanied by traces of
chalcopyrite or sphalerite. The
latter two are associated with sheared quartzose streaks (Milovanovic, 1980).
A thin (3m) zone of very weak supergene copper accumulation (0.5% Cu),
comprising carbonates and oxides, is known from drilling.
Faulting near the mine has been mapped as N-S and displaces the
NNW-trending contact between the Chesney Formation and Great Cobar Slate.
4. CSA - SPOTTED LEOPARD AREA
The CSA
mining locality, a low topographic rise (known as Elouera Trigonometrical
Station or CSA Hill) with promising gossanous outcrop, was named after the
nationalities of H. Cornish and his associates (Gibb and Conley) who took the
first leaseholding there. From the
latitude of the CSA mine south, ore deposits and prospects increase in
frequency, and it becomes convenient to define smaller groupings which more
closely reflect stratigraphic or structural alignments.
One of these is the CSA-Spotted-Leopard group, which lies within the
CSA Siltstone and along the same magnetic ridge as the Benowa-Kendi group.
Russel and Lewis (1965) referred to this alignment as the CSA Line, and
Andrews (1913) earlier referred to it as the CSA-Tinto Spotted Leopard line.
It is distinguished by its occurrence within the CSA Siltstone.
A distinct linear magnetic anomaly (the "Cobar magnetic
ridge") continues south from Spotted Leopoard mine and passes west of
Cobar. Some old workings and
former leases, and occasional base metals anomalism, are known along this
magnetic feature. Although no
distinct mineralization has been confirmed from drilling, the entire
ridge-like magnetic high remains prospective.
The CSA line lies 1500 m east of the axis of the Myrt Syncline.
Syngenetic theories of the 1970s prompted unsucessful search for
mineralization west of the synclinal axis.
The deposits in the CSA-Spotted Leopard area are:
No.
Deposit Name
Commodities
51
Block 19 prospect
Cu
52
CSA mine, Tinto, QTS Cu,
Pb, Zn (Pb, Zn, Ag)
53
Spotted Leopard
Pb, Cu, Zn
The CSA, with its adjacent settlement of Elouera, was an important
centre of mining during the Great Cobar period.
In the latest and ongoing period of mining it was where the Cobar
copper industry was revitalized. Thus,
at the CSA mine there have been two distinct and long-separated periods of
development. Of these, which the
current one is by far the larger. Although
a small amount of ore was mined in the Great Cobar period, ending 1920,
production commenced in earnest in 1965. From
that time until the end of 1987, some 12.5 Mt of ore with a head grade of
1.7% Cu, 0.8% Pb, 2.4% Zn and 24g/t Ag have been treated to yield 193 000 t
Cu, 48 000t Pb, 211 000 t Zn and 169 000 kg in
concentrates. Production in 1987
was 887 000 t of ore with a head grade of 1.8% Cu, 0.5% Pb, 3.9% Zn
and 22 g/t Ag from which concentrates containing 14 993 t Cu,
1993 t Pb, 28 217 t Zn and 11 827 kg Ag were
obtained. The total remaining
resource to 11 level (a depth of 990 m) is about 5 Mt of similar
grade.
The gossanous outcrop was discovered about 1871, and the first
extraction was a ton of copper ore taken to Bourke from a small copper
carbonate vein at the site of the later CSA underlay shaft.
Although the depth of oxidation is great at CSA and there was no quick
ore discovery, the general prospects of the `CSA hill' caused such excitement
that numerous surrounding leases were taken up, totalling over 1000 acres.
Many of these were let lapse but in 1905, when the deepening CSA
prospecting reached the wonderfully rich supergene zone, a second rush of
lease applications occurred. In
this, a great number of leases aggregating 5198 acres were applied for in the
area, and the CSA locality became fully leased all around.
All the more important blocks (e.g. Block No. 19, CSA North, CSA South,
CSA Central, etc.) have since passed to the control of Cobar Mines Pty Ltd.
The lengthy phase of prospecting led eventually to successful mining
and smelting but that was abruptly terminated by a disastrous mine fire in
1920, and the collapsed value of copper after World War I.
The prevailing economic conditions ensured that there could be no
strong attempt by the early CSA company to re-open the mine after the 1920
fire.
The onset of a second period of mining at CSA in the 1960s marked the
revival of the Cobar field. Many
new ore bodies and entire groups (`systems') of ore bodies have been
discovered at CSA since the first period of mining and the known outline
around all the mineralization has been greatly expanded.
Of the `systems' now recognized only the shallowest of these, the
Western system, has surface expression. The
small open cut, pits in gossan, and the early underground workings from the
Great Cobar period (old CSA and Tinto mines) are all confined to the Western
system.
The CSA deposit was initially worked by two mines, the CSA and the
Tinto, and the surrounding vicinity was earlier covered by numerous mining
leases. Further east, beneath an
unproductive early shaft, the QTS orebodies are present.
Only a small fraction of the overall CSA ore deposit was known to the
initial CSA and Tinto companies, which mainly worked secondary enrichments in
western orebodies with some surface indications.
The CSA mined secondarly lead ore, with significant silver and gold
content, in 1905-1907. After
further successful underground prospecting in 1910, it recovered its first
copper, also from secondary enrichment ores (22.5% Cu average), in 1911.
The underlying primary ores first encountered, however, were so lean in
copper that the long term viability of the mine appeared doubtful.
At that time the CSA ore reserve was thought to be the largest of the
Cobar field, yet the mine appeared to have no future as a copper mine unless
it could join with the Cobar Tinto copper mine, on the adjoining Gardiner's
Blocks, where copper prospects were more firmly established, and where copper
had already been produced in 1907-1911 from enrichment zone ore (Andrews
1913). Below water level the
change to lean sulphide ore was typically an abrupt one.
The great depth of leaching (130-139m) had given rise to thin zones of
very rich supergene ores near the water table, which were profitable, but
neither mine alone was thought to be payable in the primary zone.
The CSA subsequently acquired the Tinto mine.
Further east the QTS zone and other parts of the CSA ore system are
later deep ore discoveries, of which the QTS zone was initially suspected as a
different (older) ore system merely lying in close proximity to the CSA.
Since the reopening and great expansion of the CSA mine, much
information has become available on the ores and their setting.
Information on the geological setting, mineralogy, wallrock alteration,
orebody distribution and composition, geomechanics, mining methods, and
various other aspects of the C.S.A. mine and its ore system is contained in
many unpublished and published reports. These
include the following: Besley (1966), Binns & Appleyard (1986), Bouffler
(1981), Brill (1988, 1989, 1991), Cobar Mines Pty Ltd (1965, 1972, 1979,
1981), Doe et al. (1990), Gow (1965), Kappelle (1970), Marshall (1991),
Marshall et al. (1981), McLeod (1973), Robertson (1968), Robertson (1974),
Russell and Lewis (1965), Sangameshwar and Marshall (1980), Scott and Phillips
(1990), Warburton (1977), Worotnicki (1977), Worotnicki and Alexander (1977).
Occurrences in the CSA-Spotted Leopard area besides the CSA ore system
itself are quite minor. They
comprise the Block 19 prospect, the Spotted Leopard shafts, and further very
minor prospects. The latter are
represented by old workings high in the CSA Siltstone northeast of the Water
Tower (2.4 km WNW of Cobar) where trace gold values have been obtained, and
fairly widespread weak metal traces reported in drilling returns.
Formerly much of the area was under small mining leases, with
negligible recorded result. Faint
surface indications in the area have been found to lack depth continuity.
One example is a drilled gossan outcrop west of the CSA mine (Singer,
1983). This carries surface and
near-surface values of up to 2600 ppm Cu and 3590 ppm Pb but below 10m depth
values decrease markedly to around 500 ppm for both Cu and Pb.
Zinc is uniformly low at 70-130 ppm Zn in this gossan.
The strata of the CSA-Spotted Leopard area, belonging to the CSA
Siltstone, are thin bedded and rhythmically banded siltstones containing a
fine to medium grained turbidite sandstone beds ("greywackes") at
irregular intervals. Over
intervals of 2-15 c beds may gradem from fine grained sandstone at the
base to slate at the top. The
graded beds are often carbonaceous at the top.
Sandstones, which occur in beds up to 1 m thick, constitute less
than 5% of the rock mass. This
proportion increases westward towards the Biddabirra Formation, the base of
which lies about 500 m stratigraphically above the mineralization at CSA
mine. Tuffs are believed to occur
in the sequence and if they could be characterized, perhaps by trace element
studies, might prove useful for correlation.
One 30 m thick, structureless cherty bed exposed about 200 m
west of the workings (C.S.A.) consists of cryptocrystalline quartz, feldspar,
and sericite. Robertson (1974)
considered this chert a likely ash fall tuff, and believed it to be
recognisable for a strike length of 20 km.
In view of the recognition of similar rock types to the north, at Elura
mine and elsewhere, detailed study of suspected ashfall layers should be
useful.
Block 19 Prospect
The Block
19 shaft, formerly one of the CSA North holdings, is situated 500 m
north-northwest of the CSA mine alongside a 100 m long strip of ferruginous
siliceous sediments and minor gossan. It
is likely that this aligns with a weakly mineralized zone west of CSA mine,
containing the Western Gossan and a western pyritic orebody which is unmined
(Cobar Mines Pty Ltd, 1985). Gossan
samples from this zone west of the CSA mine have returned moderate values for
Cu, Pb, Zn, Sb. The Block 19 shaft
intersected trace copper carbonate staining but no depth extension of the
"gossan" has been found from drilling.
CSA Mine (Cornish, Scottish & Australian Mine)
(Incorporating
CSA North, CSA South, CSA Central, Tinto, and QTS)
CSA
mine - small winder (old main shaft), ventilation outlets and later main hoist
shaft
The
mine in 1966.
The
CSA mine is situated at CSA hill 12 km north of Cobar (14 km by road along the
Louth Road), and gossan was
discovered on this hill by 1871 shortly after the foundation of Cobar around
the Great Cobar mine. The CSA
has been the major Cobar copper mine since the closure of the Great Cobar
mine. It has in many years been
the major copper producer in New South Wales.
It
was in 961 that significant deep resource was defined by a BHP subsidiary,
leading to underground mining commence on a much larger scale in 1965. In 1980
CRA acquired the mine. It was run as Cobar Mines Pty Ltd (CMPL).
When CRA withdrew from Cobar it sold the mine to Golden Shamrock Mines Ltd in
1992. Golden Shamrock on-sold it to Ashanti Gold Fields in
1996. Finances collapsed in the mid 1997 Asian Financial Crisis
and the in January 1998 CSA was in receivership.
Glencore
International AG. Glencore, one of the World’s largest commodities traders
based in Switzerland, acquired the mine from receivers and re-opened it in
1999 under the name Cobar Management Pty Limited, retaining the old 'CMPL'
designation.
The
CSA mine continues mining
base metal, mainly copper, ores at a rate of about 1,200,00 tonnes per
annum. The sulphide concentrates, containing approximately 29% Cu, are
sold to smelters in India, China and SE Asia. In 2009, the CSA Mine
recorded its highest ever output. That year it produced over 182,000
tonnes of concentrates containing in excess of 51,000 tonnes of copper metal,
and 400,000 ounces of silver. The mine workforce is currently
(2010) about 350 personnel.
There have been
two major phases of mine development. In the initial phase mining of oxidised lead and copper ores progressed
intermittently until a disastrous underground fire caused closure of the mine in 1920.
Ore raised totalled 114 000 t during this period.
Up till 1920 there was produced some 4,640t Cu, 4,047t Pb, 2,972 kg Ag.
After the
fire, the mine then fell dormant and saw little further activity until the
1960s. From 1965 to 1990, the
yield has been 223 746t Cu, 53 947t Pb, 234 529t Zn, and 190 509 kg Ag from 13
823 830t ore mined. Proved and
probable ore reserves of the CSA mine at present (1990) are 5.0 million
tonnes. Estimates of overall metal
grades have varied as further orebodies were discovered, and have been in the
following ranges: 1.5-2.0% Cu,
0.5-1.0% Pb, 2.5-4.5% Zn. The ore
reserve estimate to a depth of 100 m at the commencement of mining in
1965 was 18 Mt at a grade of 2.7% Cu, 0.6% Pb and 2.0% Zn and was based
on 22 drill intersections. Later
discoveries increased the estimated size of the overall CSA ore deposit to 30 Mt
or more.
The CSA mine
has a long history as steady producer with little sign of being exhausted
although continued discovery of new orebodies, although costs increase as the
mine deepens. This continued until mine ownership changes and cessation
of operations at the CSA mine, which occurred in early 1998. Operations
were recommenced in March 1999 but uncertainties still remained. The
mine produced a little over 2,000 tonnes of copper metal in 1998-99. The
closing of the CSA was followed by a slump in confidence and employment and
around that time over 300 people left the community.
Initially the original CSA property was
quite small. Over time it grew to
encompass the old CSA and Tinto mines, the Block 19 and Gardiner's Block
propsects, and other tenements. New
orebodies were discovered which extended the known assemblage of related
orebodies, particularly to the east. In
1960 diamond drilling by Cobar Mines Proprietary Limited confirmed earlier
indications that the C.S.A. deposit contained two major lode systems.
These are the "Western system" which reached the surface and
had been mined in the early days; and
the copper-bearing "Eastern System" lying 150 m to the east, and
which does not crop out. The most
eastern mineralized zone is named from the old QTS shaft (Queensland, Tasmania
and South Australia propsect). It
was accepted into the CSA family of orebodies only after some debate on its
origin.
Numerous sulphide
bodies occur at CSA, varying from vein complexes to sub-massive and massive
lenses, all hosted in thin-bedded rhythmically banded sediments of the CSA
Siltstone. The ore system was
early determined to lie within the imbricated hanging wall of a fault zone
(Footwall Fault of Barton, 1977) and just north of west-northwest trending
beds in the short limb of a sinistral south-plunging fold.
The spatial relationship of the ore deposit to a sinistral warp still
dominates structural considerations, although further work has produced a mass
of structural data which is capable of interpretation in a variety of ways and
is suggestive of a complex history. The
sulphide bodies, variously termed orebodies or shoots by different writers,
are grossly transgressive to bedding and are subparallel to regional cleavage
(S1). An orebody may comprise a
nexus of mineralized veins that run approximately parallel to the principal
north-trending, steeply east-dipping cleavage.
Both dilational and replacive veins are present some of the ore is
massive in places. The orebodies
are often 60-120m in length, and may sometimes be correlated through weakly
mineralized blank segments for lengths in excess of 300m.
Typical width is about 10m but well mineralized intervals as great as
90m are known. Boundaries of many
orebodies are indistinct but often the footwall shows greater shearing than
the hanging wall and may thus appear to be more distinctive.
The lenticular orebodies or shoots sometimes plunge parallel to the
down-dip extension lineation within cleavage.
As ore margins are usually gradational, stope designs are based on
grade and structural data.
The main minerals in the ores are chalcopyrite, pyrite, hexagonal
pyrrhotite, sphalerite and galena, with minor amounts of bismuthinite,
galenobismutite, boulangerite, stromeyerite, arsenopyrite, clausthalite,
tetrahedrite, bismuth, stannite and retrograde orthorhombic cubanite and
mackinawite. Monoclinic pyrrhotite
occurs as an alteration product along grain boundaries and fractures.
Brookes (1974) defined a number of ore types in the CSA mine according
to sulphide assemblage, fabric, and host rock association.
The sulphides are accompanied by quartz, carbonate and chlorite gangue
minerals. Quartz and carbonate
veins and segregations, often with sulphide minerals, occur throughout the
mine but are more abundant in the mineralized shears which carry the orebodies.
Most veins are subparallel to cleavage but many occur in other
directions, sometimes in random or stockwork pattern.
Clasts of variably silicified host rocks, are common in the orebodies.
These are sometimes rather rounded and transgressive "pebble
breccia" zones are present in the ore systems.
At CSA mine many ore lenses contain different ore types that grade into
one another. Nonetheless, Kappelle
(1970) broadly characterised the ores into copper and copper-zinc types.
The copper ores have an abundance of pyrite, pyrrhotite, and
chalcopyrite with only minor quantities of iron-rich sphalerite (marmatite)
and galena. The copper-zinc ores
consist mainly or pyrite, marmatite, galena, pyrrhotite, and chalcopyrite.
Copper-zinc ores generally have a higher total-sulphide content and
tend to be more deeply affected by weathering.
The copper ores are lower in total sulphide content and have a large
proportion of siliceous gangue. The
ore lenses are not necessarily made up of only one ore type, and some vary
considerably with depth. One
cluster of ore bodies is sufficiently dominated by copper-zinc ore to be
termed the "CZ orebodies", but even one of these grades downwards
into copper ore. The copper-poor
ore bodies are typically smaller than the copper-rich ones, which is a
tendency well known elsewhere along the Cobar belt.
Averaged metal contents over some years showed 2+% Cu in copper ores;
and about 4.5% Zn, 1.5% Pb, and 1% Cu in the copper-zinc ores.
Individual copper lens intersections may be much above this 2+% Cu
average, e.g. 3m of 7.5% Cu, 0.7% Pb and 1.3% Zn in primary deep ore (QTS
Central). The blend of ores mined
has varied from time to time in accordance with ruling metal prices.
Wallrock alteration is moderate and similar as in other parts of the
Cobar belt. A broad chloritization
halo surrounds the ore zones, and ti may extend outwards from ore for up to 50 m.
It is more or less coincident with a zone of reduced muscovite content.
Robertson and Taylor (1987) noted in the alteration halo significant
depletion in a number of elements, particularly lithium, sodium, potassium,
rubidium, strontium and barium. Most
of these effects extend for 10 to 20 m around the ore, but the low
lithium and barium zones appear to be much wider.
As at other Cobar belt mines, silica enrichment is prominent at C.S.A.
Typically a broad zone of weak pervasive silicification surrounds
mineralization. The Western System
is characterised by patches of more intense silicification
("elvan"). Although
elvan and ore are intimately associated in many places in the Western System
they are by no means co-extensive. Black
chlorite and talc are also common. As
they are associated with linear zones and sometimes display a massive
(undeformed) fabric, they seem likely to be of subsequent formation to the
broad alteration effects.
There appears to be a constant-vergence small angular discrepancy
between the orebodies and S1, reminescent of the New Occidental deposit.
Marshall & Sangameshwar (1982) interpret such a relationship as a
compressed angular discordance between feeder orebodies and bedding,
progressively overprinted by cleavage (S1).
Foliation and banding in the ores is generally parallel or subparallel
to cleavage, although some laminations and disseminations also reflect
bedding. Brill (1989) concluded
that mineralization commenced early in the deformational history and continued
into a late-kinematic stage. If
so, overprinting of the mineralization by successive deformation can be
expected to have produced complex paragenetic relationships.
Brill (1989b) presented a paragenesis with many ore minerals beginning
deposition during early metamorphism, sphalerite and galena being deposited in
two separate phases of which the later is late synkinematic, and only two ore
minerals (cubanite and mackinawite) being solely post kinematic or retrograde.
A late tectonic mesoscopic "Durchbewegung" ore fabric, being
a chaotic mixture of ore clasts in matrix, is sometimes present in proximity
of black chlorite shear zones which show crosscutting relationships with S1
and earlier Cu-mineralization. Brill
(1989) described different vein systems. All
of these, including tension veins and late shear veins, are mineralized, and
this supports an extended period of mineralization.
The orebodies were early believed to be clustered, en echelon, as
elsewhere along the Cobar belt (e.g. Great Cobar), but increasing knowledge of
the ore pattern has diminished this view.
Mineralisation occurs in numerous vein complexes and sub-massive to
massive bodies, all of which have been locally called orebodies or lenses.
These are depicted in Figs xx and xx.
They have been are grouped under the following names:
Wetern System, CZ orebodies, Eastern System, QTS North, QTS Central and
QTS South. The Eastern System and
QTS North zones have ncopper dominant, whereas the Western System, QTS Central
and QTS South zones contain both copper and zinc-lead, separately and in
combination. The CZ (copper-zinc)
zone contains zinc rich orebodies with very minor lead and copper.
The Western System contains the orebodies first encountered and mined.
It was suspected from early times that the initial workings lie at the
western side of the overall deposit. Since
1910 or earlier, the emphasis of prospecting has been on search to the east of
the "lead stope" and other early CSA-Tinto workings.
This was prompted to some extent by surface indications.
Early cross-cutting to the east had swift success in finding further
ore. No great further extent of
mineralization eastwards was anticipated until underground disoveries diamond
in 1912 gave encouragement to expect multiple orebodies.
The Western System which contains the early workings is relatively poor
in copper and the admixed zinc-lead was very deleterious to its recovery.
Hence in 1911 it was still doubted that the CSA could ever be worked as
a copper mine. In 1912, however,
further discoveries of improved copper ore to the east were made.
It was then correctly predicted that the CSA would become one of the
future big mines of the Cobar district.
The old workings of the early CSA and Tinto mines (Great Cobar period)
are confined to the Western system. From
the Western to Eastern systems, orebodies are found over a strike length of
400m and a width of about 250m. The
separation of the mineralization into Eastern and Western systems is clearest
at about 200m depth. The
distinction becomes less clear at lower levels, where intervening orebodies of
Eastern system type approach to near the hanging wall of the Western system.
Continued drilling added a third group of orebodies, the D Zone,
located some 120m east of the deep Eastern system.
Later inclusion of the QTS zone increases the area containing
mineralization in plan to around 1000 m long and 600m wide.
The mineralization has been drilled to depths in excess of 1000m down
plunge. Although the mineralized
shear zones are themselves readily traced, the outlines of individual ore
masses within them are often not clearly defined, except that most are
subvertically elongate. The plan
of the ore bodies thus varies from level to level.
On the 830m level for example, fifteen minable lenses occur in six
shears, averaging 12m in width and 50m in length.
As is the case for other systems near Cobar, the orebodies trend
roughly north-south, plunge steeply north and dip steeply (e.g. 85o) to the
east (figure CSAVTS). The broad
ore zones themselves behave similarly as their component orebodies.
They dip to the east between 70o and vertical, and plunge northerly at
about 80o. This was for some time
uncertain in respect of the QTS zone when it had been little drilled.
Being steeply plunging lenses of Cu-Pb-Zn mineralization within zones
of strong shearing and alteration in fine-grained hostrocks, the orebodies are
similar to those elsewhere in the Cobar belt.
The structural features at CSA have been studied by Kapelle (1970) and
other geologists. The main
structural features and their relations to mineralisation are shown in Figs xx
and xx. In general the sedimentary
sequence faces west and has an average westerly dip of 75o, although dips can
vary from about 50oW to 80oE. The
ore lenses are hosted in roughly north-trending quartz-enriched chloritic
shear zones, which are broadly conformable to cleavage but in a few places
strongly cut across the extrapolated trends of bedding and cleavage.
The CSA cluster of grouped ore lenses, or ore systems, appears to be
located in a zone of southeast trending cross-structures (Scott and Phillips,
in prep; Robertson 1976, Glen 19xx). Sinistral
deflection of the Great Cobar Slate-Chesney Formation contact is reported in
this zone. At some levels the ore
systems are distinctly concave eastwards in plan, and coupled with swings in
bedding trends in the southern part of the mine area this too gives strong
impression of broad sinistral deformation.
The overall flexure or warp of bedding at CSA is such that strata from
the western side of the Western system may pass between the No. 1 and No. 2
shafts to continue southwards and lie at least as far east as the Eastern
system (e.g. No. 5 level). This
pronounced sinistral warp or monoclinal flexure displaces the strata over 200m
east-west in the space of about 100m north-south.
The warp of broad monoclinal flexure may be vertical.
The axes of individual bands have not been traced between mine levels
but the general pattern appears to persist subvertically, perhaps with steep
southerly plunge. Conjugate
north-east trending dextral flexure also occurs and overall orebody
arrangement suggests a right echelon pattern.
Throughout the mine, bedding generally dips steeply west and faces
west, whereas cleavage dips steeply (75-85o) east.
On some mine levels (e.g. 1200 level) weakly sinistral patterns of
bedding trend swing can be interpreted, albeit with considerable extrapolation
between observation points; and these bedding swings may steeply transect the
more uniform north trending cleavage. The
hinge lines of the some folds plunge southward, and a strong extension or
down-dip lineation plunges very steeply within the cleavage.
The majority of faults, shear zones and orebodies follow the main
cleavage. The ore zones are
lenticular and are controlled both by shearing and foliation.
The subvertical monoclinal flexure is clearly cross-cut by several
prominent shears which consist of black chlorite in 1 to 2 m wide zones
dipping steeply to the east. Small
parasitic drag folds with subvertical axes occur adjacent to the shears.
Typically in any ore lens a large number of the quartz-sulphide veins
lie subparallel to cleavage or foliation.
As generally the case along the Cobar belt, cleavage intensifies in
close association with ore and bedding is obliterated.
Cleavage and bedding are both prominent in unaltered rocks but within
about 10 m of the orebodies cleavage is very strong and bedding virtually
absent. In the vicinity of
mineralisation cleavage is commonly curvilinear to anastomosing.
The cleavage averages 009ostrike and dips 80o east.
According to Kapelle (1970) the main axis of folding plunges at 70o to
164o. A second period of
deformation weakly deformed the cleavage according to Kappelle, with the
direction changes in the resultant foliation being favoured loci of
mineralization.
In addition to the principal structural features mentioned above, there
is recorded a great amount of structural detail.
A major low-angle defect set is oriented normal to cleavage, with a
strike of 340-360oT and a dip of 14-25oW.
It may be similar to the "flat-makes" recorded elsewhere
along the belt, and along which supergene or earlier growth-dilational
processes are sometimes seen to have been active.
Bedding shows a sinistral swing across cleavage in the southern part of
the mine. Elsewhere it generally
strikes 340-345oT and dips steeply (80o) west.
It is occasionally overturned. To
the south the bedding swings to 330oT and trends easterly into an area with
cyclindrical fold axes plunging 70o to 150-165oT.
Elsewhere, other small scale flexures plunge 50-90o in south to
southwest directions. The folding,
which predates latest cleavage, suggests a sinistral or left-lateral shear
couple. However, departures from
all these general trends are not uncommon, especially within the shear zones
of the Eastern system. Cleavage is
generally quite constant but may be flexed into open folds with axes striking
75oT. Although there may be a main
down-dip cleavage lineation which plunges similarly as orebodies, as commonly
reported from other Cobar belt mines, the situation is not clear-cut.
Lineations occur on many surfaces and indicate movements in many
diverse directions. The lineations
are often roughly normal to the low angle joints.
If there were a strict relationship then the sub-horizontal defects
could be older than simple unloading tension joints, and might have some
relationship to the "dilational layering" deduced at Elura.
In support of this it is observed that the sub-horizontal fractures may
contain black chlorite unfilling up to 100mm thick, and these fillings may be
continuous over distances of 30m or more.
Robertson (1968) also recorded shallow (10o) south-dipping lineations
which occur on both cleavage and bedding planes.
The significance of this on other detailed structural data from the
mine is not clear but complex episodic movements seem likely, such as have
been postulated from other parts of the Cobar belt (e.g. the Peak area).
There is relatively
minor surface expression of the extensive CSA ore systems.
Only the Western system in part reaches surface.
There gossans with native copper, cerussite and minor anglesite occur
on a small hill, and are surrounded by elongate siliceous ironstained
alteration aureoles. Sinking began
as two mines, the CSA and later Tinto, a short distance along strike to the
south. Gossan near the early CSA
shaft contains prisms of cerussite, specks of native copper, occasional
anglesite, trace leadhillite(?), and rare cerargyrite.
Despite such excellent indications at and continuing below the surface,
shaft-sinking development progressed slowly over many years before rich
supergene ore was met with. The
leached zone contains only minor amounts of secondary ore minerals,
concentration of these being limited to a thin zone near the water table.
Prospecting at the CSA mine was commenced in 1871, shortly after Great
Cobar, but lacking similar near-surface economic ore as the latter, the CSA
mine was much slower to be developed. Brown
(1983) and other state that limited production commenced in 1871.
This seems unlikely, and figures are available only from after 1904.
The Great Cobar Company partook in the exploration at CSA in the late
1880s and at that time reported no payable result.
By 1899 there were several shafts in the vicinity and work from the
shaft on the main gossan zone proved a wide leached lode with a little gold
throughout and occasional cerussite veins in porous gossan.
Strong
secondary enrichment below the leached zone occurs at CSA between
137m and 147m, and was reached in 1904-1905.
The rich supergene ore was tested to water level in 1906, a sizeable
orebody proved and an adequately capitalized company formed.
The mine then commenced more rapid expansion and soon employed 50 men.
Early production came chiefly from low wide stopes in the high grade
enrichment ores. The Tinto mine,
also referred to as the Rio Tinto, commenced in 1905, was acquired by the CSA
company in 1913, and proved to be in the same ore zone now known as the
Western system. Beneath the lead
carbonate cap at CSA an enriched copper sulphide orebody was entered
(140-147m). Significant annual
tonnages of Cu-Pb-Ag ore were mined. A
smelting works was erected in 1913 and expanded in 1917.
The lead carbonate zone (137-139m) at CSA yielded at least 9355t of 35%
Pb ore. Large parcels of lead ore
railed to eastern smelters ranged in richness up to 50% Pb.
This ore was poor in copper, rich in silver and also auriferous.
Native silver was abundant in some parts of the capping.
The underlying secondary copper enrichment zone probably contained
about 155,000t ore, which averaged 4.7% Pb and 7.1% Cu in extensive sampling
done by Godfrey (1916). Similar
copper ore (6-11% Cu) was produced from the Tinto mine.
Below this, at or near water-level, the primary ore was encountered.
It consisted largely of massive pyrite and pyrrhotite enclosing
unreplaced slate, with subordinate fine-grained sphalerite, galena and
chalcopyrite. Pyritic ore at the
170m level averaged 0.4% Cu, 5.1% Pb and 15.7% Zn (Godfrey, 1916).
This Zn-Pb pyritic mineralization was considered to continue south to
the Tinto property, where the gossan zone was succeeded below by rich masses
of chalcocite. The known primary
ore zone width in both mines was then less than 6m.
The primary zone was little worked in the Tinto mine.
During
much of the Great Cobar period the CSA was not generally foreseen to become a
copper mine equal to the Great Cobar. It
had rich secondary lead and copper ores but below these the CSA-Tinto primary
ore first known was pyritic basic ore far richer in Zn-Pb than Cu.
The zinc content was a major impediment to smelting for contained
copper values. The success of the
first CSA company was initially in doubt, as it depended on the existence of
sufficient copper sulphides to form matte in blast furnace treatment.
Early interest in the primary ore was as potential basic iron sulphide
flux much needed by Great Cobar Ltd. Early
ore from CSA and Tinto mines was smelted at the Great Cobar works.
However, parcels evaluated at Great Cobar contained up to 20% Zn-Pb.
This was quite inappropriate for furnace treatment and Great Cobar Ltd
consequently declined an option to purchase the CSA mine at a low price.
The first CSA company, however, did survive and came to employ up to
about 200 men. It had a successful
smelting run over the period of raised copper price in WWI.
It smelted a blend of rich secondary ore and primary basic ore.
Peak annual production was reached in 1918 with 55,000t of ore treated.
With the 1920 collapse in copper prices, and consequent closure of the
Great Cobar Company mines, the CSA and its smelter became the focus and hope
of continued copper mining in the Cobar field.
This hope was lost when an underground fire broke out near the main
shaft and led to mine closure. The
CSA in 1919 had fair remaining reserves of high grade ore, having reached the
supergene zone much later than the mines of the Cobar "central
area". The CSA, Gladstone and
Chesney mines were in continuing production in March 1920 when the underground
fire broke out at CSA. This fire,
which continued to burn underground for ten years, and smoulder for a further
fourteen, burnt out and collapsed the main shaft down to water level (152m).
It brought copper mining in the entire district to a halt, as the
prevailing low copper price would not repay long distance transport to Port
Kembla or elsewhere. The cause of
the 1920 fire is unknown but could have been spontaneous heating.
The new CSA mine also suffered a fire (No. 1 shaft, 1980) but this was
not severe and caused only developmental delays.
Based largely on CSA mine, considerable research has since been carried
out on the oxidation reactivity (incendivity, explosiveness, etc.) of sulphide
ores and dusts of the Cobar field.
Although
CSA orebodies cluster into two major zones, the Western and Eastern systems,
other additional bodies occur still further east and west of these major
zones, as well as between them. A
small western pyritic orebody, occurs well to the west of the Western system
and a larger group known as D Zone lies east of the Eastern system.
Further east, with a long time uncertain relationship to the main CSA
systems, occurs the QTS zone of mineralization.
Initially this was thought to be locally parallel to bedding, and to
hence comprise a distinctive zone dipping in an opposing sense to the other
orebodies. Such fundamental
distinction is not supported by later knowledge.
Small
parcels of CSA ore were forwarded to Port Kembla in 1946-1948.
In 1947 Enterprise Exploration Co Pty Ltd commenced work toward
bringing the CSA mine back into full production.
Underground rehabilitation, minor exploratory development and diamond
drilling was commenced by 1949 and continued to 1957.
This drilling discovered the Eastern system of mineralization, beneath
a strong magnetic anomaly. The
leases were transferred for an interest in Cobar Mines Pty Ltd in 1957 and in
1960 deep drilling recommenced. The
Western system was drilled to 945m and the Eastern System to 762 m depth prior
to new shaft sinking. An
exploratory/return airway shaft (No. 1) was commenced in 1962.
A production shaft (No. 2) was collared soon afterwards, and production
commenced in 1965. The mine is
served by two shafts, 998 m and 1026 m deep.
The crusher and other ore handling facilities are located on 9 level
(810 m deep) and mining is proceeding between 7 level (630 m) and 9
level. Ore extraction is now
carried out entirely by long hole open stoping.
The Western and Eastern systems as delineated to date represent an
estimated 30 Mt of ore. From 1964
to 1984 the ore mined (10.4 Mt) averaged 1.6% Cu, 0.8% Pb, 2.2% Zn, 2.4 g/t Au
and 24.4 g/t Ag. The mine is
currently managed by Enterprise Metals Pty Ltd for CRA Ltd.
The No. 2 haulage shaft operates to 1000m and the old CSA shaft has
been extended to No. 4 level.
In both of the major ore systems the trends of the primary
mineralization in plan view are gently concave to the east on some levels.
This curvature may diminish at depth.
The Western system is in silicified and chloritized country rock cut by
quartz veins. The Eastern system
is in less silicified hostrock but contains a larger number of quartz veins.
Although run of the mine primary ore may average less then 5% base
metals, some of the individual sulphide veins are quite rich, containing up to
11% Cu, or 20% Zn, and/or 14% Pb. Generally,
the ore lenses in the Eastern, QTS North and QTS South systems shears comprise
chalcopyrite and pyrrhotite as splashes, narrow cross-cutting veins and
disseminations in a quartz-chlorite gangue.
Ore in the Western system and the CZ Lens zone comprises chalcopyrite,
galena and sphalerite associated mainly with banded pyrite.
Alteration in the Western system is characterised by pervasive cherty
silicification.
Both the
Western and Eastern systems contain lenses of varying metal contents.
Major orebodies in both the Eastern and Western systems are copper rich
but western stopes have been principally for copper whilst eastern ones have
been both for copper and copper, lead and zinc.
Although the mine's zinc production has often been somewhat in excess
of copper, copper was the major metal of interest when the CSA mine was
reopened. At that time many
millions of tonnes of copper ore had been delineated in reserve, at a
published grade of 3.5% Cu. In
subsequent mining, however, most copper stopes have not matched this expected
grade. Lead-zinc-copper lenses
occur between the main copper zones and also within them, especially in the
Western system. The Pb/Zn-rich
podiform lenses between the Western and Eastern copper zones, known as the CZ
orebodies, or CZ zone, contain intervals up to 3.6m wide which carry rich ore
(up to 20% Zn and 14% Pb). Average
grade is about 10% Zn, 1%Pb and less than 1% Cu.
These relatively short lenses are of varied composition.
Most are pyrite-sphalerite with smaller amounts of galena and minor
chalcopyrite. One (B orebody) is
this type higher up but grades downwards into pyrite-chalcopyrite ore.
The largest of the CZ lenses is a banded sub-massive to massive body up
to 40m wide. Gangue includes minor
talc in some of the lead-zinc-copper ore lenses.
The westernmost mineralization is a pyritic orebody, which has been
little mined and is distinctly separate from and west of the Western system.
This was encountered on the 171 m level, 91m west of the old CSA main
shaft. It is 30m long and 6m wide,
averaging 0.6% Cu, 1.7% Pb and 8.3% Zn.
The
Western system is up to 60m wide. It
consists of a relatively continuous zone of low grade mineralization, in which
are higher grade steeply north-plunging lenses which vary from copper-rich to
lead-rich, the latter being less abundant.
The plunge of the ore system overall is less steep than that of the
higher grade ore lenses within it. Copper
lenses consist of vein type pyrrhotite-chalcopyrite and sub-massive to massive
pyrite-chalcopyrite, often banded. Some
lenses contain both veins and quite massive ore (pyrite - chalcopyrite with
lesser pyrrhotite + galena + sphalerite in siliceous gangue).
However, the massive and semi-massive bodies are typically subordinate.
The mineralized lenses average 45m length, 7m width, and most contain
2.5-3.0% copper. Some are
distinctly more copper-rich, e.g. 11.3m width of 5.1% Cu.
Lenses which are copper poor tend to be rich in zinc.
Bodies of cherty silicification ("elvan") occur, being the
`flint' of Andrews (1913), and are 80% or more silica.
These are commonly cut by later quartz veining.
A mass of such "flint" is recorded to have impeded progress
of drive extension in the Tinto mine in 1911.
Angular to rounded patches or fragments of quartz, elvan, black
chlorite and chloritic siltstone are commonly enclosed by massive sulphides.
The western system is further distinguished by a strong and continuous
zone of shearing(?), rich in talc and chlorite, which forms its footwall.
The zone is complex and appears to be one of multiple deformation.
Within it, an ill-defined variable interval of particularly broken and
sheared rock, imbricated by talc-chlorite bands, is loosely known as the
foot-wall shear. The talc appears
to have developed from, or at least within, intervals of strong chlorite
growth. Bodies of talc, often
containing euhedral pyrite, are closely associated with black chloritic shear
zones. Although the talc-chlorite
footwall shear zone of the Western system is particularly noteworthy, its
features are not unique. Small
black chloritic shears, common throughout the mine, are typically curvillnear,
foliated, slickensided, branching zones consisting of closely spaced
alternating layers of siltstone and chlorite.
they may contains large amounts of sulphide, espeically pyrite and
sphalerite, and are sometimes close. Although
black chloritic shears are extremely variable in location and attitude it is
apparent that many ore lenses are bounded on, ore lcose to, their footwall
side by one or more such shears. The
black chloritic zones contain magnesium rich chlorite (Robertson, 1974).
The CZ zone mineralisation occurs in podiform lenses.
The largest begins at about 6 level (540 m deep) and extends down
below 9 level, whilst other lenses occur at higher levels between the Western
and Eastern Systmes. Average grade
is about 10% Zn, 1% lead and less than 1% Cu.
The largest lens of CZ mineralisation contains pyrite and sphalerite
with smaller amounts of pyrrhotite and galena and minor chalcopyrite, in a
banded sub-massive to massive body up to 40 m wide.
Gangue is black chloirte, chloritic siltstone, and quartz in angular to
rounded patches. The edges of the
ore lenses are fairly sharp, and the footwall often marked by black chloritic
shears.
The
Eastern system may be locally as varied as the Western system but copper
dominates overall. Run of the mine
ore grade is around 2.5% Cu from the Eastern system stopes.
This is slightly lower than from western stopes and the Eastern system
overall may be more weakly mineralized. Lenses
within the Eastern system vary from copper type (chalcopyrite + pyrrhotite +
quartz) to copper-zinc-lead type (pyrite + pyrrhotite + chalcopyrite +
sphalerite + galena). Both types
may be massive, banded or associated with quartz veins.
The sulphide minerals are mainly chalcopyrite and pyrrhotite, with
minor pyrite. These mainly occur
in one or two thick veins surrounded by a swarm of veinlets and
disseminations, with a gangue of vein quartz.
Less commonly, sulphides are massive and contain granular to nodular
quartz with little or no quartz veining. The
Eastern system orebodies typically have less well-defined walls than the
Western system orebodies. The
Eastern system in plan shows at least three lenses 50-80m in length, of
variable width averaging 10m. Each
lens may contain a number of vein sets. Large
veins of chalcopyrite + pyrrhotite occur.
They are up to 1 m thick and commonly are surrounded by stringer
mineralization of chalcopyrite and pyrrhotite in quartz veinlets.
Lenses may be linked along strike by chalcopyrite veins.
Stoping of the previously unmined Eastern system was commenced via
access cross-cuts off a zig-zag incline put down alongside the western flank.
Horizontal cut and fill stoping was later replaced by open stope
methods. Stopes are 5-25m wide,
and stand open for up to 120m height and 100m length.
The dip and irregularity of the open stopes renders them potentially
more difficult to maintain than the large vertical open stopes at Elura,
especially as the CSA orebodies tend to have weak chloritic shear zones in
their footwalls. The weak
chloritic zones and the relatively high horizontal east-west virgin stress
aggravate the overbreak problem, which can contribute up to 15% waste rock to
the total tonnage extracted.
The
QTS prospect is a zone of mineralization first prospected 600m east of the
other CSA mine ore systems, and later found to converge at north (QTS North)
to within 100m of the Eastern system. The
QTS prospect was early regarded as an ore zone adjacent to the CSA ore zones
but potentially distinct in time and space, perhaps disconformable.
It was initially interpreted as vertical to west-dipping.
With continued exploration, however, the QTS zone orebodies have come
to be regarded as part of the overall CSA system.
All lenses display the same relations to bedding and cleavage as those
of the Eastern and Western Systems, in contrast to the assertions of Marshall
et al. (1981). The QTS
mineralization lacks any magnetic anomaly at surface, but does have a positive
gravity expression. It is
characterised by a strong gravity anomaly about 900m long, delineated in 1971.
Irregular high Cu, Pb, Zn geochemistry pertains towards the southern
end of the anomaly, where there are two old shafts sunk on gossans.
In the vicinity of the old QTS shaft there
is gossan with up to 1.8% Cu, and containing boxworks interpreted as
being derived from chalcopyrite. Exploratory
drlling of the whole QTS zone commenced in 1972 and has continued
intermittently ever since. The
zone has been well tested at depths of 700-900m.
It is largely a low grade Cu orebody, with minor Pb-Zn ore, but is
insufficiently known for tonnage estimates to be made.
Best copper intersections has been 15m of 6.1% Cu.
Some significant lead-zinc intervals are present, e.g. 7.7m of 7.5% Zn,
5.1% Pb. Silver content of this
ore is significant (19-41 g/t Ag). The
zone may vary from Cu rich at the lower side to Pb-Zn rich near its upper
surface. The most recent drilling
evaluation of the lenses known as QTS South indicates some 1.2Mt of higher
grade copper ore (4-5% Cu) in a geological setting similar to that of the
Eastern system. At QTS North
sulphides other than chalcopyrite are uncommon, quartz veining is uncommon,
there is patchy silicification, and chloritization is intense.
Infill drilling in 1986-87 proved the existence of four lenses (M.,P.Q.
and R) below a depth of about 700 m.
Chloritisation is intense but quartz veining is uncommon.
Silicification is patchy and sulphides other than chalcopyrite are
rare. The westernmost lens
(formerly known as D zone) contains veins and splashes of chalcopyrite
associated with an unusual network of fine quartz veinlets in a zone of
intense cleavage. The others,
which consist of sub-massive unbanded chalcopyrite in an undeformed gangue of
black chlorite, are characterised by some spectacular intersections containing
up to 20% Cu over mineable widths.
The QTS
zone, first tested 1.2km SE of CSA No. 2 shaft, was initially regarded as
separate mineralization, and thought to possibly be of earlier formation than
the CSA system. It lies outside
most of the depletion effects recognized around the CSA systems (Robertson and
Taylor, 1987), and extends further to the north and south.
It was, at first, thought to have opposite dip to the CSA systems.
An early theory was that it might be an unusually well-preserved syn-sedimentary
deposit whereas the earlier known CSA orebodies were all variably remobilized
through the influence of shear zones. A
stratiform interpretation was supported by the presence of numerous soft
sediment fabrics in mineralized QTS zone strata (Bouffler 1981).
The syn-sedimentary QTS zone was considered as one supplier of metals
to later fluids, from metamorphic dewatering, which deposited the more
westerly orebodies. However, later
work did not confirm opposite dip and the QTS zone is now thought to be
conformable with the other CSA systems.
The orebodies in the CSA system have been subject of laboratory
investigations by a number of workers (viz Brill 1988-1991; Harris 1965;
Rayner 1969; Robertson 1968, 1974; Ramsden
et al. 1979; Marshall et al, 1981; Sangameshwar and Marshall 1980; Seccombe
and Brill 1989). These studies are
considered in discussion of genetic interpretations at the end of Cobar belt
descriptions. They largely support
a synmetamorphic timing of metallic deposition.
Brill (1989b) determined the trace element content of CSA or minerals
for Se, Cd, Mn, Sn, Ag, Co and Ni. The
temperature of ore formation and the partitioning of Se, Mn, and Cd between
sphalerite and galena were found not to show any correlation.
Partitioning of Fe and Zn between coexisting sphalerite and stannite
gave an ore-for-mation temperature of approximately 260oC.
This value is lower than temperatures interpreted from fluid inclusions
and compositions of chlorite, which average 350oC.
The apparent lowering effect is attributed to ongoing metamorphism
during and after ore deposition. The
FeS contents of sphalerite indicate a pressure range from 4.8 to 9 kbar, which
is too high for metamorphic conditions at Cobar.
The sphalerite is therefore inferred to have re-equilibrated at
temperatures below the metamorphic peak. The
Co/Ni values of pyrite from Cobar ore are lower than those of most Cu-rich
exhalative deposits, but are similar to those from remobilized vein deposits,
and hence support formation of the deposit during metamorphism.
Earlier authors proposed a number of incompatible modes of origin e.g.
epigenetic (Rayner 1969), through syngenetic (Sangster 1979), to remobilized
syngentic (Brooke 1964, 1975; Robertson 1974; Gilligan and Suppel 1978;
Marshall and Sangameshwar 1982).
Spotted Leopard Prospect
This
prospect lies in CSA Siltstone along strike from the CSA/QTS mineralization.
The Spotted Leopard No. 1 shaft is 4.2 km south of the CSA mine and the
No. 2 shaft is 0.5 km to the north of it.
The area has an outcropping "lode quartzite" thought to mark
a line of shearing (Thomson, 1953). Bedding
dips are steep westerly, whereas faults and the mineralized zone dip easterly
at more moderate angles. The No. 1
shaft was sunk in red ferruginous slates with boxwork gossan patches assaying
500 ppm Cu, 400 ppm Pb. The shaft
and cross-cuts exposed low grade lead mineralization in crushed and silicified
slate with quartz veining. The
exploratory workings exposed occasional pyromorphite, beaudantite(?), galena,
and gold spot values to 9 g/t. Underground
sampling by Enterprise Exploration gave maximum lead values of 6.9% Pb on the
7.6m level and 2.3% Pb on the 30.5m level.
The low grade lead-bearing mineralized zone may be as wide as 30m and
may average up to 0.3% Pb (Cobar Mines Pty Ltd, 1985).
Pyrrhotite, chalcopyrite, pyrite, galena and sphalerite are further
noted from drill cores.
The
mineralization is vaguely delineated at surface by broad north-northwest bands
of anomalous Cu and Pb values, and by discontinuous exposures of quartz veins
in a ferruginous slatey siltstone sequence.
Anomalous surface values range up to 1450 ppm Cu and 2500 ppm Pb.
Various diamond drilling, between 1947 and 1981, has intersected zones
of quartz veining and alteration but base metal sulphides have generally
proved to be quite scarce. The
best intersections are 0.2m of 1.2% Cu, 8.7% Pb, 7.1% Zn and 1.3 m of 1.18% Pb,
0.68% Zn.
Alteration
has been investigated by Cobar Mines Pty Ltd (1985).
Chlorite is the major alteration mineral, present both as discrete
zones and as selvages around quartz veins.
Potassic metasomatism is suggested by clusters of brown biotite around
pyrite grains. Analyses of drill
core samples for Li, Na, Ti, Sr and Ba have defined zones of Sr and Na
depletion. The depletion zones
correlate well with the zones of chloritic and/or siliceous alteration with
sparse base metal sulphides, and with the quartz veining seen at surface.
All these zones are close together and have been regarded as a single
lode, here very weakly mineralized, with greatest similarity to the QTS zone.
The mine
site was early described as a quartz shear zone with silicification
("lode quartzite"), seen to truncate an inverval of mixed sediments
(sandstone, siltstone, mudstone) to the north and west of No. 1 shaft (e.g.
Andrews 1913). Cross-faulting is
south block east, with up to 300m dislocation along 100oT.
The association of mineralization with sinistral deflection is
reminiscent of other nearby occurrences (e.g. Stoney Tank prospect).
All could be sites of stronger deposition along a linear trend of
mineralization (QTS?). That the
mineralized zone through the Spotted Leopard shafts is a long linear one like
the QTS is supported by about 4.5 km of near-coincident signatures of gravity,
magnetics, resistivity, IP and bedrock geochemistry (English & Apthorpe,
1985).
5.
MOPONE-BLUEBELL AREA
This area
include a zone of northerly lead anomaly trends "Bluebell-Ringneck
zone" east of the "Cobar magnetic ridge" and the CSA-Spotted
Leopard area. The zone is
ill-defined and about 2km wide. Cobar
Mines Pty Ltd and CRA Exploration Pty Ltd had, by 1987, located about fifty
Cu/Pb anomalies sites along it (Cu 100+ ppm, Pb 500+ ppm).
The mineralization is relatively little known and likely includes a
variety of types. Probably of
significance to the northern prospects in the Mopone-Bluebell area is a zone
of southeasterly cross-structures which extends across from CSA mine.
The Bluebell-Ringneck zone extends from Cobar as far north as the zone
of southeasterly cross-structures. The
deposits which comprise the Mopone-Bluebell group are:
No.
Deposit Name
Commodities
55
Red Tank prospect
Cu (Pb)
56
Stoney Tank prospect
Zn, Pb, Cu,
57
Mopone prospect
Fe
58
Mopone South prospect
Fe (Pb, Zn, Cu)
59
Ringneck prospect
Cu, Pb, Zn
60
Ringneck East prospect
Cu, Pb
61
Bluebell West prospect
Pb, Cu
62
Powerline prospect
Cu, Pb
63
Bluebell prospect
Cu, (Zn)
64
Royal George
Fe (?Au, Pb)
These
mineralized sites are of variable stratigraphic setting, ranging from near
basement up into Great Cobar Slate. The
most easterly prospect is the Royal George, situated in a silicified zone
close to the Devonian/basement contact. More
widespread prospective mineralization occurs in the vicinity of the Great
Cobar Slate-Chesney Formation contact. Here
lie several prospects, between Mopone homestead and Tharsis mine, which have
been of interest in the search for northern continuation of the New Cobar-New
Occidental line of mineralization. A
feature of the area is the northerly trending 2.5 km long Teralba magnetic
anomaly near Stoney Tank (Schmidt et al, 1980).
Another anomaly is near Red Tank. Drilling
at the Red Tank anomaly penetrated Chesney Formation with trace pyrite,
pyrrhotite and chalcopyrite. The
magnetic response was attributed to disseminated magnetite in concentrations
parallel to bedding (Bromley, 1983b). The
Teralba anomaly may be associated with an off-set and faulted contact between
the Great Cobar Slate and Chesney Formation.
The Mopone and Mopone South prospects occur in this vicinity,
associated with disseminated magnetite. Quartz
veining is prominent at surface and drilling has shown the magnetic sources
generally to be disseminated magnetite with negligible pyrrhotite.
Base metal values are usually negligible and the only evidence of base
metals associated with the magnetite is rare subhedral clots of
quartz-carbonate-pyrite-chalcopyrite (Bromely, 1983b).
In this area of widespread disseminated magnetite, the Stoney Tank
prospect shows the only ore development known.
It occurs in Chesney Formation just north of a strong east-northeast
sinistral deflection in the Great Cobar Slate-Chesney Formation contact.
At the
Stoney Tank prospect bedrock geochemical anomalies exist in the Chesney
Formation on the northern side of its fault offset to the west.
The mineralized zones trend north-northeast and the best values are
from between Stoney Tank and Red Tank. Metal
values in the area range up to 3860 ppm Cu, 2.28% Pb, 5100 ppm Zn, 3000 ppm
As. Drilling near Stoney Tank has
identified narrow pods of cleavage-parallel weak mineralization associated
with quartz veining and chloritization. The
sulphides present are largely pyrite, with traces of galena, sphalerite,
chalcopyrite and pyrrhotite. The
best intercept is 2 m of 2.17% Cu, 0.18% Pb, 16.7% Zn and 23 g/t Ag.
This included 0.2 m of massive sulphides.
A second hole along the Stoney Tank anomalous geochemical zone made an
intercept of 0.4 m of 0.1% Cu, 2.4% Pb, 3% Zn and 19 g/t Ag.
A third hole intersected 1.5 m of 0.74% Cu, 3.9% Pb, 8.1% Zn and 26 g/t
Ag, which included 0.5 m of massive sulphides.
Downhole geophysics suggests the mineralization is in north plunging
shoots (Bromley, 1983b). The
mineralization is associated with narrow quartz and quartz-carbonate veins,
chlorite alteration and silicification. The
veins contain pyrite, pyrrhotite and sphalerite with lesser chalcopyrite and
galena. This mineralisation
appears cleavage-orientated, poddy and discontinuous, although electrical
techniques suggest the homogenity of the broad zone.
The lack of immediately associated magnetic and gravity response is
considered to indicate the absence of significant tonnage, at least at
moderate depth. The Stoney Tank
situation suggests that other geochemical trends in the area also represent
sulphides at depth (Bromely, 1983b).
The
Ringneck, Bluebell and Powerline prospects lie within the Great Cobar Slate.
They are in an area located west to north-northwest of the Tharsis
shaft, north of Cobar. The area
contains a cluster of bedrock geochemical trends and quartz veining, with
minor old shafts and pits mainly sunk on ironstone or ferruginized slate and
siltstone. Samples from these
prospects return up to 835 ppm Cu, 2220 ppm Pb, 710 ppm Zn and 120 ppm Bi.
The area is approximately along strike from the Great Cobar-Gladstone
group. A weak aeromagnetic high
(15 gamma) passes through the area, extending from the Great Cobar anomaly to
about half as far north as the CSA mine. Drilling
along this trend, at the Bluebell geochemical anomaly intersected a sequence
with minor pyrite and pyrrhotite throughout.
In this sequence sparse chalcopyrite and galena occur in
quartz-carbonate veinlets (Bromley 1983a, CRA Exploration Pty Ltd, 1984).
THE COBAR
CENTRAL AREA
Cobar
central area, also known as 'Cobar goldfield', consolidated as two CMLs,
Consolidated
Mining Leases, under Peak Gold Mines
Over the
first 132 years of this area, about 8 km long, it gave a production in excess
of 2.5 million ounces of gold and 140,000 tonnes of copper, making it one of
the largest goldfields in NSW, and at one time the largest copper mine in
Australia.
Following Mulholland and Rayner (1947), the area term
"Central" has been loosely and frequently applied to the Cobar field
mines which are described herein under the group headings "Great
Cobar-Gladstone area (Western Line)" and "New Cobar-New Occidental
area (Eastern Line)". The
Great Cobar Company, under diverse changes in ownership and capital, was the
principal leaseholder and operator prior to closure of its smelting works,
mines and other operations in 1919-1921.
The Great Cobar deposit was discovered, and mining preparations
commenced, in 1869-1870. Within a
few years all of the important central area sites were under lease and were
being prospected. The Great Cobar
Company progressively gained control over other properties and at its peak
employed 1100 men in Cobar. Its
major capitalization was in 1906 in London, of which little had been recovered
prior to the copper price collapse in 1919.
The Great Cobar Company acquired the Chesney mine in 1904, mines at the
Peak in 1904, and the Cobar Gold Mine in 1910.
The siliceous auriferous ores from these mines were blended with Great
Cobar ore for smelting. The direct
smelting of gold-rich ores from the Cobar Gold Mine and the Chesney mine
boosted gold and silver production and gave improved recovery of these metals.
The Great Cobar company built its own refinery in Lithgow but also sent
blister copper to the Electrolytic Company at Port Kembla.
When the price of copper declined over 40% in the first quarter of
1919, the mines controlled by the Great Cobar company were the first to close.
Even prior to the fall in copper price, however, the Great Cobar
company had been working under extreme difficulties for some considerable
period. One major concern had been
the fact that deep development after 1914 indicated the three ore lenses of
the main Great Cobar lode to be pinching out rapidly downwards.
Moreover the proportion of siliceous ore appeared to be increasing and
basic ore decreasing. The
situation thus arose that the ore raised at Great Cobar became merely
self-fluxing, and no longer possessed the chemical characteristics for fluxing
New Cobar (Cobar Gold Mine) or other ores over and above its own needs.
Apart from the Great Cobar mine itself, the major central area mines
(Cobar Gold, Chesney, Occidental) were re-opened during the New Occidental
period (as the New Cobar, Chesney and New Occidental mines).
Rising costs forced the New Occidental company to close the New Cobar
mine in 1948, and the Chesney and New Occidental mines in 1952.
Cobar Mines Pty Ltd sank a new shaft at the Chesney in 1972-1976, and
extracted a quantity of near surface auriferous stone from the New Cobar open
cut in the late 1980s. Tailings
accumulated from the New Occidental Company's operations have also been
reprocessed for further major gold extraction. None of the central area mines, however,
had been returned to major
production since the 1952 closing of the New Occidental until the rise of the
the Peak Mines phase of the field.
Peak Gold
Mines Pty Limited arose as a wholly owned subsidiary of Rio Tinto Ltd after
discovery of major deep ore resources beneath the Peak. As the Peak
mine's reserves began to be exhausted the mine drove north to work the
downwards continuation of the New Occidental deposit. After that it
began an open cut at the New Cobar mine. Then major new reserves
were defined south of the Peak mine, beneath the old Perseverance
mine.
Rio Tinto
Group had begun exploring at Cobar in the early 1940s when The Zinc
Corporation formed an exploration joint venture with New Occidental Gold Mines
NL. Since then, Rio Tinto’s exploration efforts have waxed, reaching a
peak when its then subsidiary Cobar Mines Pty Ltd discovered the Peak deposit
in 1981. Since 1987, when the commitment to mine the Peak deposit was
made, CRA Exploration Pty Limited (CRAE) and Cobar Mines Pty Ltd (CMPL)
undertook exploration in partnership. After 1995 CRAE withdrew and Peak
Gold Mines has been the sole explorer and developer. Exploration
activity was intense from 1995 to 1999 when replacement reserves for the Peak
orebody were urgently sought. The success of that program added an
additional one million ounces of resources in addition to the Peak orebody.
Had it not succeeded, the Peak mine had been foreshadowed to close operations
in 2002. Instead, 2002 saw an extension to the South
Exploration Drive constructed in order to facilitate exploration of the
southern extensions of the Perseverance mineralisation.
Despite the
seeming great success of the Peak mine, with spectacular duplication of
resources by further ore discoveries or confirmations by 2002, Rio Tinto sold
Peak Gold Mines to Wheaton River Minerals, a Canadian company, in 2003.
In January 2003,Wheaton River Minerals signed a letter of intent to acquire
Rio Tinto's 25% interest in the Bajo de la Alumbrera gold-copper mine in
northwest Argentina and its 100% interest in the Peak gold mine near Cobar,
New South Wales, Australia for $210 million. Rio purported that it was
rationalising assets and considered Peak Gold a non-core
asset. Wheaton Minerals not long after this was swallowed by
its fellow Canadian competitor the giant Goldcorp, which purchased Wheaton for
over $2 billion in late 2005. Goldcorp did not hold the mine for
long and sold Peak Gold Mines in early 2007 for $US200 million ($254 million),
three months after it strongly denied a Herald report it was
considering a sale (Sydney Morning Herald, 21 February 2007 - http://www.smh.com.au/news/business/goldcorp-sells-peak-mine-for-254m/2007/02/20/1171733762517.html
)
Goldcorp
sold the mine to a Canadian-listed shell company, GPJ Ventures, linked to
investment group Endeavour Financial. GPJ also purchased Goldcorp's
Amapari mine in Brazil for $US100 million in equity as part of the deal.
It planed to change its name to Peak Gold and appoint the Peak mine's already
existing manager, Jim Simpson, as its chief operating officer. Mr
Simpson said Goldcorp had been considering the sale for about six months, even
though two other representatives had denied the Herald's report of a
possible sale in November
(fide the Herald, 21 February 2007).
In 2007-2008
ownership name would again change, with Peak Gold acquired by New Gold,
another Canadian entity, along with similar parallel business merger involving
Metallica Resources apparently.
New Gold
announced in March 2007
that further to its announcements of February that year, it anticipated that
the C$326.25 million financing would close that month and that the proceeds
would be placed into escrow till released to the company upon completion of
its purchase of the Amapari Mine in
Brazil
. The closing of the acquisition of the Amapari Mine was also expected
that month, and thereafter the Company's shares would trade under the name
"Peak Gold Ltd". Consideration for acquisition of the Amapari
Mine would consist of 155,000,000 shares of the Company. Under its
agreement with Goldcorp, US$200 million of the proceeds of the financing would
be used for the acquisition of the Peak Mine. New Gold's 17 February
2007 announcement had been that GPJ Ventures Ltd. (the "Company") had
entered into an arms' length agreement that month with Goldcorp Inc. for the
acquisition of Goldcorp's Peak Mine in Australia and its Amapari Mine in
Brazil. Consideration for these mines and the related assets would be US$300
million payable as to US$200 million in cash and US$100 million through the
issuance of 155,000,000 common shares of the Company. The Company's name would
be changed to "Peak Gold Ltd." in connection with the transaction.
It was anticipated that the transaction would be effected through the
amalgamation of a subsidiary of the Company which would be formed to complete
the subscription receipt financing and a subsidiary of Goldcorp which would
indirectly own the assets. On closing, Endeavour Financial would receive
a fee of 5,000,000 common shares of the Company. In order to finance the
acquisition and provide working capital, the Company would complete a
financing of 370,000,000 subscription receipts at a price of $0.75 per
subscription receipt for gross proceeds of $277,500,000. Each
subscription receipt would be convertible into units of the Company at closing
of the acquisition. The units would consist of a common share of the Company
and half of a transferable share purchase warrant. Each whole share purchase
warrant would entitle the holder to purchase an additional common share for
five years at a price to be determined. The use of proceeds would be for the
acquisition costs, for "setting up an office and employees for Peak Gold"
[although such surely existed already at Cobar so this might mean in Canada?],
and for general working capital.
Although the
name Peak Gold was 'reverse engineered' to continue as owner-operator, it was
soon regarded in Cobar town as a wholly owned subsidiary of New Gold.
In 2008, Peak
Gold (New Gold) produced 100,493 ounces of gold and 8.25 million pounds of
copper (down somewhat from 116,488 gold ounces and 7.5 million pounds of
copper in 2007). In 2008, Peak Gold produced its two millionth ounce of
gold since the commissioning of the mine. The operation achieved record mill
throughput of 768,727 tonnes. PGM planned to re-commence production from
the Chesney as well as continuing work on the Perseverance (Zone D)
orebodies. PGM has been very successful in continuously replaced reserves
over more than sixteen years of mining.
Peak Gold's
reserves total as at 2008 was 514,000 ounces of gold and 76 million pounds of
copper. Year 2009 saw the highest ever mine production and highest
copper production for Peak Gold.
In 1910,
Peak Gold announced that it would also move towards re-commencing mining at
the Great Cobar deposit on the outskirts of Cobar. PGM general
manager Peter Lloyd outlined the company’s plans in Cobar in April (The
Cobar Weekly, 29 April 2010). He said the company was planning to
ramp up its exploration in the area in the coming year with a focus on the
Great Cobar mine’s ore body - "We believe there is an exciting future
for this orebody. The Great Cobar mine orebody is open at depth - we
haven’t found the bottom of the mine yet.”
6. GREAT COBAR - GLADSTONE AREA
(Western
Line)
The
group of deposits in the Great Cobar-Gladstone area has also been referred to
as the Western Line (Russell and Lewis, 1965).
It has also on occasion been referred to as the central line, thereby
making the line of deposits herein called `CSA-Spotted Leopard area' the
western line in some descriptions of the Cobar mining field.
The initial mining land securement at Cobar, by a Mineral Conditional
Purchase, was a block of 40 acres named the Cobar block.
Two other 40 acre blocks were secured in 1871, adjoining the original
block to the north and south. These
were known as Great Cobar North and Great Cobar South.
The Cobar and Great Cobar South holdings were later amalgamated as the
Great Cobar mine.
This north-northwest trending cluster of deposits is situated in about
the middle of the outcrop width of the Great Cobar Slate where it passes
through Cobar. The broad alignment
is approximately parallel to that of the New Cobar-New Occidental group
(Eastern Line of Russell and Lewis, 1965).
En echelon features have been described and the deposits of the Great
Cobar-Gladstone area do not all lie along one single fault line.
Rather, the deposits are thought to occur where shorter more northerly
shears leave a better developed north-northwest fracture trend of uncertain
origin. Thus the four deposits of
this group might comprise a left echelon arrangement corresponding to four
separate shears which are variably silicified and mineralized.
Alternatively, Brooke (1975) placed Great Cobar and Dapville on the
same shear, and Gladstone on a most easterly parallel shear.
The Gladstone mineralization, at the easternmost shear, may be
transitional between Great Cobar and Eastern Line (New Cobar-Chesney-New
Occidental) types. In its low
sphalerite content, the Gladstone chalcopyrite ore is unlike Great Cobar ore
and more resembles Eastern Line ores. Some
writers have postulated a role in the Great Cobar-Gladstone area for
anticlinal folds, by analogy with the supposed attentuated underlimb stretch
folds of the New Cobar - New Occidental area (e.g. Thomson 1953).
However, any such structures within the relatively monotonous Great
Cobar Slate could be difficult to recognize.
The
deposits in the Great Cobar-Gladstone area are:
No.
Deposit Name
Commodities
96
Great Cobar North
Cu (Zn, Pb, Ag)
97
Great Cobar mine
Cu, Pb, Zn (Au)
98
Dapville
Cu
99
Gladstone mine
Cu (Ag)
Of the
above deposits the strongest similarity is between Great Cobar and Dapville,
which both contain massive magnetic ores.
The Gladstone deposit contains more siliceous ore, and is situated
further away from the main north-northwesterly trend.
It has insufficient pyrrhotite or magnetite to produce a magnetic
anomaly.
All four
deposits are primarily copper deposits and the Great Cobar copper mine was for
many years the principal mine on the field.
The Great Cobar sulphide ore contains significant gold, and the
oxidized cap of the main lode produced some rich gold ore.
By contrast, ore from the Gladstone mine carried negligible gold.
Great Cobar North
Great
Cobar North Limited shares certficate, 1912. The company was
registered in 1906 to acquire 286 acres Freehold and
400
acres Leasehold at Cobar,
adjoining the property of the Great Cobar Ltd. It entered into
voluntary liquidation 1913.
The Great
Cobar North mine, also known as North Cobar mine, was a site that commenced
prospecting about 1873. It was adjoining the Great Cobar mine to
the north. The
property changed ownership several times and there were developed four
prospecting shafts. Several strong
siliceous gossan lines outcropped on the property and the easternmost of these
was easily traceable onto the Great Cobar property, passing east of the Great
Cobar main lode. The position of
the Great Cobar main lode could be traced north only with difficulty, as a belt
with siliceous fragments in the soil. Sinking
at the gossans revealed zones of quartz veins and veinlets with steep easterly
underlay, similar as known for the eastern lodes on the Great Cobar property.
The plunge of ore lenses along the Great Cobar main lode is such that
the Great Cobar northern lens would be excessively deep on the Great Cobar
North property. Nonetheless, the
northern mine hoped to locate the Great Cobar main lode interval, drive along
it and perhaps find further northerly new ore lenses.
Some deep ore was located but development was not sufficient to
establish if this is the deep top of a new ore lens or a low grade envelope of
the Great Cobar northern lens. Sinking
did not extend deep enough to cut the projected axis of the Great Cobar
northern lens. Major development
was by the Great Cobar North Copper Mining Company Ltd, a company floated in
London in 1906. This company
developed a deep but ultimately unsuccessful exploratory mine along the
inferred northern continuation of the Great Cobar main lode.
In most of the development workings no ore was encountered, although
occasional picked samples of clean sulphides gave assays of up to 8.7% Cu.
Below 442m the company believed it had encountered the Great Cobar lode
and more encouraging traces of chalcopyrite with magnetite and pyrrhotite were
present. Diamond drilling was
employed in 1910. The main
mineralization found lies to the east of the projected trend of the Great
Cobar's Northern Lens, which latter does show a distinct NNE deflection at its
northern end, but which was believed by the Great Cobar staff to have been
faulted out. Grades were low in
the supposed lode but at 457m copper ore was met with.
It was reported in 1911 as "an important body of ore" but
surviving details of it are few. One
report stated it to be over 30m wide, including 3m of 5% Cu "basic"
ore similar to the best Great Cobar ore. At
518m a cross-cut intersected 22.2m of 1.5% Cu.
A 1913 reference to 2.25% Cu over 30m (Annual Report Dept. Mines for
1913, p. 105) has been doubted, as it appears an excessive estimate in light
of assays appearing on the level plan (Mine Record 7).
Nevertheless drilling at still greater depth below this area by New
Occidental Gold Mines NL in the 1950s confirmed the presence of a low grade
lode, possibly the disturbed downwards continuation of the Northern Lens mined
on the Great Cobar property. The
lode below 680m appears to be over 35m width and to have a significant medial
interval of 3.5-4% Cu ore. It can
be assumed as part of the Great
Cobar ore system, and has similar easterly underlay as the Great Cobar ore
bodies.
Great Cobar Mine
Lines running
from the Great Cobar mine to plants on the nearby plains,
1880s. (Photo: Mines Department)
This mine, one of the most celebrated in Australia, was long the centre
of ore raising and treatment on the Cobar field.
The companies which mined the Great Cobar ore system have been the
Cobar and Southern Cobar Copper-mining Companies Limited (1870-1876), Great
Cobar Copper Mining Company Ltd (1876-1889), Great Cobar Syndicate
(1893-1906)(a.k.a. Cobar Syndicate, Longworth Bros. Syndicate, Great Cobar
Mining Syndicate), and Great Cobar Ltd (1907-1919).
Some 4.1 Mt of ore was treated on the site, and the large remnant slag
heap has subsequently proved useful as a crushed aggregate source.
Between 3.5 Mt and 3.9 Mt of ore was raised from the Great Cobar
deposit itself, surviving records being inadequate for a precise account of
operations. The Great Cobar ore
output averaged 2.3% Cu and 1.4 g/t Au, the copper grade generally declining
through time as the mine deepened. The
average of the ore above and close below water-level was about 25% Cu, while
the value of the primary ore (commencing around 120m) in most places plunged
tenfold to only 2.5% Cu. The
average working assay published by Great Cobar Ltd in 1911 was 2.60% Cu.
Various references to the primary ore grade averaging 4-4.5% Cu across
the main lode are unusual, or might be typical of the shallower enriched
levels only. The remaining
quantity of low grade ore (2.0% Cu, 0.2g/t Au) is uncertain, variously
estimated between 1.5 Mt and 3.5 Mt. The
upper limit of speculation is 12 Mt (Mulholland & Rayner, 1952).
The
Great Cobar mine is the site of the first copper discovery at Cobar, in 1869.
The site was leased in 1870 as a mineral conditional purchase and
full-time prospecting commenced. Rich
near-surface ore was initially sent to South Australia for smelting, via
Bourke and Louth. The first load
of copper ore was dispatched in 1871 and after that deepening mine development
continued until 1919. Subsequent
to 1909 the ore mined was probably made up more from remnants at upper levels
than from deepening development. The
operations at Great Cobar reached a very impressive peak.
For a time Great Cobar was one of the biggest industrial undertakings
in the State and the largest copper mining venture in Australia.
The Great
Cobar mine came about by amalgamation of early smaller holdings along the line
of lode. Production was increased
substantially following the erection of smelting furnaces in 1875-1876, prior
to which ore was transported to Adelaide via the Darling River.
The Great Cobar Copper Mining Company Ltd was formed in 1876 after
agreements by the Cobar Copper-mining Co., Southern Cobar Copper-mining Co.
(or Great Cobar South) and other smaller copper prospecting parties.
Of these two the Cobar company was apparently the larger and in some
later references the name Cobar company continues to be used when the Great
Cobar company is being referred to. Parties
to the north, particularly the Great Northern (later Great Cobar North), were
not part of this amalgamation. By
1880 the Great Cobar gained the reputation of being the largest and richest
metalliferous mine in the Colony. There
were 550 men and boys employed at the mine in 1882.
A strong British re-capitalization occured in 1906.
The mine has been worked to a depth of 470 m in 14 levels.
Levels 13-14 were not fully developed at the time of closure.
Peak activity followed the mine's takeover by an English company, Great
Cobar Ltd, in 1906. British
capital was then used to bring other Cobar mines under consolidated ownership,
for purpose of group smelting practice. The
group smelting strategy is described in early publications (e.g. Carne
1899,1906). By 1911 the Great
Cobar Ltd was giving direct employment to between 1300 and 1400 men in and
beyond Cobar.
The mineralization is thought to lie in multiple shears which are
northerly-trending spurs off a broad north-northwest shear zone extending
south adjacent to the Dapville, and possibly Gladstone, lodes (Thomson 1953).
The Great Cobar ore system is extensive (365x30m) and was reported to
comprise five subparallel lines of lode. However
only the Western or Main Lode was productive to any significant extent.
Mine record descriptions support the inference of shearing, as they
note a strong distinction between the "channel slate" in the Main
Lode shear and the normal Great Cobar Slate country rock.
The latter, especially on the western side, was often described as
being more jointed and blocky. The
main distinction is between the "channel" of sheared slate and the
country rock, but some workers have also described differences between the
slate west and east of the Main Lode. Apparent
changes across the lode have been attributed either to different alteration
styles or to fault juxtaposition of different units within the Great Cobar
Slate. The four eastern lines of
lode are incompletely known. They
consist of quartz veins and breccias with minor to rare chalcopyrite and
pyrrhotite. The eastern lodes are
siliceous, with drillhole intersections recording up to 3.3m wide intervals of
barren quartz. Relatively little
was done to test or open up the eastern lodes at depth as the main lode itself
already contained a vast amount of siliceous ore which the smelters were
ultimately unable to handle. The
limited amount of basic ore present required selectivity in mining siliceous
ore, and after a time only auriferous siliceous ore was in demand for the
blast furnaces. A little stoping
was carried out east of the Main Lode near Harvey's shaft, probably in the
zone of secondary enrichment only. The
next lode east of the Main Lode, called the East Lode, was about 6 m wide and
low grade where cross-cut on different levels.
The supposed lines of the lode further east were not explored at depth.
The Main
Lode was traced 365m, through the original Cobar and Southern Cobar mine
holdings. It was also considered
to extend into the Northern Cobar block but later prospecting (Great Cobar
North mine) failed to confirm any extension of strong mineralization in that
direction.
The Main
Lode is of variable width, in places exceeding 30m (e.g. 48.7m width averaging
1.2% Cu). Typical working width
was 15m (Cropper 1906). In general
the mineralization dips vertically or steeply to the west, although some
buckles in the main lode are thought to dip east.
Along its length three lenticular orebodies or shoots were worked,
known as the Northern, Central or Southern Lenses (Fig. CCLS,B).
These pitch steeply to the north and are connected by tracts of
variable silicification, chloritization and weak mineralization.
Some accounts suggest left echelon or sinistral offset between the
lenses. The northern lens is
remarkable for its great thickness at depth.
At 305m (No. 10 level) it is up to 49m wide but grade and width
thereafter decrease downwards. At
the same level (No.10) the Central Lens is 23m wide, having narrowed down from
37m maximum width at 244m depth. From
No. 12 level downwards mining was confined to a diminishing area of basic ore
in the Central lens. On levels
Nos. 1-11 all three lenses were stoped. For
many years only the more massive pyrrhotitic basic ore was taken to be
smelted. This depletion of the
easier smelted ore left the last company which ran the mine, Great Cobar Ltd,
as the inheritor of a large amount of equally cupriferous yet siliceous or
sulphur-deficient ores. These were
difficult or impossible to treat in the blast furnaces.
In treating the more magnetite-rich sulphur deficient ore the furnaces
lost a larger proportion of the copper content to the slags.
In its last year before liquidation, Great Cobar Ltd was reduced to
mining such less desirable ore, and installed a reverberatory settler to
improve recovery.
Although
the Main Lode ores vary somewhat from lens to lens, and changed with depth, a
general east-west zonation holds. Basic
ore of low silica content, consisting of massive
pyrrhotite-chalcopyrite-magnetite, tends to occur at the western or footwall
side of the lode with siliceous ore fringing it to the east.
A strong zone of central magnetite between the pyrrhotitic or basic ore
and the siliceous ore is stressed in some reports whereas other writers
scarcely mention magnetite concentrations.
A "mullock band" cleanly divided the basic from the siliceous
ore on Nos. 10 and 11 levels. Both
basic and siliceous ore width could be as great as 12m.
Magnetite occur in large quantity in some parts of the Great Cobar
mine, and small pieces of native bismuth were best known from the more
magnetite-rich intervals. In these
intervals magnetite occurs as numerous veins, up to 5 cm thick and
sub-parallel to slaty cleavage. Fine-grained
disseminated magnetite has also been noted in the waste-rock intervals.
The siliceous copper ore consists of veins and disseminations of
pyrrhotite, chalcopyrite and magnetite in quartz veins or "elvan".
Concentration of galena and sphalerite occurs along the western wall,
and this banded Pb-Zn ore was considered a late stage feature by Andrews
(1913). A typical section across
the main lode from west to east is: 1-3.5m
of banded fine-grained massive Pb/Zn/Cu ore (galena + sphalerite bands with
fewer chalcopyrite bands) with chalcopyrite veins along the eastern side only,
6m of massive chalcopyrite-pyrrhotite ore with remnant slate inclusions and
variable magnetite, 10m of chert-like silicified slate ("elvan")
with myriad quartz veins carrying chalcopyrite + pyrrhotite + magnetite
(Andrews 1913, Thomson 1953). In
some parts of the mine quartz veins were exceptionally wide (1-1.8m).
Many exceptions to the generalized zonation could be quoted.
Occasionally the eastern wall was not quartz veined silicified slate
but dense masses of sulphides. Some
large masses of banded chalcopyrite-pyrrhotite ore diverted off as spurs
leaving the general lode trend at anything up to 030oT.
Such anomalous ore masses were interpreted as replacements along
buckles or faults in the slate hostrocks, and the largest examples were at
levels 10-12.
Whole lode sampling revealed up to 15% magnetite at the lower levels.
Typical average grade across the lode was about 2.5% Cu, with the
higher values (e.g. 4.5% Cu) tending to be more often in the siliceous ore and
the lowest copper content (1.5% Cu) being in the heaviest magnetite ore.
Although siliceous ore tended to more often show sizeable rich patches,
of up to 8% Cu, the basic ore was at times equally rich (e.g. 1.4m width
averaging 7% Cu, Level 11). A very
rough averaged estimate for the lode in the lower levels is that it contained
2.6% Cu and comprised subequal thicknesses of 2.4% Cu basic ore and 2.8% Cu
siliceous ore. The reported
"metal contents" of the main lode below No.9 ranged as high as 16%
Cu, these being presumably spot values in series of assays taken across the
lode to arrive at average composition. The
"lead veins" found as footwall selvage assayed up to 18% Pb and 20%
Zn but for metallurgical reasons were considered worthless.
The first ores raised from the Great Cobar were gossanous, compact or
complex crystalline secondary ores; and they were hand-sorted into different
types and grades. Cuprite ore
occurred in massive bunches of great purity near the surface.
Rich bagged ore was exported for early cash flow to enable deep
exploratory sinking, and the lower grade ores were let accumulate on site for
later smelting. Much secondary ore
averaging 10-14% copper was available, and the earliest despatches were hand
sorted to high grade (25-44% Cu). The
ores contained mammillary masses and coarsely crystallized masses of
malachite, azurite as isolated crystals and radiating aggregates, native
copper as arborescent and dendritic masses or occasional sheets along host
rock cleavage planes, cuprite, chalcocite, covellite, and minor native gold
(not early detected). Malachite
was common and occasionally pseudomorphed other secondary minerals,
particularly crystalline cuprite.
The base of the oxidized zone was irregular, often about 75m.
The gossanous and oxidized ores extended strongly to 60m, whilst rich
carbonate patches were sometimes followed down to 85m.
Below the oxidized zone was a zone of secondary enrichment, from which
most of the high grade ore production came.
This zone, often with massive chalcopyrite and chalcocite, in places
persisted as deep as 150 m. Most
secondary enrichment was at 75-120m depth, with enriched sulphide ore assaying
as high as 22% Cu. Between levels
3 and 4 rich chalcopyrite ore with up to 17.2% Cu was stoped, and its top
found at 76m. Below level 4 there
was an increase in cupriferous pyrrhotite ore, of which 2 Mt was mined for an
average yield of 2.5% Cu. Below
150m the primary ore was recorded as predominantly pyrrhotite, magnetite and
chalcopyrite. It contains minor
pyrite, marcasite, galena, sphalerite, bornite, tetrahedrite, arsenopyrite,
cobaltite, gold and bismuth minerals. Quartz,
calcite, siderite, chlorite and stilpnomelane are important gangue minerals.
The Great Cobar ore was noted for the minor bismuth
it carried, which caused treatment troubles prior to the introduction of
electrolytic refining. Early
samples of Great Cobar ore assayed up to 2.58% Bi, and content of 1-2% Bi was
not exceptional. At times, high
magnetite content also caused trouble, and was held responsible in 1913 for an
abnormal loss of copper to the slags. The
magnetite was early though to be an oxidation product, until it was found to
persist to great depth and co-exist with fresh sulphides.
Of the minor elements, gold was of great importance.
The oxidized ore mined, and the sulphide concentrates, contained very
significant gold, up to 2-5 g/t Au, and some still richer gold patches were
found at later date after copper mining ceased.
The copper sold by the company contained 15-90 g/t Au, although this
did not come to general attention before 1894.
The rich
secondary ores of the Great Cobar mine were largely exhausted by 1888 and
timber fuel had by then been depleted for many kilometres around.
The working of primary ore was not economic at the time and the mine
was closed during most of the 1889-1893 economic recession.
However, in 1892 the railway line reached Cobar, offering the
possibility of coke-based blast furnance smelting of the primary ore.
The mine was re-opened in 1893, and coke subsequently railed to Cobar.
The plant at the mine was greatly increased and by the turn of the
century the company was employing 700 men, which figure later reached a peak
somewhere around 1300. Following
the formation of the heavily capitalized Great Cobar Ltd in 1906, a massive
increase in plant took place at Great Cobar mine.
The company acquired as many as possible of the nearby copper-gold
mines, or purchased their ores, in order to better feed its new plant and more
optimally blend the charge to the smelters.
Great Cobar Ltd was decidedly over-capitalized with respect to the
established ore resources of the Great Cobar mine alone, whence its need for
expansionary interests. Building
works reached their peak after purchase by the English company.
The Great Cobar company then absorbed the New Cobar and Chesney mines
and engaged in various other ventures. During
its operation it was the principal copper producer on the field.
Its smelting plant treated 4,104,362 tons of ore from Great Cobar,
Chesney, New Cobar, and The Peak with an estimated 3,450,000 tons from Great
Cobar. The estimated overall
recovered grade of the ore treated is 2.8% Cu with 2.1 g/t Au.
The production figures for individual ore deposits exploited by the
Great Cobar Ltd remain incompletely resolved.
The Great Cobar mine closed in March 1919.
Its headframe and main shaft timbers were later destroyed by a fire in
1933.
Considerable
ore must remain in the Great Cobar ore system but quantity, grade and depth
all appear unfavourable to foreseeable further deep mining.
The highest ore reserve estimate from the 1910s, minus ore mined before
closure, could leave a figure of around 1.5 Mt of ore.
Later reserve estimates, assisted by drilling, range up to 3.5 Mt of
2.8% Cu and 0.2 g/t Au but reliability is uncertain.
Although the three lenses of the main lode were all tapering downwards
before mine closure, there is no doubt that the ore system should persist to
great depth, with high likelihood of new lenses developing.
The main lode channel has been intersected to great depth by diamond
drilling: (i) by New Occidental
Gold Mines NL between 716m and 975m, and (ii) by Cobar Mines Pty Ltd down to
1036m. This drilling, however, did
not disclose appreciable new or unexpected ore reserves but confirms the
persistence of the ore system. The
lode channel at such depths contains up to 88m of 1.0% Cu or 46m of 1.4% Cu,
with higher grade bands to 4% Cu. A
large width of mineralization was reported from No. 12 level of the mine,
where duplicate assaying along a cross-cut just north of the main shaft gave
averages of 3.6% Cu and 3.8% Cu over 36m width.
The maximum width mined was about 30m.
The stated reserves around the time of closure totalled 200,000t at
2.3% Cu. This probably included
about 100,000t of secondary ore which had not been mined from the upper levels
due to the fact that the treatment plant had been constructed directly over
the lode.
After the cessation of major copper mining at the Great Cobar, some
good surface gold values were discovered in gossan in 1935.
Some of the gossan was mined for gold by G. Wright and Associates in
1936-1937 and 1947, and by Great Western Mines Pty Ltd in 1940-1942.
Gold recovery was largely from gossan out of the southern open cut.
The gossan ore there contains gold and silver with traces of copper
oxide, native bismuth, bismite and bismutite.
The Manager of this operation believed the best gold concentrations in
the gossan to cross the main lode diagonally.
Gold content averaged 57 g/t Au over 248t and 46 g/t over 813t of
selected gossan. Small parcels of
picked copper ore were also produced from near surface in the 1940s and
likewise sent to Port Kembla. New
Occidental Gold Mines N.L. acquired the mine in 1950 and transfered it to
Cobar Mines Pty. Ltd. in 1957.
Dapville Prospect
The
Dapville shaft was sunk at a 210 m long zone of siliceous ironstone, quartz
and crushed slate. This zone shows
a concentration of quartz stringers, traces of limonite boxwork, and in places
weak silicification of the slate. It
is coincident with a 50 gamma magnetic anomaly.
Cross-cuts in the mine encountered gold traces in the quartz stringers,
but no copper values. Rare quartz
veins further west show traces of base metal sulphides, probably related to
the western shear zone shown in Thomson (1953).
The exploratory mine yielded minute pieces of native bismuth similar as
at Great Cobar.
Diamond
drilling by Enterprise Exploration Co Pty Ltd revealed an estimated 127,000t
of primary sulphide ore above 245m. Drillholes
cut up to 4.3m width of massive ore with 5.3% Cu, similar to ore at Great
Cobar. The ore lens is about 80 m
long and pinches out before reaching surface.
Gladstone
Mine
The Gladstone mine encountered rich copper ore at water-level in 1909.
It produced 0.03 Mt of high grade secondary ore averaging 6.3% Cu.
Most of the primary ore deposit remains unmined and is estimated as 2.2
Mt at 2.5% Cu and 0.5g/t Au. The
gangue is siliceous.
The Gladstone mine is located about 450m west of the Chesney.
It is not to be confused with earlier activity further east on the
Gladstone holding and in line with the Chesney workings on the latter's
southern side. The first
indications of ore were repeated thin intervals of rich secondary sulphides
encountered whilst cross-cutting at 55 m depth.
The primary orebody appears to be a mineralized shear zone containing
narrow chalcopyrite veins in association with brecciated slate and
silicification. Unmined primary
ore is amenable to extraction via the Chesney mine No.3 shaft.
The Gladstone mine commenced in 1908, encountered a rich body of
supergene copper ore in 1909 and reached its peak production in 1915.
The rich ore mined in 1911-1917 came from mostly 64-90m depth.
Some of this ore was 10% Cu. About
7.5 m above primary ore (at about 79 m depth) was a layer up to 1.2 m deep of
cavernous limonite rich in native copper.
Just above this rich chalcocite occured.
Chalcocite remained a main ore mineral up to water-level (59 m) where
malachite, azurite and cuprite were admixed.
Above water-level some small patches of carbonate ore persist but most
of the siliceous gossan is rather barren.
Bodies of rich black chalcocite ore (65-79 m) assayed 7-22% Cu.
Deepening development, by drives and winzes followed the mineralized
shear zone and was reported to have remained in mineralization almost
continuously. The early driving
(96 m) reportedly did not reach any termination, nor did the short
cross-cuts touch any walls. Pockets
of high grade secondary enrichment encountered were up to 6 m wide.
Exceptionally pure masses of secondary ore, chalcocite with some
carbondates, assaying up to 63% Cu, were possibly joint or other cavity
fillings. Parcels assaying 15%,
18% and even 46% Cu were sold to the Great Cobar smelting works.
The mine progressed to a depth of 91 m and sold ore to the Great Cobar
and CSA smelters. It was forced to
cease operations when the CSA smelters closed in 1920, leaving about 5000t of
6-10% Cu ore in sight. Most of the
ore mined came from the zone of secondary enrichment, which yielded over 2000
tons. The mine yielded the finest
azurite crystal groups obtained from any of the Cobar mines during the Great
Cobar period (Australian Museum specimens).
A little native silver was also obtained.
The Gladstone lode occurs in a siliceous `channel', and is located
where this meets an inferred wider shear zone (Great Cobar-Dapville-Gladstone
trend) striking about 342oT (Gray 1942, Thomson 1953).
The Gladstone lode channel or shear zone strikes 350oT.
It is marked by a very long line of brecciated quartz lode outcrops,
traced over a length of 1.6km to the north.
The Gladstone lode shear is sub-parallel to the Dapville lode shear,
and the Gladstone shear passes 365m east of the Dapville shaft.
The
Gladstone shear zone where mined is about 30m in width and carried weakly
mineralized lodes dipping 80oE. The
main orebody pitches steeply north and reaches a maximum thickness of 6m on
the No. 4 level. It is a siliceous
pyritic copper deposit with minor silver and little gold.
The primary ore is chalcopyrite and pyrite in silicified and
quartz-veined chloritic slate (70% SiO2).
Production was largely from high grade chalcocite
and chalcopyrite ores within and close below the enrichment zone, yielding an
average recovered grade of 6.3% Cu. Gossan
above the enrichment zone also yielded quantities of ore containing
chalcocite, nature copper, cuprite, azurite and malachite.
Massive chalcopyrite patches mined were up to 0.3m wide.
Weak chalcopyrite-bearing quartz vein mineralization occurs in an
envelope about 12m wide around the main orebody.
In 1942
New Occidental Gold Mines N.L. diamond drilled the Gladstone deposit, the best
intersection being a 5.8m true width of 5.18% Cu at 124m depth.
7. NEW COBAR-NEW OCCIDENTAL AREA
(Eastern Line)
This
linear group of deposits has long been distinguished.
It was called either the Occidental-Chesney-Tharsis line, or more
separately the Chesney line plus the Occidental line, by Andrews (1913); and
the Eastern Line by Russell and Lewis (1965).
The deposits along this line include three of the major producers of
the Cobar field (New Cobar, Chesney, New Occidental).
The term "Eastern Line" is broadly acceptable for these
deposits but it should be noted that different writers have used Eastern Line
to variously include all the easternmost Cobar belt deposits.
Many include the Peak as Eastern Line, and others have suggested Queen
Bee and Mt. Drysdale as extended repetitions.
The ores are more siliceous than those of the Great Cobar type.
The siliceous ore composition is typically about 80% silica and 8%
iron, compared with 17% silica and 41% iron in Great Cobar ore.
The Eastern Line has produced both gold and copper, but is more
renowned for gold than copper. The
presence of gold was known from as early as 1871 at the Chesney and New
Occidental mine areas, and the latter yielded a little good value stone in the
1870s. Not until the precious
metals boom of 1886-1887 did the gold prospects of the Eastern Line attract
any strong investment, and successful mining then soon commenced.
The larger mines now developed along the line are the New Cobar
(earlier Cobar Gold), Chesney and New Occidental.
The Mount Pleasant - Wood Duck segment of the line was also important
in gold production. The largest of
the mines, the New Occidental, incorporates three earlier mines: Great
Western, Albion and Occidental (in turn earlier known as the United gold
mine).
The New Cobar - New Occidental tract has a complex history with many
changes to the name, configuration and control of mining tenements.
Virtually the entire line had been prospected and variously secured by
the end of 1871. However interest
in the northern end (Fort Bourke hill) was relatively slight until 1887.
Individual mine production records are incomplete and are complicated
by group treatment practices. Early
workings along the Eastern Line were for gold, with about 30% of the gold
contents recovered by stamper battery, and most of the remainder by
cyanidation of battery sands and plant slimes.
Multiple cyanidation of tailings, with some degree of further crushing,
has taken place to date. In the
late 1980s separate tailings piles from former plants at New Cobar and Chesney
were removed by the New Occidental Tailings joint venture project (Ranger
Exploration NL & Cobar South Pty Ltd) to the New Occidental mine site
where the large dumps were already of a composite nature.
Significant annual production was derived from the tailings.
For example, retreatment of 802,187 t of these tailings in 1990
produced 788.92 kg of Dore bullion (70.9% Au, 13.9% Ag).
At the commencement of retreatment in 1987 the reserve was estimated as
2.7 Mt with 0.95 g/t Au recoverable.
The main centres along this line of
semi-continuous workings are listed below in the form of the latest or current
names:
No.
Deposit Name Commodities
66 Burns prospect Au
67 Tharsis mine Zn,
Pb, Cu (Au)
68 East Cobar Freehold mine Cu, (Au, Ag)
100 East Cobar mine Cu (Au)
101 Fort Bourke mine Cu (Au)
102 New Cobar mine Cu, Au (Pb, Zn)
103 Chesney mine Cu, Au
104 Burrabungie mine Cu
105 Mount Pleasant mine Cu (Au) (Pb, Ag)
106 Young Australian mine Au, Cu (Pb, Ag)
107
Wood Duck mine Au, Cu
108 New Occidental Au, (Cu, Pb, Zn, Bi)
(Occidental, Albion and
Great Western)
The major ore systems, New Cobar, Chesney and
New Occidental, roughly correspond to three topographic rises, Fort Bourke
hill, Tabor hill, and United hill respectively.
The mines and prospects of the Eastern Line
are close to the contact between the Great Cobar Slate and the Chesney
Formation. The importance of this
contact, known locally as the slate-sandstone contact, has been stressed from
the earliest years of development. As
the Great Cobar Slate is more prone to weathering than the Chesney Formation
sandstones, all the deposits along this contact occur on the western flanks of
the abovementioned hills. The
contact is commonly marked by either a massive veined and silicified zone 1-2
m wide, or by clusters of thin quartz veins emplaced over widths of some
metres. Although most have
regarded the contact as faulted, there is much difference of opinion in the
literature. Some writers have
suggested or implied that the contact is basically stratigraphic, with only
limited local movement (e.g. Gilligan and Suppel 1978, Sangster 1979).
Conolly (1946) and Thomson (1953) regarded the contact as essentially
concordant but attenuated by faulting along the western underlimb of an
anticline ("stretch thrust"). Rayner
(1969) stated that sandstone beds at Chesney mine can be seen dipping
southwestwards into the contact thrust plane, where they become truncated or
sheared out by extreme attenuation. Andrews
(1913) gathered much evidence of large scale faulting.
He noted that all the way between the New Occidental and Young
Australian mines, the strike of the Chesney Formation sandstones is inclined
at 5-15o to the general directions of the lodes.
Even where the sandstone beds almost parallel the lodes in strike, the
dips are opposed. Mapping by Glen
(1987), likewise showed the mineralized shears of the New Cobar-New Occidental
group as definitely truncating the bedding in the western limb of the
anticline (Chesney-Narri Anticline), with the fault line cutting the axial
trace in the Tharsis mine area. Glen
(1987) estimated that along the length of the Great Chesney fault zone at
least 1 km of Chesney Formation section is lost.
Considerable loss of Great Cobar Slate may also have occurred.
In general
the host rocks dip west-southwest at steep angles (60o-85o), whereas cleavage
dips easterly at steep angles (80o). Shear
zones which host mineralization tend to parallel the cleavage.
Thrust faulting along the shears has been suggested (Mulholland and
Rayner, 1953) with upthrust on the eastern side.
The bedding planes of the sandstones within the Chesney Formation are
in places seen dipping into, and being truncated by the shear zones.
The Chesney Formation sandstones along the Chesney fault zone commonly
strike a little west of the fault direction.
Bedding traces within the slate near the contact are generally obscure
or obliterated by shearing. Some
traces which may represent bedding, and which dip south, have been observed
along the fault zone. The
mineralization is largely confined to the Great Cobar Slate side of the
contact but this could hardly be mistaken for a syn-sedimentary effect.
Although assymetrical about it, mineralization is clearly related to
the disturbed contact. For
example, disseminated sulphides and quartz-sulphide veinlets around the
Chesney deposit may form an aureole 60-90 m wide in both the Great Cobar Slate
and the Chesney Formation. Many
orebodies are situated short distances to the west of the contact (Tharsis,
Fort Bourke, most New Cobar lenses, Chesney, most New Occidental lodes).
Others lie along the contact, or within the Chesney Formation.
This is further detailed under the individual deposit descriptions.
Although the Eastern Line mineralization is by no means confined to the
Great Cobar Slate immediately west of its contact with Chesney Formation, much
of the best ore is in that position and it is probably true that the ore
systems are narrower overall than those of the lines of mineralization which
occur further west in younger host rocks.
A
particularly large example of sinistral buckling produces a westwards step in
the slate-sandstone contact at New Cobar.
Using this, some writers (e.g. Brooke 1975) divide the Eastern Line
into a northern Tharsis - New Cobar segment and a southern Chesney - New
Occidental segment; the latter being a parallel line of shearing offset a
little to the east. Others have
subdivided the Eastern Line even further in this manner, into a series of en
echelon segments. In detail at
particular sites, several sub-parallel shear zones may be recognizable (e.g.
New Occidental). The "lode
shears" of the southern part of the Eastern Line have been variously
correlated by different geologists. The
one which has been correlated to greatest extent north-south is that which was
named the Gossan Lode at new Occidental mine.
Some workers have seen this as a principal line of movement, traceable
north as far as the Mount Pleasant mine and to the south extrapolated to pass
just west of the Peak mines.
The mineralization present in the New
Cobar-New Occidental area is varied. It
includes disseminations and veins but the chief lodes occur as irregularly
shaped lenses of altered and mineralized slate.
The dip of all major lodes is steeply to the east, parallel to
cleavage, and pitch is steeply north. Low
grade mineralized zones are widest in the northern part of the area, up to 90
m at Chesney and 122 m at New Cobar.
Orebodies are gold-bearing along the full
length of the Eastern Line. Individual
gold grains range from 950 fineness through to a minority which are
gold-silver alloy or electrum. Early
workings were mostly commenced for gold and some encountered significant
copper orebodies. As workings
along the Eastern Line extended below the depth of oxidation and leaching,
copper gradually came to seriously impair the efficiency of gold extraction.
Some of the original companies failed in this transition and others
were newly formed to adopt more appropriate technology.
A general course of events, first faced at the Chesney, was that the
free milling and cyaniding lodestuff gave place at depth to ore which was
unamenable to previous methods of treatment for gold recovery and yet not
payable as copper ore. In some of
the smaller operations (Mount Pleasant, Young Australian, Wood Duck, etc),
natural copper enrichment did furnish a temporary payable alternative to gold
in the supergene zone. The future
for the larger deposits, the Chesney and New Cobar, was more complex and saw
costly experiments in leaching and mechanical concentration processes.
The first accumulating cupriferous tailings were leached with sulphuric
acid and later cyanided.
Zinc, lead, silver and bismuth are also
present in small but noteworthy amounts. Mineragraphic
studies for both the New Cobar and New Occidental mines have found close
associations between gold and bismuth minerals, and also gold concentrated in
sphalerite-galena veins. Concentrates
produced from direct cyanidation by New Occidental Gold Mines NL contained
appreciable bismuth (Hart 1936a) and the New Occidental Mine in particular
recorded the presence of thin bismuth-rich veins.
The deposits consist largely of siliceous replacements, and the
silicification is best developed within the Great Cobar Slate.
The slate in the mineralized zones is frequently chloritized or
converted to chert ("elvan"), and some brecciation is observed at
most mines. Veins are anastomosing
or stockwork-like in places and are commonly associated with areas of
pervasive silicification. Wisps of
chloritized slate are common in some of the quartz veins near the Great Cobar
Slate-Chesney Formation contact. Chlorite
also occurs concentrated as pods and veins.
The microscopic structure of the vein chlorite is typically vermicular.
Lines of inferred movement and mineralization
have been variously mapped and correlated along the Eastern Line by different
workers. Some have mapped a number
of en-echelon shears, in contrast to the mapping by Andrews (1913) which
depicted one continuous plane of principal movement (Great Chesney Fault)
extending all the way from East Cobar Freehold mine to the New Occidental
mine. Andrews viewed this as one
traceable mineralized line of powerful dislocation, lost only where it crosses
a major broad watercourse between the Burrabungie and Mount Pleasant mines.
North of this break he referred to the line of dislocation as the
Chesney Lode (extending through the Chesney and New Cobar mining areas), and
south of the break he identified the main fault with the Gossan Lode.
He also grouped the succession of mines south of the break as the
Occidental Line (the early Mount Pleasant, Young Australian, Wood Duck,
Occidental North, Great Western, Albion and Occidental properties), regarding
this as a line of movements probably supordinate to that along which the
Gossan Lode lies.
Tharsis Mine and Nearby Shafts
The
Tharsis mine and nearby shafts, at the northern end of the New Cobar-New
Occidental area, are prospects which tested auriferous quartz veins in shear
zones. The northernmost workings
consist of several shafts extending north of the main Tharsis shaft, along the
western side of the Great Cobar Slate-Chesney Formation slate-sandstone
contact. The workings are along a
quartz-veined zone of shearing and brecciation. They were primarily for gold although the Tharsis mine did encounter
sulphide ore in small amounts. At
the Burns prospect steeply east dipping veins 20-25 cm wide assayed up to 15
g/t Au.
The Tharsis workings encountered minor
sulphide mineralization below 79 m, in a 2.4 x 18 m ore shoot plunging steeply
north. The ore shoot has
approximately the same planar orientation as the cleavage. Mineralized quartz veins within the shoot are also oriented subparallel
to cleavage, although some transgressive veins occur. Records indicate that ore was sold to the Great Cobar smelter but this
was probably in small quantity only. Ore
consists partly of quartz veins with pyrite and minor chalcopyrite, and partly
of massive sulphide intervals. The
latter includes thin bands consisting almost entirely of sphalerite.
The Tharis mine has been recorded under a
number of names: "Ivanhoe",
"East Cobar Copper-mine Extended", "Cobar Tharsis" (Carne
1908); the "Cobar-King George Syndicate (late Tharsis)" (Australian
Mining Standard 1911); the "King Edward (late Tharsis)" (Department
of Mines Annual Report, for 1911), the "Phoenix" (also listed as
Tharsis or Royal George) (Department of Mines Annual Report, for 1912), and
the "Diane" mine (Nickel Mines Ltd, 1970).
The main shaft is collared in Great Cobar Slate 120 m west of its
boundary with the Chesney Formation. Lesser
workings are situated within the Chesney Formation, on quartz veins assaying
up to 17 g/t Au.
In the main lode zone, crushing and
brecciation may be prominent, indicating that mineralization is probably
localized in a shear zone. Workings
extend to a depth of 106 m and erratic ore was encountered below 70 m.
The primary ore consists of sphalerite, galena, chalcopyrite, pyrite,
marcasite, and pyrrhotite, with minor covellite, digenite, bornite and
arsenopyrite. Mineralization is
distributed irregularly in bands in the host rock and in patches and veins of
quartz. Carbonate veinlets are
also common in the chloritic host rocks. The
planar ore shoot dips approximately 85o to the west, and plunges north at
about 75o.
At the 232-foot (70.7 m) level there is
marked supergene enrichment, with covellite, chalcocite, and copper and iron
oxides associated with residual primary sulphides.
Malachite and azurite are present on this level away from the main ore
concentrations, and malachite also ranges through higher levels.
East
Cobar Freehold Mine
South of
the Tharsis mine the slate-sandstone contact was prospected at the East Cobar
Freehold mine, close to the Nyngan-Cobar railway line.
A lode dipping steeply to the east was encountered.
It was 1.2-1.8 m wide, comprising carbonates and secondary sulphides in
slate. A sample of best selected
ore assayed 7.75% Cu, 49 g/t Ag and trace gold. Later
drilling encountered no significant primary ore.
The secondary mineralization probably derived from minor sulphides in a
quartz-veined slate interval.
New Cobar Mine
New
Cobar open cut showing lode in the northern wall, and entry portal at
base. (Photo: Yegor Korzh)
Close-ups
of northern wall showing old drives at the lode. Also note
the strata (near-vertical) changing
downwards
from weathered reddish to fresh and dark grey. (Photos: Ken
McQueen)
Work
at the New Cobar underground portal at base of the New Cobar open cut in
2004. (Photo: per Copper City Motel)
Most of the early small holdings with
distinct prospects along Fort Bourke hill (the Jubilee, Fort Bourke Tunnel
Co., etc.) were amalgamated under a company floated in England, Cobar Gold
Mines Ltd. The principal workings
then became known as Cobar Gold Mine, which was at a still later period
renamed the New Cobar mine. Not
incorporated in the Cobar Gold amalgamation was the Fort Bourke copper mine
(Fort Bourke Copper-mining Company), which prospected the actual
slate-sandstone contact as the then inferred northern continuation of the
Chesney line.
The
New Cobar deposit has been a major past producer and was for a time the major
gold mine in the State. It is
estimated to contain a further 1.3 Mt ore at 6.4g/t Au and 0.9% Cu recoverable
underground, and 0.7 Mt at 2.7 g/t Au and 0.4% Cu recoverable by open pit
mining (Cobar Mines Pty Ltd, 1990). The
deposit in the past has produced about 1.0 Mt of ore averaging 7.0 g/t Au and
1.0% Cu. The deposit has been open
cut in the past and this excavation later re-filled.
Shallow drilling by CRA (Cobar Mines Pty Ltd) found that not all of the
near surface gold has been recovered and obtained some good intersections
(e.g. 9.00m of ll.08 g/t Au, 5.12m of 13.17 g/t Au).
Gold assays from the deposit range to 30 g/t Au and in richness some of
the gold ore encountered is comparable to that of the New Occidental mine.
Occasional colloform ore texture, elvan, and presence of magnetite
strengthens the comparison with the New Occidental ore system.
The principal ore type mined consisted of
slightly silicified slate with masses of coarse white slate and elvan.
Less often, payable gold was present in little silicified slate,
particularly at shallower depths. Sulphides
typically comprised 10-15% by weight of ore mined during the years that the
mine was worked by Great Cobar Ltd. Chalcopyrite
was widely distributed, whereas galena and sphalerite occurred as more
localized concentrations. Ore from
the zone of secondary enrichment could average about 2% Cu and some went
directly to the smelters.
Most
of the orebodies in the New Cobar deposit lie just south of a sinistral step
in the faulted Great Cobar Slate-Chesney Formation contact (Fig. NCC,A).
Despite the strong surface indications, the siliceous line of lode,
about 210 m long by 12 m wide, was not strongly prospected for gold until
1887. The hill was first worked by
the Fort Bourke Tunnel Company, and later taken over by Cobar Gold Mines Ltd.
In the early workings payable stone was obtained at intervals along the
line of lode. A 100 head stamp
battery and cyanide plant was operating by the close of the century.
Early operations was generally successful down to the 69 m level but
below this unoxidized sulphides and increasing copper content were a cause for
concern, with free milling becoming less and less productive.
In an effort to more economically treat the underlying primary ore
various means of concentrating the sulphides were tried, including an
unsuccessful Elmore concentration plant installed in 1907.
The mine
was purchased by Great Cobar Ltd in 1910, and was then better able to treat
the primary gold-copper ore below 100 m depth.
Ore extracted at that time averaged 1.3% Cu.
After the 1919 Great Cobar closure, Great Cobar Ltd miners continued
exploring the Cobar Gold mine into 1920 under a receivership plan for the
company to consider switching from copper to gold production.
This plan did not eventuate and the mine closed.
The mine was next worked in 1937-1948 by New Occidental Gold Mines NL.
The latter company changed the mine name to New Cobar.
Production for 1937-1948 was from ore averaging 5.7 g/t Au and 0.9% Cu.
The mine is developed to a depth of 342 m, in nine levels.
Some near-surface auriferous stone remains at New Cobar, and Cobar
Mines Pty Ltd 1989 re-opened the southern end of the open cut for bulk
sampling.
The New
Cobar zone of mineralization is up to 125 m wide at depth, and low grade gold
values have been reported as continuous widths as great as 75 m.
The main mineralized shear zone has three cross-fractured orebodies
developed along it. From north to
south these are the Jubilee, Northern and Southern orebodies.
A 200 m length of mineralized zone was worked almost continuously in
the upper levels, encompassing the Northern and Southern orebodies.
This was known as the Main Lode. Secondary
enrichment is presumed and higher gold and copper values were recorded to
occur in horizontal layers. The
Jubilee orebody is the shortest lens, about 46 m long.
The Main Lode was mined over widths as great as 6-12 m along much of
its length. Below No. 3 level it
resolved into the separate Northern and Southern primary orebodies which
averaged 1.3% Cu and 6-8 g/t Au. These
orebodies commenced to pinch out below No. 6 level.
The Jubilee shoot, some 76 m further north, is 3-5.5 m wide.
It was much less auriferous, grading 1-2% Cu and 1.5 g/t Au.
Below No. 8 level a fourth orebody, the Western Lode, was discovered
about 60 m west of the Southern orebody. This
narrow Lode, apparently localized by cross-fractures, carried 9-12 g/t Au.
It was proved for a length of 45 m and delineated on the No 8, 9 and 10
levels. A deep drill hole
intersected it 274 m below No 9
level as 4.5 m of 13.9 g/t Au. Most
of the mineralization occurs in replaced, silicified and brecciated slate
masses; apparently with strong
development of ore and quartz veining in cross fractures (Gray 1918).
Cross fractures are also mineralized away from the lode channels.
Patches of ore occur along cross fractures between the Southern and
Western lodes. Major fracture/lode
intersections pitch steeply and are marked either by ore development or bulges
of silicification, sometimes forming shoots of siliceous ore with considerable
continuity. Predominant ore
minerals are chalcopyrite, and native gold.
Pyrrhotite is prevalent over pyrite,
and there is minor sphalerite, galena, bismuthinite and bismuth.
Disseminated pyrrhotite and chalcopyrite, with small quartz-sulphide
veinlets, occur around the lodes. One
drillhole penetrated a 122 m zone with disseminated sulphides, averaging 0.7%
Cu.
Much
of the New Cobar mill tailings were retreated at New Occidental mine in 1945
for further gold recovery. The
remainder of the New Cobar dumps was likewise removed and retreated by Ranger
Exploration NL in 1990.
The
New Cobar deposit was further mined by Peak Mines, by an open cut commenced in
2000. Open cut operations were completed in February 2004, providing a
total of 1.0 million tonnes of ore. Following that, mining
progressed underground in July 2004, to continue to provide mill feed for the
copper/gold processing plant at the Peak mine. Re-commencement of
futher mining at the Chesney mine to the south was also planned. .
Chesney
Mine
Chesney
mine, 1908. (Postcard)
The Chesney mine
s located approximately 0.6 km south of New Cobar and approximately 1.8 km
futher north than the New Occidental deposit.
The Chesney is one of the major Cobar
deposits and probably one of the least exploited.
It was noted (1989) as likely to be further mined in future.
The Chesney mine, also known as the Chesney-Cobar, has produced about
0.7 Mt of 1.7% Cu and 2.5 g/t Au ore. The
reserve to 830 m depth is 5.0 Mt at 2.2% Cu and 0.2 g/t
Au. Great Cobar Ltd
selectively extracted richer copper ore grading about 2.8% Cu.
During the New Occidental period of mining history, the Chesney deposit
was generally exploited less than the New Cobar because of the emphasis on
gold at that time. The Chesney
mine was originally worked for gold alone, and no trace of copper ore was
discovered till 46m depth. With
increasing depth it became better known as a copper mine.
Both Chesney and New Cobar produced appreciable quantities of copper
required during World War II.
As
with other major ore systems along the Eastern Line, the main bodies of ore at
the Chesney mine are located in the Great Cobar Slate close to its faulted
contact with the Chesney Formation. The
ore is strongly siliceous in large part and is hosted in sheared, crushed and
altered slate. The sulphides are
essentially disseminated throughout, with little or no development of strong
sulphide bodies like those found in the Great Cobar mine.
Although the ore system is essentially confined to the Great Cobar
slate, deep drilling has also found a little chalcopyrite-quartz ore on
eastern side of the contact, well within the Chesney Formation.
Ore potential is some 11Mt of copper ore with minor gold values.
The gold content appears to decrease at depth.
As elsewhere along the eastern line, the orebodies plunge steeply
north. They dip steeply east
parallel to cleavage, shearing and the slate-sandstone contact thrust plane.
Compared with New Cobar ore, the average ore mined from Chesney showed
less replacement of the Great Cobar Slate, was considerably poorer in gold and
richer in copper. Much ore was
mined at about 2.5% Cu grade, similar as in the deeper levels of the Great
Cobar mine. In the zone of
secondary enrichment, Chesney ore graded about 4 % Cu.
The Chesney was the first of the major Cobar
mines worked for gold. Prospecting
for gold began at the Chesney in 1871. It
was invigorated in 1887 when mining proved successful elsewhere along the
Eastern Line. Two tons sent from
outcrop to the Sydney Mint in 1887 yielded 138 g/t Au.
The Chesney-Cobar Gold-mining Company N.L. was floated in 1888.
A large quantity of auriferous silicified or otherwise impregnated
slate from the oxidized zone was treated by battery and amalgamation.
After copper was encountered at 46m a new company was formed (Cobar
Chesney Company) in 1898 and the mine underwent extensive development in 1899.
About 4500t of ore treated at the Great Cobar smelter in 1901 yielded
3% Cu. The Chesney was absorbed by
the Great Cobar company (then Great Cobar Syndicate) in 1903.
This occurred after vain expenditure of capital fighting the
difficulties of a separate existence after the free-milling gold ores had been
exhausted. During the period 1904
to 1918, the primary gold-copper ore from Chesney mine was mostly blended as a
siliceous flux in smelting at Great Cobar.
Use of Chesney ore as flux was at first high (27%) but it was used in
decreasing amounts after Great Cobar Ltd also acquired Cobar Gold Mine (New
Cobar). Some tailings from the
Chesney Company mine were also smelted. However,
a concentrator was also built to replace sending the entire ore to the Great
Cobar smelting works. This step
was taken after the rich gold-copper shoots were encountered at depth in
1910-1911. The importance of
concentration rested on the fact that the siliceous ores availabe to Great
Cobar Ltd after its purchase of Cobar Gold Mine increased well beyond the
amount able to be smelted with the basic ore obtainable from Great Cobar and
other minor sources.
After the Cobar Gold Mine purchase, Great
Cobar Ltd selectively mined the Chesney ore to average 3.5% Cu.
Chesney closed in 1920, not to re-open until 1943, under New Occidental
Gold Mines NL. From 1943 to 1952,
the mine produced 338,137 tons at a recovered grade of 1.9% Cu and 2.6 g/t Au.
Employment was 144 men in 1950. The
mine has been well maintained since is 1952 closure and drilling indicates a
resource in excess of 4 Mt of 2.7% Cu. In
anticipation of resuming production, Cobar Mines Pty Ltd sank a new shaft in
1972-1976 to a depth of 1005 m.
Two
lines of lode exist, about 15-20m apart. The
East Lode comprises low grade mineralized sheared slate and quartz along or
near the faulted contact of the slate and sandstone.
Massive quartz developed along the East Lode is up to 1.2 m thick for
considerable lengths, and the East Lode itself may be strongly siliceous over
widths as great as 2.5 m. Gold was
said to have occurred in shoots, and at depth up to 3.5% Cu develops in the
Lode. This line was worked to
shallow depth by the East Chesney gold mine.
The principal vein in the East Lode, the Chesney East Vein (1.2m),
was not worked in the primary zone and was probably only payable down
to water level.
The Main Lode occurs 30m west of the Great
Cobar Slate-Chesney Formation faulted contact and is situated entirely in
sheared slate. The Main Lode was
typically 12m wide and chip sampling along cross-cuts through the lode
averaged 3-8% Cu in the upper levels. Lower
level working average was 3% Cu in 1907. The
overall average is 3-4% Cu. Greatest
width reported in the main lode is 28m. The
Main Lode comprises for much of its length a north-northwest trending (345o)
mineralized shear zone which dips steeply to the east (85o).
At each end there are gold-rich shoots where tensional breaks of more
westerly trend occur. Approaching
either end of the Main Lode there is observed a sinistral swing of cleavage
and a thickening of veining and mineralization into the pipe-like gold-copper
orebodies, known as the Northern and Southern gold shoots (Fig. NCC).
Each of these is about 30 m long and pitches steeply north.
In each gold shoot northwest-trending cross-fractures, the gash
fractures of Sullivan (1950, 1951), are commonly filled by vein quartz.
The zones of mineralization are widest, and the gold content is
greatest, where these fractures reach their major development.
These Au-Cu shoots have attributes expected in mineralized fault jog
tension areas. The total length of
the Main Lode system is 244 m. The
Au-Cu shoots are up to 24 m wide but are usually 6-9 m in width.
The Northern gold shoot persists to the surface but the Southern gold
shoot does not make any noticeable appearance until No. 6 level.
The Main Lode was worked to 282m depth.
Greatest width was 24m, but most workings do not exceed 9m.
The mining records suggest that the pipe-like Northern gold shoot was
followed down directly from the surface, pursuing a downwardly continuous
thread of very rich stone (90 g/t Au) procurable at its northern tip.
The Chesney ores are siliceous.
Chalcopyrite is the dominant ore mineral, with minor sphalerite,
galena, iron sulphides, arsenopyrite and native gold.
The gangue is chiefly quartz, chert, and chloritic slate host rock.
Pyrite, pyrrhotite, and subordinate magnetite occur in the slate
between ore developments. Ore
minerals are disseminated and in addition chalcopyrite forms irregular veins
up to 8cm thick.
Oxidation
extends to below 76m. Copper
carbonate ores were obtained at 45-75m and secondary enriched chalcocite ore
at 75-107m. The grade and tonnage
of primary copper ore mined at Chesney by New Occidental Gold Mines NL was
initially low but gradually improved with depth.
At the No. 8 level, length, width, and assay grade had increased to
2,100 tons per vertical foot at 2.55% Cu.
Deeper drillhole intersections have shown up to 11.3 m of 3.3% Cu
(Russell and Lewis, 1965). Driving
along the Main Lode, beyond the Northern gold shoot on No 4 and No 8 levels,
revealed possible ore further north. Two
cross-cuts averaged 11 m of 1.7% Cu and 12 m of 1.4% Cu.
The upper levels of the mine also produced a little 6% Cu secondary
(carbonate) copper ore.
The mine closure in 1952 halted further
underground exploration in the vicinity of the Chesney ore system.
However, in 1966 Cobar Mines Pty Ltd announced plans to sink a deep
shaft (900 m) near the Chesney mine. It
was envisaged that such a shaft would be useful both to develop the Chesney or
other nearby Cu-Au ore bodies and to allow possible deep level investigative
driving towards the other nearby mines (New Cobar, Great Cobar, Gladstone,
Dapville, New Occidental) (World Mining, December 1966). The new shaft (No.3) was sunk in 1971-1972, but promising discoveries
first at CSA mine and later at The Peak took precedence over any exploratory
driving beneath the Cobar field central area.
Following the
development of the New Cobar open cut, the Chesney mine was further accessed
from the New Cobar workings via a 700m long decline. The Chesney orebody was
brought back into production in April 2009.
Burrabungie
Mine
The
non-productive Burrabungie workings (also referred to as the Berribungie or
Phoenix workings) were commenced about 1885 and are 91 m deep.
They were sunk to cross-cut under a wide gossan outcrop along the
Burrabungie-Mt Pleasant shear zone. The
primary mineralization was found to comprise about 3 m of pyritic ore, partly
massive, with only sparse chalcopyrite. Copper
was met with in 1897, some veins of good copper ore were discovered in 1901 at
a depth of 88m, and a minor quantity of 3.0% Cu ore probably exist.
No output is recorded. A
cross-cut in the Great Cobar Slate east of the ore encountered narrow
intervals rich in malleable copper. Further
east, a moderately strong siliceous lode developed alongside the Great Cobar
Slate-Chesney Formation contact contains brecciated slate and quartz, with
minor pyrrhotite and chalcopyrite at depth.
Mt Pleasant Mine
Winze
in the Mt Pleaant mine early workings. (Photo: Mines
Department)
Commenced
in about 1884, this mine was first worked for gold but copper was met with at
48m depth and secondary copper ore was being raised there on tribute by the
Copper Mine Syndicate in 1898. The
mine was for a time known as Cobar Copper Mine, after a company (Cobar Copper
Limited) formed to work the Mount Pleasant and Young Australian mines.
The oxidized lode material mined for gold by the Mt Pleasant Gold
Mining Company up till 1895 averaged 6-9 g/t Au and plant included a 10-head
battery. The mine was let on
tribute to the Great Cobar syndicate after 1899, which company extracted a
considerable quantity of smelting ore in 1900-1901.
The workforce was about 20 men. Peak
copper production was 152t of copper in 1902, from ore averaging 6.4% Cu.
Earlier picked ore was as richer, and the first trial parcel of copper
ore returned 27% Cu at Cockle Creek smelting works in 1898.
The
workings are 161 m deep and they exposed three lodes over a width of 91m.
The middle lode was mined for gold in the oxidized zone.
The eastern and middle lodes lie in the Chesney Formation and in the
primary zone typically contain up to 1.2 m of 1.5% Cu.
Best drilling intersections are 1.5% Cu over 4.6 m and 3.4% Cu over 5.5
m. The lodes contain seams of
massive chalcopyrite and ore averaged 2.6% Cu where driven on in the primary
zone along the eastern lode. The
western lode is a massive siliceous development along the slate-sandstone
contact. It is about 2.4 m wide
and contains around 2% Cu. The
best drilling intersection is 2.67% Cu over 6.1 m.
Indications of extensive low grade mineralization occur between the
western and middle lodes. One
drillhole encountered a zone of strong quartz veining which extends from just
within the slate for some 21 m into the sandstone, and contains 0.65% Cu
overall. This may correspond with
one of the western cross-courses in the mine which averaged 1.89% Cu over 24m.
Young Australian Mine
The Young Australian (a.k.a. Young Australia)
mine produced 0.01 Mt of 4.2% Cu and 4.3 g/t Au ore.
It was first worked for gold and encountered copper ore at depth (48m)
which was sold to the Great Cobar smelting works.
Lead and silver were also reported to occur.
Three lodes were recognized over an interval of 90m.
This
mine was commenced about 1891 and developed a deep (30 m) pipe-like open cut.
A gold shoot lay immediately west of the main gossanous lode.
The lode yielded some good specimen gold, some large pieces of ore
being on rare occassion thickly covered with visible gold.
A peak annual production, 30.7 kg Au from rich (16.2 g/t Au) siliceous
ore, was reached in 1896 and plant included a 15-head battery.
The mine was taken over by Cobar Copper Limited in 1906 or 1907.
Not all the tailings from the plant are from the mine's own ores, as
the Young Australia Gold Mining Co. at times treated material from the Chesney
mine.
The siliceous lode (Main lode or Middle
lode), situated just west of the contact with the Chesney Formation, is 6 m
wide and in places showed silicified brecciated slate fragments.
Chalcopyrite and pyrrhotite are disseminated throughout but economic
copper grades are probably restricted to the secondary zone.
An 18 m strike length of secondary copper ore lay to the east of the
gold mineralization, in a narrow lode (East lode) extending towards the Wood
Duck shaft. The East lode is about
1.2m thick. The West lode, about
2.4m thick, was also mined for copper. Oxidized
copper ore sold to the Great Cobar smelter assayed up to 8.71% Cu.
Some of the copper ore produced was rich in gold, with 13.2 g/t Au in
one assay of 4.4% Cu ore.
Wood
Duck Mine
This is on
the same mineralized and partially brecciated shear zone as the Young
Australian mine, and the two workings are connected at depth.
Records of the Wood Duck are sparse.
It was sunk on a 1.2 m wide ill-defined gossanous siliceous lode,
locally containing about 25 g/t Au. The
lode was considered to be the northern continuation of the Gossan lode of the
New Occidental mine. The mine
probably produced over 3 kg Au before being combined with the Young
Australian. The lode channel is
quartz veined, brecciated and silicified.
It is 1-3 m wide and lies within the Great Cobar Slate close to the
contact with Chesney Formation. Copper
content is 2-3% in the zone of enrichment.
New Occidental Mine
The New
Occidental mine, formerly named the Occidental, was one of the State's major
gold mines. It was developed in
two major phases, by Occidental Gold Mines N.L. and the subsequent New
Occidental Gold Mines Ltd. The
mine is estimated to have produced a total of 2,070,000 t of ore at a
recovered grade of 9.6 g/t Au over the two operating periods 1889-1921 and
1935-1952. In the 1935-1952 period
it produced 1,440,989 t at a recovered grade of 10.1 g/t Au.
The New Occidental ore system continues at depth.
Factors causing the New Occidental mine closure were rising costs, and
decreases in ore system grades and cross sectional area.
The company, in foreseeing the perils of accelerating costs against
fixed gold price, applied to the Federal Treasurer in 1945 for assistance in
major diamond drilling, with a view to diversification and recommencing major
copper production at Cobar. In
1946 its Cobar field Central area drilling programme was commenced.
Evaluation of reserves indicated strong promise for recommencing major
base metal production at Cobar, but the company's failing cash flow forced it
to cease all mining and related activity in 1952.
Propecting
at United Hill commenced in 1871. The
Occidental mine area was taken up under lease for copper in 1872 and became
known later as the United gold mine. The
United was the southern of three gold mines commenced along a zone of crushing
on the western fall of a low ridge (United Hill, Andrews 1913).
Prospecting work has been done on leases continuing further south,
recorded variously as South Occidental mine, Hartley's mine, or Great Cobar
Gold Mining Co. Details are
unknown but the work is presumed to have returned negligible prospects.
Mines north of the United (Occidental) mine, the Great Western and the
Albion, were acquired after 1904 by the Occidental Company and the underground
workings later connected. The
United mine was renamed the Occidental in 1889, in which year it commenced
major long term production. Almost
continuous production was maintained up to the closure in 1921.
It was re-opened in 1935 as the New Occidental.
The principal gold orebody of the
Albion-Great Western-United group of mines proved to be on the United property
and it is here that the large Occidental open cut was developed.
The major mineralization occurs immediately south of a sinistral step
in the slate-sandstone contact, with the Great Western and Albion
mineralization being located along the northward continuation of shearing
where it enters the Chesney Formation (Fig. NO).
The ore system comprises six lodes or orebodies, and the main workings
are south of the sinistral step in the faulted contact of Great Cobar Slate
and Chesney Formation. The step is
possibly a cross-fault between two shear segments.
Lodes
in the Chesney Formation
The main
mineralization of the New Occidental ore system occurs in the Great Cobar
Slate. However, a line of
relatively minor but locally rich lodes occurs along the easternmost line of
shearing which proceeds northerly across the north-northeast formational
contact, into Chesney Formation. This
is the Albion-Great Western trend, near the southern end of which is a small
lode known as Bowman's. Further
north is the Great Western or Albion lode which was worked above No. 2 level
over a length of 30m and a width of 3m. The
Albion lode is recorded to plunge 40oS, possibly a supergene effect.
The Albion lode does not extend south as far as the Main Lode of the
New Occidental Mine, although its containing shear can be traced south.
The shear connecting the Albion lode and passing along the eastern side
of the Occidental (New Occidental) main lode is gently concave to the east, as
are others in the vicinity.
The Albion mine itself reached a peak annual
production of 33.06 kg Au in 1895, and worked ore as rich as 35 g/t Au.
Similarly, the Great Western Mine obtained 13.58 kg Au from a quantity
of ore averaging 28 g/t Au in 1899. Minor
native copper occurred in the gold ore. Later,
the Albion lode was worked by the Occidential mine via its open cut, in the
sandstones to the north and east of the cut.
Although gold values were not found to be payable as far south as the
Occidental's Main Lode (Big Lode), it was considered that the Albion shear
could be traced south. Ore showing
strongest native copper (2-3%), as leaves deposited in the slaty cleavage, was
handpicked and sent to Dapto smelting works.
This class of ore, probably containing supergene gold was very rich and
small parcels averaged around 115 g/t Au.
Andrews (1913) considered the Albion lode to
be a separate small rich gold shoot, and not a lode continuing south to the
Occidental workings. Nonetheless
it is along the southern extension of this easterly shear line that the
richest gold values of the New Occidental property were recorded.
A narrow series of subparallel magnetite veins to the east of the New
Occidental main lode was known as Bowman's (a.k.a. East lode) lode in the
Great Western mine holding, and as the "Indicator" or Jack Johnson
lode in the Occidental mine. Gold,
galena, native bismuth, bismuthinite, tetradymite, galenobismutite and pyrite
accompanied magnetite in that part of the workings.
This vein set pinches out well north of the New Occidental main shaft.
At that point it is tangential to the tapering lens of the Big Lode,
trending north-south while the Big Lode trends 340oT.
The Indicator or Bownan's lode comprises thin magnetite-rich veins
occurring over a maximum of 1 m width of slate, with individual veins up to 15
cm wide. The entire 1 m interval
may be strongly enriched in both gold and bismuth.
Samples have assayed up to 14.4% Bi and 1053 g/t Au.
Much of this gold is visible whereas the gold in the Main Lode is
generally invisible. In the New
Occidental workings, this unusual lode was opened down to No. 9 level and
recognised in drillholes on No. 10 level (340 m).
Lodes in the Great Cobar Slate
The main
New Occidental mineralization lies south of the cross-fault deflection of the
slate-sandstone contact. Here
three lodes are present, known from west to east as the Gossan Lode, the West
Leg and the East Leg. The southern
half of the latter two almost join together on several levels underground to
form the Main Lode (a.k.a. Big Lode). It
was this lode plus the northern extension of the West and East Legs that was
mined. The Gossan Lode (a.k.a.
West Lode) is low grade. It has
been little prospected by underground workings, and to a depth of 73m only.
The lodes dip 85o to 90oE, with oreshoots pitching 90oN.
The Eastern Leg occurs within the slate at the contact of sandstone and
slate, and is separated from the Western Leg by highly cleaved slate.
The lode has been mined over a strike length of 150 m.
The legs are almost parallel in upper sections of the mine, but
converge to the south below No. 7 level to form an orebody some 21 m in width.
This wide Main Lode ore development persists in depth, and has been
mined to the No. 14 level 568 m deep. The
Main Lode is up to 28 m wide.
At its
northern end the Eastern Leg swings 20o westerly and appears to follow the
sinistral step in the slate-sandstone contact (Sullivan 1951, Robertson 1974).
This direction parallels prominent-quartz filled fractures which cut
across the Main Lode at angles of about 20o in plan view (Mulholland and
Rayner 1961). The eastern margin
of the mineralization is sharp and can be visually located to within a metre
in diamond drill cores. The
western margin is less obvious. The
most westerly mineralization, west of the Gossan Lode, consists of galena and
sphalerite in an interval of intense calcite replacement of slate west of the
main shaft. This minor lode
extends between No. 4 and No. 5 levels.
The bulk
of the mineralization, in the Main Lode, is fairly obviously delineated by
high silica content. Silicification
takes two forms - quartz veining and alteration.
Drill cores show multiple phases of quartz veining.
Veins can be tabular, rounded, ptygmatic, colloform or vuggy.
Alteration varies from weak to intense, the latter giving rise to
"elvan" like that of the CSA mine Western system.
The silicified rock contains patches of incompletely replaced slate and
chloritised slate. These may be
cuspate or wispy, and they vary in size up to a few centimetres.
Intense veining in places gives rise to a breccia texture which can
exhibit multiple phases of fracturing and/or veining.
The primary ore consists of "elvan" and chloritic slate
impregnated with fine-grained native gold and sparse sulphides.
The elvan was more common in the western part of the Main Lode.
Unoxidized sulphides become noticeable in the ore below 100m depth.
Pyrrhotite is the most common sulphide, up to 3%, followed by
chalcopyrite. Other sulphides are
pyrite, galena and sphalerite. The
main visible features in the siliceous ore are zones of quartz veining but
these do not necessarily correlate with gold values.
Less common are brecciated zones and these invariably carry enhanced
gold values. Drilling below the
workings shows copper as 0.1% in the siliceous ore and 0.8% in the Gossan
Lode. A little low grade supergene
copper ore produced by New Occidental mine was probably recovered from stopes
in the Gossan Lode on the No. 1 level. Magnetite
is minor except for the Indicator veins beyond the eastern edge of the main
silicified mass. Vein quartz is
abundant in association with the dominant sulphides, and calcite and dolomite
occur with the galena and sphalerite. Much
of the Indicator gold is visible. In
general the gold in the main shoots is invisible.
The mine
is developed to a depth of 565 m (No. 14 level) and the main shaft is 609 m
deep. One of the main features of
the orebody was its consistency in grade over much of the depth worked.
After a prolonged period of difficulty due to accelerating costs and
fixed gold price the mine ceased operating after the collapse of a large
pillar of waste into the main stope in 1953.
The inability of the single shaft to ensure adequate ventilation, and
handle the increased tonnage needed to maintain profitability, was a
significant fact. Also, drilling
suggested the orebody to be dying at depth.
A new shaft was considered necessary by 1947 but to justify its
construction much larger reserves were needed than those disclosed by
drilling.
Diamond
drilling beneath the workings (Fig. CCLSA) has confirmed downward continuation
of the deposit but suggests diminishing size and grade (Russell and Lewis,
1965, CRA Exploration Pty Ltd 1986). The
Main Lode maintained high grade (10-12 g/t) at No. 9 level and still averaged
10.5 g/t at No. 15 level, but the orebody area diminishes markedly below No.
11 level and is halved by No. 14 level. Below
the workings, early drilling (1947) suggested decline to an average grade of
5.3 g/t Au for the combined East and West Legs, or 5.0 g/t for the Gossan Lode
plus the East and West Legs. Later
drilling has confirmed this, and average intersections as low as 3.3 g/t Au
occur in the deepest probes. The
ore reserve to 965 m depth is estimated as 1.6 Mt.
Averaged over a 400 m interval, a resource of 0.4 Mt/100 m is
obtainable at average grade of 5.7 g/t Au.
8. THE PEAK AREA
The Peak gold
mine, 1995. ( Photo: Land Learn - www.landlearnnsw.org.au
)
Same view at
night, c. 2007
The Peak
area, named from "The Peak" hill, is one in which old mine and
prospect workings occur over a length of some 5 km. The area has been worked since 1887, with many changes of ownership and
amalgamation of small holdings. An
estimated minimum 30,070 t of ore has been mined, for a recovered grade of
20.5 g/t Au and 311 g/t Ag. Last
century the Peak followed the Billagoe area in winning renown for very rich
silver assays. Spasmodic
remarkably high silver assays, up to 39.8 kg/t (Brown Lode), were still being
obtained as late as 1919 when closure of the Great Cobar treatment plant
curtailed mining. As for other
Cobar belt ore systems, mineralization is expected to continue to great depth.
Gold present at the Peak could total as much as 4.5 Mt at 7 g/t Au
(i.e. 30 t of gold). The prospects
at The Peak have had many ownership changes. The area is now controlled solely by Peak Gold Mines Pty Limited,
operating through the CRA subsidiary Enterprise Metals Limited.
Although The Peak is an old mining centre,
much more remains to be won than had been obtained earlier in relatively
shallow workings. Early mining was
limited to depths above 92 m. The
indicated resource at the early stage of deep development is 4.5 Mt
grading 0.7% Cu, 1.5% Pb, 1.7% Zn, 21 g/t Ag and 7 g/t Au (Hinman
and Scott, 1990).
The principal mining centre is 8.5 km SSE of
Cobar at the southern end of The Peak hill (site of Cobar Peak Trigonometrical
Station). Early workings on the
Blue, Brown, Conqueror and Big Lodes were established over a strike of
approximately 550m. Besides
precious metals, considerable lead has been produced.
The Peak mines are on the western limb of a broad anticlinal structure
which plunges approximately 30o towards 160oT.
The relationship between structure and mineralization is no doubt
important at the Peak but interpretation has varied considerably.
The Peak is similar to the area further north (New Cobar - New
Occidental area or `Eastern Line') in that a major shear, known as Peak Shear
(= Great Peak Shear of early references) separates Great Cobar Slate on the
west from a west-dipping sandstone-siltstone sequence on the east.
Plibersek (1982) determined the general westernly dip of the eastern
strata as the enveloping surface to mesofolds, being 58o to 246oT.
A difference at the Peak area from the Eastern Line mines, further
north is that so much of the mineralization lies along further shears to the
east of the main sheared slate-sandstone junction.
In this respect the Peak mineralization more resembles that at Queen
Bee where the mineralization also occurs in sandier strata east of the sheared
slate junction. Whether these
sandier facies hostrocks at the Peak and Queen Bee are best equated with the
Chesney Formation or regarded as a basin margin facies of the Great Cobar
Slate remains unresolved and no fossils have been recovered from the
questionable strata.
Plibersek (1982) and others have proposed
multiple folding at the Peak. At
least two deformation phases are envisaged.
Following Plibersek (1982) the first fold generation (D1) comprises the
majority of mesofolds and has S1 as axial plane foliation.
The second generation (D2) comprises only rare kink folds (e.g. Blue
Lode vicinity) and has no discernible axial plane foliation.
Shear zones are usually dated between Dl and D2 but by their very
nature of strong overprinting they remain open to a wider range of
interpretation. The D2 folds at
the Blue Lode plunge subvertically and suggest late continuation of N-S
transcurrent "shear-couple" movement.
As early mapped (e.g. Rayner 1969), the main
mineralization at the Peak, comprising the Big Lode, and the nearby
Brown-Conqueror Lode (which overlies deeper orebodies later discovered), is
situated on the intersection of the northwesterly trending slate (west) -
sandstone (east) regional trend with the Peak Shear.
The Peak Shear, like other shears to the east of it, trends about 20o
clockwise of the regional slate-sandstone junction.
The East shear and Big Lode shear may be more parallel to the junction.
Along the shears which are in coarser metasediments to the north of the
slate-sandstone boundary, the mineralization is less intense than near where
the Peak shear cuts that boundary. Although
the scale is different, this is the same principle as early investigators
derived elsewhere (e.g. New Occidental, New Cobar).
The Peak shear passes north via the Silver Peak Lode.
The workings further east (Lady Greaves, Blue Lode, etc.) lie along
other subparallel anastomosing shears which have been mapped differently in
detail by various workers. Major
review and re-interpretation of the entire Peak area geology can be
anticipated from the development of deep underground gold mining in the 1990s.
The Peak area was mined and prospected
extensively from late last century through the Great Cobar period of the
present century. The presence of
gold and silver in the vicinity of The Peak came to prominence in the silver
boom of 1887, although significant mining did not commence until 1896.
Major development was prompted by spectacular assays reported from
finds by Charles Barrass and James Conley, who discovered the Blue Lode in
1895. The Conqueror-Brown Lodes
were also secured in the same year and proved payable by 1896.
Barrass and Conley built a ten stamp battery in 1896, and this was last
used in 1946. The Great Cobar
Copper Mining Syndicate acquired large interests in The Peak mines in 1896,
and by 1897 the Blue and Conqueror mines were producing gold and silver ore
for treatment at the Great Cobar smelter.
The richer gold ores were still treated locally in the battery, whilst
the silver and lower grade gold ores were carted to the smelting furnaces for
fluxing purposes. The Silver Peak
and Lady Greaves mines, further north along the Peak ridge, were opened in
1899 and 1900 respectively. The
early selective mining was of ores averaging about 30 g/t Au.
The principal leases were first held and operated by various
individuals and syndicates, followed by the Great Peak Gold Mines Company,
Great Cobar Ltd (1906-1914), Peak Mines NL (1924-1940), then by various small
operators until the 1950s when mining ceased.
Most interest has focussed on the Conqueror,
Brown and Blue Lodes. The greatest
period of activity occurred between 1896 and 1911 when the Blue, Brown and
Conqueror lodes produced about 23 000 t of ore at a recovered grade
of 22 g/t gold and 175 g/t silver.
In 1906, the Conqueror, Brown and Blue Lodes were taken over by the
Great Cobar Limited. After 1913
production ceased from the Silver Peak and for some years the only parcels of
ore from The Peak area were produced by tributors on the Conqueror, Brown and
Blue Lodes. The mines continued to
be worked intermittently by small companies and syndicated from up to the
early 1950s, but operations were on a small scale and probably less than 600t
of ore were extracted by these operators.
In 1924 Peak Mines N.L. took over the Conqueror and Brown Lode leases
and their operations continued intermittently until 1940.
In 1940 the Blue Peak Syndicate took over the Blue Lode mine with E.K.
and C. Freeman gaining control of the Conqueror in 1942.
Prior to the current phase of operations by Peak Gold Mines Pty Ltd,
the last significant production was 6.59 kg Au from the Conqueror in
1951-1953.
The production from the Peak area to end of
the New Occidental period has been considerable.
The combined production from the Blue and Conqueror-Brown lodes was
582.5 kg Au and 12364 kg Ag from 28610t ore.
The Silver Peak mine in 1906-1913 also contributed 3.4 kg Au, 1016 kg
Ag, and 110.2t Pb from 1343t ore. These
were the main producers, and contributions from the other workings are
relatively small or unknown.
After the cessation of mining at the Peak in
the 1950s, exploration work was continued by Enterprise Exploration, McIntyre
Mines, Cobar South Pty Ltd, Cobar Mines Pty Ltd, and currently Peak Gold Mines
Pty Ltd of the CRA companies group. Extensive
underground reserves have been delineated by Cobar Mines Pty Ltd and Peak Gold
Mines Pty. Ltd since commencement in 1981 of deep drilling to probe for
extensions of the Conqueror, Brown and Big Lode mineralization.
Diamond drilling as early as 1942 had confirmed the presence of deep
mineralization but values were disappointing until Cobar Mines Proprietary
Limited in 1981 discovered high grade gold and base metal mineralisation over
a horizontal width of 19 m at a vertical depth of 300 m in the first
hole of a new diamond drilling programme.
The next six holes investigated the zone between 100 and 250 m
depth, but located only narrow mineralised stringers.
The eighth hole again intersected the lode at about 270 m depth.
Between 1982 and 1985 about 31 000 m were drilled from
surface in 68 cored holes to delineate the deposit.
In 1987 the deep Peak deposit was estimated to contain 4-5 Mt of ore
with a grade of 6.5 g/t Au and 3.0% base metals.
A 740 m deep haulage shaft was commenced and the resource was later
re-evaluated as 4.0 Mt of 6.8g/t at 3 g/t Au cut-off value.
The highest gold values are associated usually with the sulphide
minerals (chalcopyrite, galena, pyrrhotite etc.).
The deep mineralization is mainly in veins.
These are variable in both mineralogy and form, but usually entail
fracture filling by quartz and sulphides.
Veins range from 1 mm to more than 1 m in width and occur as sets with
fairly uniform vertical to steep dips approximately parallel to the cleavage.
Other vein orientations are present but parallelism to cleavage
predominates. The vein densities
vary from 5 per metre to very dense complexes of submassive or massive
appearance.
The various named deposits which comprise the
Peak group are:
No.
Deposit Name
Commodities
109
Silver Peak mine
Pb, Ag, Zn (Au, Cu)
110
Lady Greaves
Au
111
Peak Northeast prospect
Cu
112
Blue Lode mine
Pb, Au, Zn
113
The Peak
Au, Ag (Cu, Pb, Zn)
113
Conqueror-Brown Lodes
Au, Ag (Cu, Pb, Zn)
113
Big Lode
Cu, Pb, Zn (Ag, Au)
114
Perseverence shafts
Cu, Zn
115
Great South Peak shaft
Au
The above
nine names are the main ones associated with the Peak area but this is not an
exhaustive list. There is in
excess of sixty sites of old workings, which may at one time have been
independent entities, along the tract between the Great South Peak Shaft and
the New Occidental mine. Some of
these shafts now show no dump indication of mineralization, and they may have
been sunk as purely speculative ventures.
The details of the old workings thought unimportant were not sought
during the present study. This
includes some deep shafts possibly on the Peak shear north of The Peak, and
some old workings which possibly trace the Gossan Lode southwards from New
Occidental to the West shear at the Peak.
The principal sites are concentrated in a
distance of 2.2 km along a zone of multiple shearing, parallel to the Chesney-New
Occidental shear zone but offset further east.
The Peak line of shearing is in Chesney Formation to the north and cuts
across Great Cobar Slate to the south. Mineralized
shears zones have been searched for still further east but without success.
A significant zone of anomalous copper geochemistry extends for over
700 m strike length on the eastern side of the area near the Hillston Road but
the Peak northeast prospect is the most easterly mineralization drilled.
Past
production was very largely from lodes close to Cobar Peak (Blue Lode and
Brown-Conqueror-Big Lode ore systems). Drilling
in the early 1980s by Cobar Mines Pty. Ltd. indicated the presence of major
gold and base metal orebodies at depth. By
1986 available ore was estimated as 3-4 Mt grading 5-7 g/t Au, with a base
metal content averaging about 3% Cu+Pb+Zn.
Lead and zinc contents are subequal, and about twice the copper
content. The main mass of higher
grade mineralization, about 300 m in length and 40 m maximum width, occurs
between 300 and 600m below surface. It
consist largely of a network of quartz and sulphide veins surrounding a
siliceous breccia lens. The CRA
subsidiary Peak Gold Mines Pty Ltd was formed to continue exploration of this
and to initiate deep mining.
It is estimated that The Peak area has
yielded over 629 kg Au and 14,448 kg Ag. In
its early history the area was renowned for its reported rich silver assays.
Silver occurs as native silver, cerargyrite, pyrargyrite, in gold
(electrum) and in galena. The
water table was met around 80 m and most of the early gold ore won was from
the enriched oxidized zone. Production
has been intermittent and did not finally cease until after the New Occidental
mine closed in 1952. A
considerable amount of tailings from The Peak was retreated at the New
Occidental mine cyanide plant.
The Peak
area is broadly along strike from and aligned with the New Cobar-New
Occidental group of mines. It is
associated with a line of magnetic anomalies continuing south from New Cobar
mine. However, in greater detail
the mines at The Peak lie along shear zones (Peak shear system) making an
appreciate angle to the general trend of the Great Cobar Slate-Chesney
formation contact (figure PG). Rayner
(1961) considered that a similar shearing couple was operant for the Peak
shear system as for the shears of the New Cobar-New Occidental area.
The Peak area structure, and correlation of
hostrocks, have been interpreted differently by various workers but most agree
that a number of subvertical mineralized lenses occur which are orientated
subparallel to cleavage and either constitute a broad shear zone or are
developed at selective places within it. Silica
and chlorite alteration is widespread. In
The Peak shear system at least three zones of stronger shearing and/or
alteration have been envisaged. From
west to east these are the Peak shear zone, the Blue Lode shear zone and Lady
Greaves shear zone. Smaller
suspected shears have been mapped but not named.
The main shears which have been named at the Peak are as follows:
Peak
Shear - Main shear zone extending via Silver Peak, Brown-Conqueror, Great
South Peak and Perseverance workings.
West
shear - Located 350m west of Conqueror-Brown workings (compare with Gossan
Lode of the Eastern Line further north).
Big
Lode shear - Western element of the Peak shear zone at Big Lode west of
Conqueror-Brown workings. Undoubted
Great Cobar Slate occurs immediately to the west.
Great
Peak shear - Eastern element of the Peak shear zone, well exposed in the
Conqueror workings.
Blue
shear - A zone comprising three narrow shears at surface, where it passes
through the Blue Lode workings. Occurs
east of the Peak shear.
Lady
Greaves shear - Poorly mineralized shear extending via the Lady Greaves
open-cut, north of the Blue shear with which it possibly anastomoses.
Over much of its length the Lady Greaves shear lies further to the east
of the Peak shear than does the Blue shear.
East
shear - Very poorly mineralized shear on the eastern side of The Peak hill.
These
shears, with the possible exceptions of the West shear and the Big Lode shear,
make a strong angular divergence clockwise from the inferred regional
slate-sandstone junction. The
latter, boundary however, is very poorly exposed west of the Peak.
Less intense shearing is probably extensive
between the Peak and Blue Lode shears. The
shears trend around 330-340o, locally 350o near mineralization.
Most have been thought to dip vertical to steeply east.
The main "Peak Shear", or set of shears in the vicinity of
The Peak deposit, dips steeply to the west.
In this zone the individual shears discerned in drill core or
underground are 0.5-5 m wide. Generally
the associated sulphides are dispersed or vein-related, but where black
chlorite is prominent along shears small crudely banded massive sulphide
orebodies may occur. From detailed
structural logging of drill core, close-sapced shearing is confirmed in the
Great Cobar Slate. These minor
shears have a vertical to steep westerly dip approximately parallel to
cleavage and are spaced 30-50 m apart. A
left lateral strike slip movement can interpreted from drag folding.
The shears appear to truncate the western limbs of south plunging folds
and effect subvertical drag. Base
metal and gold mineralization occurs within these minor shear zones, which may
be taken as smaller equivalents of the structural setting inferred for the
main sulphide zones (Kirk, 1984).
A favoured interpretation is that the
mineralization at the Conqueror, Brown and Big Lode occurs in both the Great
Cobar Slate and the Chesney Formation but with the bulk of mineralization
being in arenaceous sediments of the latter.
This contrasts with the gold bearing area further north where a
favoured locus of mineralization is in the Great Cobar Slate at or adjacent to
faulted contact with the Chesney Formation.
At The Peak a "slate wedge" appeared to cut across the main
Chesney Formation sandstone interval along the shear system (between the Peak
shear and the East shear) but interpretations of this differ.
The pelitic wedge may in part be Great Cobar Slate but it could also
include sheared Chesney Formation (e.g. Rayner 1961).
The basic structure between the Peak and East
shears is anticlinal, plunging south, and probably represents a regional
parasitic fold. In such
interpretation the Peak area occurs on the western limb of a broad anticlinal
structure plunging south at about 30o and with a wavelength of about 2 km.
The stratigraphy and structure at The Peak are complex and
interpretations to date have differed markedly.
Current interpretation is that the principal ore system is located on
the western limb of the south plunging Chesney-Narri anticline.
At the Peak a group of south plunging folds parasitic to this anticline
are cut by a series of shears and faults subparallel to cleavage.
These folds have axes plunging uniformly at 30o to 175o-180o.
The macroscopic cleavage strikes 165o and dips steeply west.
It can be seen to cut obliquely across the fold axes.
The stretching lineation plunge is very steep to the north.
Evidence of a dilation regime is provided by crack-seal features lying
normal to the stretching lineation in areas of inhomogeneous high strain (Hinman
& Scott, 1990).
Some workers have regarded sandy slates in
The Peak shear system as sheared Chesney formation and others have inferred
facies change within either the Great Cobar Slate or Chesney Formation;
a diversity of interpretations which also applies further south, to
beyond the Queen Bee mine. One
interpretation of facies variation in the The Peak area is that the upper part
of the Chesney Formation can be subdivided into two members.
The lower member, with a thickness in excess of 300 m, is composed of
thinly to thickly bedded fine to medium grained sandstone, lesser siltstone,
shale and sedimentary breccias. The
upper member consists of thinly bedded siltstone and shale and is estimated to
be 60 m thick. The Great Cobar
Slate is also locally divisible into two members.
The lower member consists of shale with minor thin laminae of highly
calcareous siltstone and occasional calcite ellipsoids ranging in size to 3
cm. Minor chlorite and trace
sphalerite are common accessory minerals in the ellipsoids.
A true thickness of approximately 300 m is estimated.
The upper member is composed of shale with rare to very minor
non-calcareous siltstone laminae. The
estimated true thickness is about 200 m giving a total local thickness of 500
m for the Great Cobar Slate. Variable
minor pyrrhotite and pyrite disseminations occur in both members (Kirk, 1984).
The Great Cobar Slate is strongly folded in contrast to the more
competent Chesney Formation. A
slaty cleavage is well developed in the fine grained sediments.
The cleavage orientation is 345o - 355o T with steep westerly dip of
80o-85o west of the Big Lode changing to 80o-85o E to the east of the
Conqueror-Brown Lode. Minor kink
folding of cleavage is common in the Great Cobar Slate.
Boudinage and ptygmatic folding of quartz and sulphide veins is common
in the fine grained sediments but more poorly developed in the competent
sandstones.
Undoubted Great Cobar Slate occurs on the
west side of the shear system and hosts the Big Lode.
The chief mineralization of The Peak area, particularly with respect to
defined reserves, is the Brown-Conqueror-Big Lode system developed along The
Peak shear zone. The hostrock
sequence for this ore system probably straddles the boundary between Chesney
Formation and Great Cobar Slate. Peak
Gold Mines Pty Ltd also interpret the presence of upwardly introduced
brecciated volcanics.
Mineralization
developed along The Peak shear system (Great Peak Fault, etc.) has two main
surface expressions: lodes of
ferruginous and quartz veined crushed slates, and silicified zones in places
grading to "elvan" (chert). Andrews
(1913) noted the siliceous gold ore bodies at the Peak to be essentially
hanging wall lodes. The high grade
gold-silver ore shoots initially worked as small individual holdings at The
Peak are in some case recorded as distinctly pipe-like.
These shoots pitch steeply south and some were very rich within the
oxidized zone. Wide low grade base
metal orebodies occur west of the gold-silver lodes in two areas (Blue Lode
and Conqueror-Brown ore system). In
addition to the steeply dipping ore lenses, quartz `flat makes' are also
recorded from the mines of The Peak area.
Silver
Peak Mine
This mine,
also known as the Great Silver Peak Mine or the Cobar Peak Silver Mine was
opened in 1899 and worked until 1915. It
was situated a little east of a major dislocation, against which strata are
deflected almost east-west. It was
developed to 107 m depth and produced silver-lead ore carrying appreciable
gold. The mine yielded over 1,097
kg Ag, 3 673 g Au and 110.2 t Pb. Some
smaller workings (Silver Peak North) occur north of the main shaft.
The shaft was sunk on gossanous slate and
massive gossan, assaying up to 0.9% Cu, 1.75% Pb, 1.0% Zn.
At around 45 m depth vughs with silver chloride were encountered.
The gossan was then worked for small discontinuous shoots containing
native silver, cerargyrite, embolite and cerussite.
Samples of the gossan assayed up to 792 g/t Ag.
Recovery grade for 3200 t of ore extracted was about 7 g/t Au
and 360 g/t Ag. Below 46m
large patches of remnant primary ore were encountered which were rich in
argentiferous galena. This galena
ore assayed up to 457 g/t Ag. Some
copper enrichment was present (40% Cu) but was of insufficient extent to mine
as copper ore.
The ore
occurred as lenses or patches. The
primary ore varies from vein style (chalcopyrite + pyrite + galena +
sphalerite) to disseminated-banded (galena + sphalerite + pyrite +
pyrrhotite). A little massive ore
was also present. Sphalerite is
common and gold grains have been noted in sphalerite.
Dolomite occurs in the gangue and as veinlets in talcose rock.
A record of "magnesian limestone" from the mine suggests that
former carbonate presence may have been considerable.
Galena deposition favouring a carbonate environment is well known and a
comparison may be drawn with the small lead-zinc lode at New Occidental mine,
which is in a zone of carbonate-healed slate breccia.
The Silver Peak orebodies are two co-linear lenses with a total length
of 37 m and an average width of 1.8 m. The
plunge is steeply to the north, about 70o.
The orebodies are leached to below 84 m and the galena-rich ore mined
appears to have been remnant patches not destroyed by oxidation.
Lady Greaves Mine
The Lady
Greaves line a mineralization, regarded as a northwards continuation of the
Blue Lode line, was commenced work on by The Peak Prospecting No Liability
syndicate about 1890. Quartz veins
with high arsenopyrite content mark the line but better gold values were
obtainable from adjacent siliceous slate intervals than from within the solid
reef quartz. After the area was
abandoned by the Peak Prospecting company the subsequent focus of activities
was slightly to the south along the same line, which has become known as the
Lady Greaves lode. Production is
unknown but ore parcels were despatched for trial treatment. Prospecting tested the narrow Lady Greaves shear zone to about 88m
depth. The zone contains crushed
slate and quartz with gold, pyrite and arsenopyrite. The gossanous quartz contains anomalous base metal values, e.g. 265 ppm
Cu, 500 ppm Pb and 245 ppm Zn. Gold
content is up to 31 g/t but is usually low. The mine may have recovered some ore from a narrow lensoidal zone of
gold values about 2m wide by 40m long, adjacent to the shear zone which itself
proved valueless. The shear zone
carries quartz veins along most of its length, and dips easterly at 70o.
The quartz veins within the shear carry arsenopyrite and pyrite but
little gold. To the north, the
silicified zone narrows, and eventually it splits into diverging quartz veins.
Blue Lode Mine
The Blue
Lode mine, also known as the Conley and Barrass mine or the Blue Peak, was
discovered about 1887. Production
records commence in 1895. The mine
yielded rich ore near surface from two small gold-silver shoots. A considerable portion of the ore mined was high grade (up to 162 g/t
Au). The deepest workings are at
73 m. Structurally, the ore shoots
are found where the Blue Lode shear cuts through sandstone units of a
south-plunging anticline. The
shoots plunge south.
The mine
commenced by following a very small (0.8 m) pipe of rich gold ore, worked for
an average yield of 155 g/t Au. At
depth the shoot pitched south. Gold
was also found to be present in the country rock and two indistinct orebodies
were developed, each averaging 1.8 m wide. These dip steeply east to vertical and pitch south as at the
Conqueror-Brown-Big Lode system. Highly
cleaved greenish-black talcose slate occurs in shears and silicification is
strong in places. A zoned
alteration halo may surround the Blue Lode, and silicification is decidedly
stronger on the eastern flank.
At surface three subparallel chloritic shears
are discernible at the Blue Lode workings, within an interval 12m wide. These are the main Blue shear, an eastern shear and a western shear.
These three sheer lines appear to converge together at the southern end
of the Blue Lode workings. Talc
development appears to be strongest in the Blue shear. Stronger quartz veining and silicification commences immediately east
of the eastern shear and is thus on the hanging wall side of the composite
zone of shearing. As elsewhere,
the upwards travel of siliceous solutions into the hanging wall may have been
along both the subvertical cleavage and within the west-dipping arenaceous
beds. Graded medium grained
sandstone beds with sole-markings (i.e. turbidite beds) are noteworthy at the
Peak, in the vicinity of the Blue Lode and elsewhere.
Some of this sandstone, the "green sandstone" of Rayner
(1961) has become highly silicified and quartz veined.
In
1896 the Blue Lode was acquired by the Great Cobar company and the maximum
annual production was reached, being 99.7 kg Au.
Silver chloride was a marked feature of the oxidized ore, with silver
content ranging up to 933 g/t Ag. An
impressive amount of silver (3 771 kg) produced from The Peak mines in 1897
was probably in large part from the Blue Lode.
High assays, to 3826 g/t Ag, were locally reported in 1899 to the north
of the Conley and Barrass workings. Some
of the ore taken to the Great Cobar smelters was very rich.
For example, the 305 t supplied by tributors from "The Peak
Mine" in the first half of 1911 averaged 117 g/t Au and 263 g/t Ag.
The exact sources of this ore are unknown but may have included the
Blue Lode. The Cobar Herald in
1899 reported that Conley and Barrass's "blue lode" had also been
picked up in Mallett's Crown of The Peak lease.
Later work
at the Blue Lode into the 1940s is not well recorded.
The Blue Peak Syndicate held the Blue Lode and erected a 5 head battern
on the site. Andrews (1913) noted
that the gold values had been lost above water-level.
It is not clear if the Blue Lode gold-silver shoots were later directly
traced below the water table in the old workings, and the nature of the
primary mineralization was uncertain. Sparse
records suggest it was quartz and crushed slate carrying pyrite, pyrrhotite,
galena, sphalerite and minor chalcopyrite.
Later drilling supports this. Derivation
of the silver from galena is possible as traces of mineralization further
north along the Blue Lode shear are rich in lead.
Some early prospecting occurred to the north.
In 1902 an adit was driven north of the Blue Lode.
The Premier shaft, sited nearby, was sunk in 1902-1907 and encountered
only minor gold values.
There are a number of diamond drill holes in
the vicinity of the Blue Lode workings, and CRA Exploration confirmed
continuity of the lode in an intersection some 350m below surface (hole DD88
PK40). The lode at depth is a zone
of particularly intense alteration (silicification, brecciation, chlorite
development) with variable vein development.
The major sulphide in the veins in pyrrhotite, accompanied by lesser
chalcopyrite, galena and sphalerite. About
500 m below Blue Lode, Cobar Mines Pty Ltd intersected a 3 m zone grading
18.8g/t Au and 1.6% Cu, associated with silicification and intense quartz
veining. Weak mineralization is
also known 300 m north of Blue Lode.
Conqueror-Brown
and Big Lodes (The Peak or Great Peak)
Opening of stamper battery
at the Peak mine in 1896.
These
lodes form the principal ore system at The Peak, which is also known simply as
The Peak deposit. About 23,000
tonnes of ore has been extracted for a recovered grade of 22 g/t Au and 175
g/t Ag. Mining commenced from two
properties, the Brown mine in the north and the Conqueror mine in the south.
As with the mines further north in The Peak group, near surface
prospecting of the Brown Lode and the Conqueror mine lode brought to notice
exceedingly rich slugs of silver ore and rich patches of gold ore.
Some of the material yielding rich gold values was almost
indistinguishable from country rock. The
gold-silver orebodies early discovered occur in silicified Great Cobar Slate
along the eastern side of the Peak shear zone over a length of 230 m.
Open cut and underground workings were developed as the Brown Lode mine
in the north (Brown shaft; East and West open cuts) and the Conqueror mine
(Conqueror shaft and open cut) in the south.
The workings proved to be on the one ore system and were connected
underground when under the control of Great Peak Gold Mines NL.
The deepest workings, in the south, were at 91 m.
The Peak shaft (Great Peak Co.) is situated on the Conqueror-Brown line
between the Conqueror shaft and Conqueror open cut, and the new Peak Gold Mine
shaft is situated west of the Conqueror shaft.
Kirk (1984) gave the following dimensions for shallow named lodes and
deeper unnamed ore lenses:
Name
Strike length | Vertical extent
| Depth
| Width (range)
Big
Lode
100m
80m
0-180m
6-16m
Conqueror-Brown 190m
90m
0-90m
20m
Unnamed
>250m
300m
300-600m
10-40m
Unnamed
150m
150m
250-400m
20m
Unnamed
70m
60m
-
40m
Unnamed
80m
80m
-
10m
The
unnamed 10-40m wide zone contains the major tonnage of gold mineralization.
The near surface mineralization is within the
Great Cobar Slate. As the
mineralised system passes downwards into the Chesney Formation it broadens to
over 160 m wide while the silica alteration zone broadens to over 400 m wide.
This effect has been interpreted in a number of ways.
It could relate to the increased fracturing within the more competent
sandstones of the Chesney Formation compared to the less competent slates
which fail in a more ductile fashion hence restricting dilational zones to
narrower widths. Alternatively,
the breccia which appears in the anticlinal core with increasing depth may
have somehow itself dilated the entire ore system.
Two lode sets were early recognized, a
west-dipping set and a less distinct east-dipping set.
The west-dipping lodes included the main line of Conqueror-Brown ore
lenses, averaging 1.3 m width, and also the much wider Big Lode which occurs
further to the west. East-dipping
lodes are not as distinct and have been less extensively mined.
The main lode (Western lode) extends the length of the Conqueror-Brown
workings. It dips west at 60o or
steeper and comprises a number of ore shoots pitching southerly to vertical.
Assay values range to 61 g/t Au and 3.6 kg/t Ag.
In some years the value of the silver in the ore raised exceeded many
fold that of the gold. It is
likely that significant parcels of ore averaged above 30 g/t Au but individual
mine records are deficient and overall returns were partially included with
the Great Cobar company figures. The
rich ore shoots, up to 1.2 m wide, remain subvertical (70o-90oS) even
where enclosing ore lenses pitch as gently as 35oS.
Along much of the main lode the gold mineralization is weak.
It broadly coincides with disseminated pyrite, sparse galena and
chalcopyrite traces.
The
Conqueror-Brown ore lenses lie in or near the Peak shear.
Contacts with country rock have been recorded as sharp.
The Big Lode occurs on the western side of the Peak shear, and lies
10-25 m west of the main Conqueror-Brown line of lode.
The Big Lode outcrops for more than 200 m length, as an ironstained
siliceous body or gossan up to 15 m wide which contains significant base metal
values, e.g. 1680 ppm Cu, 7700 ppm Pb, 3100 ppm Zn, 28 ppm Ag.
Pitch is probably to the south. The
Big Lode was penetrated in cross-cuts from the Conqueror mine and found to be
very low grade, e.g. 1-2.5% Cu + Pb + Zn.
Exceptions include 6.8% Cu over 1.5 m, 10.2% Pb over 4.6m, and a
reported output of 102 t of gold ore averaging 8.4 g/t Au.
Higher assays of up to 11 g/t Au probably represent patches of
secondary enrichment. Small silver
shoots (up to 185 g/t Ag) have been found associated with primary sulphides.
Diamond drilling for the New Occidental company in 1942-1943 showed the
mineralized interval to increase with depth, up to a width of 61 m at the
greatest depth penetrated, assaying 0.56% Cu, 0.47% Pb, 1.45% Zn, and 4 g/t
Ag. Higher grade values
encountered range up to 3.02% Cu over 6 m, 0.94 % Pb over 15m, 2.9% Zn over
16.8 m, 17.6 g/t Ag over 7.6 m, and 1.4 g/t Au over 7.6 m.
The
Deep
Peak ore bodies
In 1981 Cobar Mines Pty Ltd commenced a
programme to continue deep exploration below the entire system of lodes, to
pass through the Big Lode and the Peak Shear and test the depth extension of
the Conqueror-Brown Lodes.
As the principal commodity
then sought was copper, the 1942-1943 drilling of the Big Lode by New
Occidental Gold Mines did not extend far enough east to test below the
Conqueror-Brown Lodes. The Cobar
Mines Pty Ltd and other CRA subsidiary deep drilling below the Conqueror mine
area since 1981 has resulted in the discovery of a widened ore system at depth
and has delineated a resource of 4.5 Mt of ore grading 0.7% Cu, 1.5% Pb, 1.6%
Zn, 21 g/t Ag and 7 g/t Au. Drilling
evaluation of this resource had progressed to 600m depth by 1985.
A new 510 m shaft was commenced in 1987.
The mineralization occurs as a network of quartz and sulphide veins
generally subparallel to cleavage. Sulphide
density ranges from sparse veins to submassive coalesced concentrations of
veins. The dominant sulphides are
pyrrhotite, chalcopyrite, pyrite, sphalerite and galena with minor
arsenopyrite, tetrahedrite, argentite (?) and galenobismutite (?).
Gold is present as native gold and electrum, and may comprise
relatively coarse grains of up to 0.5 mm.
It is interesting that below the
Brown-Conqueror workings in Great Cobar Slate, the main deep Peak
mineralization straddles the contact between the Chesney Formation and the
Great Cobar Slate. Although local
faulting is evident, the contact is seen to be gradational from a sandstone
dominated sequence through a 50-100 m thick transitional phase into a
shaly sequence. The contact in
this locality could be essentially intact, whereas further north (New
Cobar-New Occidental area) deformation is so strong that major dislocation
cannot be disallowed. Growth of a
strongly siliceous core to the mineralization system at depth in the
Brown-Conqueror segment of the Peak shear has perhaps afforded protection from
the shearing which has elsewhere commonly rendered the Great Cobar Slate -
Chesney Formation contact discordant.
Below 500 m five ore lenses wrap around a
core of siliceous + brecciated material (Figs. TPLS, TPTS).
The siliceous breccia body is developed below 450m depth and extends to
at least 900m. It expands rapidly
downwards, increasing from narrow zones 10 - 50 m wide at 450 m depth into a
massive zone more than 300 m wide at 600 m depth.
The breccia contains only sporadic sulphides even though high grade
sulphide lenses occur on its upper western flank and in association with
apophyses on its upper surface. Some
sulphide veining postdates the breccia. The
genetic relationship between the mineralization and the Peak breccia is
speculative. Hydrobrecciation is a
suspected causative process but the timing of this with respect to formation
of the surrounding ore lenses is unclear.
The breccia is largely surrounded by an alteration zone characterized
by silicification, chloritization, montmorillonite formation and alkali
depletion. Chlorite alteration
extends out furthest from breccia where there is obvious fracture control, and
stilpnomelane is present in quartz veins.
Considerable potash feldspar is present in both late stage quartz veins
and parts of the breccia mass. The
K20 content of the breccia (avr. 4.5%) is greater than that of the Nurri Group
hostrocks. Alkali depletion is
most apparent in the sparseness of sericite or muscovite in the sedimentary
rocks. Lithium depletion occurs in
general proximity of mineralization. The
brecciation may have occurred during deformation around a more competent core,
possibly involving underlying volcanics (Cobar Mines Pty Ltd, pers. comm.
1988).
The main zones of mineralization, with or
without accompanying strong silicification, occur as steeply dipping lenses.
The several deep lenses lie subparallel to cleavage and plunge steeply
north in the direction of the stretching lineation.
The maximum strike length of the mineralised zone defined by the lenses
is about 300 m (at a depth of 400 m).
The width ranges from 60 m in the upper part to more than 150 m
at 500 m depth. The top of
the mineralisation lies at a depth of 250 m and the economic bottom
occurs at about 600 m, although subeconomic intersections of some lenses
exist down to 200 m below this level.
Economic mineralization has been found mainly in three westerly-dipping
lenses designated as the Western lenses (Nos. 1,2) and the Copper lens (No.
3); and also in three smaller lenses (No. 4,5 and 6) which dip steeply east.
The lenses extend over a strike distance of 300m, with true widths of
5-10 m. They are proven over a
vertical distance of 400 m, from 250 to 650 m below surface.
The distinctly arcuate nature of some of the ore lenses in plan
("Copper lens" and "W20" lens) suggests possible dilation
postdating the major Peak Shear during the development of this ore system.
The Peak ore system invites analogy with Elura in its possible
dilational and almost intrusive-like features.
There is some suggestion of structural doming over the main Peak
deposit. Regional bedding-cleavage
intersections in the vicinity generally plunge 20-40oS, but at surface over
the deep orebodies display dramatic plunge reversals.
The western lenses (Nos.1,2) of the deep Peak
Prospect ore system are sphalerite-galena rich, and correspond with the Big
Lode. Gold grades are moderate,
about 3.5 g/t Au. The lenses
comprise two indistinct subparallel zones of mixed quartz-sulphide veining
associated with silicified sediments. Both
quartz-sphalerite-galena and quartz-pyrite-chalcopyrite form 1-200mm wide
veins. The mineralization dips
vertically to steeply west and may aggregate a width of up to 15 m.
The next easterly lens, No 3, is a 10-25 m wide copper lens
(chalcopyrite + pyrrhotite + sphalerite + galena), which contains most of the
gold present in the ore system. It
is 13 m wide on average and may correspond with the main
(westerly-dipping) Conqueror-Brown lode; although
the Conqueror-Brown mineralization has also been correlated, alternatively,
with one of the deep eastern lenses. The
copper lens mineralization is associated with strong quartz veining,
silicification and chloritisation. The
copper lens has an average grade of 11 g/t gold and about 4% combined
copper, lead and zinc, and is estimated to contain two-thirds of the gold in
the deposit. visible gold is
common. The lens appears to be
truncated by the Peak shear. The
No 4 lens (8 m wide) diverges from No 3 and dips easterly, passing down the
eastern side of the siliceous breccia pipe.
In its easterly dip it is like the ill-defined easterly-dipping
mineralization worked in the Conqueror mine.
The No. 4 lens contains 14% Pb + Zn and 150 g/t Ag with low values
of copper and gold. It is about 8 m
wide. The mineralisation and
associated alteration are truncated by the Peak shear.
The No. 4 lens contains two styles of mineralisation in close
association. Strong disseminated
sphalerite-galena mineralisation is preserved in remnant blocks adjacent to
large amounts of silicified sediment. This
mineralization is silver poor. Overprinting
it is banded to massive black chlorite-sphalerite-galena-pyrrhotite(-pyrite)
in which banding is chaotic. This
minerization is silver rich. The
No 5 and No 6 lens are a concentration of mineralized quartz veins (with
pyrrhotite + galena + sphalerite). Gold
occurs with iron sulphides, and base metal sulphides are sparse in these
eastern lenses.
A zone of siliceous breccia is developed
below a depth of 450m and extends to at least 900 m.
The breccia increases rapidly in size downwards, changing from narrow
10-50 m wide zones at 450 m depth to a massive body at greater depth.
At 650 m depth the siliceous body is about 500 m long and 150 m
wide. The breccia body is sheathed
in sulphide mineralization, as by the high grade sulphide lenses on its
western shoulder, and may be laterally continuous with sulphides in an upwards
direction, yet the main body of the breccia contains only sporadic sulphide
mineralization.
The genetic implications of the extensive
brecciation present at the Peak remain uncertain.
Within the breccia body angular or subangular fragments of country rock
are commonly 1-2 cm in size, and are variably silicified.
The dip of some ore lenses to the east, divergent away from The Peak
shear, suggests compression around an early silicified breccia pipe, rather
than brecciation being subsequent to shearing.
The breccia may have geometric differences to the Peak ore lenses.
Whereas most Peak area orebodies are south-plunging the breccia pipe is
north-plunging. There may be
considerable diversity associated with the brecciated rocks.
Below the 350 m level a large body of rock is variable brecciated,
pseudobrecciated and partially chloritised.
Pseudobreccia textures range from vein network breccias to massive
silica replacement textures. Locally
these effects extended for a width of 300 m, of which half or more is
strongly silicified. The siliceous
material consists of juxtaposed fault slices of three different rock types.
The western lobe is composed largely of pseudobrecciated and
chloritised, devitrified, flow banded acid volcanic rock, probably of
rhyolitic to rhyodacitic composition according to Hinman (Hinman 1989, Hinman
& Scott 1990). The least
altered examples display a strong banding, possible vugh fillings and rare
feldspar phenocrusts.
Hinman (1989) considered the mineralization
to be related to syn-D1 high-strain zones on the contact between felsic
volcanics and the Chesney formation along the Great Peak Fault.
Hinman suggested that mineralization predated late faulting or shearing
although the higher mined deposits appear to lie in these structures.
The deep reserves
estimated at The Peak in 1990) were 3.9 Mt of ore with average grade of 7.1 g/t Au, 0.8% Cu, 1.3% Pb,
1.5% Zn and 14 g/t Ag. This is
located primarily between depths of 270 and 730 m.
The geological resource extrapolated to 800 m comprises about 4.5 Mt of
ore.
Drilling
away from the Peak ore system has not revealed further significant
mineralization on the shears which lie further east, although weakly
disseminated pyrrhotite extends well east of the Lady Greaves shear zone.
Drilling along a geochemical trend which passes 350 m east of the
current Peak Prospect encountered sporadic quartz veining, silicification, and
minor mineralization (CRA Exploration Pty Ltd, 1986). The quartz-chlorite vein system carries sporadic calcite, pyrrhotite
and minor chalcopyrite. The best
intersection is 1 m of 1.3% Cu.
The large Peak
deep ore reserves first drilled in 1980 were commenced to be mined in 1991 and
were near exhaustion by 2002. Over one million ounces was produced
from the new Peak mine during that period. Mining moved in 2006 to the
top of the deep ore below the Persevence mine to the south. Also,
in 2007, Peak Mines completed development of a decline connecting
the underground workings at Peak with the New Occidental ore body to the
north.
Workings south of The Peak
(Great
South Peak, Perseverence and unnamed workings)
The Peak
shear zone continues south of the Brown-Conqueror-Big Lode ore system, and may
cut completely across the Great Cobar Slate.
Along this continued line of shearing there occurs the Perseverence-Great
South Peak line of mineralization. This
carries only weak surface values, up to 130 ppm Cu and 550 ppm Pb.
Shafts, drives and cross-cuts have explored along the shear zone to
depths of up to 73 m, and there has been subsequent drilling, all for
negligible economic result. The
exploratory workings, bedrock geochemistry and drilling have confirmed weak
metal values along this zone (up to 0.2% Pb and 440 ppm Zn, but mostly below
300 ppm Pb or Zn).
The exploratory workings are reported to have
encountered low but significant gold values, although no production is known.
The Great South Peak mine intersected quartz reefs thought to be within
the Peak shear (`Great Peak fault'). Gold
values are unrecorded (presumably negligible) in the thicker veins.
However, some thin veins (1-3cm) on more than one level returned good
assays of around 60 g/t Au. One
interesting occurrence noted was a quartz cross leader 0.5m thick which
contained coarse visible gold. This
cross-vein, dipping 65o in a NNE direction, was apparently found to be
auriferous only where it crosses the shear.
There is little other significant record of gold along the southern
extension of the Peak shear zone. Many
quartz veins have been sampled and found barren.
Nonetheless, one mullock sample from a shaft in the vicinity of the
Perseverence mine did carry gold (180 g/t Au) in vein quartz (CRA Exploration,
pers. comm. 1990), confirming older reports that the southern workings did
encounter gold values.
All indication of the Peak Shear is lost a
little to the south of the Great South Peak workings.
Projected several kilomtres south of the Great South Peak shaft the
trend of the Peak Shear would meet the Cobar-Hillston main road west of Narri.
Along this projection, and ranging that far south, are a number of
shafts and diggings about which very little is known.
Samples from this vicinity are generally unremarkable, and are only
occasionally anomalous (values ranging up to 1000 ppm Cu, 400 ppm Pb, 1200 ppm
Zn, 45 ppm Ag). Coherent anomalism
or traces of strong mineralization are not recorded from this area of
southernmost workings. For these
old prospected sites broadly along trend of the "Eastern Line" via
the Peak no other special features of interest can yet be discerned, and the
original basis of the prospecting is unknown.
The new deep
deposition under the Perseverance shaft, located 900 m south of the Peak mine,
was discovered via exploratory underground driving and drilling from the Peak
mine. Significant mineralisation was first discovered at
Perseverance in 1994 when PGM drilled a hole targeted 950 m below the historic
workings. The target was based on an apparent plunge to the
stratigraphic controls to the Peak Orebody. Plunging mineralisation was
encountered 600m south of the Peak orebody and followed down plunge and down
dip to c. 1000m south of the Peak orebody. Access for extraction was
then developed via a decline ramp from the Peak workings, from a near-shaft
base 630m below surface, to the Zone A orebody. Development of the
Perseverance resource commenced in mid 2002 and the first production stope was
brought on line in December 2003. The first mining was a long hole open
stoping operation at 25 m level spacing. Development then
continued down dip and plunge from the Zone A ore body to the deeper Zone
D ore body. Zone D is a relatively high grade deposit and has the bulk
of the known gold resource for the Perseverence deposit.
9.
CORONATION-ROOKERY AREA
Compared to The Peak, the mines of the
Coronation-Rookery trend are concentrated along faulting which lies further to
the east. It may be generally
remarked that the lines of mineralization lie further east, the further south
they lie, in a left echelon pattern (i.e. compare the Great Cobar-Gladstone,
New Cobar-New Occidental, The Peak, and the Coronation-Rookery lines).
A peculiarity along the Coronation-Rookery line is the presence of
westerly underlay. Mineralization
further north commonly underlies to the east (e.g. CSA, Great Cobar, New
Cobar, Chesney, Mt. Pleasant, New Occidental).
The sparsity or lack of magnetite might also be a group characteristic
of some significance.
The Coronation-Rookery group for the most
part comprises a line of mines and prospects concentrated along the western
side of a largely fault-bounded strip of Chesney Formation.
The deposit which comprise the Cornonation-Rookery group are:
No. Deposit Name
Commodities
119 Coronation mine
Cu, Pb, Zn
121 Beechworth shaft
Cu, Pb, Zn
122 The Central
Cu
123 Central South prospect
Cu, Zn
124 Queen Bee North
Cu
125 Queen Bee
Cu, Pb, Zn (Ag,Au)
126 Queen Bee South
Cu
127 Wright prospect
Zn (Cu, Pb, Au)
128 Illewong anomaly prospect
Cu
129 The Secret shaft
Zn, Cu (Pb)
130 Carissa shaft
Pb, As (Au)
131 East Ridge prospect
Pb
132 Mt Nurri prospect
Au? (Pb)
133 Langfords shaft
?Au
134 Narri prospect
Zn, Pb (Cu)
135 Rookery prospect
Zn, Pb (Cu)
136 Anomaly C1 prospect
Cu, Zn (Pb)
217 Anomaly D prospect
Pb
As is also
the case for mineral areas closer to Cobar, the mineralization is largely
restricted to shear zones on the slate side of a faulted contact between sandy
slates and sandstones (the "discordant contact").
However, in this part of the Cobar belt the Chesney Formation
sandstones are too severely cleaved for ready detection of bedding.
It is thus unresolved whether or not the sandstones dip discordantly
into the shear zones as they do further north (e.g. New Cobar-New Occidental
area). The stronger cleavage
possibly reflects association with a deep-seated master or boundary fault
system (Rookery fault zone), in contrast with the New Cobar-New Occidental
line being a postulated back fault system.
The back faults dip steeply east whereas the Rookery fault zone is
thought to dip west. Where
determined in mine workings, or by drilling, the zones of mineralization in
the Coronation-Rookery area do dip steeply to the west (e.g. Coronation,
Beechworth, Queen Bee). Further
south, however, the occurrence of porphyries on the eastern side of the
"discordant contact" has been taken to suggest an east-dipping
shear.
The main
workings of the area are the Coronation, Beechworth, The Central, and Queen
Bee. Of these, only the Queen Bee
mine was a significant producer. The
identity of some old workings (e.g. Central South and Queen Been North) is not
absolutely resolved. There are at
least three shafts between The Central and the Queen Bee.
There are many other weak indications of mineralization widespread in
the area. For some kilometres
south of Illewong there are sparse old workings, in the general vicinity of
the "discordant contact", for which little information is available.
Joklik (1948) shows the positions of 5 shafts along this favourable
prospecting zone.
Most of the workings in the
Coronation-Rookery area were probably commenced for gold.
Quartz veining is prominent at the Coronation, Beechworth and The
Central shafts. The Queen Bee was
initially prospected, unsuccessfully, for gold.
Base metals are present in small amount at many places, sometimes over
considerable width (e.g. Narri prospect), and the geochemical anomalies mapped
are up to 400 m wide. The Narri
prospect area is also of interest from the reported presence of
"elvan" in association with trace base metal sulphides.
The area certainly warrants continued exploration and deeper drilling
of any incompletely explained geophysical anomalism.
From Narri south to Wire Yard Tank several quartz vein and ironstone
trends have been prospected in the past. Surface
and mullock sample values are generally low (below 200 ppm Pb) but at Carissa
shaft several samples have returned around 1% Pb.
Coronation
mine
The
Coronation mine is in a weakly mineralized zone about 20 m wide, containing
minor gossans. At the mine a
cross-linking fault appears to truncate beds between two thrust or shear lines
(Thomson, 1953). Workings up to 66
m deep lie along strongly outcropping quartz veins and encouraging gold values
were obtained at various times. The
gossans have returned significant assays, with values up to 0.36% Cu, 1.31% Pb,
1100 ppm Zn and 7.2 g/t Ag. Drilling
beneath the old workings intersected a zone of low grade
pyrrhotite-chalcopyrite-sphalerite-galena dissemination, and
quartz-calcite-chlorite stockworks, within Great Cobar Slate.
Beechworth shaft
Strong
quartz veining, similar to that at Coronation mine, occurs at the Beechworth
shaft. The zone of silicification,
with disseminated pyrrhotite, lies in Great Cobar Slate within 30 m of the
contact with Chesney Formation. The
zone of quartz veining has been drilled and contains minor sphalerite,
chalcopyrite and galena.
The Central
The deep
(94 m) exploratory workings known as The Central mine were sunk in Great Cobar
Slate close to the contact with Chesney Formation, at a site with strong
ferruginization and copper carbonate staining in quartz veins.
A very low grade lode with small veins of pyrite is present.
It contains a little copper and is 11 m wide.
This was cross-cut at 94 m after the shaft intersected weak
chalcopyrite-elvan mineralization. No
assays are recorded. Surface
values in the vicinity range up to 135 ppm Cu and 600 ppm Pb (CRA Exploration
Pty Ltd, 1983).
Queen Bee mine
The Queen Bee mine at its peak was one of the
well known Cobar copper mines. It
produced about 0.04 Mt of ore averaging 7.9% Cu.
The main period of mining was 1903-1909, and the mine employed up to
160 men. About 3660t Cu and 1.37
kg Ag were produced. The gold
contents of the mined ore and the smelted copper is unknown.
Gold content may have been generally low and not noteworthy, as sparse
assay records mention only trace gold. The
estimated remaining reserve to 600 m depth is 1.2 Mt of 2.4% Cu ore.
The
Queen Bee is the only deposit of economic grade in the Coronation-Rookery
group. Like some of the other
sites, it is close to the faulted Great Cobar Slate-Chesney Formation contact
and had only minor surface expression. It
was first prospected for gold in 1872-1895, with little success and little
activity after the 1870s. The area
was again taken up under lease in 1902 to prospect for copper, and rich copper
carbonate ore was struck at 26 m depth after sinking through leached limonitic
gossan. The Queen Bee Copper
Mining Company NL was floated in 1903. Early
production was of rich ore from around 46 m depth.
Much of the early ore assayed 10-15% Cu, although several patches were
as rich as 40-50% Cu. The ore was
subjected to a two week roast in low 6m wide heaps, before smelting in
reverberatory furnaces.
The 76 m level was mainly in chalcopyrite ore
although some areas of copper carbonates and other secondary minerals peristed
downwards, with bodies of chalcocite obtained from as deep as 113 m.
The mine developed six levels, down to 204 m, and production until 1909
yielded 3049 t Cu. Average
recovered grade was 7.5% Cu. Later
production (1912-1919) was small scale. Little
ore was raised after 1915. The
Queen Bee became dependent upon the CSA mine for treatment and hence was
forced to close in the same year as the latter, 1920.
Small parcels of ore were produced in 1947 by a later party.
In 1950-1957 the main shaft was repaired by Eagle Ford Co. Pty. Ltd.,
and some stoping was carried out. Only
the richest of the broken ore (dressed to 20% Cu) was raised for transport, to
Port Kembla, since underground leaching proved more economical.
Subsequent small production came largely from leaching operations by B.
McLernon and associates in 1961-1969. Thereafter
the mine was acquired by Cobar Mines Pty Ltd.
The Queen
Bee ore system is 20-70 m wide and the ores are hosted mainly in sandy slate,
accompanied by occasional coarser beds, in a zone of crushing. The
crushing and dislocation was early thought to be along the contact of the
Great Cobar Slate and the Chesney Formation (Andrews, 1913).
However, the discernment of highly sheared Chesney Formation and Great
Cobar Slate may present a problem at Queen Bee (Kelso 1982).
The intensity of shearing may be locally very strong.
Descriptions of shear zone materials include reference to cobbles
flattened to cardboard thickness and sand-sized clasts drawn out into threads.
The orebodies lie 30-60 m west of the faulted contact with recognizable
coarse Chesney Formation sandstone. This
contact is the Queen Bee Fault, marked by concentration of quartz veining.
The ore
system hostrocks can be interpreted in various ways.
The chief alternatives have mostly been regarded as a facies change in
Great Cobar Slate, or Chesney Formation sediments rendered more slate-like
within the mineralized shear zone (Andrews 1913, Joklik 1948, Iten and Carter
1951, Thomson 1953, Kelso 1982). The
situation is similar as in The Peak area.
Fold styles on opposite sides of the Queen Bee fault are possibly quite
dissimilar. There appear to be
steeper fold axes on the western side, within a strip containing multiple
shear zones. In the vicinity of
the mine the slaty beds are strongly folded, with folds plunging vertically.
Kelso (1982) mapped and described the mineralized shear zone as a
80-200 m wide ductile-brittle complex containing remnants of an anticline.
The mineralized zone has shears at low angles to regional cleavage.
In places the shearing passes into more brittle faulting and areas of
brecciation are recorded from the mine and drillholes.
Locally there is pervasive silicification.
Some of the ore itself displays cataclastic texture, mainly at
microscopic scale. The orebodies,
faults and cleavage all dip steeply southwest (80o).
The ore
system comprises three major lodes: Main
Lode, East Lode and West Lode. A
small subsidiary lead-zinc lode is also recognized.
The mine produced mainly from the one principal lode channel, the Main
Lode. This was followed down from
near the surface and varied from 0.3 m to 7 m in width, inclusive of barren
intervals. Patches of solid ore
were 1.5-3 m wide. In the primary
zone the lode contained copper-rich seams or "channels" separated by
copper-poor pyritic mineralization. The
lode interval usually contained two or more rich channels up to 0.9 m wide.
These were generally siliceous, with about 3% Cu.
Richer patches existed and chalcopyrite-rich siliceous ore as rich as
6-11% Cu was obtained in considerable quantity.
Diamond drilling has confirmed the down-dip continuation of the Main
Lode, with intersections of up to 10.3 m true width at 2.9% Cu.
The massive sulphide in the main lode is commonly banded parallel to
cleavage. It contains less than
0.1% Pb and consists of pyrite, chalcopyrite, sphalerite and pyrrhotite.
Three
lenses were mined along the Main Lode: Main
Lens, South Lens and Lead-Zinc Lens. These
lenses plunge northwest at 65-70o. In
the Main Lens mineable ore extends for a length of 122 m and has been stoped
from 27 m to 172 m. Stope width
averaged 1.8 m to 110 m depth, below which the orebody decreases in width.
The lens contained a number of high grade 0.9-1.2 m wide seams which
averaged up to 10% Cu. The South
Lens is a blind orebody whose top reaches No. 3 level (110 m).
It is largely a low grade siliceous orebody up to 1.8 m wide and
containing less than 2% Cu. As in
the Main Lens, there is local development of a high grade (11% Cu) ore seam up
to a metre wide.
The Main
Lode mineralization is accompanied by silicification of the slates, with
quartz veining and sulphide stringers. Disseminated
sulphides and low grade veinlet mineralization occur in an envelope which
extends for up to 9 m around the Main Lode channel, and is particularly
well-developed on the western wide. On
the 110 m level this comprises an ill-defined Lead-Zinc Lode, best developed
on the southwestern or hangingwall side of South Lens.
The Lead-Zinc Lode contains disseminated galena, sphalerite,
chalcopyrite, pyrite and arsenopyrite in silicified slate.
An average assay is 0.4% Cu, 1.4% Pb, 3.3% Zn, 12.2 g/t Ag.
At the 137
m level, the East Lode was struck in cross-cutting about 5 m east of the Main
Lode. The East Lode has ore widths
of up to 0.6 m, and grades of up to 5% Cu.
The low
grade West Lode is defined from cross-cuts.
It lies 21 m west of the Main Lode and comprises disseminated pyrite
with traces of base metal sulphides. It
averages up to 0.5% Cu. The West
Lode is strongly brecciated in places.
10. VICTORIA TANK-NYMAGEE AREA
This area
is taken to be a direct continuation of the Cobar belt although prospects are
relatively minor. Some dense
ironstones occur but the area has yielded only weak indications of
mineralization. It may be a
relatively barren segment of the Cobar belt but is probably mineralized to
some extent. A relatively recent
gold discovery near Stones Tank confirms that it is not without promise.
The prospects within the Victoria Tank-Nymagee area are.
No.
Deposit Name
Commodity
214 Unnamed
Fe
215 Square Tank prospect
Zn (Cu, Pb)
216 Unnamed
Zn (Cu)
218 Unnamed
Fe
219 Victoria Tank vein
Pb (Cu, Zn)
220 Unnamed
Cu
241 Stones Tank prospect
Au (Ag)
The largely fault-bounded strip of rocks
designated as Chesney Formation in the north continues through this area,
although the strata have instead been assigned to the Mouramba Formation
(Canbelego sheet 8134). The strip
has had reconnaissance exploration by several companies, without encouraging
results. The Mouramba Formation is
considered equivalent to the Chesney Formation, and probably the Great Cobar
Slate. It has a shallow marine
arkosic basal facies developed where it overlies the Nymagee Igneous Complex.
This contact zone, which is probably faulted, appears to have faint
geochemical anomalism but available data is not definitive.
The boundary of the rhyolitic Stones Tank Tuff is also anomalous.
This had been considered as possibly an intrusive body but is now
regarded wholly as a volcanic member of the Mouramba Formation.
Mineralization traces are largely associated
with quartz veins. The Victoria
Tank quartz vein yields sample assays of up to 680 ppm Cu, 1560 ppm Pb and 310
ppm Zn. The vein has a marginal
laminated structure, perhaps associated with shearing.
The
Stones Tank prospect, which is the most promising prospect in the Victoria
Tank-Nymagee area, is also associated with a shear zone containing quartz
veins. However, the Stones Tank
prospect differs in the reported presence there of "elvan" type
silicification.
Stones
Tank prospect
The Stones Tank prospect was recognized only
in the 1980s as an area of distinct low order Au-As-(Ag?) anomalism spatially
related to the margin of a rhyolitic area near Stones Tank. Initial
gold anomalism showed close spatial relationship to the contact between
volcanic and sedimentary rocks. Arsenic
anomalism had a similar distribution while silver anomalism was more
restricted.
The highest gold concentration appears to be
along shear zones parallel to the regional cleavage, at or near the contacts.
It is accompanied by variable silicification, sericitization and quartz
veining. The main mineralized
shear (Stones Tank shear) dips steeply east (80oE) and is located on the east
limb of a south plunging anticline in the sedimentary rocks.
Elsewhere near the boundary of the rhyolitic area, fold plunges in the
enclosing sedimentary rocks are subvertical, indicating possible
"aureole" of high strain affects.
The prospect drew initial comparisons with
The Peak, where silicified volcanics(?) had then been recently reported from
the core of The Peak ore system. However,
further work at Stones Tank prospect did not bear out a strong comparison, and
the Stones Tank prospect may be simple mineralization in shear zones not
unlike many others in the Cobar region.
The highest gold values are from 1.8 km west
of Stones Tank, at the volcanics' western boundary shear zone, which is near
the northwestern known limit of the volcanics.
gold is also anomalous over small intervals within another shear zone a
little further east (e.g. 2m at 3.54 g/t Au and 7 g/t Ag).
Along the main western shear ("Stones Tank Shear") the best
intersection is 3m of 71.8 g/t Au with 11.9 g/t Ag.
The shear is mineralized for at least 100m length and another good
intersection was 4m at 5.3 g/t Au. The
gold may be patchy supergene enrichment but its depth continuity has not been
tested. Costean sampling obtained
25m at 1.22 g/t Au across the sandstone-volcanics western contact, with a high
(5m at 1.88 g/t Au) marking the Stones Tank shear zone.
Elsewhere near the rhyolite boundary a 40m rock chip traverse in
sandstone averaged 1.1 g/t Au. Spot
anomalous lead values, up to 4450 ppm Pb, have also been obtained in the area.
The rhyolitic Stones 'Tank' Tuff Member
(Felton, 1981) contains laminated and welded rhyodacitic crystal-vitric tuff
and flow-banded porphyritic rhyolite. Felton
(1981) did not suspect any of the rocks as intrusive.
Non-core drilling did not report sediments interbedded with the
rhyolite and hence did not confirm the assumed nature of the rhyolite as
volcanics. Norgold Ltd (Peko
Exploration Ltd & Norgold Ltd, 1990) nonetheless considered that
depositional features can be recognized within the igneous rocks, confirming
them as volcanics. Rocks reported
as tuffaceous occur with the rhyolitic varieties at surface, as well as highly
irregular siliceous sandstone-like bands of unknown origin.
Later diamond drilling also revealed breccias and sandy tuffs with
coarse fragtments, and confirmed the presence of epiclastic interbeds.
The Stones Tank area became more strongly
prospective when Electrolytic Zinc Co. (1986a) followed up weak bedrock
geochemical anomalism known there from reconnaissance with more detailed
grid-based work. An area
straddling the sheared contact between rhyolite and Chesney Formation
sandstone was found to be anomalous in gold.
The first detected gold anomalism, with accompanying arsenic to 240 ppm
As, lies 300m west of the Rookery fault, along the rhyolite-sandstone contact
which there trends boadly east-west. Later
prospects were found along the western boundary of the volcanics.
The siliceous to chalcedonic nature of some of the rhyolite may be
evidence of hydrothermal alteration. As
further encouragement it was found that gold grains to 1 mm could be panned
from colluvium at the site.
The western contact between the Mouramba
Formation and rhyolite is in places a 2-10m zone of moderately silicified
sheared rock (Stones Tank shear) with minor quartz veins and patches of
"elvan". Drill testing
from 1988 onwards upgraded the prospect. Anomalous
values for other metals are not strong and range to 195 ppm Cu, 1100 ppm Pb,
350 ppm Zn. Drilling has revealed
fine-grained pyrite (2-5%) to be common in the rhylolite.
Trace disseminated pyrrhotite, chalcopyrite and sphalerite are
associated with calcite and quartz veinlets in the rhyolite.
The rhyolitic volcanics are concluded to be host rocks only, and not
the source of the metals detected.
COBAR
BELT DISCUSSION
The mineralisation of the Cobar area has been
studied by numerous workers since copper was discovered in 1870.
Important background reading on the area includes Andrews (1913),
Sullivan et al. (1947), Sullivan (1950), Thomson (1953), Mulholland and Rayner
(1958), Russell and Lewis (1965), Rayner (1969), Robertson (1974), Brooke
(1975), Gilligan and Suppel (1978), Kirk (1983) and Glen (1987).
There are many other studies available and in all there have been
numerous genetic models devised or applied for the Cobar belt.
The three aspects which have been stressed most in relation to the
pattern of metalliferous occurrence in the Cobar belt are:
stratigraphic distribution of deposits, association of mineralization
with shears or faults, and cleavage; and
the steeply plunging lenticular to pipe-like nature of ore-bodies.
It was early appreciated that mineralizaion occurs along structural
planes, and that pipe-like ore bodies could be localized at the intersections
of planar structural elements of varied origin.
With
the exception of Elura, all deposits in figure XX occur in a D1 high-strain
zone (see section on Tectonism). Various
laboratory determined parameters are dependent upon pressure at time of ore
formation. This has been estimated
as 2-3 kbar (6-10 km burial) (e.g. Sangameshwar and Marshall 1980, Brill
1989b). Because of the high strain
effects, possible syndeformational ore deposition, and the
removal/non-deposition of the Mulga Downs Group, pressure remains one of the
more critical unknowns with a strong bearing of genetic interpretations.
In the
Cobar belt all significant deposits occur in carbonaceous turbidites of the
Nurri Group or in the CSA Siltstone of the Amphitheatre Group (figure xx).
Major mineralization occurs in three linear zones.
These are situated in the CSA Siltstone, in the middle Great Cobar
Slate, and in the Great Cobar Slate at its contact with the Chesney Formation.
Some occurrences of adjacent and subjacent minor mineralization within
Girilambone Group rocks are also known, at Mount Drysdale and elsewhere.
Except in the Warrego-Elura area, the major deposits lie near the
faulted preservational edge of the Cobar Basin, in a D1 high strain zone
(figure xx). The Warrego-Elura
area is also within a strain zone, a D3 high-strain zone (Glen 19xx), and
there appears to be strong fault/shear control of many deposits along the
Cobar belt. Probable fault control
is apparent at many mines (Great Cobar, New Cobar, Chesney, New Occidental,
Queen Bee, CSA, etc.).
Deposits at Queen Bee, The Peak and New
Cobar-New Occidental areas may be localized on faulted sections of the contact
between the Chesney Formation and the Great Cobar Slate, or in nearby faults
within the Great Cobar Slate. The
slate-sandstone contact is now but one of many factors considered for deposits
but in the early history of the Cobar field it was the only major control then
recognized. The New Cobar, Chesney,
New Occidental, Peak, Queen Bee, Central, Beechworth, Coronation, Tharsis,
Leslie and Mount Drysdale mines were all established at or near this seeming
stratigraphic horizon, which was consequently regarded as most important.
Subsequently it was realized to be much faulted, and the
slate-sandstone contact was regarded as a favourable locus for ore deposition
because of competency differences.
Some type of discontinuity at the contact
between the Chesney Formation and the Great Cobar Slate has been recognized in
these areas since the time of Andrews (1913).
The commonly presented generalization was that mineralization occurs
along the contact in a series of en enchelon shears which step east proceeding
southwards. The shears dip steeply
east, the sandstone beds dip steeply into the shears from the east, and the
attitude of the slate beds within the shear zones in variable (commonly
indeterminate). Conolly (1946),
followed by Thomson (1953), regarded the contact in the Central area as the
attenuated underlimb of an anticline ("stretch thrust") preserving
near original (essentially concordant) relations across it.
There was assumed to have been little or no displacement of adjacent
rocks. Sullivan (1950, 1951)
envisaged greater disturbance and regarded the slate-sandstone contact as a
folded fault or series of faults (discordant contact) which subsequently
localized the formation of lode shears. Sullivan
postulated that the boundary is a high-angle reverse fault close to the axial
plane of a major anticlinal feature which he interpreted as persisting from
Cobar to Nymagee. Mulholland and
Rayner (1961) regarded it as a series of short, east-dipping thrust faults
separated by WNW-trending steps representing unfaulted parts of the contact
between the two formations.
Alongside
disagreement on the detailed nature of dislocations, some authors (e.g.
Gilligan and Suppel, 1978; Sangster,
1979) have suggested or implied that the contact is basically stratigraphic,
with only limited movement, and that ore bodies are essentially stratabound.
Later work, especially by Glen has confirmed that faulting along this
contact (e.g. Great Chesney Fault) is major.
Glen postulated numerous other important faults throughout the region,
and stressed their role in mineralization.
In some cases, geochemical exploration along these suspected fault
lines has revealed encouraging metals anomalism.
Current
interpretation has the Great Chesney Fault as a major structure which cuts off
bedding in the western limb of the Chesney-Narri Anticline, and truncates its
axial trace in the Tharsis mine area (figure xx).
At least 1œkm of section in the Chesney Formation may have been lost.
The WNW-trending steps in the contact, the postulated normal
stratigraphic contacts of Mulholland and Rayner (1961), are probably cross
faults in the Great Chesney Fault. A
major step just north of New Cobar mine can be shown from mine plans to dip
north (Sullivan, 1951) rather than south parallel to bedding in the short
limbs of F1 folds in this area. A
similar but smaller step in the contact at the New Occidental mine was shown
to be a fracture by Mulholland and Rayner (1961).
The Great Chesney Fault is steeply east-dipping.
Stratigraphic evidence would suggest a simple high angle reverse fault
(east block-up), but it may have a complex history.
In the schemes developed by Glen (1978-1991), faults of the Cobar belt
have moved at different times and in different directional senses.
As early as Joklik (1948) attempts were made to apply structural
concepts such as major fault plane curvature at great depth, low-angle
thrusting, and fault plane re-activation.
Prior to Glen's (1978-1991) work the structural evidence for such
concepts was quite inadequate at Cobar. Nonetheless,
Joklik (1948) was of the opinion that the en echelon mineralized faults of the
Cobar belt represent combinations of steep shear thrusts and low-angle
overthrusts, partially superimposed from different periods of movement.
Joklik (1948) put forward the first tectonic scheme for the Cobar area,
some of the principal elements of which are still in use although the
interpretative details have been subject of much change.
Joklik considered that ore deposition was syntectonic, in the last
stages of compression.
Besides the association of ore deposits with
faults and shears, the role of folds has long been speculated upon.
Sullivan (1950, 1951) and others have postulated major anticlinal
cross-folding. The position of
Cobar itself was thought to be determined by a cross-fold.
This style of folding was thought to have caused domes to develop at
the intersection of axes. A major
anticlinal crossfold on the continuation of the Amphitheatre Dome structure
was projected by Sullivan to pass immediately north of the Thasis mine.
Sullivan also envisaged that granites probably occur along the axis of
his Cobar-Nymagee anticline, and that domes might correspond to cupolas.
Sullivan (1951, p. 155) stated "just as the whole Cobar field is
associated with the anticlinal crossfold, each ore deposit within the field is
associated with a similar structure. It
is suggested that these may correspond to the position of minor cupolas in the
underlying granitic rock". Before
Sullivan (1950, 1951) postulated a line of granitic intrusions beneath the
Cobar belt, Lloyd (1936, 1937) and Mulholland and Rayner (1947, 1961) had also
considered a granitic source. Rayner
(1969) later tended to reject any implied connection between the small
intrusives along the belt and ore deposition, because of evidence for the
Nymagee Granite predating the Cobar Supergroup.
The postulated role of granites has not been confirmed but the
relationship between orebodies and stratal domes which Sullivan introduced
continues to remain of interest (cf. Elura, The Peak).
The mode of development of this structural association is still subject
of investigation and controversy.
Sullivan's (1950) consideration of a possible genetic relationship
between mineralization and subjacent granite, in the same conceived broad
anticlinal fold structure as the Cobar deposits, was made at a time of
different regional geological interpretations.
The host rocks at Cobar were then thought to be early Silurian, and two
small plugs of porphyritic microgranite located 1 km and 2 km SSE of the Queen
Bee mine were thought to likely indicate the northwards continuation at depth
of the granite near Nymagee. Also
as a result of studying the igneous rocks in the Nymagee area, LLoyd (1936,
1937) had earlier reached the same view, that the Cobar ore deposits were of
igneous origin. However later
workers, from Rayner (1969) onwards, showed the district granites to generally
pre-date the Cobar Supergroup and to therefore be unrelated to major Cobar
mineralization. Although some have
maintained otherwise (e.g. Longman and Meares 1972) it has now become
generally accepted that all the well known granites predate the Cobar
Supergroup.
Possible
structural controls (shears, faults, folds, diapirs)
In numerous specific cases, and for the Cobar
belt overall, attention has been drawn to the possible role of structures in
localizing mineral deposition. Most
discussion focusses on the role of faults and shears.
Broader speculation has also been made on the possible localizing role
of upwarps and diapir-like physical movement of material in their cores.
As already mentioned, a general idea was that along the axes of long
regional anticlinal trends (e.g. "Cobar-Nymagee" anticline of
Sullivan 1951) there might be further domal structural highs or crests, caused
perhaps by interference cross-folds. It
was thought that such sites would be favoured by ore deposition because of
increased fracturing and the possiblity of intrusives in their cores (e.g.
Sullivan's hypothetical granite cupolas).
Subsequent to the general formulation of such ideas, the Elura
mineralization was found to be in tight domal culminations along a fold crest.
Other earlier known examples include the Peak.
There the major lodes appeared to be in or near the axial plane of a
shear-disrupted sub-vertical anticlinal structure, later found to have a
siliceous breccia core. Steeply
dipping lodes at the Brown-Conqueror mining locality, representing the main
Peak ore system, were early known to diverge at depth with the strongest
mineralization dipping west and weaker mineralization dipping east.
This suggested an anticline, although stratigraphic evidence was
lacking. Later deep drilling here
indicated a zone of inhomogeneous high strain adjacent to and above infaulted
slices of basement lithologies, further supporting the possibility of a
disrupted anticlinal structure or even a diapiric growth feature.
For both Elura and The Peak some initial consideration was given to the
possibility of diapiric intrusions. Mineralization
at diapirs is not an uncommon phenomenon.
Many diapirs in the Flinders Ranges, for example, carry post-intrusive
copper mineralization (e.g. Hall et al., 1986).
However, for the Cobar belt such speculation was short lived and
alterative interpretations soon prevailed.
Faults and shears have long been believed to
provide passage for fluids and favourable loci for metals deposition along the
Cobar belt (e.g. Andrews 1913, Sullivan 1951, Mulholland and Rayner 1961).
Mulholland (1941), Joklik (1948), and others, have regarded structural
geology as all-important in the Cobar area.
They have stressed the need for very detailed structural mapping, and
much structural work has indeed been carried out.
The early suggestions that Cobar ore deposits are of epigenetic
replacement origin all rested largely upon structural observations.
Structural features, especially the more obvious faults and shear
zones, have long been held to play a dominant role in localizing the ore
deposits of the Cobar belt. Many
writers have regarded the mineralized shear zones as major structures.
Rayner (1969) referred to them as "tecto-lineament zones".
The structures could be major ones in both time and space.
Some of the larger ones could mark long lived tectonic features that
controlled development of the Cobar Basin.
Glen (19xx-19xx) has extensively developed this line of investigation.
Sigmoidal trend deflections in bedding and
fault traces were also held to be important.
These were well recognized near Cobar by the late Great Cobar period.
It was early determined that the faulted contact between the Great
Cobar Slate and the Chesney Formation ("slate-sandstone line") is in
close proximity to the mineralization (e.g. Tharsis, New Cobar, Chesney, New
Occidental, Peak, Coronation, Beechworth, and Queen Bee mines).
In 1948 Zinc Corporation mapping also suggested the CSA deposit to be
associated with a marked swing or curvature of bedding in the CSA Siltstone;
and a strong sigmoidal deflection was later there confirmed by mapping bedding
trends within new mine workings.
Recognised quite early was the tendency for
ore to be deposited south of distortions in the Great Cobar Slate-Chesney
Formation contact (variously termed folds, deflections, warps, bends, kinks,
buckles, etc). The interpretation
of these features, like the structural interpretation of the contact itself
has varied greatly. For example,
Joklik (1950) stated that deposits are localized in shears arranged en echelon
along a "discordant contact" between competent and incompetent beds,
the Chesney Formation and Great Cobar Slate; whereas others (e.g. Thomson
1953) favoured a concordant contact. Writers
who envisaged major movement along the contact often inferred, from the
deflections along it, that a regional sinistral shearing couple had operated.
Andrews (1913), Sullivan (1951), Thomson
(1953), Mullholland & Rayner (1961), and others have noted the deposits
along the Queen Bee and Great Chesney Faults to consist of steep lenses
generally oblique to bedding. For
the CSA deposit, Robertson (1974) and O'Connor (1980) showed that
mineralization lay close to east-dipping cleavage and oblique to the
west-dipping bedding. Similar
relationship pertains in the Central Area mines near Cobar, with cleavage
dipping east and bedding usually dipping steeply west when discernable.
In addition to general conformity with cleavage rather than bedding,
individual ore lenses and entire ore systems appear to be preferentially
associated with faults or shears. Within
the shear zones themselves the relationships are not so clear cut.
Bedding is often obscured, but at places along the Chesney Fault it is
has been dragged into strong conformity with the shear direction and hence
dips in approximately the same direction as cleavage.
Associated alteration patterns may also show alignment upon, or
symmetry about, shears and faults. Recent
work (e.g. Glen, 1987a; Brill,
1988; de Roo, 1989;
Hinman, 1989) favours a syn-deformational, metahydrothermal origin,
with deposits occupying dilatant sites in the country rock along these
important structures.
Many workers have considered the
possibilities of Cobar Belt orebodies being syndeformational.
The more recent work on the genesis of these orebodies supports the
conclusion that they are syn-deformational metahydrothermal.
Since the degree of metamorphism varies considerably, and generally
decreases westwards from the preserved edge of the Cobar Basin, it is possible
that ore deposition occurred penecontemporaneously at different places under
differing severity of metamorphic conditions.
It has been speculated that the hostrock conditions prevailing during
mineralization at Elura, and perhaps CSA, during mineralization might be best
considered as diagenetic. The
possible structural controls on fluid movement include faults, shears, steeply
inclined dilatant jog structures, more horizontal partings due to vertical
dilation, and unusually deforming anticlines with central piercement by
hydraulic fracturing or other forms of deformation growth.
Some orebodies likely occupied either dilatant sites in the deforming
country rock (Glen 1987) or mixed sites of dilation and replacement (de Roo
1989). In zone 1, orebodies lie
adjacent to or within fault and shear zones, and occupy imbricate structures
formed in the Nurri Group rocks. Some
orebodies occur on or adjacent to "short cut faults" (Glen 1988).
In zone 2 the only ore system known to date is Elura.
Its pipe-like orebodies are localized in folds but nontheless form a
linear array. It is believed that
an underlying thrust may have provided the fluid pathway and focussing
mechanism needed to form the orebodies. Because
of the lesser deformation, genetic studies are expected to prove especially
fruitful at Elura. The rocks
surrounding the CSA and other
deposits close to the eastern margin of the Cobar Basin are stongly folded and
cleaved, but those to the west and around the Elura mine are characterised by
more open folds and less prominent cleavage.
Brecciation
is an important feature along many faults, especially in high strain zones.
Numerous breccia bodies have been recorded along the mineral deposit
belts, especially the Cobar Belt. Some
have been debated as possible sedimentary bodies but many are clearly tectonic
in origin. In some cases, as at
CSA mine, the tectonic clasts have become rounded and a rock of
pseudo-conglomeratic nature is the result.
Other bodies have been interpreted as dilational hydrobreccias, and
some may have formed at fault jogs. Some
of the breccia bodies have been established as having a sub-vertical steeply
plunging orientation, with lenticular to pipe-like cross-sections.
One such body at The Peak is about 150œm in diameter.
Felsic volcanic clasts, now silicified, have been recognized in this
breccia (Electrolytic Zinc Co A/asia, 1988) (ELœ2605).
It is therefore thought to have carried up volcanic rock not
represented in the surrounding Cobar Supergroup hostrocks.
Breccia pipes and lenses could be products of similar vertical
dilational tectonic processes as are envisaged for the growth of some of the
orebodies (e.g. Elura ore system). Numerous
syntectonic aspects of mineralization have been considered by various workers
(see xxx Section). Ideas range
from tectonically passive mineralization, including that in dilation zones, to
more interactive variations. Marshall
and Sangameshwar (19xx), for example, considered the Elura ore system as an
early domal structure nucleated about mineralization, which then in turn
focussed an on-going tectonically driven piercement process.
The association of mineralization with major
structures has suggested ore genesis models involving the migration of metal
bearing fluids towards and along major faults, with the precipitation at
dilatant sites therein of gangue minerals, sulphides and native metals.
Fault jogs and other mechanisms have been envisaged as a means of
producing favourable dilations, pressure relief sites, and hydrobrecciation.
Glen (1987a) suggested that the fluids involved were metamorphic in
origin and Brill (1988) attempted to demonstrate this for the CSA deposit.
Ore along the Great Chesney Fault maybe
associated with left-stepping fault jogs.
Dilations could have formed either as cross tear faults during
thrusting or as dilational jogs during left-lateral strike-slip movement.
The Chesney deposit, with gold ore pipes located at each end of a
linear lode, is a good example of the possible selective mineralization of
cylindroidal masses of rock disturbed in tear or jog zones at the of fault
length terminations. As noted by
Sullivan (1951), Mulholland and Rayner (1961) and Glen (1987b), these areas
are marked by intersections between north-northwest and
west-northwest-trending major fracture systems, with consequent development of
significant fault-induced permeability in the imbricated footwall of the fault
itself. Whatever the mechanism for
forming the apparent fault-end gold shoots at Chesney mine, analogies could be
expected at a variety of scales. Thus
Lloyd (1943) thought that the entire New Cobar-New Occidental line of
mineralization, with two major gold mines at its extremities, might somehow
mirror at grander scale the situation within the Chesney mine.
Mulholland and Rayner (1961) stated that
cross fractures near Cobar developed with a northwesterly trend and a
northerly dip. The intersection of
these with the cleavage planes formed a northerly pitching "grain"
to control the pitch of ore shoots. Rayner
(1969) again mentioned that the principal deposits (New Cobar, Chesney, New
Occidental and the Peak) are each associated with a notable buckle in the
contact, providing a favourable locus for ore deposition.
Rayner considered the flattened cylindrical shape of the ore bodies to
result from impregnation and replacement following the actual attenuation and
shattering of cylinders of rock, particularly slate, by
tensional rotation. That is
to say, such columns formed between overlapping thrust planes moving under the
action of wrench couples. The
sense of coupling movement, so far as a horizontal component is concerned, is
generally interpreted as sinistral or east block north.
Rayner's ideas on tension and attentuation may be compared to more
recent models of dilational structures by de Roo and others.
Although details differ considerably there is long-standing focus on
mechanisms for ore deposition at areas of structural dilation.
Models allowing both vertical and horizontal dilation have been invoked
for the Cobar belt.
Structural controls must play a dominant role
in any interpretation of Cobar belt mineralization patterns.
The Cobar belt of deposits is an empirical grouping.
Its geological correspondence mainly takes the form of a high strain
zone forming the eastern edge of the deformed Cobar Basin.
Following Glen (19xx-19xx) it is envisaged that the Cobar Basin was an
intracratonic basin formed by left-lateral transtension and inverted under
right-lateral transpression. A
high-strain "flower zone" which developed along the eastern margin
is characterized by near meridional thrusts, folds and regional cleavage
(structural zone 1). The lower
strain zones 2 and 3 to the west are characterized by widely spaced
WNW-trending F1 folds, thrusts which sole on a detachment, NE-trending F2
folds and uncommon S1 and S2 cleavages. Much
of the Cobar belt falls in Glen's zone 1.
All major deposits of the Cobar belt, except
Elura, occur in a regional high-strain zone (Glen's Zone 1) characterised by a
subvertical cleavage and down-dip elongation lineation.
Deposits within Zone 1 cluster along three lines of strong deformation
interpreted as major thrust systems. Gold-copper
deposits (New Cobar, New Occidental) lie on the Great Chesney Fault, an
east-dipping backthrust, or imbricates in the immediate footwall (Glen 1987b).
Copper mineralisation lies in east-dipping thrusts within the Great
Cobar Slate (e.g. Great Cobar - Gladstone area).
Copper-lead-zinc-silver deposits lie in steeply east-dipping
imbricatations (chloritic shear zones) in the CSA Siltstone east of the
Footwall Fault of Kapelle (1970). The
high-strain portion of the Cobar belt containing these deposits likely reached
lower greenschist grade and pressures of approximately 3 kbar during Early
Carboniferous time (Brill 1988). CSA
mine ore formation temperatures from various methods (fluid-inclusion
analyses, chlorite compositions, etc.) are in the range 300-350oC and this is
thought compatible with synmetamorphic deposition (Brill 1989b).
Many of the ore deposits are prominently
aligned along faults. Moreover,
ore deposition also appears to show a spatial association with anticlinal
axes. Mineralized breccia at The
Peak and the massive sulphide orebodies at Elura well demonstrate this
association. In simplistic terms,
explanations have varied from the folding of strata around a pre-existent ore
system to the hydrothermal or even diapiric emplacement of sulphides into
pre-existent anticlinal structures. These
extremes relate to the extremes of genetic modelling, discussed below.
In a purely syngenetic model, for example, the Elura orebodes which
occur in domal cores may be postulated as diapirs emplaced via physical
remobilization of earlier deposited sulphides.
The correlating of regional structural
elements along the Cobar belt, and also between a given ore system and its
surrounds, has received a considerable amount of attention but the deformation
phases discerned in one deposit can not yet be related to those in another
with any certainty. In all cases
studied, however, the sulphide minerals are affected significantly by
deformation and none of the deposits can be considered as post-tectonic.
The most detailed ore system structural studies published have been for
Elura, where four phases of ductile deformation are recognized in the mine
area (de Roo 1987, 1989), and for The Peak (Hinman, 1989).
The results of structural studies, already
referred to, give considerable support to the Cobar belt deposits being
syntectonic. De Roo (1987,1989
formulated a detailed syntectonic growth model for Elura.
The Peak mineralization system is equally noteworthy in a structural
sense. Hinman, (1989) suggested
that The Peak deposits occur where deformation has been partitioned around a
contact between sediments of the Nurri Group and silicified felsic volcanics.
The Peak mineralization may have been emplaced at a structurally
pre-conditioned site and/or generated structural anomalism after the orebodies
and associated silicification were formed.
The Peak mineralization is situated within a structurally anomalous
volume of rock that presents a target to the explorationist some 2 to 3 times
the size of any alteration or mineralization (Hinman, 1991).
If ore deposition was synetectonic, a range
of structural mechanisms may have existed for influencing the behaviour of the
heated fluids. Hinman's studies
have explored these possibilities, which generally involve multiple local
cycles of ore deposition rather than one single wide-ranging paragenetic
history at Cobar. Abrupt
reductions in fluid pressure induced by rupturing could have played a key role
in ore deposition. Rapid slip
transfer across dilation fault jogs or bends may cause abrupt local reductions
in fluid pressure. At high crustal
levels in geothermal systems, such pressure reductions may directly induce
mineral deposition. At mesothermal
depths the processes are less direct. One
process applicable to Cobar possibilities is for reverse faults to cap
over-pressured fluid systems, and allow cyclic interplay between lithostatic
and hydrostatic pressures. Frictional
shear failure can occur only after supralithostatic fluid pressures are
attained. Crustal rupturing can
then allow sudden drainage of an over-pressured reservoir with rapid fluid
pressure drop towards hydrostatic values.
Phase separation of carbon dioxide, rapid mineral precipitation and
hydrothermal self-sealing might occur. In
a syntectonic setting of ore deposition, fluid pressures might then rebuild
towards the supralithostatic values needed to trigger the next episode of
cyclic deposition.
The timing of the deformation or inversion of
the Cobar Basin is still open to question and multiphase deformation is
distinctly possible. Sediments of
the Cobar Basin were metamorphosed to lower greenschist facies during a major
period of defromation often interpreted as Carboniferous.
The Cobar Supergroup rocks have long been considered to have undergone
one major deformation. This was
earlier regarded as Tabberabberan (Middle Devonian) such as by Russel and
Lewis (1965) and Rayner (1969); and later on as Kanimblan (Carboniferous) as
by Pogson and Felton (1978) and Glen (1982,1985).
As Glen (1985) and others noted, Carboniferous deformation appears
likely on the basis of a general absence of unconformable relations below the
Mulga Downs Group, and the congruence of folds between the Mulga Downs and
Amphitheatre Groups. However,
evidence from isotope geochronology (Binns, 1985;
Glen, Dallmeyer and Black, 1986) suggests that deformation may have
occurred relatively soon after sedimentation in the Early Devonian.
Age dating (Black & Glen 1983, Glen et al. 1986) suggests that the
cleavage deformation in Zone 1 is late Early Devonian (~400Ma), with no sign
of any Carboniferous overprint. The
data suggest that the eastern margin and main bulk of the Cobar Basin
underwent regional inversion in the late Early Devonian, in an event which
largely failed to affect the basin further west except for block faulting.
Such earlier deformation within the high strain zone may not have been
orogenic or sufficiently intense to preclude the depositon of the Mulga Downs
Group, as P-T estimates from mineralogical studies require the Mulga Downs
Group presence to satisfy the required lithostatic load (Brill 1988).
The main inversion event along the western edge of the Cobar Basin and
in the adjoining Winduck Shelf is still regarded as Carboniferous by Glen
(1991). It was accompanied by
deformation of the overlying Mulga Downs Group.
This interpretation has the opposite basin margins undergoing inverion
at different times.
For the Cobar belt, the Rb-Sr isotopic ages
predating the Devonian show a regional isochron of 442 +/- 4 Ma.
K-Ar dates from Zone 1 of ca. 405 Ma suggest Early Devonian cleavage
formation. The Ar/Ar spectra (Glen
et al. 1983, 1991) give intermedate temperature plateaux for Zone 1 around 400
Ma and total gas ages ranging from 378 to 397 Ma.
For Zone 2, total gas ages range from 401 to 423 Ma, and
intermediate-temperature plateaux around 450 Ma and 437 Ma have been obtained.
Glen et al. (1991) interpret these data as reflecting low grade
cleavage formation at around 400 Ma. Older
dates from Zone 2 could reflect basement provenance age of preserved detrital
micas and feldspars which did not undergo complete re-equilibration during the
400 Ma event. These ages
correspond with metamorphism of the Girilambone Group and intrusion of
Silurian granitoids. The greater
preservation of detrital characteristers for the northern part of Zone 2
suggests lower deformational strain and/or temperature of metamorphism during
the 400 Ma event. This can be
applied to Elura. Further south
and east in Zone 1, the isotopic data indicate more complete re-equilibration
of detrital phases during the 400 Ma event.
Some investigators have concluded that most
of the Cobar belt mineralization was emplaced duing cleavage development.
Binns (1985) dated muscovite intimately intergrown with quartz and
sulphides at the C.S.A. deposit at about 390 My (K-Ar and Rb-Sr).
The whole rock 40Ar-39Ar age determinations of about 385 to 400 My
(Early devonian) on samples of Great Cobar Slate and CSA Siltstone are
believed to date the end of cleavage formation (Glen et al. 1991).
Since these strata are, on meagre palaeontological grounds, also
assigned an Early Devonian age (e.g. Baker, Schmidt and Sherwin, 1975), it is
currently consistent that mineralization took place during deformation in the
Early Devonian. Most of the Cobar
belt ore is regarded as deformed, such that tectonic deformation would appear
to have continued beyond that time by which the bulk of the sulphide ore had
been deposited. At several
deposits, however, there are minor phases of mineral deposition discerned for
which the sulphides appear much less deformed.
Thus some primary sulphide deposition at the major deposits is thought
to have continued through the waning phases of host rock deformation.
Where bedding lies oblique to S1 cleavage,
all the major deposits in the Cobar belt consist of lenses or lodes lying
oblique to bedding. The best
example comes from the CSA mine (O'Connor 1980) and also from Elura mine
(Schmidt 1980). At the New Cobar
mine, the ore body strikes NNW, oblique to bedding and occurs in an area
marked by the intersection of NNW-trending faults and veins with NW-trending
faults and veins.
Most Cobar
belt deposits for which good data exist, consist of one or more subvertically
plunging lenses or lodes. The
maximum extent of all primary mineralisation bodies is subvertical.
Vertical elongation may be discordant to both bedding and cleavage, and
in the case of the Great Cobar main lode it may bisect the angle between
bedding and cleavage.
Structures attributed to the inversion of the
Cobar Basin have been extensively mapped and described by Glen (1982-1991).
Glen interprets Basin inversion as caused by right-lateral tranpression,
with the structural zones of varying intensity
and geometry reflecting the partitioning of deformation into
strike-slip and compressional components.
The Cobar belt largely coincides with Glen's Zone 1 along the eastern
edge of the former basin. Its
structural style is characterised by steep thrust faults which are inferred to
shallow with depth as part of a linked thrust system and merge into a floor
thrust which itself steepens into a strike-slip fault.
Glen (1988-1990) has described three thrust plates.
From west to east, these plates are the steeply west-dipping Cobar
Plate (imbricated at CSA mine) and bounded to the east by the west-dipping
Cobar Fault; the strongly
imbricated Chesney Plate between the Cobar Fault and the east-dipping Great
Chesney Fault; and eastern pop-up
zones including the Queen Bee Plate.
Of the various major fault zones, the Myrt
and Rookery faults are interpreted as reactivated syn-sedimentary faults.
The other faults have no demonstrable early history and developed as
short-cut structures during thrusting. Major
folds in Zone 1 are interpreted to be thrust related ramp (fault-bend) folds,
or fault-propagation folds which were subsequently affected during regional
shortening by a subvertical S1 cleavage overprinting (Glen 1985, 1990).
Evidence of strike-slip movement on faults within Zone 1 is indicated
by the braided pattern of the Rookery Fault, by jogs and quartz vein arrays in
and adjacent to the Great Chensey Fault (Glen 1987a), and by shallow plunging
striae on steeply east-dipping shears which overprint dip-slip striae both at
the CSA mine and at The Peak in the Blue Shear.
At the Peak, braided faults and steep folds in chlorite-talc schist
indicate a strike-slip movement. Whereas
most of the strike-slip movement in Zone 1 was left-lateral, a right-lateral
component of vertical movement has also been documented on the Great Chesney
Fault (Glen, 1987a). Some
right-lateral movement on the Myrt Fault is also suspected.
he spatial relationships of deposits to
faulting becomes more apparent as more and more faults are mapped in the Cobar
region. Relationship of
mineralization and microstructural features is also subject of ongoing
studies, with complex data sets and interpretations generated from both the
Peak and Elura mines. The
deformation and faulting is likely multiphase throughout the Cobar belt.
Hinman (1989-1991) has evolved a four stage tectonic history for
mineralization for the Peak area. The
first three of Hinman's stages are progressive elements of a continued ductile
shortening along the eastern margin of the deformed Cobar Basin.
They are: [S1] Folding
associated with a weak S1 cleavage; [S2] development of regional slatey
cleavage that postdates and transects folding; and [S3] the local development
of an ore localizing highly inhomogeneous high-strain reactivation cleavage.
The fourth stage [S4] may involve relaxational post ductile faulting
which crenulates earlier ductile fabrics.
Within his multistage framework, Hinman
further proposed a detailed paragenesis from the Peak system.
The five main paragenetic stages are as follow:
1. Late-Post S1 pyrrhotization of sedimentary pyrite and carbonate in an
assymetric halo extending in the up-dip direction around the mineralized zone.
2. Open space vein infill by coarse quartz, carbonate and pyrite.
3. Introduction of vein and replacement silica and carbonate accompanied
by galena and Fe-rich sphalerite. At
its strongest, this stage is represented by massive replacement zones that
core broader zones of `elvan' style silicification.
These are products of grain scale, crack-seal silicification driven by
ductile deformation.
4. Replacement and veining characterised by silica, chlorite and muscovite
accompanied by galena, pyrhotite, chalcopyrite, bismuth, silver and gold.
Sulphides dilate quartz grain boundaries lying at high angles to the L3
stretch direction.
5.
Stage of replacive Ag-rich banded ores comprising black chlorite with
muscovite, sphalerite, galena, pyrrhotite, pyrite and chalcopyrite.
This stage is characterized by post-ductile deformation with massive
replacement of siliceous and mineralized assemblages.
Irregular, disharmonic banding is controlled by jostling between
remnant unreplaced blocks during late [S4] faulting.
Genetic
interpretations of Cobar belt deposits
Many differing hypotheses have been offered
in relation to the genesis of ore deposits in the Cobar belt.
Some have already been mentioned here. Very broadly, an epigenetic hydrothermal origin has been the most
favoured, with challenge from syngenetic and remobilization theories
particularly in the 1970s. Fault-controlled
epigenetic hydrothermal origin, possibly from a very deep parental magma, was
firmly favoured in earlier years and into the 1960s. Following a period of syngenetic and other considerations, hydrothermal
(metahydrothermal) origin again became the dominant theory, and remains such
at present. Corbett (2002) stated in regard to the
"clear" epigenetic relationships to the basin turbidite host rocks
throughout the Cobar Au-Cu-Pb-Zn district (e.g., Peak, CSA, Perseverance, New
Cobar) that whilst Stegman (2001) stressed the difficulty in classifying the
Cobar deposits, intrusion-related origin cannot be ruled out.
The hypotheses which have been invoked at
Cobar range from classical epigenetic to syngenetic (e.g. Sangster 1979), and
remobilized syngenetic (e.g. Brooke 1975, Gilligan & Suppel 1978). Syngenetic models came into vogue in the 1970s.
Robertson (1974), Gilligan and Suppel (1978), Sangster (1979) and
others envisaged a submarine exhalative origin for the orebodies. Metals zoning hypotheses in syngenetic terms and the interpretation of
elvan as exhalative chert were two features of this period.
Early authors generally regarded the deposits
as epigenetic hydrothermal bodies localised by faults or shear zones parallel
to cleavage. Andrews (1911)
believed that igeneous activity played no part in the process, but others
invoked magmatism either as a direct source of metals or as a heat engine for
high temperature fluid circulation to mobilize metals (Sullivan, 1950;
Mulholland and Rayner, 1958; Rayner,
1969). Prior to the 1980s the
genetic theories could be neatly grouped into three categories:
a)
Epigenetic hydrothermal replacement of unspecified source (e.g. Andrews
1913, Connolly 1946, Mulholland and Rayner 1947, Thomson 1953).
b)
Granite-sourced hydrothermal (e.g. Lloyd 1936-1939; Sullivan 1948,
1950; Joklik 1948, 1950).
c)
Syngenetic, volcanic and submarine exhalative (e.g. Besley 1966,
Robertson 1974, Brooke 1975, Gilligan 1978, Sangster 1979).
Theories
subsequently developed or elaborated are not so readily grouped, and may mix
earlier concepts with more recent ones including processes of metamorphic
dewatering, hydrofracturing, and tectonic modification.
Demonstration of apparent inadequacies of the simpler models has often
led to more complex modifications. Syngenetic
sulphides were seen to be modified by subsequent metamorphic or structural
events. Some of the later theories
would recognize in the Cobar belt, or in individual deposits, all three types
of ore deposition discernible with respect to metamorphic cleavage (pre-, syn-,
post-). Such a model, if admitted,
is so versatile that virtually anything can be explained:
syngenetic ore subjected to ductile remobilization, syn-metamorphic or
syn-deformational ore, and post-cleavage vein style ore.
For example, one line of more complex hypothesis involves remobilized
subhalative and exhalative mineralization being overprinted by
metahydrothermal mineralization (Marshall & Sangameshwar 1980, 1982;
Marshall et al. 1983). Most
of the genetic theories have envisaged heated ascending fluids for the
transport of the metals, with channeling and/or deposition along structural
features. A range of ideas still
exists as to where the metals came from and the tectonic environment and
timing of their various movements.
Although the syngenetic exhalative theory has
fallen from favour at Cobar, much of the later theory still bears a strong
sedimentary basin emphasis, the idea being that the Cobar basin somehow
remains germane to the mineralization even if the latter is accepted as
postdating basin infilling. The
genetic link through time which is often envisaged entails basinal fluids and
their expulsion. This is the
concept that the genesis of many metalliferous deposits, like hydrocarbons
concentrations, can be fitted into a context of normal sedimentary basin
evolution. Effort has been made to
borrow principles from petroleum geology:
e.g. structural controls, maturation of source rocks, migration routes
for fluids, distribution of favourable "reservoir" rocks and traps,
seismic pumping, basinal dewatering, etc.
The generation and "maturation" of the metal-bearing fluids
could be generally in reverse to well-known principles of ore mineral
deposition. Thus the typically
last-deposited metals (Pb-Zn) might be the first to enter the concentrating
fluids as the sediments passed through the appropriate post-burial temperature
window. Basinal brine models have
been proposed for many years, such as for Mississippi Valley-type and other Pb-Zn
deposits. They suggests that both
hydrocarbon fluids and briney metalliferous fluids are expelled from the basin
fill as it subsides, thickens and becomes compacted.
All manner of fluid migrations have been considered, from continuous to
episodic, and from vertical to strongly lateral.
In the case of the Cobar Basin, and perhaps drawing from even further
west in the Darling Basin, it may be conceived that cover remained intact to
the west whereas rupture and fluid escape was extreme along the eastern
margin. There, in the east, basin
inversion may have accentuated metal maturation and generated syn-deformational
deposits. As a variation to this
model, deep intrusions may be incorporated merely as a further heat engine
rather than a prime metals source. Studies
commissioned on the Cobar basin to investigate themes similar to the above
currently remain proprietary in detail but have not directly lead to any
discoveries.
As with most New South Wales hardrock
metalliferous deposits, those at Cobar were early regarded as hydrothermal and
epigenetic. Embryonic aspects of
metahydrothermal theory, basin dewatering, and flow focussing along deep
seated fault systems are present in Andrews (1913) and other early work.
Andrews concluded that deep seated mineralizing solutions rose by way
of a long series of en echelon faults extending from Mount Hope and Nymagee to
north of Cobar, by way of Bee Mountain and the Peak.
He conceived two phases of ascending ore-forming fluids influx.
The initial solutions were conceived as derived in their entirety from
a deeply-buried complex, and concentrated during ascent along the great planes
of dislocation and shearing evident along the Cobar belt.
Andrews thought that this initial phase formed the pyrrhotitic ores.
For the introduction of the later ores, considerably richer in
lead-zinc and lacking pyrrhotite and magnetite, Andrews postulated that some
deep-seated intrusive activity occasioned both a rejuvenescence of movement
along old fault planes and a complete change in ore character.
Early workers generally regarded the Cobar
mineralization as vein type and closely related to faults or other features of
deformation. The depositional
environment has commonly been regarded as mesothermal, with some tendency
towards limited epithermal-type aspects suspected along the eastern edge of
the belt. No igneous source has
been readily apparent. Many early
descriptions interpret replacement, and less often dilation, in association
with the quartz vein mineralization which the writers accepted as hydrothermal
and epigenetic. Of the early Cobar
authors only Chesney (1889) included syngenetic speculations.
A phase of strong interest in attempting syngenetic interpretations, in
the 1970s, was followed by many laboratory studies from which the results
currently favour return to hydrothermal metahydrothermal models of
mineralization.
Results from fluid inclusion work, isotopic
studies, and various geothermometric approaches have all been broadly
consistent with the structural and textural evidence for dominant
hydrothermal-metamorphic influence on the ore systems.
Current thought is that these influences were formative and not related
merely to remobilization effects. Dominant
recognition has been given to metamorphic processes by Glen (1984, 1987), de
Roo (1989), Himan (1989), Brill (1989) and others.
Sangameshwar and Marshall (1980) early advanced this current trend with
their inferences from CSA mine sphalerite.
From the untenable pressure results obtained (7.7-9.0 kbars) they
concluded that the sphalerite had reequilibrated and that there had been FeS
dissolution during decay of a protracted cleavage-producing structuro-metamorphic
event. The CSA system is
undoubtedly not unique in this respect. Application
of the spahalerite geobarometer to sulphide deposits from apparently prograde
greenschist facies terrains has often yielded anomalously high pressure
determinations. Even where
sphalerite-derived pressure determinations from such deposits conform with
stratigraphic and silicate-based determinations, they may exhibit greater
standard deviations for a given deposit than those from high grade terrains
(Scott, 1976). Sphalerite may
reequilibrate concurrent with inversion of hexagonal pyrrhotite to monoclinic
pyrrhotite. Retrograde
reequilibration of sphalerite with pyrite and monoclinic pyrrhotite, following
initial equilibration with pyrite and hexagonal pyrrhotite at peak
metamorphism was the explanation of the low FeS contents of sphalerites at CSA
mine which Sangameshwar and Marshall (1980) advanced.
The degree and importance
of physical or chemical remobilization processes for the ore systems in the
Cobar region has been debated. In
general, remobilization might occur through transfer from previous positions
by plastic flow in a rheid or near-solid state; or it could involve saltatory
chemical dispersal and redeposition of metals by fluids moving through
temperature and pressure gradients. Cobar
viewpoints on this have ranged from envisaging some orebodies as having been
completely remobilized into their present settings (e.g. Elura) to an almost
complete dismissal of remobilization. Debate
centres on the scale and importance of remobilization in the Cobar belt;
whether it has been a major and formative process for the ore sytems,
or something causing only minor modifications to them.
given the complexity of the ore systems, and the eveidence of
deformation and ore deposition during a prolonged time interval, it seems
likely that some of the texturally younger features may be after
remobilization. Late stage
features are most frequent in veins, breccias and shears but htere are also
recorded small intervals within sulphide orebodies where quartz remains
equigranular and cleavage traces are absent.
This may suggest remobilization as both a hear source sufficient for
recrystallization or the preservation of unmetamorphosed enclaves within shear
zones seem less likely options. When
the Elura ore system was little known, it could be conceived as a possible
emplacement of a physically remobilized mass of syngenetic sulphides.
The concentric zoning was able to be explained by the pyrrhotitic core
comprising original basal layers. Thus
pyrrhotitic ore could have been originally overlain by pyritic ore, in
accordance with some syngenetic exhalative theory.
Subsequent detailed studies at Elura, however, do not support a
remobilized syngenetic origin of this sort.
Supporters of remobilization have often
expected it to have transposed stratabound or stratiform orebodies in
accordance with, and in small scale mimickry of, the broader regional
structures. The geometry of the
ore lenses may in turn be mimicked on a smaller scale by a number of strain
markers, including pressure shadows and the elongate steeply pitching
morphology of Cobar belt orebodies has been compared with other elongate
pitching features along the belt, such as pyrrhotite lineation,
flattened-elongate load casts, and deformed pebbles or concretions.
Many have made such comparisons, with Gilligan and Suppel (1978)
considering the issue at some length. Gilligan
and Suppel (1978) supported remobilization and concluded that the elongation
and general geometry of the Cobar belt orebodies is best explained by mass
remobilization parallel to the maximum extensive strain during cleavage
formation. Relations between
massive sulphides, hostrock fabrics, and mineralised vein systems have
suggested to a number of worker that the sulphides were deformed and
remobilized over a protracted period, during which the host rocks may have
changed from ductile to brittle. The
processes and concepts grouped under remobilization are so numerous that
remobilization to some degree may be accepted as important to ore formation.
Textural evidence presented is often at microscale.
Evidence for the gross remobilization of any pre-existing Cobar belt
orebodies remains less compelling.
The broad idea of physical remobilization of
stratiform ore deposits in the Cobar belt is that strong regional shortening
perpendicular to the belt trend has produced folding in the Cobar Basin host
strata and a quite disparate response in the sulphide bodies.
The sulphides were prone to be squeezed into near vertical lenticular
pipes. The proponents of such
remobilization have usually also held chemical remobilization to be very
important and only the Elura ore system with its more massive ores could be
thought amenable to a pure "toothpaste" conceptualization of
physical remobilization.
A horizontal ore system squeezed and
transformed into a vertical one by regional shortening could be expected to be
overlain by domal and underlain by synclinal features.
Such could be expected at Elura, where regional flattening may be in
the range of thirty to forty percent. Further
south the Cobar belt ore systems are in a high strain zone, with as much as
seventy percent regional flattening. Any
pattern of overlying anticline and underlying syncline formed in response to
the vertical flattening of a stratiform ore system might be expected to have
been obliterated by the strength of regional flattening, wrench shearing and
other deformation along most of the Cobar belt.
Thus Elura is generally accepted as the best test site for physical
mobilization. Corrolaries of the
model include the need for extraordinarily high strain in fold limbs drawn out
parallel to the sulphide plugs squeezed from horizontal to vertical.
Fold limbs might thin perhaps tenfold towards the sulphide interface,
and an influx of silica towards the ore is also expected.
The massive sulphides in extending parallel to the vertical cleavage
growth were also prone to sub-horizontal fracture and infill.
The more ductile sphalerite may have been able to elongate vertically
without sub-horizontal fracture, whence the prominent vertical lineation
defined by sphalerite concentration.
Whatever the degree of remobilization, there
is current fairly wide acceptance that the metals and sulphur which are now
components of orebodies throughout the Cobar belt were released from Cobar
Supergroup sediments during early metamorphic dewatering.
The mineralization has a lead isotope Devonian crustal signature
(206Pb/204Pb of 18.07-18.16) in common with that hosted by felsic volcanics in
the Mt Hope and Rast Troughs further south (Carr et al. 1991).
Comparing further afield, sulphur and lead isotopic compositions are
similar between Cobar belt deposits and Lachlan Fold Belt conformable volcanic
mineralization, e.g. Colo Creek, Woodlawn and Captains Flat (Ostic et al.
1967, Marshall et al. 1981, Gulson & Vaasjoki 1982).
The Devonian "crustal signature" isotopic compositions from
the Cobar belt plot close to the orogene model curve and can be distinguished
from both the Ordovician and Silurian crustal signatures. (Carr et al., 1991).
Fluid inclusion work (e.g. Seccombe 1990)
suggests that metals were likely transported by chloride complexing in carbon
dioxide and methane bearing fluids. Saline
CO2-bearing fluids present during early metamorphism may have interacted with
or have derived from relict basinal brines.
This could explain the early syntectonic formation of Elura and other
Cobar-style deposits. The metal
bearing fluids were concentrated along the major faults of the Cobar belt high
strain zone, and along such linear pathways further focussing or preferential
deposition may have occurred at local features including fault jogs and other
permeability controls. Dilatant
structures played a prominent role in metal deposition (Glen 1988, de Roo
1989b) and some associated breccias may represent hydraulic fracturing.
Evidence suggesting possible sudden pressure and temperature drop as a
means of metal deposition is widespread, and almost epithermal-like colloform
fabrics are known from the New Cobar-New Occidental area.
Such processes, however, may have been merely auxilliary when compared
with the very large mass movement of material in solution which must be
inferred from the large scale replacement or alteration of host r
Alternation zone carbonate formation, particularly prominent at Elura,
presumably depleted the ore-forming fluids of carbon dioxide and thus changed
their pH. Such chemical effects
may have been more important than the rather more obvious physical effects
observable within and around the orebodies.
Early observers of mine geology in the Cobar
mining field noted discordance between ore lenses and host strata and
suggested that the orebodies were of epigenetic replacement origin (e.g.
Andrews 1913; Thomson 1953; Sullivan 1950, 1951; Mulholland and Rayner 1961;
Rayner 1966, 1969). In the Central
area, mineralized veins commonly belong to NNW and NW-trending veins sets.
This is especially the case at the Chesney mine and as first suggested
by Sullivan (1951), Mulholland and Rayner (1961) and less clearly by Conolly
(1946), the plunge of higher mineralization values may represent the
intersection of these vein sets.
Following
recognition of a world-wide class of sediment-hosted syngenetic deposits,
attempts were made to interpret the Cobar deposits as syngenetic deposits
which had been subsequently physically remobilized during regional deformation
- especially given their location near the syn-faulted edge of a carbonaceous-turbidite
filled sedimentary basin (e.g. Brooke 1975, Gilligan & Suppel 1978,
Sangster 1979). Sangster (1979)
argued that Cobar belt ores (citing both the Cobar mineral field and Nymagee)
consist of remobilized submarine exhalative deposits.
Gilligan and Suppel suggested that the present great vertical extent of
the Cobar orebodies was caused by elongation during deformation of originally
tabular syngenetic orebodies; with the elongation direction being comparable
with mesoscopic metamorphic lineation. Many
of the 1970s syngenetic authors, and some in the early 1980s (e.g. Bouffler
1981), envisaged proximal exhalative systems as the source of metal-rich
brines. Although syngenetic theory
was at peak popularity in the 1970s a much earlier concept of a large
sedimentary sulphide orebody is that which Dr E.D. Peters formulated for the
Mount Lyell deposit in Tasmania (e.g. Mount Lyell Mining & Railway Company
Ltd., 1893 annual report). Peters
conceived of the immense Mount Lyell pyritic deposit as having formed as a
sedimentary basin deposit, with the sulphides precipitated on the floor of a
depression in concentrated form. Such
ideas presumably predate the 1890s and their application at Mt. Lyell.
Chesney (1889) appears to have been the only writer last century to
envisage sedimentary deposition for Cobar belt ores.
Chesney's brief mention of such a possible origin appears to be merely
an application of a theory elsewhere then in vogue, and it does not draw on
any local evidence.
Typically in the 1970s phase of syngenetic
popularity, the syngenetic interpretation of the main Cobar belt ore deposits
was combined with the concept of lateral facies equivalence between the CSA
Siltstone, Great Cobar Slate and Chesney Formation, allowing the Elura, CSA
and central Cobar area deposits to be positioned more or less along the same
time line. Other syngenetic
orebodies in the Cobar Supergroup were also postulated at that time further to
the east, such as the Rabbit Hill, Baal Gammon, Pipeline Ridge and Glens Hill
deposits at about mid section level, and the Mount Boppy gold mine and Mount
Boppy copper mine at the base. The
envisaged correlations were that stratiform mineralization broadly shifted up
sequence from east to west (e.g. Gilligan 1979).
In terms of the earlier accepted stratigraphic column, the degree of
suggested lateral equivalence was in some cases remarkable.
For example, Gilligan (1979) depicted the hostrock of the Mt Drysdale
deposit, a conglomerate long regarded as the base of the local Devonian
sequence, as coeval with the CSA deposit whose hostrocks had long been thought
to entirely postdate the Nurri Group rocks in an earlier stage of geological
interpretation.
In the initial metallogenic study of the
Cobar 1:250,000 sheet area (Gilligan 1978a) a syngenetic approach was favoured
and a primarily stratigraphic layout was adopted for the Notes.
Such a layout, with the first order division being according to
hostrock stratigraphic unit, has been rejected here in favour of the more
stratotectonic division into belts. This
is done because a syngenetic origin for the Cobar deposits was not sustained
by further work. Syngenetic, or
remobilized-syngenetic, ore deposition theory held moderately well for some
deposits in the Cobar belt (e.g. CSA, Nymagee).
Plibersek (1982) investigated the Peak area mines and concluded that
the mineralization there was entirely epigenetic, and probably deposited in
the Early Carboniferous prior to the close of deformation.
At the same time (1980-1982) mine development was underway at Elura,
bringing to light features which are equally persuasive against the 1970s
thinking as are those of the Peak.
In the 1980s (following O'Connor 1980, Adams
& Schmidt 1980) most explorationists returned to favouring discordant
hydrothermal emplacement, often evisaged as having taking place during the
main phase of regional metramorphism. Hydrothermal,
or metahydrothermal processes returned to favour throughout the 1980s.
They were supported by O'Connor (1980), Plibersek (1982), Kirk (1983),
Schmidt (1983), Binns and Appleyard (1986), Glen (1987), and Robertson and
Taylor (1987). The ideas of
exhalative syngenesis were not entirely given up in the 1980s.
Some continued to pursue theories based on remobilisation of
synsedimentary exhalative acucmulations (e.g. Marshall and Sangameshwar 1982,
Marshall et al. 1983, Pogson 1983,
Suppell, 1984). Since discordant
ores from the Cobar belt have similar S and Pb isotope composition to the
conformable volcanic mineralization at Colo Creek, Woodlawn and Captains Flat,
a shallow crustal (e.g. Cobar Supergroup) source is envisaged and not a
granitic source as postulated for Cobar by some early writers.
The possibility that the source sediments themselves contain exhalative
syngenetic sulphides cannot be excluded. However,
exploration geochemistry to date has not revealed any stratigraphic horizon in
the region which is particularly enhanced in metals.
Such a horizon, if found, could reflect the existence of unknown
syngenetic ores.
A great
deal of accumulated structural observation demonstrates that orebodies along
the Cobar belt cannot be considered conformable to the bedding, nor the
mineralization syngenetic to its present sites.
Thus those attracted to syngenetic theories for the Cobar region need
to invoke mechanisms of remobilization to explain some of the more distinctly
epigenetic ores. Some increasingly
complex models have been developed to accommodate both syngenesis and
epigenesis. Several ideas have
been put forward to explain the presence of both massive (mainly Pb-Zn) and
vein-type (mainly Cu-Au) mineralization: At
the CSA mine, Robertson (1974) suggested both chemical and mechanical
remobilization of a pre-existing syngenetic orebody with deposition of
sulphides, mainly in veins, controlled by hydraulic fracturing, and their
subsequent solid-state deformation. Sangster
(1979) suggested stratiform Pb-Zn tops to Cu-Au stockwork feeder zones.
Marshall and Sangameshwar (1982) suggested that CSA-type deposits
developed by a combination of stratiform and feeder syngenetic mineralization
which were mechanically and chemically remobilized and then overprinted by
veining from metahydrothermal fluids.
A
structural study of veins around the Cobar central area deposits, carried out
by Glen (19xx), recognized five sets of quartz veins.
From the presence of quartz fibres parallel to L1 in the surrounding
sediments, in veins of sets 2 and 3, Glen inferred that these veins are
syntectonic, formed during vertical extension of the rock mass.
He suggested that all sets of veins formed progressively during the D1
deformation. Glen further
suggested that the mineralized veins in the central area deposits and at the
CSA mine could be placed into these five sets, thereby implying the
syntectonic introduction of mineralization.
Note, however, that these suggestions apply strictly to the
mineralization in (quartz) veins and also in silicified sediments;
it is not known how far they apply to the more massive mineralization.
At CSA mine, there is some evidence that Pb-Zn veins cross-cut Cu veins
and were thus of later introduction. Rayner
(1969) and others have advanced evidence for a classical paragenetic sequence
of ore mineral deposition: pyrite,
magnetite, pyrrhotite, chalcopyrite, sphalerite, galena.
Moreover, Rayner evisaged that ore deposition was syndeformational.
He noted evidence of on-going stress between early sulphide and the
main copper-zinc deposition, and thought that some further structural movement
probably separated the main copper-zinc mineralization from the main phase of
galena deposition. The early
evidence for ongoing structural movement during ore deposition came from the
CSA and Cobar central area mines.
The
favoured interpretation of Cobar belt ores having oscillated broadly from
epigenetic to syngenetic and back again, some genetic discussions have
envisaged combination of the two. Harris
(1965) provided a lead isotope analysis of galena from the Silver Peak mine,
corresponding to an age of 267+/-50 m.y. (Early Carboniferous), suggesting
that the deformation at Cobar is no later than Kanimblan in age.
Other early lead isotope study (Ostic et al., 1967) found that
sulphides from the somewhat older host rocks, as at Great Cobar, Tharsis,
Silver Peak and Queen Bee deposits, have 206Pb/204Pb values about 0.15% lower
than CSA ores. Such a difference
would be consistent with emplacement of mineralization at CSA and Elura some
30 million or more years later on for these western or up sequence mine sites.
One explanation advanced was that the younger deposits may represent
hydrothermal remobilization of much earlier stratabound exhalative
mineralization. On a more compact
scale, it was likewise thought by some that much of the sulphide at CSA might
have been remobilized out of underlying conformable exhalative ores in the QTS
zone. Such earlier distinctions
have diminished with further knowledge. The
QTS ores, for example, are no longer viewed as fundamentally different and are
now classed as an integral part of the CSA ore system.
Except for the Warrego-Elura deposits, all
deposits in the Cobar belt lie in a D1 high-strain zone.
The sulphides and metals in veins were deposited from solution.
Given the low solubilities of silica and metals in solution, the large
tonnages of metals in the deposits around Cobar require vast amounts of fluid.
Multipass flow (convection) may have been required to supply the amount
of fluid. In order to concentrate
material precipitated from solution (in response to changing P-T-X conditions)
it is necessary for fluid flow to be focused up conduits.
The strong association between faults and the mineral deposits suggests
that fluid flow was effectively focused up these faults (shears) and fractures
associated with them. If metals
and sulphur, scavenged from crustal rocks by metamorphic fluids, moved along
the major structures then deposition could have occurred syntectonically from
a variety of transient pressure reduction mechanisms.
These could involve fault jogs formed largely independent of local
structure, as well as intrusive or stoping hydraulic brecciation which could
be favoured by the stresses built up in deforming anticlines.
Given the inferred depths of burial of rocks now exposed in the Cobar
area during deformation (ca. 7 km), fractures and faults were probably
hydraulically opened, in response to fluid pressure build-up.
Rapid escape of fluid to the surface (and thus loss of fluid pressure)
was prevented by a low permeability barrier closer to the surface - perhaps
the Mulga Downs Group. The solid
state deformation of sulphides visible in the Cobar deposits implies
precipitation before the end of deformation.
As the ore-associated elvans of the Cobar belt lack obvious foliation
relicts, they might support a phase of dilational ore system growth which
generally predates ductile shearing. Mineralizing
fluids, however, were still active following the formation of shear zones as
these commonly carry enhanced metal values in areas of mineralization.
The heat required to set up the hydrothermal
systems was possibly provided by the same energy source as produced regional
deformation and low grade metamorphism. The
silica source may have been provided by corrosion and dissolution of detrital
quartz grains during the cleavage-forming process.
The metal source may have been trace metals present throughout the
cover sequence and underlying basement.
In such a
hydrothermal model, metal sources do not need to be pre-existing high grade
orebodies, and given the dominance of fluid rather than mechanical processes
in orebody formation, it is unlikely that in situ remobilization (Brooke 1975,
Gilligan and Suppel 1978, Sangster 1979) occurred.
One cannot rule out the probability of a pre-existing orebody being
intersected by flow lines in a convective hydrothermal system (Robertson's
1974 model), but its presence is not required in a convective hydrothermal
model.
At Elura,
Schmidt (1980, 1983) found evidence for mineralization and wall-rock
alteration before the end of deformation and metamorphism.
He favoured an epigenetic model in which rupturing of a small anticline
allowed the escape of metal-bearing, over-pressured solutions (Schmidt 1983).
De Roo (pers. comm. 1985) also favours an epigenetic model, and
suggests a genesis as a syn-deformational replacement orebody.
The majority of recent studies have favoured
the derivation of the metals in Cobar belt ores from within the depositional
realm of the now-deformed sedimentary rocks (e.g. Sun 1983, Seccombe &
Brill 1989, Seccombe 1990, 1991). It
has been suggested that dispersed metal ions expelled from compacting and
dewatering sediments were concentrated and channelled along features of higher
porposity such as faults, shear zones or the like (Kirk 1983, Glen 1987,
Robertson & Taylor 1987). Mineralising
fluids from great depths may have moved upwards along the major tectonic
structures which now contral the preservational edge of the Cobar Basin.
The prominence of pyrrhotite and magnetite along the Cobar belt, and
the detection of methane in Elura fluid inclusions (Seccombe 1990) is
suggestive of reducing conditions in the syndeformational mineralizing fluids
which may have evolved by compressional dewatering and later metamorphic
dehydration of sediments in the Darling Basin (Cobar Sub-basin).
In this respect the record of `bitumen' in limestone of the Kerrigundi
area, and the apparent widespread distribution of framboidal pyrite, could be
relevant. The Basin may have been
anoxic away from its fringing shelves. Apart
from the fringing shelf areas, the shelly fauna of the Darling Basin is meagre
and commonly fragmental, perhaps in part being detritus carried in from afar
by turbidity currents. Early
forcing out of the reducing connate waters could be expected, and these early
fluids may have become less reducing as metamophic dehydration began
contributing to the metals transporting system.
This could explain why the earlier parts of ore systems (e.g. cores of
the Elura ore pipes) are more pyrrhotitic.
This is an alternative explanation to early suggestions of physical
remobilization with the pyrrhotitic core representing an earlier (down
sequence) sulphur-deficient stratiform sulphide phase.
Brill (1989b) has pointed out a number of
anomalies from CSA ore mineral studies, such as low apparent temperature of
formation derived from partitioning of Fe and Zn between sphalerite and
stannite, which could be explained as the disequilibrating effects of ongoing
metamorphism during and after ore deposition.
This contrasts with the high temperature of formation (400-600oC)
initially inferred for Cobar belt ores from laboratory studies by Rayner
(1969). Brill (1989b) cited many studies of trace elements which have related
the ratio Co/Ni to the genetic type of an ore deposit.
Pyrite of volcanic association commonly shows Co/Ni values between 5
and 50. Pyrite of sedimentary
origin yields a value of less that 1. The
ratio in hydrothermal (vein) pyrite averages close to 1.7, and is invariably
less than 5. In pyrite from the
CSA mine Co/Ni values range from 1.07 to 4.22 and average 2.4 (Brill 1989b).
On the basis of Co and Ni content, Brill compared CSA pyrite to that of
certain remobilized vein deposits elsewhere, and ruled out magmatic,
sedimentary and volcanic origins.
The O,C,H and S isotope data from CSA
indicate depletion within the ore zones. There
are two possible explanations: (i)
fluids interacted in the ore zone (meteoric and metamorphic water);
or (ii) a pre-existing discordant vein system was modified and
overprinted during the metamorphism. Fluid
inclusion data and chlorite compositional data indicate a possible resetting
of early, low-temperature Pb-Zn veins, but not early, higher temperature Cu
veins (Brill 1990). They also
indicate a lower temperature for late shear zone mineralization.
The classical model of exhalative massive sulfide deposits implies a
higher temperature, discordant Cu-rich feeder (or stringer) vein system and an
overlying lower temperature, concordant Pb-Zn mineralization.
Brill (1990) noted that although possibility of a pre-existing vein
system at CSA cannot be disregarded, this need not be an exhalative feeder.
She has interpreted a prolonged phase of early to late tectonic fluid
activity. If this is so any
pre-existing vein system that may have been present has likely been strongly
remobilised and overprinted by metamorphism.
As for the CSA system, a lengthy period of mineralization, which is
largely or entirely syntectonic, has been envisaged by Hinman (1989) for The
Peak. Hinman postulated four
phases intimately associated with activation and reactivation of shears.
The final phase of deformation produced north-north west trending
shears which dislocate the ore zones and these structures may also contain
minor base metal and gold mineralization.
The interpretation of laboratory results from
Cobar belt has not been unequivocal, and is largely dependent for final
interpretation upon the geological and tectonic assumptions made.
Nonetheless, as Secombe (1991) has stressed, major differences in metal
content of individual Cobar belt deposits do not necessarily require major
differences in temperature or salinity of the transporting fluids.
Rather, control by variations in pH may be important.
Sulphur-saturated Au-Cu deposits contrast with those where pyrrhotite
is a major iron sulphide. Differences
in metal content may relate to source rock and structural pathways along which
fluids migrated during metamorpshism. Seccombe
(1991) considered that access from Girilabmone Group basement along the
E-dipping Great Chesney Fault might explain the high Au and Cu content of the
deposits localised along this structure. Mixed
Girilambone Group basement and Cobar Supergroup sources, tapped by fluids
focused along the W-dipping Cobar and Myrt Faults, could be inferred for Au,
Ag and Cu mineralization at the Great Cobar, Dapville and Gladstone mines.
Gold-deficient base metal mineralization distant from the Central area
(e.g. CSA mine) might then be related to W-dipping structures that penetrated
Cobar Supergroup sequences exclusively.
As should be apparent from the foregoing
discussion, a great range of processes and genetic models has been invoked in
seeking to understand the mineralization of the Cobar belt.
Numerous possible combinations have been considered between magmatic
sources, volcanic sources, basin sediment dewatering processes, and tectonic
structures or events which might serve as conduits, focussing mechanisms or
pumps. Indeed many different
mechanisms may have been active at different times and in different places and
it is not yet possible to confidently conclude which were the dominant genetic
processes along the Cobar belt.
In comparing the mineralized Cobar belt with
other provinces, the following should be borne in mind.
The deposits in their present form are clearly impossible to term syn-sedimentary.
They have strong tectonic associations and could be syn-tectonic.
Deposits cluster along the eastern side of a large structural
depression (Darling Depression). The
dewatering of the host rocks, the Cobar Basin's Early Devonian sediments, was
likely influential upon metals transport regardless of any earlier sources for
these metals. The Cobar Basin is
interpreted as an extensional trough or rift basin.
The equivalent sequence further west is poorly known because of
extensive Mulga Downs Group cover and the eastern shelf of the Cobar Basin
holds the westernmost examples of strong carbonate facies in the NSW
Palaeozoic. Intermittent land
masses may have existed further east in Cobar Basin time, as well as major
marine rises (e.g. Molong Rise) of a somewhat continental character.
The conglomerates of the Nurri Group and Meryula Formation are
suggestive of a sizeable land mass to the east.
Thus the general setting of the Cobar Basin, although marine, is
closely tied to continental crust, including granites intruded largely in the
Silurian.
A useful analogy for metalliferous dewatering
of a rift basin of continental crust, such as the Cobar Basin is interpreted
to be, may be with the Salton Trough even though the analogy is far from an
exact one. The Salton Trough,
below the Salton Sea of southern California, is a continental rift formed by
impingement of the East Pacific Rise spreading system on the North American
continent. Heat from intrusions at
depths as shallow as 3 km causes diapiric rise of basinal brines that
accumulated within the last 5 Ma.
The Salton Sea area subsurface is
comparatively well known from geothermal drilling.
The domal top of a brine diapir has been mapped using fluid geochemical
and temperature data from 50 or more commercial geothermal wells.
The rising brine diapir has stripped base metals from the sediments
with which it has interacted. Transient
fluid mixing across the interface localizes incipient base metal deposition.
The presently-exploited metalliferous brine volume is estimated at 7
km3, and heat flow data imply that the brine is convectively upflowing at a
rate of 0.02 km3 per year. The
exploited brine reservoir contains at least 13.2 Mt of Fe, 10.6 Mt of Mn, 3.3
Mt of Zn, 660,000 t of Pb, 66,000 t of Cu, 212 M oz of Ag, and 106,000 oz each
of Au, Pt and Pd. If the currently
unexplored brine volume at depths greater than 2 km is considered, the amount
of metals stored in brine may be an order of magnitude higher (Centre for
Geothermal Resources Research, University of California at Riverside, 1989).
It has also been postulated that if sufficiently thick lakes or seas
cover such brine diapirs, they may advect to the sediment-water interface to
form "exhalative" stratiform ore deposits.
Thus metalliferous transport and deposition from intra-basinal fluids
could feasibly be driven either by pressure in compressive tectonic phases,
contemporaneous with and following cleavage formation, or by magmatic heating
in early extensional stages. Suppel
and others, from the 1970s onwards, postulated fluid convection cells over
intrusions as a metals transporting mechanism within the undeformed fill of
the Cobar Basin. More recent
hypotheses mainly concentrate on late tectonic fluid activity.
In the high strain zone along the eastern side of the Cobar Basin there
is certainly evidence of late tectonic deposition, and traces of any earlier
activity could be largely obliterated. Evidence
of any pre-deformational metals transport would best be sought to the west of
the high strain zone.
Ore
zonation in the Cobar belt
Although metals zonation is not particularly
prominent in the Cobar belt, apparent zonation patterns at a number of scales,
have been pointed out or remarked upon for the Cobar belt by many authors.
Some have conceived of a south-north zonation, others have thought
east-west; in both cases
attempting to relate the imagined zonation to stratigraphic younging.
It was early noted that major ore deposits hosted in the Cobar
Supergroup occur progressively higher in the sequence towards the north.
Thus the Queen Bee deposit is hosted near the top of the Chesney
Formation, the Great Cobar in the Great Cobar slate, and CSA and Elura
deposits in the CSA Siltstone.
Two mineralization end members feature in
considerations of zonation. One is
gold + chalcopyrite-pyrrhotite in frequently siliceous ores.
The other is a lead-zinc type, massive or foliated
pyrite-sphalerite-galena, which tends to be hosted in more chloritic r
The first of these end members typically contains less than 25%
sulphides, and in some gold deposits sulphides are very minor (e.g. Mt
Drysdale, New Occidental). The
second type usually contains more than 50% sulphides.
Kappelle (1970) noted that lead-zinc mineralization is more common in
the Cobar belt than was previously been appreciated.
It is present at the CSA, Great Cobar, New cobar, New Occidental, Peak,
Queen Bee, Tharsis and other mines to varied extent.
In some mines (e.g. New Cobar, CSA) galena veins have been recorded to
cut across copper ore masses. In
some ore systems lead-zinc ore appears to show a preferred occurrence west of,
or up-sequence from, chalcopyrite-pyrrhotite siliceous ore.
Lead-zinc ore bands are recorded to lie mostly on the western side of
copper and gold mineralization, as at the CSA, Great Cobar, Chesney, New
Occidental and the deep Peak ore systems.
In other cases (e.g. New Cobar) pockets of moderate to strong galena
development appear to be of haphazard distribution.
Pyrrhotite was early interpreted as a high temperate phase, with
lead-zinc deposition as lower temperature mineralization.
This generalization was tailored to fit geological options,
particularly in syngenetic theories. Later
work, however, suggests much more complicated paragenesis at the deposits
which have now been studied in detail (Elura, CSA, Peak).
At Elura the zoning is unique in being concentric but otherwise shows
definite similarities.
Higher values for silver and lead are known in the outer zone, whereas
the inner pyrrhotitic zone has a slight but distinctive copper enrichment
(0.2% versus 0.15% Cu).
It has at times been questioned whether Cobar
belt metals zoning is real or if it is a conceptual illusion brought about by
selective description and over-generalization.
Some of the internal ore system patterns certainly seem real and some
are based on data from thousands of samples (e.g. Elura), but comparisons
between deposits are more questionable and may not have a uniform framework.
Russell and Lewis (1965), Robertson (1977) and many later workers have
commented on the distribution of metal dominances.
They have suggested both local and regional stratigraphic zonations of
metals within the Cobar belt. Deposits
hosted low in the Cobar Supergroup, and located to the south, tend to be
copper dominant (e.g. Queen Bee). Higher
in the sequence, and further north, lead-zinc becomes more important with this
trend culminating at Elura. If the
pattern is envisaged as gold and copper to the east, lead-zinc increasing to
the west, it is generally an up-sequence trend and has been suggested to occur
both on the regional scale as well as on the scale of individual ore systems
or even single lodes. The
stratigraphically highest host rocks are expected west of the Cobar belt.
There gold mineralization is known as well as trace base metals.
Of the latter, lead appears to dominate.
These minor occurrences, spaced up to 100 km west of Cobar, are poorly
known and likely irrelevant to the Cobar belt.
Stratigraphically, the trace mineralization ranges as high as the
Winduck Group (e.g. Buckambool Sandstone).
No mineralization traces are known in the Mulga Downs Group.
Basically, a crude zoning at various
different scales may be suggested for the Cobar belt, in which zinc-lead ore
types occur to the west of copper types, pyritic ore types occur to the west
of pyrrhotitic ore types or magnetite enrichments (i.e. sulphur increase
westwards), and massive ore occurs to the west of disseminated/stockwork/vein
types of mineralization. Examples
can be given at various scales, in many respects up to whole-belt scale except
for sulphur. An alternative
expression of sulphur zoning might be to say that it has a tendency of medial
deficiency. Noteworthy sulphur
deficiency may be central or medial in individual deposits, such as at Elura
(inner pyrrhotitic zone) or Great Cobar (medial magnetite bands) as well as
for the whole belt. In the
whole-belt case the greatest sulphur deficiency may be medial (e.g. a
considerable proportion of the ca. 40% iron in the Great Cobar main lode is
present as stilpnomelane, magnetite and pyrrhotite at Great Cobar).
At the eastern deposits, including the sulphide-rich Queen Bee,
pyrrhotite may be minor or absent. For
deposits west of Great Cobar there is a corresponding absence or scarcity of
magnetite.
It is also questionable whether or not `gold
to the east' should be considered a general zonation feature, especially for
the purpose of syngenetic hypothesis. In
the case of the deposits along the Great Chesney Fault (Eastern Line
deposits), a greater presence of carbon in the footwall rocks may have been
responsible for the precipitation of gold (Glen 1991).
Nonetheless gold appears to be concentrated in general along the
eastern side of the Cobar belt (Mt Drysdale, New Cobar, New Occidental, Peak).
Bismuth also appears to be enhanced somewhat along the Eastern Line
deposits. Other elements besides
Cu, Pb, Zn, Fe, S, Au could be investigated when fully considering the
apparent metals zonation, e.g. selenium (Brill 1989b).
Partial data are available for minor metals, especially for CSA and
Elura, and suggest significant differences for trace elements in the same ore
mineral between different ore systems. Given
the mesoscopic near-concordance of bedding and cleavage in many deposits,
western orebodies will commonly appear to be situated stratigraphically above
nearby eastern ones.
The ores at Great Cobar may exemplify the
pattern of Pb-Zn-S increasing to the west (cf. up sequence) with Au/Cu-Si
enrichment and S depletion along the eastern side.
The generalized zonation at the Great Cobar main zone is one of minor
erratic Pb-Zn to the west, then a major pyrrhotite-rich interval, then thin
bands of magnetite concentration, and finally a wide siliceous interval to the
east. Rich gold concentration,
which is rare at Great Cobar, is known only along the east of the lode.
There, erratic rich gold is on record, with grab sample assays in
excess of 60 g/t Au (similar to the richest shoot grades at the Chesney mine).
Reiterating the typical Great Cobar main lode
zonation in greater detail, the Pb-Zn enrichment along the western side was
erratic but in some places took the form of banded ore rich in
galena-sphalerite. This is
recorded to have comprised thickness of up to 3.5m along the western wall.
East of it there generally lay massive chalcopyrite-pyrrhotite ore,
thence silicified slate with quartz veins carrying chalcopyrite + pyrrhotite +
magnetite. Sphalerite is generally
low or absent to the east, although rare "skins" of sphalerite were
found along the eastern wall of the lode in much the same manner as sphalerite
was common along the western wall. Also
at the Gladstone mine there was a concentration of thin veins containing
sphalerite noted along the western footwall side of the workings.
At New Occidental, the most westerly lode is likewise a
galena-sphalerite one (very minor). To
its east is a very low grade base metals lode, then the main siliceous gold
orebody, and beyond that some thin magnetite veins.
The New Occidental thus conforms adequately to the envisaged
generalization. At Queen Bee a
small subsidiary lead-zinc lode is known on the southwestern or side of South
Lens in the Main Lode. In the
continuation of the Cobar belt further south, orebodies at Nymagee also show
an overall metals zonation, from Cu rich in eastern and stratigraphically
lowest orebody to Pb-Zn rich ore in the west (Jones 1979).
A pattern of copper enrichment and sulphur
deficiency to the east, and lead-zinc prominence to the west, has also been
envisaged at CSA mine. The
situation is less clear cut overall at CSA than elsewhere, although there is
clearly zonation within some of the bodies of mineralization.
The QTS zone was early reported to show good zonation, from Cu rich at
"stratigraphic" base to Pb-Zn rich at the opposing upper side.
It was early noted that in the eastern system ores, equally variable as
those of the western system, copper dominates overall and pyrrhotite may be
more abundant. The entire large
magnetic anomaly over CSA was initially attributed to the eastern system
(Russell and Lewis 1965). Such
early "zonation" assumptions, however, overlooked the QTS zone which
was little known at the time.
Detailed laboratory studies of ore minerals
might be expected to shed light on zoning by revealing different formation
temperatures for different ore compositions or fabrics, or between the same
type of ore at different localities. Results
to date have been somewhat equivocal. If
the ores are of synmetamorphic origin it may be the case that many
disequilibrium features have been acquired which render inoperative the
various partitioning type geothermometers developed elsewhere.
In particular, effects of low temperature overprinting have been
elaborated by Brill (1989b). Relatively
subtle differences between members of proposed zonations, temporally from
south to north or east to west, or within ore systems, could remain obscured
if it is not possible to disentangle the efects of falling ore formation
temperatures from those of retrograde regional metamorphism.
Zoning has been described from the Elura,
CSA, Great Cobar, Gladstone, New Occidental, The Peak, Queen Bee and Nymagee
ore systems (e.g. Paterson 1974, Robertson 1977, Suppel 1978).
Some have doubted the validity or usefulness of the generalizations
attempted, whereas others have considered the apparent zonation to be highly
significant. Sangster (1979,
1980), one of the latter, regarded the apparent zoning as a meaningful natural
phenomenon. He and others have
stressed it as a principal basis for supporting syngenesis of the Cobar type
deposits.
Rayner (1969) and a few others (Joklik,
Sullivan etc.) have at time laid aside the stress on lateral zonation and have
focussed instead on possible connected series of vertical changes.
Rayner suggested that apparent lateral variations might merely reflect
more important general vertical zoning patterns produced by the geothermal
gradient during ore deposition or the differing depths to
hydrothermal/magmatic sources. Rayner
envisaged that at about the position of Cobar the highest temperatures
existed, and that the temperature of ore deposition decreased to the north and
south. In Rayner's interpretation,
deep zone high-temperature ore (pyrrhotite-magnetite-chalcopyrite) was
deposited at the levels presently exposed in the Great Cobar mine, while lower
temperature ores richer in gold, bismuth, lead and zinc were formed at sites
to the north and south. In one
scheme, Great Cobar ore was considered as hypothermal, New Occidental ore as
mesothermal, and The Peak as mesothermal to epithermal (e.g. Joklik 1948).
Joklik predicted that the main Peak deposit should be underlain
successively by orebodies of the New Occidental, Chesney and Great Cobar type.
However, agreement on this point has been absent amongst those
favouring vertical zonation, with a minority inclined to view the Peak
mineralization as representing the deepest instead of the shallowest ore
depositional conditions. Similar
interpretation of Cobar as a structural, geothermal and/or igneous intrusive
crest along the Cobar belt, also features in the work of other writers (e.g.
Sullivan 1950). Sullivan and
others have suggested that the ores along the Cobar belt might vary
(hypothermal, mesothermal and even epithermal) according to the depths to the
tops of postulated granite cupolas beneath the belt.
The New Occidental deposit, and less often the New Cobar, has usually
been quoted as the distal example in such theories, with supposed presence of
epithermal or hot spring features. Such
interpretation, from the 1950s onwards, has never been widely accepted
although the Cobar gold deposits of the "eastern line" continue to
be occasionally referred to epithermal or hot spring activity (e.g. anonymous
1983, Minfo 2).
According to the broad vertical zonation
theory, with its increasing depth to metal sources both north and south of
Cobar, Elura would be expected to be richer in Pb-Zn than CSA.
Rayner's (1969) vertical zoning suggestions thus continue to explain
some of the later observed facts. Although
his overall scheme is generally not now in favour, some recent writers do
speculate on the differences between Cobar belt deposits in terms of vertical
structural levels. Conner (1985)
arranged deposit types in ascending sequence, from deep dynamic deposits (high
confining pressure) to passive replacement deposits (low confining pressure).
In this sequence Conner placed the deposits as:
Peak, New Occidental, New Cobar, Chesney and Great Cobar, CSA, and
Elura. In this model, the size of
the ore system may increase upwards and metal dominance is from Au to Cu to Pb-Zn.
Vertical zonation was also favoured by early beliefs that the CSA mine
is richer in zinc and lead towards surface and in copper at depth.
Fuller statistics and later development do not clearly confirm this,
yet there is a weak, vertical zonation in so far as some of the individual
orebodies are clearly Pb-Zn rich at the top and Cu rich at depth (Bouffler
1981).
Somewhat related to the ideas on vertical
zonation is the partly alternative theme of metal relationships with the tips
or terminations of orebodies in a strike lateral zonation.
There are remarks, largely now unconfirmable, of particular metals
being consistently higher near either the upper (often northern) or lower
(often southern) limits of plunging lenticular or pipelike bodies.
Gold would seem to sometimes concentrate along the steep crests (e.g.
Chesney) and copper near the basal limits (e.g. Great Cobar central orebody).
Such observations could often be accomodated in the ideas of zonation
involving vertical changes, already referred to.
However the alternative in some cases might be some form of
strike-lateral segregation. Chesney
is a good example. There the ore
pipes formed dilationally at the end of a straight mineralized fracture are
much enriched in gold relative to the straight course mineralization.
Furthermore the very tips of the system could should greatest
enrichment if gold deposition outlived silicification and other self-sealing
processes. At least the crest of
the northern pipe appears to have been so enriched at Chesney according to
records. This could still be
explained as vertical zonation but if the southern pipe limit at Chesney also
has peak enrichment then a strike-lateral element might have to enter zonation
theory. Along such lines, but
purely speculatively, authors have wondered if the New Cobar-Chesney Fault-New
Occidental combination (highest gold, mineralized fault, highest gold) might
reflect along-strike movements.
Theories
for ore zonation
Within Cobar sheet ore systems, Elura has a
unique high degree of concentric zonation.
When the Elura ore system was little known, and conceived as a possible
emplacement of a physically remobilized mass of syngenetic sulphides, the
concentric zoning could be explained by the pyrrhotitic core comprising
original basal layers. Thus
pyrrhotitic ore would have been originally overlain by pyritic ore.
Subsequent detailed studies of Elura, however, do not support a
remobilized syngenetic origin.
In some cases the suspected west to east
zonation (e.g. sphalerite-galena, pyrrhotite, chalcopyrite, gold,
magnetite-bismuth, selenium) could be interpreted as the result of fluids
rising from dipping conducts such as faults.
Progressive sealing of the fault systems could displace the locus of
fluid travel and changing composition and temperature of the ore-forming
fluids might then have given a succession of differing ore compositions.
Commonly along the Cobar belt the mineralized shears and the cleavage
dip steeply east whereas bedding abuts and dips steeply west.
This would give good opportunity for silicifying fault-controlled
fluids to leak eastwards and up-dip. Instances
of this are likely both at The Peak and the `Eastern Line' of deposits (New
Cobar to New Occidental). At many
places silicification effects have been recorded as stronger along the eastern
side of an ore system, propect or individual lode (e.g. Blue Lode, The Peak).
It was early remarked that the main Cobar lodes were easily located by
means of slight eminences, upon the western sides of which they outcrop.
The rises were attributed to the greater weathering resistence of the
silicified metasediments, or of sandstone relative to slate.
As early writers pointed out, the concentration of mines on the western
sides of hills (e.g. Fort Bourke Hill, Mt. Tabor, United Hill, The Peak)
provide examples. The east side
strata, once silicified, may have become less likely to fracture than the
rocks (commonly slate) immediately to the west.
Thus sealing by silicification along the eastern side of some ore
systems, may have helped localise the last active fractures along the western
side where lead-zinc concentrations are found.
Many writers (e.g. Andrews 1913, Joklik 1948) have thought this.
They have envisaged slippage of the orebodies along their footwalls,
followed by introduction of low temperature Pb-Zn ores along the fractures.
Joklik (1948) regarded the Cobar ore deposits as mostly hanging-wall
deposits. The western
galena-sphalerite pyrite-chalcopyrite lodes were early remarked to have more
clearly defined walls and were hence considered younger than any more diffuse
pyrrhotite mineralization to their east (e.g. Andrews 1913b).
It was suggested that this ore type was deposited in a subsequent phase
when "the lodes appear to have slipped upon themselves.."
(Andrews 1913b, p.9). Another
feature of fractures which have been inferred as the last-active is that they
may contain minerals in nodular form, usually nodules of marcasite or pyrite.
Nodular or concretionary growth form is perhaps suggestive of
lower-pressure later stage growth than pertained generally in the ore systems.
Early theories to explain zoning in base
metal deposits are divided into ones which ascribe the cause of zoning to the
chemical behaviour of the ore-forming fluids, and those which stress the
physical behaviour of the fluids. Added
to these are later theories on ore zoning that have been applied specifically
to submarine volcanogenic deposits. The
latter theories relate zoning to the chemical and/or physical behaviour of
exhalative thermal "plumes".
Change in composition of ore solutions over
time was proposed by Spence and de Rosen Spence (1975) as an explanation of
zoning in massive sulphide deposits of the Noranda district, Canada.
They noted the pattern copper-zinc-pyrite both in deposits arranged in
ascending stratigraphic order, and also in individual deposits.
They envisaged a copper to zinc cycle, possibly related to volcanic
evolution. Sangster and Scott
(1976) noted that in static brines, however, precipitation is slow enough to
allow separation of ore minerals in the reverse order of their solubilities,
i.e. chalcopyrite followed by galena and sphalerite.
Many supposed syngenetic deposits vary stratigraphically from Cu rich
in the lower portion Pb-Zn rich at the top.
This is considered the typical zonation in volcanogenic deposits.
Bouffler (1981) believed the QTS mineralization to have such
stratigraphic zonation evident, and he attributed it to multiple exhalations
of metal rich brines.
Turner and Gustafson (1978) suggested that
under suitable conditions an exhaled hydrothermal ore solution could form a
stable fluid which could flow along the sea floor over large distances with
minimal mixing. Jones (1979)
criticized the concept of a stable fluid in this environment, stating that
quenching, causing almost complete precipitation of the sulphides, occurs in
or close to feeder vents. Solomon
and Walshe (1979) attempted to show that the development of major massive
sulphide deposit types could be explained in terms of the behaviour of ore
bearing plumes entering seawater. They
suggested that quenching in feeder pipes of the plumes close to the sea floor
and immediately above feeders causes rapid deposition of pyrite and
chalcopyrite. Cooling at the base
of the plume leads to precipitation of sphalerite and galena particles which
could either fall back into the accumulating pyrite-chalcopyrite or else would
be entrained and removed in the rising plume.
The sphalerite and galena could thus be carried horizontally for some
distance before sinking back to the sea-floor.
Both Sato (1973, 1977) and Large (1976a,
1977) sought to chemically explain mineral zoning as an effect of differential
precipitation from polymetallic exhalative solutions upon mixing with
seawater. Results suggest that
chalcopyrite - pyrrhotite - pyrite + magnetite assemblages are
deposited under relatively reduced, high temperature environments, probably
typical of feeder systems and of the seawater - rock interface immediately
above feeders (Large 1976a). Thus
these assemblages are expected in stockwork systems immediately below massive
sulphide deposits, and in the lower portions of massive sulphide deposits.
Where solutions move towards the edges of pipes and towards the
periphery of the massive ore zone the solution would undergo greater cooling
before mixing with seawater, and increasing amounts of pyrite and sphalerite
(with galena) would be precipitated.
Colley (1976) suggested that deposits which
are the distal equivalents of Kuroko type deposits could occur in marine
sediments with only minor intercalations of volcanic rocks as the result of
two processes. One is the
formation of fine ore precipitates from brine-ore solutions which are lighter
than seawater. The other is the
reworking of deposits to deeper water of sedimentary basins by volcanic and
sedimentary processes (including transport by submarine slides and turbidity
currents). Plimer (1978) proposed
a scheme similar to that of Large.
The regional zoning suspected for the Cobar
field has been suggested as compatible with the proximal to distal zoning
described above. The copper-rich
pyrrhotitic deposits of the Central area would represent proximal deposits and
the increasingly lead-zinc rich deposits of CSA and Elura would represent
progressively more distal deposits. Ranged
against such interpretation, however, are the general arguments for the Cobar
deposits being metahydrothermal without ever having passed through a
syngenetic stage.
If the deposits are syntectonic, paragenesis
in Cobar belt ore systems could be quite complex (e.g. Hinman 1989-1991).
The results of boiling from episodic tectonic pressure releases
(tectonic "throttling") seem more likely to have occurred along the
high strain eastern edge of the Cobar belt where Cu-Fe-Au ores are most
noteworthy. At least some of the
associated Pb-Zn in the eastern Cobar belt tends to be late stage, perhaps
even postdating ductile deformation. For
the CSA ore system, Brill (1989b) confirmed that two different styles of ore
exist: Cu-rich ore, comprising
mainly chalcopyrite-pyrrhotite-pyrite, and Pb-Zn-rich ore, composed
principally of sphalerite-galena-pyrrhotite-pyrite.
Brill further concluded that the Pb-Zn ore occurs in two distinct
phases, the latter of which is associated with chlorite-filled shears that
dissect earlier mineralization. Others
have envisaged Pb-Zn (-Ag) being deposited late in the mineralizing sequence
in company with carbonates.
Regarding the interpretation of possible late
stage Pb-Zn, it is noteworthy that Brill (1989b) deduced a post-depositional,
low-temperature overprint of sphalerite at CSA mine.
This is inferred from trace element partitioning between sphalerite and
galena, and partitioning of Fe and Sn between stannite and sphalerite.
Similar conclusions were reached by Sangameshwar and Marshall
(1980), who calculated a pressure range of 7.7 to 9 kbars in their
study of sphalerite from the CSA mine.
Brill (1988) calculated a 2.5-3 kbar pressure
for the Cobar belt high strain zone from a study of Si content of regional
metamorphic sericite, and thus regards the 7.7-9 kbar figure of Sangameshwar
and Marshall (1980) as an artefact of a low temperature overprint of
sphalerite. Post-depositional,
low-temperature changes, seem to have likewise influenced partitioning of Fe
and Zn in coexisting sphalerite and stannite.
Calculated temperatures of 236-306oC obtained from this geothermometer
are likely to represent a late metamorphic signature.
The FeS contents in sphalerite also could have been affected by late
metamorphic re-equilibration. The
sphalerite geobarometer thus yields unreliable results in the Cobar belt
(Brill 1989b), which hinders interpretation of the envisaged metals zoning.
Microstructural correlation across the Cobar belt structural zones is
still too little advanced, to support or refute the common suspicion that the
Elura Pb-Zn ore system may be younger than the more easterly Cu-Fe-Au
deposits.
The zoning theories thus far discussed
include possible temporal variations in evolving and long-lived ore generating
regimes, spatial variations due to transport peculiarities of exhaled metals,
or relatively local variations due to factors such as episodic boiling.
To what extent the apparent ore zoning is affected by an ore system's
local environment, in comparision with deep seated(?) factors affecting the
whole region, remains uncertain. Theories
attempting a regional perspective take into account the likely source rock
differences if the metals have been somehow scavanged from within the upper
crust.
The
possibility that different metal suites could result from similar
concentrating mechanisms operating within different crustal blocks or source
basins is well recognized, although it can only be speculated upon for the
Cobar belt. In syn-dformational
metahydrothermal ore genesis, zoning may in part be explicable in terms of
fault geometry. The Queen Bee and
Great Chesney faults dip east and penetrate relatively thin basinal sediments
(Nurri Group) in addition to Ordovician basement.
Faults in the CSA Siltstone dip west and penetrate a large thickness of
the Amphitheatre Group. Faults in
the Great Cobar Slate and at The Peak cut through a lesser thickness of Nurri
Group and penetrate basement. Some
of the more easterly the Cu-Fe-Au mineralization could contain metals sourced
from the east, especially from basement rocks, whereas the deposits further
west might relect metal sources within the deeper parts of the Darling Basin
(Glen 1991). Gold in particular is
most noteworthy in deposits along the eastern edge of the Cobar belt, where
the sourcing contribution from the Girilambone Group could theoretically be
greatest.
All of the many available theories can explain some aspects of the Cobar
belt metals zoning. None, however,
offers a full explanation which is consistent with all geological inferences.
Some of the later work, such as Hinman's (1991b) proposal of both late
and early Pb-Zn paragenetic stages at The Peak, suggest that the traditional
formulation of metals zonation in the Cobar belt may in part be based on
over-generalizations. As more
specific details are being inferred for studied ore systems, the histories
interpreted for each system have been is growing decidedly more complex and
more multiphase (e.g. Hinman, 1989-1991).
This trend, if valid, may demolish some of the relatively simple
generalizations and comparisons previously drawn between deposits along the
Cobar belt.
REFERENCES
Click
here for list of Cobar 1:250,000 sheet references
