THE GIRILAMBONE
BELT - geology and mines
Helix Resources
geologist at the company's "Girilambone Eastern Basin" project area.
By
the time the Cobar 1:250,000 sheet metallogenic study was well advanced all
rocks of the sheet area that appeared older than the Cobar Supergroup were
being referred to as if they were one unit - as Girilambone Group.
Unlike
the Cobar Supergroup, in which considerable stratigraphic advancement has been
achieved, almost no reliable stratigraphy exists for these 'Girilambone Group'
rocks. Without discernable stratigraphy the unit is literally a
geological arbage bucket. Some areas consist of sequences
rich in radiolarian chert and little other variety (Ballast Beds
facies). These rocks are perhaps some of the least deformed within
the 'Girilambone Beds'. Elsewhere there are highly
crenulated phyllites, some quartzites, mafic schists, mafic and Alaskan type
intrusives, serpentinites with minor asbestos, and rare pillow basalts (Mt
Dijou volcanics). Overall it is often thought of as a deep water
marine 'sequence' although no actual sequence has anywhere been
established. Utah Exploration extensively explored the area and
attempted elaborate structural analyses. They suggested an
overall crude stratigraphic sequence but subsequent attempts to confirm the
Utah postulated sequence have not succeeded. The idea that
there is a distinctly older portion of the Girilambone Group is seen in
figures below showing "Basement schists". These are
thought to be more tightly and complexly deformed than 'overlying' Girilambone
Group rocks and perhaps not even the same 'sequence'. The
proof or details for such impression are not known - this terminology has been
used by Straits Resources geologists but much of Straits' work in the region
still remains confidential (As at 2010 some 4 exploration reports by Straits
in the Girilambone Belt are open file and 20 remain confidential, dating from
as far back as 1992 and in conjunction with Nord
Resources).
The Girilambone
area or "belt" is not as well defined or as rectilinear as the
other belts recognised from further west in the Cobar 1:25,000 sheet area, such
as the Cobar and Canbelego belts. Also, the
Girilambone belt structure is not well understood.
The broad mineral field has been conceived either as two trends (via
Budgery-Girilambone and Kurrajong mines) or one imperfect horseshoe trend
encompassing all these mines and prospect.
In either case this would seem to contrast strongly with the relatively
simple NNW trending Cobar and Canbelego belts.
The contrast is lessened somewhat in recalling that the Girilambone
belt mines have strong similarity with those of the Tottenham-Albert district.
A line passing from Tottenham-Albert between Hermidale and Girilambone
restores a NNW orientation to mind similar as for the other Cobar sheet
mineral belts. This suggests that
the Girilambone belt as currently envisaged could be part of some larger
NNW-trending feature which extends from Albert to Girilambone but can not be
recognized from current geological knowledge.
Some general information on the distribution, history and nature of
mines or mineral resources along the Girilambone belt is available in Andrews
(1915b), Carne (1899, 1908), McClatchie (1985), Scheibner and Markham (1976),
Scheibner and Pearce (1978), Smith (1974, 1976) and Suppel (1975, 1977).
More specific information is available in company reports, and includes
work by Australian Selection Pty Ltd (1966-1967, 1975-1982), Australian
Selection Pty Ltd and Seltrust Mining Corporation Pty Ltd (1980), Burdett
Explorations Pty Ltd and Anglo Range NL (1971), Gold Fields Exploration Pty
Ltd (1985), Hunter Resources Ltd (1989), Pakac Holdings Pty Ltd (1989), Pakac
Holdings Pty Ltd and Australian Gold Development NL (1988), Planet Resources
Group NL (1988), Preussag Australia Pty Ltd (1978), Sanidine Pty Ltd (1984),
Seltrust Mining Corporation Pty Ltd (1981-1982), Tai Pan Explorations Pty Ltd
(1971) and Utah Development Co (1969-1973).
Within the Girilambone belt mineralization is hosted solely within the
Girilambone Group. The hostrocks
are strongly deformed. The age of
the rocks is unknown, with speculated ages ranging from Ordovician to
Precambrian. Graphite is locally
common and if found to be of stratiform distribution could support a
Palaeozoic age. The rocks are
generally too deformed to preserve radiolarians.
Features reflecting strong deformation occur on both mesoscopic and
microscopic scales and include transposed layering, boudinage, rootless folds,
multiple foliation surfaces, and local abundance of chevron kink folds.
Very tight refolded folds have been recognized in sawn specimens.
Metamorphic mineral assemblages are typical of the greenschist facies
and grade appears to be fairly uniform throughout the belt.
Unlike suspected Girilambone Group rocks elsewhere, areas of markedly
elevated metamorphic grade have not been detected.
However, the presence of trace or minor metamorphic biotite is common
in the more labile metasediments and suspected matavolcanics, suggesting that
the Girilambone belt is largely upper greenschist facies.
A strong metavolcanic component within the Girilambone belt has been
inferred by some workers but this remains inadequately confirmed.
The Girilambone belt of mineral deposits is named after the Girilambone
copper mine. The Girilambone belt
is characterized by highly pyritic deposits which proved to be economically
cupriferous only in the oxidized zone and more particularly within thin
blankets of copper carbonate/oxide secondary ore and sulphide (chalcocite)
supergene enrichment. A typical
sequence of ore types encountered with increasing depth is: sparse cuprite
with malachite, minor azurite and native copper in leached or ferruginous ore
(ca. 0.5-1% Cu); oxide ore with
chalcocite, pyrite and malachite ore (1-3% Cu); relatively thin supergene
enrichment zone of chalcocite with variable pyrite and chalcopyrite (3-5% Cu);
primary pyritic ore with sub-granular trace chalcopyrite (1-1.5% Cu).
Although a vertical sequence of this sort might be encountered at any
point over orebodies, the superimposed blankets of different secondary ore
types are generally not simple horizontal layers.
In the example of Budgery mine, where a moderate plunging pipe-like
body of secondary ores was mined, the records suggest that cavernous gossan
was best developed along the dipping crest of the pipe, carbonate ores beneath
the gossanous crest towards the pipe axis, and supergene enrichment bodies
more towards the lower surface of the pipe-like `lode'.
At the best known deposit, Girilambone mine, the primary ore lenses are
relatively shallow dipping. This
and the presence of interference folds (NE and NW trending sets) may have
produced groundwater effects conducive to development of basin-like
concentrations of supergene ore. Generally,
leached rock extends from surface to about 25 m depth and passes into a zone
characterised by malachite and azurite, and then into a secondary enriched
zone dominated by chalcocite. Native
copper is common in parts of both the oxide and chalcocite zones.
In extracting the secondary ore large open "ballroom" stopes
were developed in the upper levels of the Girilambone copper mine.
Mining activity was greatest in the decades 1880-1910 and the whole
belt had fallen dormant by 1920-1921. The
Girilambone mine (depth 166m) was by far the largest mine, accounting for over
90% of the belt's production. The
mine experienced numerous treatment problems and a considerable reserve was
left unmined at its closure. Intense
exploration of the Girilambone belt, expecially its northern part,
re-commenced in the 1960's and has resulted in several kilometres of diamond
drill core. Utah Development Co.,
Selection Trust, Australian Selection Pty Ltd, Sanidine NL, Hunter Resources
Ltd, Nord Resources, and others, have explored the Girilambone are.
Drilling programmes by Utah (1969-1973) and Selection Trust (1975-1980)
were concentrated at the Girilambone mine and prospects several kilometres to
the north of it. Utah completed
over 64 percussion and combined percussion-cored drill holes after extensive
geochemical and geophysical surveys. A
resource of 3.32 Mt of 2.12% Cu was calculated at Girilambone mine from this
work. (Smith, 1971-1976).
In 1989 Nord Resources (Pacific) Pty. Ltd. commenced a 140 hole
re-evaluation of the Girilambone deposit with a view towards open cutting it
for copper recovery by heap leaching and electrowinning.
Nord's drilling increased the resource estimation to 7.0 Mt of 1.7% Cu.
Smaller copper orebodies which are present nearby, as at the Northeast
prospect and Hartman shaft prospect, could supplement the next phase of
production at Girilambone Mine.
Workings along the Girilambone belt were little developed within the
primary zone orebodies. The
primary orebodies, from limited mining records and subsequent drilling, appear
to be stratabound. They have been
widely postulated as syngenetic and suggested as submarine volcanogenic in
origin. However, summary mentions
of the orebodies as stratiform or stratabound omit to take into consideration
the old records describing some of them as pipe-like, or otherwise elongate
plunging. Broad envelopes of
mineralization which have not been closed off by drilling appear to plunge
southeasterly at each of the two major deposits (Girilambone, Budgery).
As with the Cobar belt, the significance attached to any uniform plunge
trend along the belt would be uncertain.
Close association with ophiolites or volcanic rocks has not been
demonstrated for the metalliferous deposits of the Girilambone belt, although
McClatchie (1985) has interpreted many fine-grained metamophic rocks in the
southern part of the belt as metavolcanic in origin, especially at the Budgery
mine. Most of the inferred
metavolcanic rocks are mafic but minor felsic metavolcanics have also been
suspected. Confirmation requires
chemical and other studies. Rocks
interpreted as sheared felsic volcanics (McClatchie, 1985) occur in the Bonnie
Dundee and Great Hermidale mine areas. The
main such interval in the Bonnie Dundee area, a possible sheared rhyodacitic
volcaniclastic coarse sandstone, is about 5 m thick.
More definite deformed felsic volcanics, or intrusives, still
preserving phenocrystic quartz and feldspar, occur in the Bylong area west of
Girilambone mine. Further north,
on the Bourke sheet, felsic volcanics or porphyry has been mentioned to occur
(south of Buckeroo Mountain and north of Sunday Morning Hill (pers. comm. C.
Plumridge) but no details are known. Ultramafics
which occur within the Girilambone belt may carry trace chalcopyrite (e.g.
Williams 1959b), and may occur in proximity to sulphide mineralization; yet
these rocks do not appear to be genetically connected with the cupriferous
pyrite mineralization.
Mining and underground prospecting along the Girilambone belt was
brought to a halt by exhaustion of supergene ore, and by the potential high
transport costs following the closure of the nearest smelters (Great Cobar,
CSA) in 1919-1920. Interest in the
Girilambone belt was renewed with intensive investigation by exploration
companies in the 1960s and 1970s, particularly by Utah Development Co. (Smith
1971, 1973). Since the 1960s over
1 km of trenching has been done along the belt to overcome lack of outcrop.
Over sixty prospects are recognized on geochemical and other grounds.
It is not possible to fully review all these here.
Recent rehabilitation works, with the bulldozing of old workings,
renders difficult the interpretation of old records.
Smith (1971,1973) and L. McClatchie (in Central West Gold N.L., 1985)
have attempted geological syntheses of extensive but fragmentary data.
McClatchie attempted to trace two hypothetical deformed stratiform ore
zones between the Bonnie Dundee and Budgery areas, of which the
"lower" is the more continuous zone.
Despite such attempted ore horizon correlations, and a large amount of
structural analysis by Utah Development Co., the stratigraphic framework is
still quite inadequate throughout the Girilambone belt.
This impedes any judgement as to the acceptability of a stratabound ore
horizon model such as advanced by McClatchie.
More such work is required as ore reserves are apparent at some sites,
mostly small and low grade except for Girilambone mine.
The Girilambone belt has yielded minor gold
production from mineralization which appears related to late stage
transgressive structures. This
includes gold in quartz veins. However,
most of the copper mineralization is not thought to be of vein type and
stratiform features are prominent in some mine areas.
The relatively insignificant mining in the primary zone hinders
understanding of the deposits. Most
copper production was from mixed oxide and carbonate ores, and from supergene
sulphides (particularly chalcocite) developed at depths of 60-90 m.
Pyritic ore was encountered at depth in the mines, which then became
uneconomic to work. Typically, ore
grade fell from 7% or better to an uneconomic 2-3% Cu as the primary zone was
being entered.
Primary orebody shapes are poorly known as few workings persisted far
into primary ore. From geological
mapping, mine records and drilling data three types of mineralization are
envisaged:
1. Stratabound pyritic
ore zones/horizons, which are perhaps syngenetic.
2. Plunging copper
orebodies which may represent a later phase of epigenetic
concentration of metals within the pyritic zones.
3. Transgressive
mineralization in cross-cutting structures and stockwork vein systems.
Metals may be remobilized from underlying
stratabound zones.
Gold and lead feature in this type of mineralization.
Copper has been produced both from moderately dipping zones of
stratabound aspect (type 1) and also from plunging orebodies (type 2) whose
relationship with pyritic ore zones requires further clarification (e.g.
Budgery mine). Suspected
cross-cutting remobilized mineralization is not copper-rich except where still
in close contact with the assumed stratabound source zone.
The possible stratabound mineralization zones are aggregates of narrow,
variably ferruginous bands which in the primary zone are found to comprise
disseminated sulphide, mostly pyrite. The
zones are up to 50m wide and in some narrow intervals the pyrite may be
densely concentrated (sub-massive).
Most
of the copper mines have been opened to below the base of oxidation but
descriptions of the primary ore are meagre.
The primary ores appear to have the following characteristics:
(a) They are dominantly pyrite, with only a small content of fine-grained
chalcopyrite; plus traces of
sphalerite, pyrrhotite and occasional galena.
Sphalerite and galena are present as rare minute grains (ca. 25
microns), included usually in pyrite. Sphalerite
plus galena seldom, if ever, total 1% of the sulphide content.
(b) Primary ore grade is typically below 2% Cu (with occasional thin
intervals to 5% Cu).
(c) The hostrock is selectively the more quartzose intervals of the country
rock at all scales. The simplest
explanation of this is that it might reflect former porosity and permeability.
An alternative explanation invokes exhalative silica.
he fine
grainsize of the chalcopyrite and the quartzose nature of hostrocks were basic
characteristics behind the numerous treatment problems experienced during the
life of the Girilambone mine.
The
Girilambone belt contains the following six mineral areas:
1.
Bylong Area
2.
Larsen-Avoca Tank Area
3.
Girilambone Area
4.
Bonnie Dundee Area
5.
Great Hermidale Area
6.
Budgery Area
1. Bylong Area
The Bylong
area contains numerous poorly recorded small gold workings, and a lesser
number of prospected gossanous outcrops, in an area west of the Girilambone
mine. Production is unknown, and
probably negligible. Some of the
ironstone was used as flux at the Girilambone mine and the area may also have
had a small quartz crushing plant. The
deposits in the Bylong area are:
No.
Deposit Name
Commodities
26
Unnamed
Fe
27
Unnamed
Fe
28
The Comet
Au
29
Finn's vein
Au
30
Unnamed
?Au
31
Unnamed
Au
32
unnamed
Au
33
Ironstone quarry
Fe
89
Riles Rise prospect
Fe
Reconnaissance
geochemical sampling has detected many places in the Bylong area where rocks
are anomalous in base metals (e.g. Australian Selection Pty Ltd, 1980).
Massive and siliceous ironstones are locally prominent.
Some of these have shafts and pits sunk along them but there has been
no drilling and their origin remains uncertain.
They contain metal trace values up to 900 ppm Cu, 295 ppm Pb, 300 ppm
Zn, 135 ppm Ni and 60 ppm Co. Deformed
felsic volcanics occur between Riles Rise prospect and The Comet.
The
principal feature of the Bylong area is a linear concentration of small gold
workings west of Bylong homestead. This
is a northerly trending zone which swings southeasterly (140oT) in the south
to possibly parallel the shear which truncates the Girilambone mine
stratabound ore zones.
The Bylong
gold workings appear to be mostly on thin quartz veins in schist.
Individual workings are mostly small, following ironstained quartz
veins and veinlets developed both within fractures and along the foliation.
The veins are commonly thin (2-5 cm) and discontinuous.
Occasional more continuous veins are 15-25 cm wide.
Shaft sinking commenced in 1897 and intermittent prospecting continued
until 1936. Lease plans include a
machine tenement at a dam, and a small crusher may have operated here.
The prospected lines of quartz veining in places carry considerable
iron and manganese oxide impregnation, and in places the host rock schists
contain limonite pseudomorphs after pyrite.
Sample assays from along the linear zone range up to 90 g/t Au,
1.8 g/t Ag, 425 ppm Cu, 2000 ppm Pb and 530 ppm Zn.
The pit reveals intense quartz veining in highly contorted schist.
2. Larsen-Avoca Tank Area
The
Larsen-Avoca Tank area contains a cluster of five deposits located 3-5 km
northerly from the Girilambone mine.
The deposits in the Larsen-Avoca Tank area are:
No.
Deposit Name
Commodities
34
Avoca Tank prospect
Cu, Au
35
Hartman shaft
Cu
36
Northeast prospect
Cu (Zn)
37
Hunter shaft
Cu
38
Larsen shaft
Cu
The
Larsen-Avoca Tank area was in part reserved for mining purposes in 1912 and
prospecting commenced last century. It
is the most likely location of "Thomson and Smith's" copper show
which was being prospected near Girilambone in 1882.
Prospecting has been on-going and a significant low grade copper
reserve is present at the Northeast prospect.
The mineralization is similar to that at Girilambone mine.
The Avoca Tank prospect is of less certain affinity.
The mineralized trend, at least from bedrock geochemistry, is
transverse to the Hartman-Larsen trend. The
Avoca Tank prospect is tentatively classed with the late stage remobilization
effects, and stratabound mineralization could be present at greater depth.
Avoca Tank
Prospect
The Avoca
Tank prospect is at a gold prospecting shaft about 2 km north from the
Hartman-Larsen group of workings. Gossan
and dump samples give values of up to 0.7% Cu, 0.22% Pb, 4.7 g/t Au, 1.2 g/t
Ag. Drilling intersected massive
haematite and magnetite gossan over a width of 34 m, containing one 3.45 m
section of 0.35% Cu. The deepest
drilling intersected 6 m of pyritic massive sulphide assaying 1.63% Cu.
Zinc values are up to 1700 ppm and gold is negligible in drill
hole samples.
Hartman-Larsen
Group of Workings (a.k.a. Girilambone North mining area)
The
Hartman-Larsen prospecting area, also known as the "north-west
prospect" with respect to the Girilambone mine, comprises a compact
association of prospects (Hartman, Hunter, Larsen, Northeast) which could be a
single stratiform zone that has been tightly deformed, or a set of co-genetic
separate ore lenses if epigenetic. Drilling
results support a stratabound although not stratiform nature to the
mineralization. Some 1-2Mt of low
grade copper ore could exist in the area.
However, drilling results are not as encouraging as for the nearby
Girilambone mine, nor is there potential for open cut mining.
Geological
mapping and the old workings appear to define two parallel mineralized zones.
These were initially regarded as the limbs of a northwesterly trending
and plunging anticline, although subsequent drilling gives little support to
this (MacDonald, 1984). The main
zone is the Hartman-Larsen trend, along which most of the old workings occur
(Hartman's, Hunter's, Larsen's). Many
holes have been drilled along this trend and it appears to comprise a zone of
weak mineralization which is more or less continuous, particularly from
Hunter's northwards. Between the
main centres of mineralization along the trend there may be encountered in the
sericite-chlorite schists intersections of 20-30 m which are weakly
cupriferous (300-800 ppm Cu). A
second shorter and less extensively explored zone of mineralization lies east
of the Hartman-Larsen trend and contains the Northeast prospect.
A feature of the Hartman-Larsen area is that along each of the parallel
zones the mineralization is stronger at the northern end.
An estimated 1.5 Mt of 1.8% Cu ore exists at Northeast prospect on
one termination, and on the other there is mineralization with a best
interestion of 26 m of 1.45% Cu near the Hartman shaft.
The old workings of the Hartman-Larsen group were sunk on bodies of
ironstone and sometimes followed copper carbonate veins.
The Hartman, Hunter and Larsen shafts were all sunk along a single
gossanous trend, which drilling reveals as the cap of an easterly dipping
broad zone of disseminated weak pyrite-chalcopyrite mineralization in
quartz-chlorite schists. The best
intersections are all in the oxidized zone.
Low copper values (0.7% Cu) may persist across most of a broad zone,
and give aggregate intersections of up to 30-50 m.
Higher grade (2% Cu) occurs over intersections as great as 8 m and
dump specimen values, probably from the zone of supergene enrichment, are up
to 8% Cu. The primary ore of
disseminated fine-grained pyrite and minor chalcopyrite contains only 0.4% Cu.
The sulphide is mostly subhedral pyrite.
Chalcopyrite typically occurs as highly irregular to skeletal patches
in the areas of greatest pyrite concentration.
The sulphides selectively favour quartz-rich bands and laminations
within the schist and rarely occur in wholly micaceous host rock.
Minor gossanous traces and old workings occur between the
Hartman-Larsen prospecting area and the Girilambone mine but drilling has not
detected any definite evidence of mineralization.
Open cut
mining began in the area in June 1996 and it began in 1997 to supply feed for
the solvent extraction electro-winning (SX-EW) processing plant at Girilambone
mine - this area being foreseen to continue plant feed as the (Murrawombie)
open cut at Girilambone would become exhausted of secondary enrichment zone
ore.
3. Girilambone Mine Area
This area has
orebodies which partially outcrop and were worked at the Girilambone mine,
about 4 km west of Girilambone railway station.
In 1883 Girilambone had a population of around 500, with 130 men and
boys working in the mine. Employment
peaked at around 200 men after the formation of the Girilambone Copper Mining
Co. Ltd, but production was spasmodic. Copper
production will likely recommenced at the mine area in the 1990's.
No.
Deposit Name
Commodities
90
Girilambone Copper Mine
Cu (Zn, Ag, Au)
Copper was
first discovered in the Girilambone belt at the Girilambone mine hill (Copper
Hill). The area was taken up under
Volunteer Land Orders in 1880 by Mssrs Hartman, Campbell and others associated
with Cobar copper mining. Hartman
first observed copper indications there in 1875.
Commencing in 1881, mining, underground prospecting and later drilling
activity in the mine area have been intensive.
Nonetheless, the ore system still remains (1980s) to be fully
delineated and traced down-plunge. A
resource of 3.32 million tons at 2.12% Cu was estimated after extensive
exploration work by Utah Development Co. in 1969-1973 (Smith 1971, 1973, 1974,
1976). This included at least 1.7
Mt of oxidized and supergene enriched ores grading up to 3% Cu, which could be
readily open cut. This, however,
was insufficient to meet the requirements of Utah's economic evaluation and
the prospect was relinquished. An
increased resource envelope figure estimated for the deposit following later
drilling by Nord Pacific Ltd was 6-7 Mt of 1.9% Cu (3.5-4.5 Mt mineable) at
0.4% cut-off grade, or 8.7 Mt of 1.4% at Cu at 0.2% cut off.
There is also present a small gold resource (ca. 0.7 Mt at 1.3 g/t Au).
This occurs in the leached cap and has low average copper content (ca.
0.06%). One metre drill interval
assay results range as high as 4 g/t Au but assays of 3 g/t or above are
exceptional. Prior to Nord
commencing work at Girilambone, trace gold had been noted at the old
Girilambone plant foundations. There
is no record of any gold treatment at the mine but minor auriferous stone may
have been brought on site from the surrounding district.
Thin
outcropping bands of copper-staining on "Copper Hill" were sampled
in 1875 by T. Hartman, one of the discoverers of copper at Cobar.
The mine, initially known as the West Bogan Mine, was commenced in
1880. Early mining was of the
richer secondary ores, of about 7% Cu. Later
grades fell to below 3.5% Cu. Numerous
problems were experienced at the mine: difficult mining conditions, disputed
ownership, shortages of water, and recurrent difficulties in ore processing.
Smelter output accounted at times for as little as 50% of the copper
raised. Extensive tailings
leaching was introduced early in the mine's history to compensate for this and
complete the recovery of copper. The
mine worked intermittently until 1917, following which only small scale copper
leaching operations have been carried out.
The main
shaft is over 165 m deep and there are six major levels.
Carbonate ore is recorded as occurring down to about 65 m and
"black ore" (chalcocite) at greater depth.
The top 25 m is relatively leached.
Mining from 1881 to 1907 raised approximately 90,000 t of copper ores
(2-5% Cu). Employment was 80 men
in 1882, rising to 200 in 1989. Mining
ceased between 1885 and 1893 when the Cobar field was also in decline.
Major production then occurred between 1897 and 1901 when 56,000t of
ore was raised. Production was
mostly from rich patches of secondary ore from the 91 m and 122 m levels.
This ore largely comprised chalcocite, with lesser tenorite, cuprite,
carbonates and other secondary minerals. Where
mined, oxidation and secondary enrichment are believed to have considerably
modified the original pyrite-chalcopyrite mineralization, yielding well
developed supergene blankets over the primary ore lenses.
The "lodes" recorded as varying from 2-30 m width were
presumably pockets of secondary ore, perhaps controlled to some extent by the
complex interference folding. Overall,
mining was restricted to the richer patches of secondary ore in the upper
levels, with much of the production perhaps coming from places where carbonate
ore was dominant (e.g. 3rd and 4th levels).
Production ceased after the known secondary orebodies had been
effectively exhausted. Low grade
primary ore was encountered at the 152 m level but was probably not mined
to any great extent. The best
grades relate to a chalcocite zone oxidized which directly overlay primary ore
and underlay the oxidized-leached zone containing variable malachite, azurite,
chalcocite, cuprite and native copper.
Chalcopyrite at depths penetrated by the past underground mining, or
amenable to future open cut mining, never occurs as monomineralic grains but
rather as irregular relict cores enclosed in chalcocite or other minerals.
The pre-replacement nature of this chalcopyrite, whether primary or
secondary, is undetermined. Chalcopyrite
appears to reach high abundance (40%) only within the chalcocite zone,
suggesting that much of it may be secondary, as is also observed from some
Cobar belt deposits. Chalcocite
ranges to 0.5mm grainsize, but is commonly very fine and "sooty".
It surrounds and replaces pyrite or other sulphides.
Most chalcocite is intricately composite with pyrite but less often the
dominant sulphide present may be chalcopyrite.
It is fine grained (0.15 mm max.) and has associated bornite and
covellite. Chalcopyrite content
can range to triple the chalcocite content and over double the pyrite content
but such chalcopyrite dominance is always of limited extent.
In
view of the obscuring features described above, present at all depths mined,
the mechanism of primary mineralization at Girilambone remains poorly
understood. The deposit lies
adjacent to a major shear zone on a regional scale but locally the apparently
stratabound nature of the orebodies has been the feature to most impress
exploration geologists. The
mineralization is hosted in the highly deformed metasediments of the
Girilambone Group. Some of the
hostrock is appreciably graphitic (ca. 0.1% C) but the distribution and
significance of the graphite is undetermined.
Quartz-chlorite-sericite schist is the dominant rock type in the mine
area but the mineralization is largely within quartzite.
No metavolcanics are postulated, in contrast with the Budgery and other
deposits further south. Serpentinite
in the vicinity is not mineralized and its emplacement might post-date the
mineralization. Serpentinite has
been intersected in several drill holes and occurs within a costean and pits
to the north-east of the mine area.
Copper mineralization is mainly within thinly laminated to massive
quartzite. Styles of
mineralization include massive, thinly laminated to banded, disseminated and
fracture-filling. The quartzite
host rocks occur as lenticular bodies assumed to be stratigraphically aligned
("Pink Quartzite" member). Below
the "Pink Quartzite", lower grade disseminated mineralization is
found in quartz-chlorite (penninite) schist.
Thus two primary ore types can be recognized:
the more massive type within the Pink Quartzite lenses, and a more
disseminated type within schist. Ore
hosted in the Pink quartzite member varies from disseminated to laminated to
massive sulphide. The massive
sulphide layers range from 0.1 m to 7 m in thickness.
Up to six thin massive sulphide layers may occur in the mine sequence.
As Smith (1976) noted, the massive ore is restricted largely to the
hanging walls of Pink Quartzite lenses. Laminated
sulphide also reduces from hanging wall to footwall.
The pyrite
laminated sulphide or pyritic bands vary in texture from massive 0.8 mm
granular to disseminated with coarse euhedral crystals up to 3 mm.
The pyritic bands are often irregular and discontinuous.
Quartz tends to be more prominent in the pyritic bands than between
them. It is thought that the
pyrite, whether of syngenetic or replacement origin, selectively favours the
more quartzose layers. Both
massive sulphide intervals and low grade chalcopyrite stringers favour the
quartzite. Chalcopyrite
occurs finely dispersed within the pyritic areas and also in fine traces
discordant to lamination. In the
schist-hosted disseminated mineralization the dissociation of chalcopyrite
from pyrite is more noticeable, with irregular blebs of chalcpyrite up to 1 cm
long in quartz segregations.
At least
two phases of folding are apparent in the Girilambone area.
A detailed study of the structure of the Girilambone mine area by Utah
recognized two generations of folding. A
third generation of deformation was suspected from slip cleavage within some
units. The first generation
folding is relatively tight and has an overall northeast trend and plunge.
The crest of ore antiform is represented by the low conical hill
("Copper Hill") at the mine area.
Second generation superimposed folds which almost mask the first phase
are broad low amplitude folds which generally strike and plunge to the
northwest. Early mine workings may
have follewed a "lode" which was a supergene blankent dipping
northeast and striking northnorthwest. The
Pink Quartzite member in the mine area trends northeasterly along the first
generation structure, which is refolded by second generation folds with an
axial plane crenulation schistosity. The
first generation northeast trending folds plunge northeast and are cut by a
major northwest striking reverse fault zone or shear which dips northeast at
45-55o (East Shear Zone). The
Girilambone mine hostrocks extend down the east flank of the northeast
trending first generation structure, as far as the East Shear Zone.
The shear zone truncates the Pink Quartzite mineralization.
Sulphide mineralization is only poorly known on the opposite side from
the mine, where the sources of electrical anomalism have more often proved to
be graphite. The shear zone itself
does not appear to be mineralized. It
is intersected in drilling typically as an interval of brecciation, fault
gouge and clay. An unmineralized
northwest trending siliceous zone which occurs on the northeastern side may be
either local silica transportation into the plane of the shear, or could be
the sheared east limb of a postulated regional arch structure.
In
comparison with the multiply deformed host rocks, the secondary orebodies at
Girilambone mine are structurally simple and tabular.
Diamond drill holes (41 by Utah Development Co, 1 by Australian
Selection Pty Ltd) outlined two moderately dipping tabular bodies containing a
combined total of 3.3 million tons of 2.1% Cu. This includes an enriched zone of 1.66 million tons of 3.06% Cu.
Substantial widths, up to 23 m of enriched ore (5-10% Cu) were
encountered at the hanging walls of the primary orebodies. Also present are minor to moderate amounts of zinc (to 3%) and silver
(to 35 g/t). Lead is present
only in very minor amounts. Gold
assays average 0.7 g/t Au in the secondary zone and 0.5 g/t Au in
the primary zone.
The original
Girilambone Copper Mine area has now been open cut as 'Murrawombie' pit and
all traces of the old mine lost. The
Murrawombie open pit was terminated in 1999, although copper production
continued for two more years from heap leaching. Nord Pacific
Limited and Straits Resources Ltd developed the open cut and processing plant
and by 2001 had produced in excess of 110,000 tonnes of 99.999% copper metal
by Solvent Extraction, Electrowinning (SXEW) processes. Mining commenced
in the Murrawombie Pit in late 1992, after an ore reserve of 78,000 tonnes of
copper metal recoverable by SXEW methods had been established. After
1992 an additional ore reserve of 41,000 tonnes of recoverable copper has was
established at the northern prospects and mining commenced at 'Girilambone
North'. Mining was suspended in late 2008 after the halving
of copper prices in that year made operations marginal. At that time the
mines and processing plants had about 170 staff. Plans for
expanding underground mining were resumed again in 2010.
Prior
to the revitalisation and open cut mining at Girilambone by Straits and its
associates, Utah Development had explored the mine area and much of the
Girilambone belt, as far south as Hermidale, for decades. This
writer was not aware that Utah had resumed mining before it finally gave up on
the district. However, a geologist who worked on the project, Tim
Hopwood ( http://timhopwood.com/pbpres5.htm
l) has referred to Utah doing
'mining', in these terms:
|
Organization
Name:
|
Utah
Development Company
|
|
Position:
|
Consulting
economic and structural geologist.
|
|
Duration:
|
January,
1966 - April, 1969
|
|
Sector:
|
Mineral
Exploration
|
|
Region:
|
Australia
& NZ
|
|
Country:
|
Australia
|
|
Terrane:
|
Lachlan
Fold Belt 3936
|
|
Property/Project:
|
Girilambone-Hermidale
Cu Deposits
|
|
Commodity:
|
Iron
Oxide Cu- Au
|
|
Duties:
|
Regional
and detailed prospect structural mapping, resulted in discovery of
Girilambone Cu Deposit, subsequently mined by Utah
|
|
Service/Skill:
|
Geology
Regional &Targeting
|
|
Referee:
|
Richard
D Ellett, former MD of Utah Development Company
|
Tim's resume
( http://www.timhopwood.com ) includes
"His earlier years were spent at the Universities of Sydney,
Adelaide
,
Melbourne
and
Illinois
where he taught and researched structural controls of ore deposits. Between
1974 and 1985 he gave eight courses on "Geological Environments of Ore
Deposits" for the Australian Mineral Foundation, in
Adelaide
. As a consultant in mineral exploration, Hopwood's work contributed
directly to the discovery of the Cleveland Tin Mine, for Aberfoyle in 1962,
Girilambone Copper Mine in 1969, Fitzpatrick Lode for North Broken Hill in
1975, and significant intersections
at the Pinnacles Mine for Pasminco in 1993". Thus Tim was a
strong part of Utah's rather remarkable structural geology approach to its
exploration around Girilambone and along the Girilambone Belt south to
Hermidale. As noted above, Tim believes the regional, plus more detailed
(at prospect scale) structural mapping resulted in the discovery of the Girilambone
secondary oxide Cu deposit which was (eventually) mined.
In its
final exploration report for Girilambone in 1974 (GS 1970/119) Utah stated
that it was applying for a prospecting licence to secure title over the
remaining seconary mineralistion around Girilambone mine. Nonetheless EL
work was able to continue but that did later give way to PLs. Those PLs
were then continued with in the area and finally ended in 1983. At
some time well before then Utah's interest had been disposed of and a number of
other joint venturing companies were exploring the area. By 1973
Utah had calculated at reserve at Girilambone mine of 3.3 Mt at 2.12%
Cu. Utah offered the possibility of open cut production to a
number of companies but without success. Seltrust Mining and joint
venture partners acquired tenements in 1974, and in 1975 proved another 1.5 Mt
of 1.8 % copper at NE prospect. One joint venturer in the area, Nord Australex Nominees Pty Ltd
in 1990 negotiated the purchase of the then exploration licence surrounding
Girilambone Copper Mine from Hunter Resources Limited. By 1991 Nord was
advancing a feasibility study for mining and Straits Mining Pty Ltd acquired a 60% share
in the project.
In October 1992 project development of the
Girilambone Copper Mine commenced. By February 1993 the stacking of the heap leach pads
was underway and in May 1993, the solvent extraction and electro-winning plant was completed,
allowing copper production to commence. At the northern prospects mining
was commenced in June 1996.
Mining ceased
in 2000 but processing activities at the Girilambone mine plant continued until 2003.
Over this period of time, four open pits (three at Girilambone North Mines -Larsens pit, Northeast pit and Hartmans
pit; and one at Girilambone mine - Murrawombie pit) were mined until the
copper oxide ore in the weathered zone was exhausted. Heap Leaching, Solvent Extraction
(SX) and Electrowinning (EW) were employed to process the oxide ore and produce a London Metal Exchange (LME) grade copper
cathode product in sheet form. The sulphide ore types which occur deeper
down were not mined until Tritton mine was developed.
The company
formed to develop the Tritton deposit, Tritton Resources Limited, in 2002 had
the resources around Girilambone transferred to it but later on the majority
equity passed back to Straits Resources.
In 2004-2005,
mining was recommenced at the base of the Murrawombie pit in order to supply
574,05 t of sulphide ore to the Tritton processing plant during the development and commissioning stages of the
Tritton mine.
In September 2008
a Copper Cementation Plant was opened at Girilambone mine. This plant was developed to
exploit the still remaining copper in the heap leach pads.
In November 2008, after a dramatic fall in the copper
price during that year, staffing was reduced and the Girilambone and Girilambone
North mining operations were suspended, with only the Copper Cementation Plant
left working so as to continue copper production. In 2010 mining
plans were resumed on for sulphide ore extraction at Girilambone and
Girilambone North.
Besides the
unusual, and so far unconfirmed, statement shown above that Utah mined at
Girilambone (i.e. the sizeable secondary copper ore deposit it did outline)
another unsual statement is that Straits has discovered "numerous +1 Mt
ore deposits in the region":
http://www.helix.net.au/index.php?page=43
Helix
Resources is a long-experienced exploration company in the Lachlan Fold Belt,
and had stated that copper and gold targets in the Cobar/Girilambone region
would be the company’s main exploration focus in the first half of
2010. It perhaps has basis for the above statement as years of
exploration work reportage in the Girilambone Belt still remain
confidential. What the "numerous
+1 Mt ore deposits in the region" are which Straits Resources discovered
is not known. Tritton is the well known one. Straits
did not discover the Girilambone and Girilambone North deposits as these had
been found and proven up long previously. Hence the statement
remains a curiosity.
4. Bonnie Dundee Area
(Tritton)
The Bonnie
Dundee area contains the following deposits:
No.
Deposit Name
Commodities
86
Bonnie Dundee mine
Cu, Au, Ag
87
Budgerygar North mine
Cu, Au (Ag)
87
Budgerygar mine
Cu (Ag, Au)
88
Budgery King
Pb, Cu
--- Tritton
Cu (postdates the metallogenic mapping)
This group
of deposits is characterized by gossanous development along what has sometimes
been mapped as a continuation of the Budgery area "mine horizon"
further south (Australian Selection Pty Ltd, 1982).
Prominent gossans may be weathered quartz-haematite-magnetite rocks.
Faint geochemical traces extend for 2.5 km north of Bonnie Dundee mine
and 1 km south of Budgerygar mine.
The
gossans of the Bonnie Dundee area were initially prospected for gold,
commencing in the 1880s, but there is no record of the quantity of gold won.
From 1907, copper was the principal commodity won from the area.
As elsewhere in the Girilambone belt, almost all of the production came
from oxidized and supergene enriched zones above 100 m depth.
The
deposits of the Bonnie Dundee area display some similarities to the larger
Budgery deposit. Appreciable gold
was produced, ore shoots pitch southerly, and the North Budgerygar secondary
orebody is recorded as pipelike. If
the Bonnie Dundee area deposits are related to a once-continuous stratabound
zone then it is likely that the Bonnie Dundee and North Budgerygar mines are
sited at deflections or cross-features along that zone, similar to the Budgery
mine being located at major kinking in the regional meridional trend.
The broad envelope of geochemical values at Bonnie Dundee trends
northeasterly, quite oblique to the stratiform trends.
Drill holes to test the alternative possibility, that the Budgerygar
mines are on a separate ore zone continuing north to the east of the Bonnie
Dundee, encountered no mineralization.
In the
Bonnie Dundee area the greater number of significant gold values are from the
Bonnie Dundee mine (max. 4m of 4.62 g/t Au), where available records support
the likelihood of gold remobilization. At
Budgerygar North there was an association of secondary copper and gold values.
The
Budgerygar mines lie to the southeast of the Bunnie Dundee mine, on the likely
continuation of the same pyritic zone which has been deflected or faulted to
the east. The North Budgerygar and
Budgerygar mines were opened near the northern and southern outcrop limits of
a narrow northerly trending gossan over 360 m in length.
Continuity of a pyritic zone at depth, between the two mines, is
suggested by SP contours (Smith, 1971). The
gossans of this zone are in places earthy, and in places finely cellular after
pyrite. They provided the initial
focus of activity in the Bonnie Dundee area, and were first taken up as gold
leases. Ferruginous material
yielded prospective values of up to 102 g/t Ag and 15 g/t Au.
The Budgerygar North was worked to 90 m depth and produced
secondary copper ore of average grade 8.7% Cu, containing at least 179 t Cu
(estimated total production 220 t Cu). The
Budgerygar mine was developed to 110 m depth, and yielded at least 58 t
Cu in ore of 5.1% Cu average grade (estimated total production 100 t Cu).
The Budgerygar mine apparently lacked as definite an ore shoot as the
pipe-like body followed in the Budgerygar North mine.
Weak gold traces of up to 1.3 g/t Au have been obtained between
Budgerygar and Great Hermidale mines.
A group of
mining leases taken out near the Budgery King prospect are at a similar
stratigraphic position as the Bonnie Dundee and Budgerygar mines according to
Australian Selection Pty Ltd (1980). The
sampling values from the Budgery King area range up to 5,800 ppm Cu, 5.4%
Pb and 200 ppm Zn. The lead
values, mostly above 4,000 ppm, are the highest recorded in the
Girilambone belt. Drilling
encountered no significant mineralization.
Bonnie Dundee Mine
Bonnie Dundee Mining Company No Liabililty (ML 17 and GLs 1,2,6 of Parish
Tritton). The area has quartz veins carrying minor gold, as
well as the more important copper ore. It was possibly first
worked for gold, down to 110 ft, and 27 tons of gold ore yielded 14 oz
gold. Of the 27 tons raised, 15 tons was sold to smelters at Dapto, and
12 tons were treated at the "Wellington stamp battery"
(Bodangora?). A syndicate of three men worked it as a gold
mine. Subsequently copper was the commodity of interest, and the change
from gold mining syndicate to copper mining company was in 1906. The
secondary ore zone was cavernous in places. Below the water table,
at 200 ft, the ore was 'basic' sulphide ore. The length of the
lode was upwards of 200ft.. Strike was a little east of north (as shown
by the stoped area outline on plan). Dip was to tha east, as in
the Budgerygar mine. By 1915 the mine had yielded about 900 tons
of secondary ore, averaging 4% copper. The ore was sold to the
Cobar smelters and up to seven men worked at the mine. The mine
hoped to erect a smelter as the lower grade primary ore was not
saleable. Only the secondary ore paid to transport to Cobar.
The Bonnie
Dundee mine was developed to a depth of over 500ft (164 m) in a narrow ore zone which,
if stratiform, is probably the continuation of the Budgerygar gossan zone.
The ore zone dips steeply east (70o) and at the lower mine levels was
about 10 m wide. The
principal primary sulphide body was 1.8 wide.
Percussion drilling by Australian Selection Pty ltd confirms northerly
extension and easterly dip of the the copper ore zone.
Significant copper values continue for at least 100 m north of the
workings but no economic grades have been intersected in drilling.
Bonnie Dunnee
mine in 1915 (MR 399)
An upper
stockwork quartz zone, which overlies the copper-bearing zone for a limited
length, is significantly enriched in gold and lead.
Assay values for surface and dump samples at Bonnie Dundee range up to
4.9 g/t Au, 70 g/t Ag, 1.1% Pb and 2.49% Zn.
Highest drill hole assays are 4 m of 4.62 g/t Au, 12.6 g/t Au over 1 m,
and 2.98% Pb over 1 m. (Central West Gold N.L., 1985).
The underlying copper ores mostly averaged 2-4% Cu.
Mines Inspector Godfrey sampled the workings in 1915 for average assay
values of 2.07% Cu, 3 g/t Au, 35.6 g/t Ag.
It was in
the stockwork zone that mining was commenced for gold, in or before 1894.
The copper-bearing zone was penetrated at 64.4 m depth and from 1907
onwards copper was the principal commodity won.
About 250 t Cu was obtained from 7183 t ore.
Some of this ore (6939 t) had a content of 19.5 kg Au and 300 kg Ag.
Little information is available on the copper orebody worked.
It was possibly a pipe-like secondary orebody plunging south within a
broader tabular ore zone, similar to the herein interpretation of
Budgerygar North orebody. It
was worked mainly between the depths of 73 m and 152 m, over widths
of up to 10 m. At the lower
levels (122 m, 152 m) the mine was in the primary zone and a 1.2-2.5 m
wide pyritic lode was present, assaying up to 4% Cu, 35% Fe, 40% S.
This lode was driven along and averaged 4-4.5% Cu for significant
lengths at a width of 1.2-1.5 m, yet did not prove economic to mine.
It was the richest interval within a pyritic zone about 10 m wide.
Budgerygar North Mine
This mine, the Bugerygar North Block No. 2 Copper Mining Co. N.L. is in
portion 2 of Parish Tritton. It was managed by James Henry
Dillon. It lies 340 m southeast of the Boonie Dundee.
James Dillon stated that he had discovered it and after sinking to a depth of
8ft he encountered carbonate ore. The orebody ran N-S, was 2-16
feet wide, and dipped east. The length of the lode at the 160 ft level
was 45 ft and a level at 225 ft may have lost the lode. Dillon extracted
712 tons of ore which was carted to the railway line at Hermidale and
presumably sold to 'Cobar Smelters'. Up to six men worked at the
mine when owned by Dillon. A plan of the mine was made by the
manager of the Budgery Mine. The orebody worked is secondary and is recorded as pipe-like.
About 220 t of copper was produced from ore which assayed up to 12.5% Cu
and 47 g/t Ag, and averaged 8.7% Cu.
Selected samples have returned values up to 35.75% Cu, 62.3 g/t
Ag, 15.2 g/t Au, 2,500 ppm Pb and 4,800 ppm Zn.
In 1912 (Ann. Rep. for 1912, Andrews 1915) the depth was 68.5 m
and neither the water table nor primary ore had then been reached.
The mine was forced to close in 1920.
Some further small ore parcels, yielding about 50 t Cu at Port Kembla,
were later raised on a trial basis (1946,1947,1951), possibly by Mr B.
McLernon who held the lease. In 1951 Mr McLernon transferred it to
Mr T. Kelly and his partner Mr. J. Whyte. In 1951 some 19
tons of heavy sulphide ore were raised. The workings were by then 300 ft
deep, with 7-10 shafts sunk.
The primary ore is assumed to be similar to that in the Budgerygar mine
and a Utah diamond drill hole, about 150 m northwest of the Budgerygar
North workings, intersected pyritic ore layers in schist (Smith, 1971).
The irregular diffuse layers contained bands of up to 40% pyrite, of
grain size up to 0.5 mm.
Budgerygar Mine
The
Budgerygar mine was commenced in 1906 or 1907 by shaft-sinking to cut a
strongly outcropping east-dipping gossan.
By 1909 the shaft was 360 ft deep (MR1133). A cross-cut at 62 m revealed minor copper sulphide enrichment
(0.3% Cu) in the mineralized zone. Below
75 m the primary sulphide zone was encountered and exploratory
development was carried down to 110 m depth.
About 100 t Cu was produced, apprently from a pipe-like shoot.
The
pyritic zone is 20-30 m wide and contains numerous irregular but parallel
layers of pyritic ore, evenly distributed throughout the entire width and
comprising about 20% of the whole. In
each ore layer there are thin bands of densely disseminated pyrite;
the concentrations being quite conformable with the contorted
metamorphic layering. The bands of
dense pyrite are up to 1 cm thick and may comprise up to half of the rock
volume in each ore layer. The
copper content of the pyritic ore is very low, about 0.8% Cu.
Utah drill hole DDH BD1 (Smith, 1971) intersected the mineralized zone
below the old workings as 17.2 m of 0.41% Cu.
Over a 19 m interval pyrite makes up 25% of the core.
Some sections are up to 60% pyrite, which has a grain size of up to 3 mm.
Chalcopyrite is noted only as minute blebs within pyrite.
The
Budgerygar mine development in the pyritic ore zone did not encounter economic
copper values. However, a
significant tonnage of copper ore was raised, averaging 5-7% Cu, presumably
from the supergene zone. The
location and orientation of the secondary orebody is uncertain but it has been
inferred to be pipe-like (McClatchie, 1985).
Tritton
Mine
The
Tritton copper mine is owned and operated by Straits Resources.
The Tritton underground mine is on a blind ore body but close to old mine
workings.
The
Tritton deposit was discovered in 1995 after a SiroTEM
survey over and near the historical Budgerygar and Bonnie Dundee copper-gold
mines, 800 m north of Tritton, indicated significant conductors beneath the
old workings. The survey was extended to the south and a similar sized anomaly
detected over the now known Tritton deposit. The top of the Tritton
deposit is approximately 180m below surface and early holes tested above this
and failed to intersect significant mineralisation. Down hole EM was used
routinely and off hole anomalies were recognized. Intensified drilling
around the anomalies defined the deposit.
Cross section at
Tritton mine after Fogarty (2001)
Massive
sulphide presence was discovered using systematic moving loop surface TEM
surveys. The use of this technique was based on previous experience of
its usefulness around the Girilambone copper mine. The deposit consists
of upper and lower zones or lenses, both highly conducting. The
lower lens was not discovered from the surface but rather through the use of
downhole EM techniques. The upper ore zone (UOZ) lies below 160 m and
the lower ore zone (LOZ) was met below 400 m. Both zones have EM time
constants of about 10 ms. Apart from having a minor associated magnetic
anomaly (of which there are numerous in the Girilambone belt) the Tritton
deposit was not easily detected from the surface by any geological,
geochemical or geophysical technique. The Tritton deposit is
similar to the Budgerygar and Bonnie Dunnie mineralisation, which being
evident above 60m depth is readily detectable by a variety of geochemical and
geophysical techniques - and also had been found and sunk in by early
prospectors. Collins (2001) states that the Tritton and Budgerygar
deposits form an interesting test site for the development of new geophysical
technology, due to the similarity in their form but difference in depth and
hence detectability. Thus a number of geophysical techniques have been
tested over the Tritton deposit since its discovery to see if they can detect
it.
The
Tritton mineralisation is in two zones, the upper (UOZ) and lower ore zone (LOZ)
[some descriptions state there is also a 'central' zone]. Each is
approximately 400m long, with strike at 028°T, dip to the east at 20 to 70°
and pitch towards 130°T. The UOZ is not weathered and is hosted within
quartzite and minor schist. The ore body varies from massive banded pyrite and
chalcopyrite to bands of sulphide laminated with silicified schist. The
UOZ contains the highest chalcopyrite:pyrite ratios, the highest gold and
silver values, minor bornite and tennantite and more numerous lenses of
hematite+magnetite+silica alteration. The LOZ occurs as massive
and banded pyrite+chalcopyrite lenses in chloritic semipelitic schist,
immediately overlying carbonate+epidote+magnetite altered mafic schist
(Fogarty, 1998) [other descriptions have stated that the lower ore is not
immediately above the mafic schist but within in]. The high grade
massive sulphide ore has been described as occurring in "pipe-like"
bodies. Narrow, sub-vertical mafic dykes cut the orebody.
The Tritton ore body is controlled by both fault and fold structures.
Deposit scale controls include domains defined by major (district scale)
steeply dipping northwest trending faults. These faults are responsible
for the overall NW-SE trend of the deposit. The overall south east plunge of
the ore body is controlled by the east to east southeast dip of bedding and a
bedding parallel fault which lies between the north-west striking faults.
The
Tritton ore body is hosted within a north-east trending, south- east dipping,
bedding parallel structure. Two sub-parallel structures have been mapped
on surface - the Budgerygar and Bonnie Dundee lodes. These lodes have
strike lengths in excess of 800m, and each has been the focus of intense
silica-sericite alteration. Sulphide mineralisation is found along the
entire length of these structures but is concentrated in the high grade
shoots.
The
mineralisation is high (with maximum values within the ore zone as defined by
the 2.5% Cu cutoff) in Cu (32%), Au (9.9g/t), Ag (470ppm), Zn (2.3%), Pb
(1400ppm), Ni(110ppm), Co (1755ppm), Sb (3.2%), As (1700ppm) and Bi
(27ppm). In addition analysis of the Tritton Assay Standard, composited
from ore on the 5085 to 5010 levels returned elevated trace elements including
Hg (6.5ppm), In (6.23ppm), Se (157ppm), Sn (35ppm), Cd (181ppm) and Tl (5ppm).
A
primary sulphide resource containing 10.25 million tonnes at 3.01 % copper
(0.31 Mt Cu) was estimated; or a feasibly mineable 14Mt at 2.7% Cu, 0.3g/t Au
and 12g/t Ag at a 1% Cu cutoff (0.38 Mt Cu ) was outlined based on 80,000m of
drilling in 241 drill holes (but cf. 4.6 Mt at 1.9% Cu announced by Tritton
Resources in 2003). Drilling density was increased to
approximately 40m by 40m in the UOZ and less (but at least 80m by 80m) in the
LOZ. The resource was modelled for mine design as a regular,
tabular, easterly dipping sulphide permeated zone with a blank between the UOZ)
and LOZ.
The
mine commenced in 2004 with the development of the access decline, and stope
production commenced in March 2005.
The
mining found the ore 'beds' to have strong mesoscopic folding, so that the two
orebodies followed proved very irregular, with considerable variations in
thickness, strike length, dip and grade distribution within the two ore
zones. This is not surprising sine the 'folded' nature of
the ore from the 'Budgerigar mines' had been well known
previously.
In
its first year of production, Tritton mine yielded 23,088 t of copper in
concentrates and it aimed to produce approximately
25,000 t of copper, 7,000 ounces of gold and 100,000 ounces of silver per
year. Production ceased during the global downturn but was recommenced
and in the 12 months to June 2009 the mine yielded 24,111 tonnes copper
concentrate.
Tritton
operates a conventional crushing and flotation plant, with capacity for
producing duces
approximately 100,000 tonnes of concentrate per year at a grade of 25% Cu,
2g/t Au and 30g/t Ag.
5. Great Hermidale Area
A number
of shafts exist around the Great Hermidale mine, about 5 km
south of the
Budgerygar mine. Little is known
about these workings, or the geology of the area, which is characterised by a
pronounced aeromagnetic anomaly. The
deposits in the Great Hermidale Area are:
No.
Deposit Name Commodity
82
Unnamed
?Au
83
Unnamed
?Au (Pb)
84
Great Hermidale Au, Cu
85
Great Hermidale East Au
Massive
gossanous ironstone crops out, in bodies up to 3 m wide and 100 m
long. Minor quartz-limonite
manganiferous breccia zones are present. Laminated
mica schist on the dumps contains abundant iron oxide pseudomorphs after
pyrite. The workings of the Great
Hermidale East are within interpreted sheared acid volcanics with stratabound
pyrite (McClatchie, 1985). The
mineralized zone appears to be very steeply dipping.
Assays from the Great Hermidale area indicate widespread trace
mineralization, with values up to 3.1 g/t Au, 4.6 g/t Ag, 3.62% Cu,
1100 ppm Pb and 650 ppm Zn.
6. Budgery Mine Area
This
southernmost mineral area along the Girilambone Belt contains the main Budgery
mine and various other shafts for which there is record has been compiled:
No.
Deposit Name
Commodities
206
Budgery mine
Cu, Au
The area encompasses the nominal North Budgery, South Budgery, South Budgery
Extended, Budgery Reward, Budgery Wattle and other mining properties, many of
which were regarded as purely speculative holdings (Danvers Power 1907, Carne
1908). In all there are about 24
shafts, for most of which there is little or no record.
Only a few shafts and pits in the immediate vicinity of the Budgery
main shaft are thought to have yielded any return.
Some of the other workings, however, do contain gold values, and these
range up to 13.2 g/t Au (McClatchie, 1985).
The gold traces range as far as 1 km west (e.g.6.49 g/t Au in gossan
400 m north of Rocky View homestead) (Budgery Reward?).
Carne (1908, p 247) stated that none other than Budgery mine
encountered copper but Danvers Power (1907) noted that a separate copper body
was discovered at the Budgery Reward. The
Budgery Reward lies west of the Budgery mine on old sketch maps but the
precise location is no longer known. Department
of Mines assay cards (1909/1791 and 1909/1792) also record good Cu, Au and Ag
values for samples collected by J.E. Carne from the "Budgery North
Mine" (North Budgery shaft, or possibly an error for Budgerygar North?).
Budgery Mine
The
Budgery mine was initially prospected and worked for gold, on a small scale,
in 1886-1895. Gold was early found
in the Red shaft, Gidding's shaft and other nearby workings where dump samples
have assayed up to 51.6 g/t Au (McClatchie, 1985) (figure xx).
There was probably only minor production (e.g. 62 g Au in 1895).
The gossans contain 1-4 g/t Au in places and early spot assays
range up to 18.1 g/t Au and 5.3 g/t Ag.
The early gold production and prospecting probably followed quartz
veins within the gossanous areas. Gold
in a small quartz leader, followed for about 6 m, is referred to by
Andrews (1915, p 112). Native
copper in the form of thin leaves was also met with in the oxidized zone but
the presence of the main copper lode went undetected until workings reached
the level of supergene enrichment.
The near-surface gold of initial interest at Budgery mine appears to be
fracture-related and is perhaps remobilized from underlying mineralization.
Drilling results have defined a 2-5 m wide gold zone trending east
through Gidding's shaft and the main Budgery shaft (figure xx).
It includes limonite-impregnated schist with up to 51.6 g/t Au, and
less ironstained schist with 10.5 g/t.
It was presumably the source of auriferous ore parcels grading 6.4 g/t
(1895) and 7.5 g/t (1915, 1923). Gold
grades are very patchy and few significant intersections have been made.
The best intersection is 4 m at 8.8 g/t Au but tonnage is
indefinable and appears to be small (McClatchie, 1985).
The copper orebody was discovered in 1905 fifty metres east of Red
shaft. It is a pipe-like secondary
orebody situated perhaps within a susidiary fold or fault trough on a second
generation anticline (figure xx). The
hostrock deformation is believed to be complex and incompletely understood.
The orebody has yielded about 1486 t of copper.
It is about 25 m in diameter (over 30 m in horizontal long axis)
and plunges about 45o south-southeast. It
was mined from various working levels to a depth of 125 m, producing
approximately 1,350 t Cu. As with
other mines of the southern Girilambone belt which depended upon the Cobar
smelters for their ore market, mine closure was forced by cessation Cobar
smelting in 1919-1920. The upper
stopes at Budgery were later reconditioned (1950-1952) but re-opening of the
mine did not go ahead. Since 1929
intermittent leaching was carried on in the Budgery mine over many years,
utilising broken ore stope filling. The
leaching program, capable of yielding up to 43 t Cu per annum, was hindered by
collapse of the main shaft in 1963.
The rock types within the mined pipe are generally not well recorded.
However, gangue of the secondary orebody was noted as a magnesian slate
which caused some difficulty at the smelters (Carne, 1908).
This "magnesian slate" record confirms that foliated mafic
metavolcanics, known from drilling, are the probable hostrocks for at least
the upper part of the orebody. Dark
green chloritic rocks encountered in drilling at and near the copper orebody,
and considered to be of mafic volcanic parentage, contain up to 700 ppm
Ni and mostly have nickel in the 100-200 ppm range.
Minor talc and actinolite-rich zones occur in these rocks.
The
pipelike copper orebody dips south-southeasterly at 45o.
It contains a 20 m vertical interval in which rich ores, both
supergene sulphides and oxide masses, were found above the water table.
The supergene accumulation is possibly located in a synclinal warp but
structural details are very uncertain. Above
the zone of enrichment the gossan was rather barren, except for small patches
of carbonate ore. Most of the
production was from the enriched zone, mined between the 55 m and 79 m
levels. Ore parcels from here
assayed up to 14% Cu and averaged about 8% Cu.
Ore averaging below 3.5% Cu was apparently left unextracted.
The richest ore, with picked samples assaying up to 63% Cu, was from
the top of the supergene zone at around 60 m depth.
Below 80 m, some oxidized ore is still present, with up to 3%
native copper in places, but it is not in large quantities.
Deeper than 80 m the ore grades fell to generally below what would
repay transport to the smelters. To
raise and transport ore averaging 4% Cu was not then considered profitable.
The
primary pyritic ore was first noted at 76 m depth.
It there contained 3.5% Cu and the general copper content declined
further to 2% by 95 m depth. Pyritic
ore (1% Cu) is still present down-plunge at 122 m level, suggesting a
wide pyritic zone if the overall setting was originally stratiform.
Strongly pyritic intervals of at least 8 m thickness are present.
Andrews (1915) noted the pyritic ore to be similar to the Bonnie Dundee
area pyritic ores, in which concentration of disseminated pyrite replicates
the contorted layering or foliation typical of the area.
Discussion of Girilambone Belt Mineralization
The
Girilambone belt copper deposits have often been generalized as
characteristically stratiform copper mineralization associated with probable
mafic volcanics. There is little
firm evidence to support this and such generalization may be invalid.
Regionally, the Girilambone Group consists of quartzo-felspathic
schist, phyllite, greywacke, slate, quartzite and minor rocks interpreted as
metamorphosed volcanics. The
latter lack definate features, like lava pillows, such as are known further
northwest (Mt Dijou Volcanics, Bourke 1:250,000 sheet).
The suspected mafic metavolcanic of the Girilambone belt could in some
cases be altered intrusive rocks. The
copper ores are associated more often with the most siliceous units in the
Caro Schist than with suspected metavolcanics, although the latter may
certainly be mineralized. As
regards the mineralization being stratiform there is little evidence for this
and most the the major deposits known appear to be elongated down pitch.
The Budgery deposit is particularly well established as pipe-like.
Pyritic zones nonetheless do appear to be stratabound and are feasibly
the host rock to later epigenetic mineralization which enriched them in other
metals. Late stage minor
remobilization of metals into transgressive structures is suspected.
Despite
reservation as to the submarine exhalative origin of their copper content, the
general geological setting and the likely stratabound nature of the pyritic
zones does favour a deep sea volcanogenic origin for the Girilambone belt ore
deposits. Or at least this is what
earlier researchers thought. Comparison has been made
with Cyprus or Besshi type deposits. Utah
Development Co. (e.g. Smith 1971, 1973, 1976) during long-term exploration of
the Girilambonie belt suggested that metals were
derived from basic volcanism and deposited within oceanic sediments, some of
which are chemical precipitates. Similarity
was noted between the Girilambone belt deposits and the Besshi type deposits
of Japan by Gilligan (1978). It
has also been suggested (Smith 1973, 1976; Suppel 1975) that Girilambone belt
cupriferous pyrite deposits are a volcanic exhalative type similar to the
Cyprus type deposits. Sawkins
(1976) described the Besshi deposit in Japan in the following terms -
"... a thin tabular ore body consisting mainly of massive pyrite and
subordinate chalcopyrite occurs in the Sanbagawa metamorphic terrain in mafic
schists intercalated with peletic schists and minor quartzose schists
(metamorphosed cherty beds?)". The
overall composition of the ores is the same as for Cyprus-type deposits but
their environment is characterized by a thick sequence of greywackes and much
less mafic volanics. The
Girilambone belt deposits are comparable to the Besshi type, although massive
pyrite is not as abundant. At
Girilambone Mine, Utah named the main quartzite unit the Pink Quartzite Member
and viewed it as a metamorphosed chemical sediment.
However, subsequent microscopic examination of grab samples has
revealed trails of heavy minerals grains, of which many have rounded shapes
indicative of sedimentary transport. This
suggests a normal clastic origin for the Pink Quartzite facies.
The
pyritic zones show features on a mesoscopic scale, and by correlation between
drill holes, which suggest they are stratabound or stratiform.
The deposits and prospects can also be grouped in horizons and
formations so that they appear to have stratigraphic restriction which extends
over long distance along strike. Where
this has been done between the Budgery and Bonnie Dundee mines it is
interpreted that the various mineralized horizons aggregate to no more than
100 m in thickness, and that most occurrences fall in a 10-30 m wide
pyritic zone extending many kilometres (McClatchie 1985).
Nonetheless, the continuity of this zone between known deposits is an
interpretation from sparse outcrop and requires to be further tested by
drilling.
Features
not supportive of a stratiform syngenetic origin include the pipe-like form of
copper orebodies and the apparent concentration of ore at relatively young
structural features. If further
work confirms the presence of very extensive pyritic zones containing sporadic
Cu/Zn/Au/Ag shoots then these metals may have been added epigenetically.
Widespread zones of syngenetic pyrite may have presented favourable
host situations.
Considering
other possibilities, there is no evidence of deposits being related to the
basic and ultrabasic intrusions which occur in their vicinity.
Except for gold the Girilambone belt mineralization shows little
relationship to post-metamorphic faults and shears.
Unlike the Cobar and Canbelego belts, the more prominent faults appear
more to truncate primary orebodies than to have acted as ore fluid channelways.
The second category of orebody in the Girilambone belt, the
transgressive or stockwork deposits with gold and lead enrichment, are
hypothesized to carry metals remobilized from underlying stratabound deposits.
There is some evidence for this at Budgery and Bonnie Dundee mines
concerning the distal expression of the envisaged process.
Physical connection to an underlying stratabound body has not been
described. However the stratabound
mineralization itself is known commonly to be cut by quartz veins which carry
very small amounts of remobilized metals.
Perhaps indicative of incipient remobilization is the fact that
chalcopyrite and sphalerite are more abundant at the Girilambone mine near the
tops of the moderately dipping orebodies, giving primary zone contents of
exceptionally up to 10% Cu and 2-3% Zn. Australian
Selection Pty Ltd (1982) considered that there has been considerable
remobilization within the stratiform ores. However, upwards increase in copper and zinc can be explained other
than by remobilization. Such
zoning is common for stratiform volcanogenic deposits, and at the NE prospect
zinc also increases stratigraphically upwards in hole DDH NGL-22 where the
sequence is logged as overturned (Australian Selection Pty Ltd, 1976; MacDonald, 1984).
More recently the possibity of syngenesis has rarely been
mentioned. Straits Resources and Tritton Resources geologists have
determined a paragenesis at Tritton suggestive a 'standard' cooling sequence
for mineralized fluids - being an overall change from quartz+oxide
(magnetite/hematite), to sulphide minerals (pyrite/pyrrhotite → copper
sulphides → lead+zinc sulphides), thence to later carbonate deposition
in places. This they believe is closely comparable to that
documented in intrusion related copper-gold systems throughout the south west
Pacific (e.g. Corbett and Leach, 1998). A contract study on
the ore by Leach reinforced this. This inferred timing sequence is
also closely comparable to that described from the Cobar area (Stegman,
2001). A magmatic component to the
Cobar
belt mineralisation has been previously excluded for the Cobar deposits
by various geologists, based on a multitude of criteria, including differences
in age dates between intrusions in the region versus the dating of deformation
and mineralisation events, and also on on isotopic evidence.
Nonetheless, the Tritton geologists concluded that "the many similarities
in alteration and mineralisation between intrusion related deposits and the
Tritton deposit, the high selenium and Se:Te ratios and the close proximity to
syn-deformation granite intrusions make a felsic-intrusion source to the
mineralisation at Tritton compelling" (Mike Erceg and Bruce Hooper of
Tritton Resources; information given to Sydney exploration geologists
group). Such conclusion of an epigenetic origin has come a long
way from the various ideas that dominated in the 1970s, which envisaged a
seafloor origin for the deposits of Girilambone type.
On the other
hand, however, Downes (2008) reported that Tritton Sulphur isotope values
range between 4.5 and 14.2 δ34S‰ (mean 10.0 δ34S‰) and support
the interpretation that sulphur was sourced from a fractionated seawater
sulphate reservoir, interpreted to be the host sequence. Also, at
least one exploration company, Helix Resources, appears to still adhere to the
older VMS/VHMS ideas for Girilambone type copper deposits which were formerly
dominant. In 2010, Helix stated that "The Quanda and Five
Ways Projects consist of two exploration licences covering the potential
southern extensions of the Girilambone-Tritton-Budgery mineralised trend. The
tenements cover an area of approximately 1100 square kilometres, with variable
cover. The Company has identified a series of magnetic trends that are
interpreted to represent mafic bodies under and intruding Girilambone Basin
sediments. These mafic bodies are considered critical features in the
mineralisation model for the region and initial exploration will be focussed
proximal to these features. In 1Q10 a 100m line spaced aeromagnetic
survey is being flown in conjunction with regional geochemical surveys and
historical prospect assessment. ELA 3900 and ELA 3901 cover an
additional 430 square kilometres of prospective Girilambone Basin sediments.
The applications cover additional areas over basal sequences of the
Girilambone Basin where VMS/VHMS systems may be present" ( http://www.helix.net.au/index.php?page=44
).
Thus
fundamental differences of opinion/theory still exist for these
deposits. Also, as
can be seen noted in the description of the Tritton deposit, different sources
have stated different things - i,e, two or three ore zones, the lower ore lying
immediately over the mafic schist or lying within it, etc.
Virtually all the detailed exploration reports of the modern phase since the
revitalisation of mining at Girilambone have remained
confidential. Hence it is not yet possible to resolve such
discrepancies.
REFERENCES
Click
here for list of Cobar 1:250,000 sheet references
