Cobar Superbasin - Mine locations and structural elements based on mapping and synthensis by the Geological Survey
(R.A.Glen and others; with later simplification and emphasis [e.g. limestones] by Vladimir David. The three main
mineral or mining belts of the Cobar 1:250,000 sheet area are the Cobar belt (Elura south to Nymagee),
Canbelego belt (Mt Boppy to Pipeline Ridge) and the Girilambone Belt (Girilambone through Tritton).
THE CANBELEGO BELT - geology and mines
The Canbelego belt is the name given to an elongate north-northwest trending
concentration of deposits traceable fully across the Nymagee 1:250,000 sheet
and across the southern half of the Cobar 1:250,000 sheet (figure xx).
This belt of mineral deposits extending south from Canbelego towards
Mineral Hill, via Bobadah has been long recognized.
It has been referred to by a variety of names (e.g.
Canbelego-Bobadah-Melrose Belt, Metals Exploration Ltd 1982).
The belt is about 160 km long.
It appears to terminate 20 km north of Canbelego but this is very
uncertain due to poor outcrop. The
early documentation on geology and mining of the Canbelego belt is largely
that of Andrews (1913a, 1915b,). Carne
(1899, 1908) and Jaquet (1905, 1906) have also provided early notes.
Fail (1976) and Felton (1976, 1981) have done extensive mapping
synthesis and stratigraphic studies, whilst Gilligan (1975-1978, 1982-1983)
has paid particular attention to the mines around Canbelego.
In addition to these workers, numerous company geologists have examined
the Canbelego belt.
The Canbelego belt is the next most intensely explored tract in
the Cobar sheet area following the Cobar belt itself.
Exploration companies which have worked along the Canbelego belt
include Amad NL and Kenneth McMahon and Partners Pty Ltd (1970), Amoco
Minerals Australia Co (1980), Amoco Minerals Australia Co (1980-1983),
Anaconda Aust Inc and Quality Earths Pty Ltd (1965), BHP Minerals Ltd
(1985-1987), Cominco Explorations Pty Ltd (1975), Cyprus Minerals Australia Co
and Arimco NL (1990), Cyprus Minerals Australia Co and Homestake Australia Ltd
(1987), Cyprus Mines Corporation (1969-1972, 1974), Cyprus Mines Corp and
Heinrichs Geoexploration Aust. Pty Ltd (1969-1970), Dowmill Pty Ltd and
Freeport of Aust Inc. (1986), Dowmill Pty Ltd and Gold Fields Exploration Pty
Ltd (1988-1989), Dowmill Pty Ltd et al. (1984, 1989), Duval Mining Aust. Ltd
(1986), Epoch Minerals Exploration NL (1987), Epoch Minerals Exploration NL
and Mt Boppy Minerals Pty Ltd (1985), Epoch Mining NL and Sons of Gwalia NL
(1989-1990), Epoch Mining NL (1987-1988), Esso Exploration and Production
Australia Inc (1976), Freeport of Australia Inc. (1986), Le Nickel (Australia)
Exploration Pty Ltd (1975), Getty Oil Development Co Ltd (1980), Pacific
Copper Ltd (1983), Homestake Australia Ltd and Amoco Minerals Australia Co
(1985), International Mining Corporation NL (1990), Mines Exploration Pty Ltd
(1976), Le Nickel (Aust) Exploration Pty Ltd (1974-1975, 1977), Metals
Exploration Ltd (1982, 1984), Mines Exploration Pty Ltd (1974-1977, 1979),
Mines Exploration Pty Ltd and Cobar Mines Pty Ltd (1971), Nickel Mines Ltd
(1971), Pacific Copper Ltd (1981, 1983, 1984), Pacific Copper Ltd and Jones
Mining Ltd (1984), Penarroya (Australia) Pty Ltd (1978-1979, 1982), Penarroya
(Australia) Pty Ltd and Le Nickel (Aust) Exploration Pty Ltd (1978), Pennzoil
of Australia Ltd (1973-1974), and Sulphide Exploration et al. (1981).
There are significant differences in mapping and stratigraphic
interpretation evident between various workers.
Successive companies in the same area have differed in their
interpretations of the weathered strata, Devonian versus basement, even when
utilizing the same drill cores; and the Canbelego district in particular
requires further detailed mapping to resolve the existing conflicts.
The Canbelego mineral field is the main mining centre along the
Canbelego belt north of the Bobadah and Mineral Hill fields.
It has been a significant producer of copper and gold.
There are dozens of old workings and gold prospecting sites in the
immediate Canbelego area. Copper
has been produced from a group of mines several kilometres further south.
The Canbelego field extends from Boppy Mountain in the north to the
Mount Boppy South Copper Mine in the south (Fig. CB).
The discovery and early history of the field is covered in detail in
Andrews (1915).
The Canbelego belt of mineral deposits is, like the Cobar belt,
strongly associated with strata of Early Devonian age but mineralization also
occurs in basement rock. A
significant distinction of the Canbelego belt is the abundance of volcanics.
The possible genetic connection between volcanism and mineralization
within this belt has been subject of much consideration.
The deposits of the Canbelego belt assume a variety of disseminated,
dipping tabular or fold-like forms. No
carrot-like, cigar-like, pipe-like or lenticular plunging ore shoots of great
persistence are suspected on current knowledge; and this might present a
contrast with the Cobar belt.
The basement rocks comprise phyllites, quartz mica schists,
slates and micaceous sandstones referred to the Girilambone Group.
They exhibit strong deformation and low grade regional metamorphism.
Mafic metavolcanics occur, as at the Canbelego Copper and Shango mines.
Overlying the Girilambone Group, in a number of tight structures within
the Mineral Hill Synclinorial Zone, is the Early Devonian Baledmund Formation.
This contains siltstone, sparse carbonate rocks, sandstone,
conglomerate and felsic volcanics. The
latter include dacite and rhyolite lavas, ash flow tuffs and minor chert.
Palaeontological evidence indicates that these rocks may be correlated
with the Cobar Supergroup (Sherwin 1973, 1974, 1975 and in Fail 1976).
The regional geology is shown in simplified form in Fig. CB.
The general Early Devonian setting is envisaged as clusters of emergent
volcanic islands separated by shallow marine basins.
The volcanics at Canbelego and Babinda are interpreted as piles
representing separate eruptive centres. The
volcanics of the Canbelego pile (Florida Volcanics) are about 1000 m thick.
The synclinal area between the piles is interpreted as a shallow basin
(Kopyje basin), and another basin is postulated further north (Canbelego
basin). These are structural
basins and perhaps also depositional basins.
Fail (1976) and Felton (1981) discuss the palaeogeography.
The structural basins do not show symmetry of deposits across their
axes, and are likely truncated in the east.
In particular, basal conglomeratic and calcareous facies widespread in
their western parts are not known to be repeated along the eastern sides of
the basins.
The chief host rocks of mineralization along
the Canbelego belt are the Early Devonian strata (Kopyje Group) of the Mineral
Hill Synclinorial Zone but mineralization also occurs in the basement rocks of
inferred Cambro-Ordovician age. Such
basement mineralization may occur in proximity to mineralization within the
Devonian rocks. Gilligan (1982)
suggested that in some cases the basement mineralization has been remobilized
downwards out of the Baledmund Formation.
The structure of the Kopyje Group rocks, especially in the Kopyje-Pipeline
Ridge area, has been interpreted by some as a regional half-graben bounded on
the east by growth faulting during the Early Devonian.
It is referred to by Leeson (1980) as the "Mineral Hill Volcanic
Rift".
The
eastern edge of the Canbelego belt can in part be delineated as the Coonara
fault (Fig. CB), which bounds the Mineral Hill Synclinorial Zone.
The axis of the Canbelego basin appears to be cut in the south by a
swing in the Coonara fault, which may also have cut off much of the eastern
side of the basin. To some extent
the same may apply to the Kopyje basin, with truncation on the eastern limb
and in the south. Synclinorium
elements are smoothly truncated in the east but extend in the west as
northwest to north-northwesterly trending spurs.
The Canbelego belt includes basement mineralization between these
spurs. Where the basement is
mineralized there tends to be appreciable shearing and a somewhat higher
metamorphic grade is suspected (Felton, 1981).
Basic metavolcanics in the basement are favoured host rocks for copper.
The history of
mining in the Canbelego belt is focussed strongly on the Canbelego field, and
in particular upon the Mount Boppy Gold Mine which was for many years the
premier gold mine in New South Wales. The
village of Canbelego, which has now largely faded away, owed its existence to
the Mt. Boppy mine.
Both gold and copper have been produced in the Canbelego belt.
Unlike the Cobar belt or the Girilambone belt, none of the early small
gold vein sites passed into copper lodes at depth.
Both copper and gold were mined or prospected at many small mines along
the belt. After the closure of the
Mount Boppy gold mine, and the dwindling of Canbelego's population, there was
little underground work carried on in the belt's smaller mines.
At a few, however, there was some very small scale subsequent copper
recovery by precipitation. Many of
the gold shows lacked permanence at depth, and some near-surface enhancement
of gold values by supergene and/or residual enrichment is assumed.
In the Canbelego mineral field early gold prospect assays ranged up to
400 g/t Au (Birthday and Boppy Boulder mines) and rich spot values (e.g. 1.2
kg/t Au) were obtained at the West Boppy mine.
Near surface enrichment is inferred for such high values and most cut
out above 12 m depth. Surficial
enrichment, to 180 g/t Au, is also recorded at the Mount Boppy lode.
A significant producer from presumed near-surface enrichment was the
Canbelego King mine, which yielded considerable gold at good grades above the
depth of 13 m. In contrast to the
lesser gold mines, the Mt. Boppy lode proved "permanent" and ceases
at depth for structural reasons.
In total there are 11 mineral areas within the Cobar sheet
portion of the Canbelego belt. These
areas are listed below from north to south.
1. Florida Area
2. Boppy Mount Area
3. Central Canbelego Area
4. East Canbelego Area
5. South Canbelego Area
6. Burra-Hermitage Area
7. Buppe-Shango Area
8. Kopyje Area
9. Mt Lewis Area
10.
Hartwood Gap Area
11.
Babinda Area
1.
Florida Area
This is a
small area extending to 13 km north of Canbelego, for which the poorly
outcropping gold mineralization has been suspected as locally
similar to the Mt Boppy Gold Mine at Canbelego, i.e. hosted in formerly
calcareous beds near the base of the Devonian sequence, now much brecciated
and silicified. This
interpretation fits what is known but the evidence is more meagre than is the
case for mineralization further south in the Canbelego belt.
An area with suspected small infaulted remnants of Devonian strata has
been outlined (Hayes, 1983) but the suspected Devonian outliers remain
unmapped on an individual basis. Also
of some interest is an equant magnetic anomaly at the railway west of Coonara
homestead, designated C9 by successive explorers (Felton et al., 1975).
This anomaly probably represents the presence of intrusive rock.
Weathered bedrock in the C9 area is slightly anomalous in lead and
zinc, and there are unconfirmed reports of gold in the vicinity.
Drilling in the C9 area has intersected weathered feldspar prophyry and
suspected more mafic rocks (weathered form only).
Although metal values deserving of prospect status have not been
obtained at C9, the combination of features reported does warrant continued
investigation of the anomaly.
The
Florida area contains a number of sites with old workings, of which the
following are prospective (figure xx):
No.
Deposit Name
Commodities
78 The 4Ms - McGuinness & Party
Au (Ag, Pb, Zn)
151
Langbein prospect
Au? (Cu, Pb, Zn)
The 4Ms (McGuinness & Party)
Prospect
The 4Ms
prospect is named after McGuinness & Party (GL12 by H.C. & T.
McGuinness, L. Mourin, C. Murray - the four "M"s).
The main site is situated at the southern end of a meridional zone of
generally shallow prospectors' workings, developed for the most part in
1932-1935. This zone is about 800 m
long and 10-30 m wide. Shallow
workings along the northern end are on weathered iron and manganese stained
sediments with minor quartz veining. Outcrop
is poor and the disposition of the mineralization is unclear.
Near the southern end workings are deeper, to 40 m, and a steeply
dipping pyrite-rich quartz-breccia lode is prominent.
The main
workings are postulated to lie along the brecciated contact of cover and
basement rocks. This is suggested by underground records and the presence of
both rock types on the main shaft dump. The
mineralization, in breccia and lensoidal quartz vein networks, is thought to
be predominantly within the Devonian rocks.
Former presence of the Devonian basal calcareous facies is suggested by
petrographic description of "calcsilicate skarn" (McClatchie, 1981b)
and rare fossil impressions in dump material.
Variably silicified crinoidal and (?) coralline siltstone has been
reported both from outcrop and mine workings.
Limestone breccia has been noted on the main dump (McClatchie, 1981b).
The formerly limy rocks, however, are by no means abundant.
Samples
from surface and shallow pits show values up to 328 ppm Cu, 2140 ppm Pb,
2100 ppm Zn, 883 ppm Ba, 57.8 ppm Au, 9.9 ppm Ag.
At a depth of 8.2 m in the main shaft the lode was 9.14 m
wide. Reported Au values
encountered by prospecting ranged from 5 g/t to 16.8 g/t Au, with
the average grade of the lode apparently in the range 5-10 g/t.
Later assays are as high as 57.8 ppm Au, 9.9 ppm Ag,
145 ppm Cu, 1000 ppm Pb and 900 ppm Zn from
siliceous outcrop of the lode (Gilligan, 1978).
Although grades of 5-10 g/t Au occur near surface these
appear not to persist at depth, suggesting that supergene enrichment has
occurred. Percussion drilling
beneath the workings obtained intersections of up to 5m of auriferous stone
but grade was low, (e.g. 1.55 g/t Au, Pacific Copper Ltd, 1984).
Near the
main workings the zone contains three gold shoots along a strike length of 250 m,
and these appear to be associated with the contact between Devonian strata and
basement (Pacific Copper Ltd, 1984). The
main shaft dump contains both fossiliferous fine grained Devonian sandstone
and Girilambone Group puckered phyllite. All
three pods of mineralization intersected carry significant gold:
e.g. 2.5 m at 2.7 g/t Au
(pod A), 4.5 m at 2.8 g/t Au (pod B), 12 m at 4.0 g/t Au
(pod C). The highest value
obtained is 1 m at 14.56 g/t in pod A.
These gold values appear to be restricted to above 30 m and to be
in west-dipping manganiferous siliceous intervals within siltstone.
The pods at surface are 20-35 m long and 5-9 m in maximum
width.
A possible
total for ore contained in the three delineated pods or shoots was estimated
by L. McClatchie for Pacific Copper Ltd. in 1982 as 77,000 t at 4-5 g/t Au.
2-5 General Canbelego Area
2. Boppy
Mount Area
The Boppy
Mount area contains a group of small gold deposits, from which production has
been very minor. As with the
central deposits at Canbelego, those in the Boppy Mount area are prospective
for gold and the mineralization is associated with brecciation close to the
junction of basement and Devonian cover rocks.
The
prospects in the Boppy Mount area are:
No.
Deposit name
Commodities
154
Venturer shaft
Au (Pb)
155
N. Boppy Boulder
Au (Pb, Zn)
156
Boppy Boulder
Au (Ag, Pb)
157
Boppy Blocks
Au (Pb, Zn)
158
Hardwick prospect
Au (Au, Pb, Zn, Cu)
159
Jack Lock shaft
Au
The
Devonian host rock sequence includes conglomerates and calcareous facies with
fragmental carbonates as in the Canbelego central area.
Fail (1976) noted the presence of calcarenites at Boppy Mount.
The primary mineralization is not as strong as in the deposits of the
East and Central Canbelego areas.
Bodies of
ironstone are prominent in the Boppy Mount area.
These bodies may contain considerable manganese, and areas of manganese
oxide are enriched in cobalt and nickel (up to 2.23% Co, Card, 1898).
Although ironstones are of considerable extent at surface they are
reported from old workings to diminish rapidly with depth.
The high gold values recorded from the Boppy Mount group of deposits
are mostly from ironstone or highly ferruginized Devonian strata.
This gold is recorded as being extremely fine-grained and can be
attributed largely to supergene enrichment.
The only primary mineralization known in the Boppy Mount area is
disseminated pyrite, and quartz-pyrite veinlets.
Freeport
of Australia (1986) has mapped the Boppy Mount area, and drew attention to a
prominent north-south zone of ferruginization and brecciation over one
kilometre long (tested by the Boppy Boulder and Boppy Blocks workings), with
peripheral zones of silicification (tested by the Hardwick and the Venturer
shaft workings). The ferruginized
zone is high in arsenic, lead, zinc and antimony but mostly has low gold
values (0.1 - 0.3 g/t). The
outcrops containing higher gold values, with 10 g/t at Hardwick's and 1.4 g/t Au
in the Venturer shaft area, were interpreted by Freeport as silicified
fragmental carbonate rock of the type seen in the basal calcareous facies
elsewhere in the Canbelego structural basin.
As well as within these "silicified calcarenites", high gold
values occur in manganiferous breccias developed along the basement contact
zone. Small areas of cover rocks
occur as tight folds within the basement.
Boppy Boulder Mine
The Boppy
Boulder mine, located at a strong outcrop of ironstone on the southern fall of
Boppy Mountain, was commenced by J. Plunkett and Party in 1909.
Surface prospects reported from the ironstone were as high as 122-398 g/t
(4-13 oz/ton). Sinking,
driving and cross-cutting was undertaken by the Boppy Boulder Gold Mining Co. N.L.
till 1913. Only irregular thin
developments of auriferous quartz were met with, the best being 60 g/t
stone of limited extent. Gold
production is unknown, probably negligible.
The mine
workings comprise several shafts sunk in Devonian sediments (quartzite,
conglomerate, etc.) which appear to be tightly infolded with basement
metasediments. The Devonian
conglomerate is weakly anomalous in gold (0.3 ppm Au. 0.2 ppm Ag).
The ferruginization appears to have developed with and above the
conglomerate. The ferruginous
rocks, with breccia texture in places, are anomalous in lead and antimony,
containing up to 470 ppm Pb and 828 ppm Sb.
North Boppy Boulder
The North
Boppy Boulder workings, due east of Mt. Boppy trig, are located near the
northern end of what Freeport regarded as a single north-south zone of
ferruginization passing through the North Boppy Boulder, Boppy Boulder and
Boppy Blocks prospecting areas.
At North
Boppy Boulder the workings are developed in heavily ferruginized strata,
including basal conglomerate. Trace
metal values reach 145 ppm Cu, 1100 ppm Pb, 1100 ppm Zn,
1350 ppm As, 3.9 ppm Ag.
Boppy Blocks
The Boppy
Blocks and Hardwick workings are located 600 m to the south of Boppy
Boulder, on the western and eastern flanks respectively of an outcrop area of
basement rocks 300 m wide. On
the western flank there is a similar northerly trending ironstone zone as at
Boppy Boulder. Numerous shallow
workings are present along this zone, which is about 150 m long and 40 m
wide. The manganiferous ironstone
is weakly auriferous (0.3 ppm Au, 0.6 ppm Ag) and contains
sporadic trace metal values up to 140 ppm Cu, 500 ppm Pb,
1800 ppm Zn. As at Boppy
Boulder, the ironstone at Boppy Blocks appears to terminate close to surface.
Both Boppy Boulder and Boppy Blocks were considered by Pascoe (1987) to
lie within the same ferruginized/silicified calcareous interval near the base
of the Devonian sequence. The main
shaft passed through Devonian strata into basement metasediments.
Hardwick Prospect (West Canbelego Mine)
This is an
area of numerous pits and about eleven shafts, where H.D. Hardwick and
associates prospected extensively in the period 1939-1961.
Leases were held for gold and manganese but no production is known.
The
workings are concentrated in an area of 250 m by 30 m, on a small
rise of Devonian siltstone, sandstone and minor conglomerate.
Compared with the strong ferruginization at the Boppy Boulder-Boppy
Blocks lines of deposits, the development of Fe/Mn oxides is apparently patchy
at Hardwick's area.
Gold
values derive mostly from ironstone scree or small ironstone bodies in
fine-grained Devonian strata. Reported
gold values range up to 6 g/t Au, with one exceptional assay of 31.1 g/t Au
(Gilligan, 1978). Silver assays
average 25 g/t. Sampling at
Hardwick's area has revealed spot values up to 2100 ppm Cu, 0.25% Pb,
1.05% Zn, 5500 ppm Cu, 540 ppm Sb, and 1500 ppm Mo.
3. Central Canbelego Area
The
deposits of the Central Canbelego group lie in a small area (1.5 km
north-south x 1 km east-west) immediately adjacent to
Canbelego village on the southwest side (figure xx).
The area lacks the Devonian volcanics such as occur to the east and
south, and it also lacks the Mount Boppy Conglomerate Member which is
prominent elsewhere. The host
rocks at the Mt Boppy gold mine are believed to lie stratigraphically
below the Mt Boppy Conglomerate Member (Puce 1985a,b).
The deposits in the Central Canbelego area are:
No.
Deposit Name
Commodities
160
Mt Boppy gold mine
Au (Zn, Pb, Cu)
161
West Boppy mine
Au (Ag)
162
North Mt Boppy
Au (Pb, Zn)
163
Hoffnung shaft
Au, Zn, Pb
164
South Mt Boppy (Nesham shaft)
Au
(Ag)
The
Central Canbelego group comprises the important Mt Boppy Gold Mine, and other
workings which were developed largely in quest of extensions of the Mt. Boppy
lode. During the operation of the
Mt Boppy mine a vigorous exploration programme was carried out, particularly
in the period 1905-1910. This was
coeval with a prospecting boom along the Cobar belt and further afield, which
followed discovery of rich ore at the CSA mine.
The Mount Boppy Gold Mining Company explored its own ground and was
instrumental in the formation of the South Mount Boppy Gold Mining Co Ltd and
North Mount Boppy Gold Mining Company. The
latter two companies, both incorporated in London, sought repetitions or
extensions of the Mt Boppy lode in immediate adjoining areas.
Extensive cross-cutting and driving was carried out but failed to
locate any further payable lode material, with the possible exception of the
West Boppy lode.
Mount Boppy Gold Mine
This mine
is the second largest gold producer in New South Wales. It was for a prolonged period the State's leading gold mine.
It was the mainstay of the Canbelego mining field, employing upwards of
200 men. The main phase of lode
extraction produced over 13.5 t Au at a recovered grade of 12.2 g/t.
Later additional production has been from tailings retreatment and
mining of lower grade portions of the lode. The deposit has been worked along a strike length of 427 m and to
a depth of 216 m. The main
ore production phase was in 1899-1922 and the first major tailings retreatment
was in 1939-1940. Major
production, largely retreatment work, resumed in the 1980s. Over 452,000 oz Au (14,201 kg) has been won from about 1.1 Mt
of ore.
The Mt Boppy lode was discovered by M. O'Grady, who commenced
work on it with T. Reid in 1896. In
1897, with a shaft down 39 m, the mine was purchased by the Anglo-Australasian
Exploration Mining Company. In
1898 this company proved the lode (eastern leg) for over 300 m in length, and
to a depth of 61 m. The lode
averaged 2.4 m in width and a 264t bulk sample sent to Dapto gave a return of
24.5 g/t Au. Following this
encouraging result the Mount Boppy Gold Mining Company Limited was floated in
England in 1900. Full scale
production was commenced in 1901 and the lode was found to yield consistently
at about 21 g/t Au. By 1904 the Mt
Boppy mine was the leading gold producer in New South Wales, with the lode
being worked along a length of 380 m and to a depth of 122 m.
In the period 1901-1923 some 1,099,121 t of ore were treated
for a recovery of 13,460 kg of gold. This
is equivalent to a recovered grade of 12.2 g/t.
Lode values were variable, but grades consistently in excess of 31 g/t
were obtained in some places. High
grade was particularly common in the oxidised zone which extended more or less
to the 91 m level. Some stopes
north of Taylor shaft at No. 2 level averaged up to 37 g/t Au.
Values decreased in the sulphide zone.
Nonetheless, the sulphide zone ore, as mined, frequently assayed 15-18
g/t Au. In the oxidized zone, free
gold was quite noticeable and was observed along host rock joint planes as
well as in the oxidized siliceous lode.
The Mt Boppy deposit has been variously interpreted but appears
similar to the auriferous vein and siliceous breccia deposits found throughout
the whole length of the Canbelego belt. If
once of syngenetic form, the final siliceous breccia/vein lode form would be a
result of remobilization along the faulted contact between Devonian and
basement rocks.
The Mt Boppy lode comprises two legs or limbs.
First recognized was a poorly outcropping siliceous lode dipping to the
west. This is the eastern leg,
also known as the main lode. As
mining progressed a western leg was discovered, and the two legs were later
found to come together as a southerly plunging trough-like or synclinal
structure. This was recognized by
Jaquet (1905, 1906), and by Andrews (1915), and was referred to as an
"inverted saddle reef". The
synformal lode structure has been generally accepted as indicative of a normal
syncline. However, the strata
between the two legs have not been logged in detail to confirm repetition.
Bedding to cleavage relationships are consistent with the strata
between the two legs comprising a syncline.
The synclinal structure suggested by the lode shape has local
overturning on the eastern limb. The
best ore was obtained along the keel and on the eastern limb, the western limb
having been worked only to a limited extent.
According to Andrews (1915) and subsequent authors, the main
lode is situated just above basal conglomerate in the east limb of a tight
syncline (figure xx). However,
excavation and drilling in recent years along the main lode did not confirm
the presence of basal conglomerate near surface.
Coarse clasts were not found to be common and those observed were more
in the nature of breccia than conglomerate.
The western contact with basement rocks is not as lode-like or
continuously mineralized as the eastern contact zone.
The eastern limb of the syncline and the nose (or trough) of the fold
may be intact, but the western limb appears to be faulted out.
Alternatively, the ore may have occupied a parasitic fold on a major
fold limb, and the "western limb" may never have existed (Gilligan,
1982). Generally, the two lodes
have been taken to represent the steep-dipping limbs of a tight south-plunging
syncline. A major open-cut was
developed at the fold nose and Henry's shaft was sunk to service deeper levels
working the south-pitching structure. Relatively
rich gold values are recorded at intervals from along much of the keel of the
syncline.
Workings extend to 247 m depth (Henry shaft).
The deepest level developed was 234 m below surface but little or no
ore would have been produced from this level.
The Mount Boppy Gold Mining Company ceased working the mine in 1922
following depletion of payable ore reserves.
During the operation of the mine the Company carried out extensive
underground work in prospecting for extensions or repetitions of the Mt Boppy
lode.
The western and eastern lodes, worked for
lengths of 100 m and 300 m respectively, are 0.3 m - 15 m
wide. The main (eastern) zone of
mineralization is characterized by much silicification and quartz veining,
some of the latter as stockwork. Massive
quartz was encountered particularly in the keel area of the syncline.
The main lode shows variable development of chert and
siliceous breccia. Gilligan
(1978) suggested that the chert is of volcanogenic exhalative origin but it
may be equally well compared with the hydrothermal replacement cherts
prominent in Cobar belt mineral deposits.
The lode at the eastern limb was stoped to a depth of 216 m and over lengths of up to 427 m. The western limb was less extensively worked, with the main openings extending discontinuously to a depth of 113 m over a length of 213 m. Average width of the lodes was about 3 m, although a width of 21 m was reported at a depth 238 m, probably in the keel area. A 5 m width of mineralization was not uncommon along the eastern limb. Several open cuts were developed in oxidised ore in the vicinity of where the keel cropped out.
Values along both the east and west legs
were variable , but grades consistently in excess of 31 g/t were obtained
in the central section of each, particularly in the oxidised zone.
Values decreased in the sulphide zone.
Nonetheless, the east limb at the 213 m level, for example, still
averaged 16.4 g/t Au over a width of 4.3 m.
The sulphides present at depth were mainly sphalerite and galena
(Andrews, 1915). Lesser pyrite,
chalcopyrite, arsenopyrite, and cubanite also occurred in the ore (Rayner
1969, Gilligan 1978). Sulphides
locally comprised massive ore patches. These
are of unrecorded size, probably quite small.
Sphalerite is the dominant sulphide and frequently occurs as discrete
coarse aggregates up to 1 cm size. Silicified
breccia contained sulphides with calcite, siderite and ferroan dolomite.
Lode material includes siliceous siltstone with net-veining of
quartz, massive white quartz, and siliceous breccias with or without
carbonates. The lode was described
by Andrews (1915) as consisting of impure quartz and silicified slate
containing small amounts of sulphide minerals such as pyrite, chalcopyrite,
sphalerite and galena and intersected by veinlets of glassy to white coloured
quartz. The "impure
quartz" is fine grained, is commonly of a waxy or chalcedonic nature and
could, in part, be described as chert. It
may be brecciated. Irregularly
distributed sulphide disseminations and small massive sulphide bodies occur
within breccia. Massive quartz was
encountered particularly in the keel area.
The siliceous breccia contains ghost outlines thought to be silicified
siltstone clasts. Their margins
may be sharp or diffuse. Gilligan
(1982) hypothesized hydraulic fracturing, with lubrication by metahydrothermal
fluids allowing some fragmented masses to move as intrusive breccia.
The ore hostrock interval is lost to the
north where the plunging syncline surfaces (figure xx).
To the south it has been suggested that lode termination is due to
faulting. In the south the gold
shoot in the eastern limb appears to be bounded by an easterly-trending
vertical plane. Values in the
western limb may have a similar abrupt curtailment as Gilligan (1977)
postulated a fault in the southwestern part of the main open cut.
It is clear that payable mineralization did not extend as far south as
the Henry shaft. The southern
limit of stoping is generally 90-120 m north of this shaft.
Although the lode termination has the appearance of a fault, this seems
not to have been strongly evident underground (to judge from the company
reports). Andrews (1915) commented
that the economic ore appeared to stop along a vertical east-west trending
plane. An alternative is that the
eastern limb siliceous lode structure does continue south but ceases to be
economic due either to facies change (syngenetic origin) or vertical
shoot-like nature of mineralization (epigenetic origin).
Structural information from crosscutting and drilling at the Hoffnung
shaft suggests that the synclinal structure at the Mt Boppy Mine, with
mineralization along the base of the east limb, does continue southward.
Typically in the Mt Boppy mine the crenulated phyllite of the basement
was separated from the siliceous main lode by up to 3 m of breccia or
conglomerate (Andrews, 1915). There
was a hanging-wall of silicified siltstone and a conglomeratic footwall.
The conglomerate appears to be lost southwards but may re-appear at the
Hoffnung shaft. South of the
economic ore shoots of the Mt Boppy Mine, conglomerate is little known along
the contact at the eastern limb. It
may merely have been removed by faulting as there is conglomerate recorded
along the western limb contact in the Hoffnung shaft workings.
Moreover, at the eastern limb contact at Hoffnung's a drillhole
(Freeport DDH 5) did intersect 1.6 m of chloritic conglomerate
immediately above basement. Conglomerate
at this position had previously been reported as absent in the Hoffnung
workings.
The main or northern open cut was developed across the plunging
synclinal trough axis where it meets the surface.
Other open cuts were developed along the eastern limb of the structure
and these were later expanded by Epoch Mining NL.
From its northernmost surface limit, the lode trough plunges to a depth
of 210 m over a horizontal distance of some 300 m.
The angle of plunge varies, so that the keel profile displays
undulations (Fig. MBLS). The
overall plunge is about 30o. The
keel undulations could suggest transverse folds.
However, there is no evidence, such as crenulation cleavage, for a
later generation of transverse folding. Minor
subhorizontal shearing has been noted in the mine area and this may bear some
relation to the deflections in the general plunge of the keel.
Mine development and the form of the lode is illustrated in
Fig. MBLS and MBVS (A,B). The
Henry shaft was sunk to service the deeper levels working the south-pitching
structure, and prior to 1917 the mine was worked from the Taylor and Henry
shafts, to depths of 129 m and 243 m respectively.
In 1917 a new main shaft was sunk east of the main workings, thus
making available for mining a considerable tonnage of ore in pillars
surrounding the Taylor shaft.
As Figs. MBLS and MBVS (A, B) show, the eastern leg of the lode
formed the main orebody. Stoping
of the eastern leg was carried out over a strike length on the 30 m level of
some 390 m. Deeper levels yielded
successively shorter strike lengths of mineralization.
The western leg, in contrast, appears to have been stoped only over a
maximum strike length of 105 m and this was restricted largely to the
shallower levels. The western leg
has discontinuous openings along a length of 213 m and to a depth of 113 m.
The western leg and the trough were not recognized at the southern end
of the mine. One possible
explanation is that the mine syncline is parasitic upon a larger fold limb
(Fig MBLPF) and actually dies out to the south, as suggested by Gilligan
(1982). In such a case the
syncline structure known at Hoffnung shaft would not be a direct continuation
of the mine syncline.
The gold of the Mt Boppy Mine is reported to have been extremely fine grained and the recovery plant tailings have warranted repeated retreatment. Much of the higher grade gold ore mined, some quite close to surface, might represent supergene enrichment although better primary gold mineralization exists than at other gold deposits in the Canbelego belt. Gilligan (1978) reported the gold to occur in quartz, breccia or country rock, along joints or cleavage (paint gold), in cavities, or in intimate association with sphalerite. Andrews (1915) noted that high gold values frequently occurred in ore containing a high proportion of sphalerite, and in mineragraphic examination 63% of the gold grains observed by Gilligan (1982) were in contact with sphalerite.
Subsequent to 1923 the Mount Boppy mine leases passed to local
interests and small scale production continued from 1929 to 1957.
This consisted of gold recovery from various waste materials and the
raising of small tonnage parcels of near-surface ore.
Appreciable lower grade reserves remained at the Mt Boppy mine when the
Mt Boppy Gold Mining Co. ceased operations in 1922.
Large tailings dumps remained for subsequent retreatment and at least
10,000 t of the eastern lode remained unstoped at shallow depth.
This lode gives spot assays up to 55 g/t Au.
A percussion hole by Ausminda in 1966, angled 24 m across the
lode, averaged 6.4 g/t Au.
West Boppy Mine
Of the
workings near the Mt Boppy mine which were developed to test for extensions or
repetitions of the rich lodes, only the West Boppy mine, about 300 m to
the southwest, is thought to have been successful in locating similar gold
ore.
The West
Boppy lode, a possible western fault or fold repetition of the similar Mount
Boppy lode, was cut at depth in exploratory workings from the Mount Boppy gold
mine. However the West Boppy lode
was not then worked and may be unpayable at depth.
Subsequently, rich near-surface assays (to 1.2 kg/t Au) were obtained
from the West Boppy lode in 1930 by W.J. Mitchell and party.
Four shafts, up to 61 m deep, were developed and Mitchell continued to
produce small ore parcels from this mine till 1940, yeilding 2.55 kg Au from
231 t ore.
The sub-vertical lode comprises massive
quartz, as well as quartz veinlets, in a zone of ferruginous and silicified
brecciated country rock. Width
reportedly ranged up to 8.2 m and averaged 2-3 m.
The lode appears to lie along the Devonion basement contact, which is
brecciated. In drill hole Pacific
Copper DDH DCB-1 the brecciation is largely within the Girilambone Group, the
main brecciated interval being of 3.5 m core length.
However, intense fracturing and brecciation is also present in the
cover rocks (McClatchie, 1981a). The
mineralization appears to favour the fractures in the Devonian rocks.
The main fracture zone in the Devonian rocks is pyritic and corresponds
with a 1.1 m surface ironstone containing 8-15 g/t Au.
The contact between basement and cover rocks can be traced in shallow
workings for about 100 m. Up
to 1 m of quartz veining associated with brecciation is evident in the
workings. Also present is the same
type of cherty silicification that is prominent in the Mount Boppy mine.
The lode
was percussion drilled by VAM in the 1970s and the partially cored hole by
Pacific Copper was drilled in 1982. The
gold values appear to be irregularly distributed.
Although assays of around 30 g/t Au were reported from near
surface, the average grade is low. The
recovered grade from mining operations is 1.1 g/t Au, although 46 t
of hand picked ore taken between the 15m and 45 m levels in 1936 averaged
18 g/t. Supergene
concentration appears to be associated with pyrite oxidised to limonite in
both quartz veins and Cobar Supergroup sediments.
As the quartz veins are generally narrow (although up to 1.2 m at
the 15 m level) much of the gold occurred within cavities and fractures
in the sediments. The nature of
the primary mineralisation is suggested from drill hole DCB-1, where pyrite is
widespread through both cover-rocks (up to an estimated 10%) and the
Girilambone Group (pyrite + pyrrhotite ranging to several percent).
Maximum base metal values encountered are lower than for the Mt Boppy
mine, the highest trace value obtained being 495 ppm Pb.
North Mount Boppy Mine
During the
peak years of the Mount Boppy Gold Mine, the North Mount Boppy and South Mount
Boppy Gold Mining companies were established to locate strike extensions of
the mineralization to the north and south, by surface pitting (largely
unrecorded), underground shafts and cross-cuts. About 450 m north of the Mount Boppy mine over 500 m of
cross-cutting was carried out from the North Mount Boppy shaft.
The North
Mount Boppy Gold Mining Company was not successful in finding a northward
continuation of the main lode of the Mt Boppy mine. A "lode" was encountered in the North Mount Boppy workings
but it is now interpreted that this lies in a fault zone within the basement,
about 250 m east of the faulted contact with Devonian rocks. The North Mount Boppy lode contained trace base metals but negligible
gold. A sample of quartz with Fe/Pb/Zn
sulphides collected from the dump assayed 1.2 ppm Ag, 0.3% Pb,
0.1% Zn, and gold undetectable (McClatchie 1981a).
Hoffnung Shaft
Rather than a separate occurrence this could be regarded as 'part' of the Mt
Boppy mine but workings never were connected between the two so it is here
regarded as a separate entity. Unlike the later much
disturbed area around the Mt Boppy mine, the tip at the Hoffnung Shaft is a
good repository of the rock types encountered underground at this site, with
mainly Devonian rock variations seen. The record of the workings
suggests they did enter the sub-Devonian 'basement' although no Girilambone
Group rocks were found during a brief examination of the dump.
The
exploratory Hoffnung workings south of the Mt Boppy mine were developed to
prospect the contact zone between the Devonian sequence and basement rocks.
Hoffnung's shaft is situated 450 m south of the Henry shaft, and
was positioned to test for possible continuation of the synclinal structure at
Mt Boppy mine. Cross-cuts were
directed towards the expected basement rocks of each limb. As at Mt Boppy mine the main mineralization proved to be in the east
limb at the contact with basement rock. The
cross-cutting was through Devonian siltstone, "limestone",
sandstone, grit and minor pebble conglomerate (figure MBVS-C). The east cross-cut traversed 7.9 metres of "lode formation",
consisting of "blue quartz, with small quantities of galena and
blende". (Mine Record). The
lode, which was driven upon, is bounded by a north-south "wall"
beyond which the cross-cut entered "clean slate". The lode was driven upon along the "wall", for a total of 14
m without encountering any gold values. Later
drilling by Freeport (Puce, 1985) confirmed that the eastern limb contact is
mineralized and that the western one is not. Conglomerate was intersected at the end of the western cross-cut, which
probably also ended in basement rock.
Black siltstone, polymictic conglomerate, calcareous
fossiliferous sandstone, phyllite and prominent quartz/calcite veining can be
noted on the dump. Discoidal
lenses of fibrous calcite occur en echelon in the slate. Gold
values from dump samples have been mostly low, except for one siltstone sample
returning a 7.2 g/t Au value. Similar
subcrop spot results have been obtained in the vicinity but no discrete gold
anomalies are outlined. The dump
material reveals minor pyrite in grits and conglomerate but other sulphides
are uncommon.
South Mt Boppy (Nesham Shaft)
At this
site, east of the Hoffnung shaft, quartz veins crop out and significant gold
values in the 5-12 g/t range have been repeatedly obtained in assays.
The best assays are from vein quartz and chert breccia somewhat similar
to material at West Boppy. Little
is known of the lode form in the Nesham workings, but it appears that the
deposit (seemingly comprising two parallel 1-1.5 m lodes) occurs at the
synclinally folded contact of basement and cover rocks.
Folding might not be tight, as most of the deeper workings appear to
have been wholly within basement rock. At
the 61 m level a strong reef, reportedly auriferous throughout, was
opened up. However, there is no
record of ore production.
4. East Canbelego Area
This area
is located close to the Devonian/basement contact near where the thick eastern
cover of acid volcanics commences. As
in the Boppy Mount and Central Canbelego areas, mineralization occurs in both
basement and cover rocks. There is
also a similar trace metal association recorded, with up to 1800 ppm Pb,
200 ppm Zn and 220 ppm Sb.
The mineralization prospected appears to be largely within the
basement, whereas in the Central Canbelego area it is largely hosted just
above basement.
Surface
gold prospects, from ironstones and siliceous breccia, yielded some remarkable
grades or spot result assays - e.g. 333 g/t (Baker's claim), 400 g/t
(Birthday mine), 660 g/t (Reid and Ranken's shaft).
However, no sizeable orebodies were located at these sites and some of
the high gold values could be supergene enrichment resultant from the erosion
of weakly mineralized Devonian/basement contact zone rocks.
As is the case for the other nearby groups of deposits, primary gold
deposition may have been in breccias, quartz veinlets, and Devonian strata
(including conglomerate).
There has
been significant production from one of the mines, the Canbelego King.
The nature of the ore is unrecorded, other than that it included
"gossan" near the surface.
The
deposits in the East Canbelego area are:
No.
Deposit Name
Commodities
165
Newhaven shaft
Au (Ag, Pb)
166
Hidden Treasure
Au
167
Canbelego King
Au (Zn)
168
Reid & Ranken shaft
Au (Ag)
169
Birthday mine
Au
170
East Mount Boppy mine
Au (Pb, Zn)
171
Bakers claim
Au (Ag)
172
Wealth of Nations
Au (Ag)
Newhaven Shaft
The
Newhaven prospecting area encompasses a line of shafts and pits along an
isolated basement outcrop area east of Canbelego.
The principal line of old workings is in basement metasediments and
lies approximately 200 m west of outcropping Devonian rocks.
These workings are in a zone of crushing (Andrews, 1915) which is
anomalous in lead (to 3400 ppm), arsenic and antimony (to 140 ppm).
The main shaft, the Newhaven, obtained high gold values within a 1.2 m
wide lode but values cut out at 5.5 m depth and could represent supergene
enrichment. Vein quartz with
limonite-filled boxworks on the Newhaven dump contains up to 4.5 g/t Au
and 10.2 g/t Ag (Gilligan, 1978).
Canbelego King Mine
This
deposit is regarded as a fault breccia zone similar to the Birthday and East
Mt Boppy deposits described below. Production
was 14.34 kg Au, much of it at a grade of over 80 g/t.
The lode is recorded as 2.4 m wide and 13 m long.
Assays of ironstone and hostrock indicate up to 165 ppm Pb,
620 ppm Zn, 690 ppm As, 150 ppm Sb.
Reid and Ranken's Shaft
Reid and
Ranken's shaft is situated on a small rise of conglomerate/quartzite and
phyllite. The underlay shaft was
sunk in the Devonian sediments, adjacent to conglomerate.
It followed generally low gold grades of up to 5 g/t, exceptionally
reaching 17 g/t Au. Although
collared in Devonian rocks,the workings appear to have extended into laminated
quartzite of the basement beds. Sphalerite,
pyrite and trace silver (16 g/t) occur in siliceous rock from the dump
but it is uncertain whether this is basement quartzite or silicified Devonian
sandstone. Conglomerate
outcropping near the main shaft has yielded occasional spot values of up to
660 ppm Au and 30 ppm Sb, which supports mineralization being best
developed within the Devonian rocks.
The Birthday Mine
The
workings are developed in a gossanous breccia zone, apparently within basement
although Devonian conglomerate lies nearby.
Associated heavy Fe and Mn replacements show enrichment in Zn (to 2000 ppm)
and Pb (to 880 ppm). Early
prospects yielded up to 400 g/t Au.
It is not known whether the high values were obtained in the Devonian
or basement rocks. Similar pyritic
conglomerate to that at Reid and Ranken's adjoins the Birthday Shaft about 200 m
to the southeast. By comparison
with the spot value of 660 g/t Au obtained from conglomerate at the
nearby Reid and Ranken shaft, the high surficial gold values could be
suspected as supergene enrichment during the removal of overlying Devonian
strata.
An early
parcel of 70 tons averaged 38 g/t. This
was probably surface stone. The
deeper workings encountered only minor gold values, up to 5.4 g/t, and
are mostly within the basement rocks. The
annual yield for 1935, by J.J. Sanders, was 27 tons which averaged 15.9
g/t Au. Being relatively high
grade ore this was perhaps a further gathering of surface gossan.
Estimated total production is 4.9 kg Au.
East Mt Boppy Mine
This
deposit, which yielded 684 g Au, is recorded as an ironstained siliceous
lode and is probably a fault breccia zone similar to that at the Birthday
mine. Assay values show up to 930 ppm
Pb, 1440 ppm Zn and 120 ppm Sb in the hostrocks.
Wealth of Nations
The
workings are developed at the Devonian/basement contact, which is much
fractured and possibly brecciated. Dump
material of ironstained conglomerate and sandstone assays up to 445 ppm
Pb, 3.1 ppm Au and 4.5 ppm Ag.
5. South Canbelego Area
The South
Canbelego area contains six minor base metal prospects aligned within deformed
Devonian strata, and situated close to a Devonian/basement contact south of
Canbelego. Brecciation near the
contact occurs in the vicinity of the Canbelego Peak and Pendergast deposits.
The mineralization appears to be more strongly confined to the Devonian
rocks in this group of deposits than in the other groups of deposits further
north. The hostrocks include
conglomerate, sandstone, and perhaps strata of the inferred basal
"calcareous" facies. From
the alignment of the deposits, the foliation of the hostrocks, and local
presence of strong chloritization, a broad shear zone several hundred metres
wide has been postulated (Gold Fields Exploration Pty Ltd, pers. comm.).
The
deposits in the South Canbelego area are:
No.
Deposit Name
Commodities
173
Puces vein
Cu, Zn
174
The Priests
Pb, Zn, Cu (Au)
175
Canbelego Peak
Cu, Pb (Au, Zn)
176
Pendergast silver-lead shaft
Pb, Zn, Cu (Ag, Au)
177
Unnamed
Pb (Cu, Ag, Zn)
178
Mouramba shaft
Cu, Pb, Zn
Whereas
the deposits further north in the Canbelego belt (East Canbelego, Central
Canbelego, Boppy Mount and Florida areas) are gold deposits with trace base
metals (0.2-0.3% Pb, Zn), the South Canbelego area is more noteworthy for
basemetal values (Cu, Pb) than for gold. In
this respect the South Canbelego area mineralization appears transitional to
that of the Burra-Hermitage area further south.
Gold values associated with the South Canbelego deposits are mostly in
the range 0.1-2.0 g/t. There
are no occasional very high assays like those of the East Canbelego area, the
highest value being 10.5 g/t.
In the
South Canbelego area the deposits have subeconomic grades of mineralization in
the oxidized and enriched zone and only extremely weak pyritic mineralization
is known at depth. The primary
mineralization is disseminated pyrite at the Priests and minor veinlet
pyrite-quartz below the Canbelego Peak workings.
The
dominant control on mineralization is almost certainly the broad
northwesterly shear but locally the secondary mineralization appears to
be related to cross-faults. This
is best seen in drilling results from Canbelego Peak.
Near the Pendergast silver-lead shaft some quartz porphyry occurs.
This has been regarded as a dyke (Andrews, 1915) but might instead be a
downfaulted sliver of the overlying acid volcanics.
If this were so, the crossfault pattern suggests dextral movement at
the main northwesterly shear. The
main shear zone appears to dip steeply southwest.
It is co-linear with shearing at the Burra mine further south, where
the dip also appears to be to the southwest.
6. Burra-Hermitage Area
This group
of deposits is notable mainly for copper ore, with lesser lead-zinc and minor
gold. The main mine is the Burra
mine, which was later combined with the Block 51 mine and worked as the Mt Boppy
Copper Mine.
The
primary copper mineralization is characterized by the association of
chalcopyrite and magnetite with chloritization.
Lead and zinc sulphides tend to be concentrated close to but separate
from chalcopyrite concentration. Calcareous
intervals appear to be favoured hostrock for the lead-zinc mineralization.
The deposits are hosted in a Devonian sheared conglomeratic and
calcareous interval, overlain by felsic volcanics.
In this respect they are similar to the other stratabound deposits
further north in the Canbelego basin. They
differ in the greater base metal content.
They are perhaps also less silicified, with better preservation of the
initial allothchonous carbonate content.
The
deposits have associated low order magnetic anomalies, attributed to
pyrrhotite and magnetite. A 2 m
mineralized interval intersected in drilling at Mt Boppy South consists
largely of magnetite, and a 5 m interval at the C2A prospect is rich in
magnetite. Chalcopyrite is the
chief primary ore mineral. It
occurs either unaccompanied or intergrown with pyrite, magnetite and trace
galena. Individual areas of
chalcopyrite seldom exceed 1 cm. Copper
production was from secondary enrichment lodes.
Other primary mineral associations apart from
chalcopyrite-pyrite-magnetite are minor. They
include pyrite-pyrrhotite in carbonaceous shales, and galena-sphalerite-pyrite
in calcareous facies. The
association of lead-zinc with calcareous facies occurs both at Burra mine (Zn/Pb
= 1.3) and at the C2A prospect (Zn/Pb = 1.5).
The C2A prospect perhaps offers the best opportunity for appreciating
the nature and setting of the Burra-Hermitage area ore types.
The C2A area is essentially free of the shearing effects present along
the Burra-Block 51-Mount Boppy South alignment of deposits.
The Burra
and Block 51 Mount Boppy South mining areas are aligned but not fully
continuous. Both areas have
indeterminate sheared sediments adjacent to (below?) coarse acid volcanics.
Intermittent quartz-haematite lodes, less than 3 m wide, occur
within the shear zones. One of
these lines of siliceous ironstones in the southern area is almost 800 m
long.
The
interpretation is that the deposits lie in the thin Devonian basal calcareous
sequence, underlying the nearby volcanics (northeast side) and close to the
basement (southwest side). However
the basal Devonian strata are here strongly deformed, and perhaps overturned,
within the shear zone. The
demarcation between the Devonian and basement rocks is not readily mapped, and
the hostrocks are not observed to dip beneath the volcanics.
The hostrocks, particularly at Burra copper mine, are difficult to
assign stratigraphically. They
have been regarded as Girilambone Group by some writers and as abnormally
deformed Devonian strata by others.
The shear zones at both the Burra and Block
51-Mt Boppy South mining areas appear to dip steeply southwest, similar to the
co-linear shear zone at the South Canbelego group of deposits.
The deposits of the Burra-Hermitage area
are:
No.
Deposit Name
Commodities
184
C2A prospect
Zn, Pb, Cu (Ag, Au)
185
Burra copper mine
Cu, Au (Ag, Pb, Zn)
186
Mount Boppy Block 51
Cu, Pb (Zn)
197
Mount Boppy South mine
Cu (Zn, Pb)
C2A Prospect
Located
about 4.5 km southeast of Canbelego, this is a 90 nT aeromagnetic anomaly
without Induced Polarization response, established to have a magnetite source.
Due to the
intensity of diamond drilling (Penarroya Aust. 1979, 1982) and absence of
strong faulting, the C2A prospect is the best known example of the group of
apparently stratabound basemetal deposits near Canbelego.
At C2A the mineralization is hosted in a thin subvolcanic sequence of
conglomerate, siltstone and fragmental dolomitic limestone close above
basement. Little or no base metal
sulphides are present in the basement, although pyrite is abundant in the
sparse matrix of a breccia at 375 m in DDH C2A6.
Eight
diamond drill holes have explored the extent of the weak stratabound
mineralization, and have shown the presence of erratic low gold values (Penarroya
Aust. 1979, 1982). The drilling
demonstrates the mineralization to overlie a basement ridge or dome covered by
acid pyroclastics and flows. The
limits of the mineralization have not been fully defined, because of high
drilling costs. One hole sited
along the basin axis (DDH CB2-1) penetrated a vertical depth of 640 m
into the volcanics in an unsuccessful attempt to reach basement.
Drilling in this area has demonstrated an inverse correlation between
magnetic intensity and volcanic
overburden thickness, suggesting a domal structure underlying the C2A magnetic
anomaly. In DDH C2A-7 on the
western flank of the structural high significant mineralization is restricted
to the basal 65 cm of the Devonian sequence.
This assays 0.35% Cu, 0.38% Pb, 0.96% Zn, 24 g/t
Au. The strata dip at 30-35o.
At the structure is ascended eastwards the limestone-rich section of
the basal beds increases to a 65 m thickness.
This section is significantly mineralized.
Drill hole DDH-6 illustrates the typical stratigraphic succession
encountered:
A.
0-278m Rhyolitic crystal
tuff (Florida Volcanics)
B.
278-291m Chloritic siltstone (Baledmund
Formation)
C. 291-356m
Fragmental dolomitic limestone
D. 356-361m
Magnetite-chlorite rock (Baledmund Formation))
E.
361-379m Metasiltstone
(Girilambone Group)
Mineralization occurs in units C and D
above. Within the interval
containing dolomitic limestone there are narrow intervals containing
sphalerite and galena. In the
magnetite-chlorite rock chalcopyrite is the dominant sulphide.
The magnetite is disseminated to massive.
The best reported intersections for each unit respectively are 2.75 m
of 2.06% Pb, 3.15% Zn, 6 g/t Ag and 1.0 m of 1.85% Cu, 0.6 g/t Au.
The
mineralized fragmental limestone interval is the best documented example of
the calcareous facies low in the Devonian sequence of the Canbelego basin.
Although
the lead-zinc mineralization (pyrite-sphalerite-galena) occurs in intervals
which are calcareous, the sulphides on a smaller scale generally avoid actual
limestone clasts. At C2A
individual coarse (5 mm) crystals of pyrite show euhedral outline where
in matrix but become anhedral where moulded against limestone fragment
outlines, without any incursion into the carbonate.
Sulphides are concentrated in the matrix between carbonate clasts, or
along clast boundaries where there is negligible matrix.
Galena shows the greatest spatial affinity with carbonate but even it
tends to avoid the limestone in preference for the veinlets of later calcite
which commonly traverse more than one limestone clast.
The textural features suggest that the limestone was well lithified at
the time of mineralization.
Burra Copper Mine
This mine
has produced ore containing 211 t Cu, 558 g Au and considerable
silver, at an average grade of 7.8% Cu. Two vertical lodes have been worked.
These are 0.9-10 m wide, 30-37 m in length, and 20 m
apart. The lodes strike 290oT,
which is oblique to the trend of the enclosing shear zone. The lodes appear to be en echelon and situated immediately above a
basal unconformity marked by breccia or conglomerate. This interpretation is undertain because of the strong deformation
present in Burra mine vicinity, but is supported by analogy with the less
deformed C2A prospect.
The Devonian hostrocks at Burra mine includes slate, silstone,
sandstone, conglomerate with schistose matrix, phyllite breccia, and
brecciated quartz. Calcareous
bands have yielded crinoids, brachiopods, and corals including favositids and
?Mucophyllum. The fossils are too
deformed for specific identification. Limestone
is reported from drilling. The
mineralized shears probably extend into basement rocks.
Little known "porphyry" in the mine vicinity has been
regarded as volcanics by some observers and as dyke rock by others.
The host rocks likely underlie the Canbelego volcanic pile and are
intruded by a porphyry dyke associated with the volcanics.
One drill
hole, BBD-40 by Swiss Aluminium Mining Aust. (1974), intersected both copper
lodes. The western lode gave 9.93 m
averaging 1.2% Cu and the eastern lode assayed 0.25% Cu over 0.6 m.
A thin interval (2 m) contained visible sphalerite and galena,
averaging 0.27% Pb and 0.35% Zn.
In this interval the lead and zinc sulphides are associated with
crinoidal limestone.
In the
primary zone chalcopyrite is the dominant sulphide, forming areas up to 7 mm
across. Although galena is not
prominent its minor presence is suggested by the silver content (13 g/t)
of the high grade secondary ore (18% Cu).
Mount Boppy Block 51 Mine
The Block
51 and Mount Boppy South leases were held on a nearly continuous line of
ferruginous lode commencing about 700 m southeast of the Burra workings.
The southern leases were worked or prospected at later dates than the
Burra mine (Carne 1908, Andrews 1915, McManus & Weber 1969).
Between the Burra and Block 51 mines the Devonian hostrock interval
appears to become progresively more sheared out from beneath the more
resistant acid volcanics.
The
geological setting of the Block 51-Mount Boppy South line of lode is similar
to that described for the Burra mine. The
thin Devonian hostrock interval along the southwestern margin of the volcanics
includes quartz sandstone and minor conglomerate.
There is brecciation and much surface ferruginization evident.
Assay values are up to 2280 ppm Cu, 2% Pb, 1990 ppm Zn and 280 ppm
Bi. Large geochemical soil
anomalies occur about the line of lode, which appears to be essentially a
quartz haematite shear breccia. Both
carbonate and sulphide secondary copper ore are present but in small amount
only. Little production has been
obtained (5.8 t Cu at average ore grade 5.2% Cu).
The primary ore in brecciated hostrock is largely pyritic, with
subordinate amounts of disseminated fine-grained galena and chalcopyrite.
Mount Boppy South Copper Mine
The workings are
situated in the quartz-haematite lode zone, 800 m long and 2-10 m
wide, which extends from Block 51 to beyond Mt Boppy South.
Dump and surface samples give values of up to 4.2% Cu, 1075 ppm Pb
5600 ppm Zn, 630 ppm Bi. However,
surface copper values rarely exceed 0.4% and there has been no ore production.
The best diamond
drilling intersection is 2.17 m of massive magnetite with chalcopyrite,
assaying 2.1% Cu. This
mineralization is in fault gouge along the contact between the Girilambone
Group and Devonian sediments.
7. Buppe-Shango Area
This is a
group of workings at copper deposits and prospects, all of which are hosted in
basement rocks (figure xx). The
principal property here was known as the Canbelego copper mine.
Ore raised from some of the other claims (e.g. the Buppe lode) was
found to barely repay transport to Port Kembla.
At most of the sites prospected the surface indications were
combinations of copper-staining, rubbly quartz and gossanous ironstone.
Large quartz veins or veined intervals (0.3-3m thick) are present.
The veins are possibly pre-Devonian and strongly boudinaged in places.
At depth, carbonate-quartz veins occur.
Maximum values from the ironstones are not very high (800 ppm Cu,
1100 ppm Pb, 590 ppm Zn) but small pockets of secondary copper ore (5-13% Cu)
were found at some prospects. Gold
appears to be negligible for these deposits.
The best of the gossanous outcrops are at the Canbelego and Shango
mines, and these are the main sites from which copper was produced.
Individual bands of gossanous ironstone are seldom more than 30 cm
wide. The Canbelego-Shango trend
of mineralization is the most important feature of this group of deposits.
Along this trend minor old prospect workings were developed over a
length of 800 m or more, encountering bodies of secondary copper ore at
Canbelego copper mine (10% Cu) and Shango mine (7% Cu).
The
deposits of the Buppe-Shango area are:
No.
Deposit Name
Commodities
179
Buppe lode
Cu (Au)
180
Rankens Reward
Cu
181
Canbelego Queen
Cu
182
Canbelego copper mine
Cu (Ag)
183
Shango
Cu
The main
area of interest, extending about 1 km along strike and about 600 m
across strike, was covered by early copper leases ("Canbelego",
"North Shango", "Shango", "Canbelego West",
"Canbelego North" leases) which were described by Andrews (1915).
Foliated metamorphosed igneous rocks variously identified as diorite,
gabbro and albitite outcrop poorly throughout this area.
The most prominent string of small outcrops of such rocks occurs to the
east of the Canbelego and Shango copper mines.
These outcrops were initially regarded as dykes or pipes of crushed
gabbro and diorite. Diamond
drilling at the Canbelego mine has shown the associated sequence to be largely
tuffaceous and many of the coarser metamorphosed basic rocks which crop out
might also be crystal tuffs rather than magmatic rocks.
The chief reason for not regarding all of them as metamorphosed tuffs
is the occasional presence of relict bulk-rock magmatic features, including
granophyric intergrowth texture. The
rocks commonly now consist of tremolite, chlorite and saussurite.
Very small unaltered areas remain with relict hornblende, clinopyroxene
and feldspar outlines in the coarser indeterminate rocks.
The drilling at the Canbelego copper mine revealed greenish porphyritic
textured basic tuffs in which no plagioclase remains but the crystal sites are
largely preserved chlorite. The
tuffaceous origin is supported by interbedded non-porphyritic foliated tuffs
which remain plagioclase-rich. The
foliated basic rocks may be significantly albitized, with up to 3.7% Na.
Thus some of the plagioclase crystal tuffs now of intermediate
composition (oligoclase) may have been formerly of more calcic nature.
The sodic alteration could feasibly be hydrothermal and coeval with
mineralization, as quartz-albite-carbonate veinlets are common in Canbelego
copper mine rocks. Suspected acid
volcanics occur in the tuffaceous sequence but the volcanic origin of these
rocks is less certain. They
contain quartz eyes (strained porphyoblasts, relict phenocrystal quartz, or
normal clastic granules) of up to 2 mm size.
The
Canbelego copper mine deposit has been suggested as volcanogenic stratiform,
and of Girilambone Group age (e.g. Cotton, 1976a).
However, there are no identified fold repetitions of mineralized
horizons, and the more certain igneous mafic rocks, such as on the Shango
dump, are nowhere known to be mineralized.
The only well known primary mineralization, at the Canbelego mine,
appears to be a lode in fine-grained highly chloritic rock which lacks any
definite igneous/pyroclastic texture and could equally well be a shear zone.
The trend of the lode appears to parallel that of faulting which
truncates folds within the Girilambone Group.
The
features of the Buppe-Shango group of basement-hosted deposits include:
high chalcopyrite/pyrite ratio, very low lead-zinc content and
negligible gold content. The
intense chloritization offers one point of comparison with the copper ores of
the nearby Hermitage-Burra group of deposits.
If the
Buppe-Shango deposits are regarded as formed in the same epigenetic episode of
mineralization as were the other Canbelego belt deposits to the east and north
then a certain very broad vertical zonation between deposits can be suggested.
In the structurally lowest setting copper was dominantly deposited.
Nearby, and in a higher structural position, copper was deposited just
above basement with significant gold accompaniment and minor bismuth.
Minor lead-zinc concentration occurs immediately higher.
Fluids which were perhaps depleted of basemetals after their deposition
at lower levels may have subsequently deposited gold and trace antimony.
Brecciation effects are strongest for the latter, consistent with
lesser cover. However, alternative
explantion is available for each element of this model.
Canbelego Copper Mine
This mine
has yielded 173.3 t Cu at an average ore grade of 9.6% Cu.
Production was from a narrow vertical lode of compact ferruginous ore
containing considerable native copper in the secondary zone.
The lode has been traced for a length of over 100 m without sign
of lensing out. It has been stoped
along a length of 23 m where enrichment extened to a depth of 90 m.
The
mineralization occurs within a tuffaceous sequence of Girilambone Group
metamorphic rocks, containing basic intervals.
The basic rocks are present on both sides of the lode.
To the east a drill hole intersected albitized porphyritic rock
regarded as metamorphosed lava or crystal tuff.
Thin intercalations of greenish porphyritic textured basic tuff also
occur to the west of the lode. Much
of the tuffaceous rock is medium grained (0.5 mm) fairly well sorted
plagioclase crystal tuff, now albitized. Also
present is tuff containing relict crystals of hornblende and clinopyroxene to
1.5 mm diameter, now almost totally replaced by chlorite, carbonate and
epidote.
Unmined
remnants of the oxidized lode have a grade, estimated from drilling, of about
8.5% Cu and 14.9 g/t Ag. Massive
oxidized pyrite ore lumps seen on the mine dumps are this grade, samples
containing 9-10% Cu. Within the
primary zone, known from diamond drilling transects, the single lode worked in
the secondary zone is resolved into vertical bands of mineralization,
separated by barren intervals of similar widths.
Intersections representing sets of these bands include the following:
6.7 m of 1.% Cu, 5.8 g/t Ag;
4.7 m of 2.7% Cu, 10.0 g/t Ag.
Richer ore bands present include 1.5 m with 7% chalcopyrite and
0.5 m with 10% chalcopyrite. The
mineralization is not sharply delimited and the main lode interval is enclosed
in a zone of weaker mineralization. An
assay of this broader zone in one hole (DDH CD-2, Cotton 1976a) shows 23 m
of 0.9% Cu and 4.8 g/t Ag. Presence
of sparse chalcopyrite is also recorded elsewhere in the drilled sequence, as
intersections of up to 30 m length. Copper
traces thus occur across a wide zone (100-200 m)
of testing, and possibly extend further west into the Canbelego West lease
(Andrews, 1915).
Diamond
drilling has shown that the dominant hostrock in the lode interval is very
dark green indurated chlorite schist. Both
quartz veining and more cherty forms of silicification pervade this rock in
the main zone of mineralization.
The
primary zone has chalcopyrite as the chief ore mineral.
The chalcopyrite is accompanied by pyrite, massive chloritization,
silicification and variable carbonate content.
Pyrite is often quite subordinate, the overall chalcopyrite/pyrite
ratio being estimated as 4:5. The
lode interval typically contains narrow submassive sulphide bands 10-30 cm
wide. These have up to 15%
chalcopyrite, with chalcopyrite blebs up to 2 cm size (often in small
quartz veins), and thin sulphide stringers.
The lode may in places contain up to 2% sphalerite but the ore is very
low overall in both zinc (0.1%) and lead (0.01%).
Specks of galena are mostly seen within quartz veinlets, and are
extremely scarce. Pyrrhotite has
not been noted within the lode interval but does occur in basic porphyritic
rock to the west of the lode.
The lode
at the Canbelego mine is parallel to clearly defined faulting and might itself
be emplaced along a shear. Faulting
at the Buppe-Shango deposits does not appear to be accompanied by such strong
brecciation as occurs at the deposits near Devonian/basement contacts.
The only noteworthy breccia at the Canbelego mine is a
siderite-cemented schist fragment breccia encountered in the workings.
Brecciation seen in the diamond drill cores is quite minor.
Evidence
is in favour of fault control for the main zone of mineralization at the
Canbelego mine. The strike of the
main fault, which changes with depth in the mine, is 330-350oT.
This is similar to the alignment of the Canbelego and Shango mines
(350oT). The connecting line
between lode intersections in drill holes trend 340-350o, parallel to the main
fault where it was driven along in the mine workings at the 61 m level.
This lode trend of about 340oT appears to be at variance with the
outcrop trend (305-310oT) of the porphyritic tuff ("norite", Andrews
1915) east of the mine. Such
outcrop trends continue as far south as Shango mine.
Either the outcrop trends are parallel to cleavage or the faulting cuts
across Girilambone Group bedding. Mine
and drill hole records suggest that the faulting does transect folded bedding.
In the absence of any demonstrated stratigraphic repetition of the
lode, the mineralization seems best regarded as epigenetic (shear-related)
rather than syngenetic (volcanogenic).
8. Kopyje Area
The Kopyje
area deposits are in Devonian strata lying between the Canbelego and Babinda
volcanic centres. The deposits of
the Kopyje area are:
No.
Deposit Name
Commodities
188
Nerang prospect
Pb, Zn
189
Unnamed
Au(?)
190
Pipeline Ridge prospect
Zn, Cu, Pb (Au, Ag)
191
North Glens Tank shaft
Au? (Pb)
192
Glens Hill prospect
Pb, Zn (Cu)
234
West Kopyje prospect
Au
235
Kopyje prospect
Pb, Zn, Cu
236
Boundary Fence prospect
Zn, Pb, Cu
237
Sarona prospect
Zn (Cu, Pb)
As with
Canbelego belt deposits further north, there is some degree of spatial
relationship between mineralization and the Devonian/basement contact.
A dissimilarity with the northern deposits is that mineralization
extends well up into the acid volcanics. The
mineralization is largely hosted in pyroclastics (e.g. Nerang, Pipeline Ridge,
Glens Hill and Kopyje prospects). The Sarona Downs Tuff Member hosts the two
main prospects, at Pipeline Ridge and Glens Hill.
It is also noted that a northeast-trending fault corridor may connect
these two prospects.
The most extensively drilled of the deposits hosted in or near pyroclastics is that at Pipeline Ridge. The controls on this mineralization are poorly understood. The volcanic-sedimentary contact was initially considered a paramount syngenetic control. Later the concept developed of mineralization being controlled by a northeast trending fault-bounded corridor. Pipeline Ridge and Glens Hill prospects were both early considered to have syngenetic (volcanic exhalative) aspects. However, subsequent drilling defined the gold values to be in shoots cross-cutting the strata and perhaps rising from the major fault zone to the east and controlled by the northeasterly fault corridor. Pipeline Ridge was then re-interpreted in terms of epithermal gold mineralization with base metal credits in favorable lithologies adjacent to syndepositional growth faulting. Any stratiform features at this deposit are minor and presumably result from selective stratiform replacement within or close to the pathways of rising fluids.
Nerang Prospect
The Nerang
prospect area is adjacent to the Coonara Fault and the easternmost old
workings may have passed through Devonian strata into basement rocks.
Early attention may have been attracted by the presence of minor fairly
strong cellular limonitic gossan but values obtained are unknown.
Bedrock geochemical values are clearly enhanced in the area but they
lack any clear trends or centre. Values
appear to be sporadic and high lead values have mostly not been relocatable on
follow-ups. Values recorded range
to 428 ppm Cu, 6500 ppm Pb and 2500 ppm Zn.
Some enhanced values are associated with a faulted contact between
siltstone and pyritic agglomeratic tuff. No
significant intersection was obtained in diamond drilling but there is
widespread disseminated fine pyrite/chalcopyrite and rare coarse galena in one
hole (DDH-N1, Leeson & Bluck 1976). Maximum
drill hole values are 1050 ppm Pb and 1100 ppm Zn.
Pipeline Ridge Prospect
Pipeline
Ridge, located 19 km south-southwest of Canbelego is one of the most
prospective sites along the Canbelego belt.
Mines Exploration Pty Ltd discovered and drilled this prospect in
1973-1977 (Bluck 1976a). It
comprises gold and base metal mineralization within siliceous crystal and
vitric tuffs in an area of little outcrop.
Interest in the locality was generated by the detection of a zone of
high metal values (up to 550 ppm Cu, 0.25% Pb) in a regional geochemical
survey. A follow-up detailed I.P.
survey outlined an associated resistivity high, with no magnetic anomalies
present.
Diamond
drilling revealed that mineralization occurs as disseminations, short thin
sulphide bands, veinlets, and infillings in open space chert breccia horizons.
The mineralization consists of chalcopyrite, sphalerite and minor
galena. It occurs in pyroclastic
units throughout which pyrite is ubiquitous, but is little concentrated in
particular beds. From the
1973-1977 drilling the mineralization was estimated to average 1.41% Cu, 0.47%
Pb, 1.3% Zn, 70 g/t Ag and 3 g/t Au.
Continuing interest subsequently focussed on the gold content.
Amoco (EL1249) fillet sampled previous drill core and detected
anomalous gold over broader intervals than previously noted (e.g. 140m of
0.23g/t Au in DDH PR8, 95m of 0.6g/t Au in DDH PR3, 34m of 0.8g/t Au in DDH
PR1).
Facies and the textures have suggested metal deposition at low
temperature in open spaces within a volcanic pile that thickens to the south.
From the textural studies both epithermal and exhalative stratiform
origins have been considered in exploration reports.
The Devonian sequence (Baledmund Formation) at Pipeline Ridge has a
largely fine-grained basal interval which abuts the Coonara fault.
It consists of massive to bedded tuffs, with lenses of black siltstone,
chert and minor phyllite conglomerate. It
is overlain, with a sharp contact, by a restricted lenticular unit of pyritic,
siliceous vitric tuff which is the host unit to the mineralization.
This passes up into cherty and pyritic lapilli tuffs which are flanked
and overlain by a variable sequence of conglomerates, agglomerates, quartzite,
chert, granular siltstone and crystal lithic tuffs.
The whole is overlain by siltstone.
The sequence is overturned, and dips steeply to the east with westerly
facings (Leeson, 1980). The
crystal tuffs and cherts are placed in the Sarona Downs Tuff Member.
The source of the cherts has been suggested as exhalative silica.
However, there is also widespread hydrothermal alteration apparent, comprising
silica, sericite, ankerite/siderite and chlorite replacement within the felsic
volcanics. The alteration silica
varies from cryptocrystalline to delicately colloform.
Mineralization is associated with the zones of more intense
silicification, as veins and bands (often colloform).
Leeson (1980) considered that the bulk of the mineralization appears to
have taken place at low temperatures before compaction.
The main mineralization is interpreted as a steep north-pitching system hosted within tuffs in the lower part of the volcanic pile. The mineralized intervals are of subeconomic size and thickness for base metals, up to 3.3% Cu, 3.7% Zn, 4.0% Pb over narrow intervals (1-5 m). The copper-rich sections appear to have higher gold values than the lead-zinc rich sections. The gold occurs in narrow shoots of material above 3 g/t Au grade, separated by areas of elevated geochemical values (0.2-0.5 g/t Au). The high grade gold intersections (e.g. 6m of 5.2g/t Au) lie within a broad easterly dipping mineralized zone, subsequently interpreted as a volcanic or epithermal stringer zone cross-cutting the indistinctly bedded coarse pyroclastics. The easterly dipping zone which encloses the gold shoots is 650 m long, 100 m wide, and has been tested to 220 m depth. A small gossan 500 m north, higher in the volcanic sequence, assayed 0.65% Pb, but appears to have formed from a pyritic crystal tuff containing only traces of fine-grained galena. Gold traces have been obtained up to 900m north of the main mineralization.
Glens Hill (Restdown) Prospect
The Glens
Hill or Restdown copper mine; an old exploratory mine, is located 1.5 km
south of Pipeline Ridge. Copper
ore is noted in Mining Division returns to have been raised here, as late as
1949, although no sales are recorded. Stringer
lead-zinc mineralization is hosted in fine to coarse pyroclastics and
associated pyritic sediments in the lower part of the Devonian sequence (Kopyje
Group). Whereas the Pipeline Ridge
prospect is near the eastern margin of the Kopyje synclinorial structure, the
Glens Hill prospect is on the western side, located on the limb of a minor
anticline. Immediately north of
the old mine, drilling by Cyprus Mines and Mines Exploration encountered minor
base metal sulphides. The
sulphides in the coarser pyroclastics facies occur preferentially in the
matrix. The best reported drill
intersection is 1 m @ 1.2% Pb, 1.8% Zn, 3.5 g/t Ag. Best sample assay from the mine workings is 3.02% Cu, 20 g/t Ag,
trace gold.
One
horizon with trace gold (0.09 g/t Au), and strongly anomalous for
basemetals, has been suggested as exhalative chert similar to Pipeline Ridge
(Amoco Minerals Aust., 1983).
Kopyje Prospect
At the
Kopyje prospect a number of old prospecting pits and a shaft have been sunk in
Devonian basal sediments consisting of conglomerate, sandstone and felsic
pyroclastics. One persistent
horizon of conglomerate and fossiliferous tuff contains a small massive base
metals gossan, which has given assays of up to 2500 ppm Cu, 16000 ppm Pb and
7500 ppm Zn. Some of the
gossan shows textures interpreted as derived from veins of coarse-grained
sulphides. The mineralization is
close to the Devonian/basement contact. An
extensive 45 m wide contact zone with gossanous quartz breccia has
anomalous base metal values to 1100 ppm Cu, 7000 ppm Pb and 3000 ppm
Zn. One long (410 m) narrow
siliceous breccia transgresses the Devonian/basement contact, with the
brecciation particularly pronounced where the shear crosses the unconformity.
Rotary
drilling at this prospect has demonstrated the presence of only weak
mineralization, of undetermined form, in the pyroclastics.
The maximum values obtained from cuttings are 0.22% Cu, 0.50% Pb and
0.30% Zn.
Boundary Fence Prospect
The
Boundary Fence prospect area is large, extending some 3-4 km south of
Boomerang Tank. It is notable for
the presence of ironstone bodies up to 60 m wide.
These occur within, or immediately overlie, a basal pyroclastic
sequence consisting mostly of well-bedded tuffs.
These beds outcrop 50-300 m east of the faulted base of the
Devonian sequence and overlie more massive quartz-veined and pyritized felsic
volcanics. There is bedded
calcarenite and conglomerate low in the sequence.
The
ironstones locally have brecciated fabric and are anomalous mainly in zinc.
Maximum values are 4500 ppm Cu, 200 ppm Pb and 5600 ppm
Zn but most of the ironstone returns negligible values other than for zinc
(Mines Exploration Pty Ltd, 1975). The
pyroclastics which host the ironstones in places carry much disseminated fine
euhedral pyrite. It is thought
that the ironstone bodies have mainly developed from ferruginization of
pyritic siltstone intervals within the pyroclastics, with very little
basemetal sulphide gossan being recognized.
9. Mount Lewis Area
The two
deposits in the Mount Lewis area are:
No.
Deposit Name
Commodities
232
Restdown gold prospect
Au (Zn, Pb, Cu)
233
Mount Lewis prospect
Cu, Pb, Au
Both deposits are in the vicinity of a fault postulated on the Canbelego 1:100,000 geological sheet. At both deposits there are trachyte occurrences, possibly co-magmatic, with an abundance of very fine pyrite. These felsic bodies, perhaps representing feeders of the Devonian volcanism, are of interest if they can be confirmed as intrusive and demonstrated as a source of silicified shear zone mineralization. This would strengthen the hypothesis of a volcanic source for much of the Canbelego belt mineralization, including some of that in basement and near-basement shears.
Restdown Gold Prospect
The
Restdown gold lode is a silicified shear zone up to 1.5 m wide, locally
brecciated, with extensive old workings along parallel quartz veining and
lesser stockwork development. At
surface the lode shows arsenopyrite, trace galena and abundant pyrite. It was discovered in 1893 and thirty men were noted as soon working on
it. Shafts to a depth of 80 m
were sunk along it over a length of 500 m. Gold values of 4.5 g/t to 15 g/t are reported to occur along
a considerable portion of the prospected length but production, if any, is
unknown.
The lode is encased on both sides by phyllite and drilling
revealed the metasediments to be graphitic below oxidation base.
Some chert breccia is rich in sphalerite.
Drilling did not confirm the continuation of rich gold values at depth
and results are typically below 0.5 g/t.
Best base metal assay values are 0.15% Cu, 0.43% Pb, 0.45% Zn.
A slightly
porphyritic albitized trachyte occurs at the southern end of the prospect and
is probably intrusive. This rock
contains abundant very fine-grained disseminated pyrite.
The pyrite may contain base metal sulphide inclusions, as a thin
surface staining on the weathered igneous rock carries up to 1% Pb, 1% Zn and
15 g/t Ag.
Mount Lewis Prospect
This site,
500 m south of Mount Lewis homestead, has only shallow pits and the
hostrocks are little known. Maximum
values from surface materials, including ironstone, are 280 ppm Cu, 268 ppm
Pb, 1100 ppm Zn and 2 g/t Au. The
source host rock appears to be a potassic or sodi-potassic trachyte very rich
in minute pyrite crystals, which invites comparison with the intrusive rock at
Restdown lode.
10. Hartwood Gap Area
This minor
area has base metal values associated with metamorphosed mafic rocks in the
basement. Mines Exploration Pty
Ltd (Bluck 1974) considered from the magnetic pattern that large masses of
basic rock may occur within the Girilambone Group along the axis of the Barrow
Syncline.
The deposits in the Hartwood Gap area are:
No.
Deposit Name
Commodities
238
Bradbury's show
Cu, Pb, Zn
239
Hartwood Gap prospect
Cu, Zn
Bradbury's Show
At this
site, on the eastern limb of the Barrow Syncline, a sulphide gossan is
associated with a marginally sheared basic mass at or near the
Devonian/basement contact. The
igneous mass is a foliated hornblende-plagioclase rock, probably gabbro rather
than diorite originally, with the hornblende now mostly altered to chlorite.
The surrounding rocks are indurated but it is unknown if this is a
contact metamorphic effect. Base
metal values at the site reach 1.2% Cu, 2.5% Pb and 4300 ppm Zn. Mines Exploration Pty Ltd recorded the host rock as magnetite-bearing
chloritic diorite (Bluck 1974).
Hartwood Gap Prospect
This
locality has trace to very weak mineralization, encountered both in drilling
and in old workings (Rankin's show). Values
are generally quite low but reach 1000 ppm Cu, 430 ppm Pb, 1200 ppm
Zn, 465 ppm Ni, 715 ppm Co. Old
workings occur both in basement rocks and in Devonian felsic volcanics. Several small shafts are collared through the sheared basal breccia of
the Barrow Range member. A narrow
gossan appears to cut across the basal phyllite-bearing breccia. One line of shallow drilling showed the low base metal values within
the basement to strengthen somewhat as the sheared contact with the overlying
Devonian rocks is approached.
The
Girilambone Group includes sheared basic igneous rock. Percussion drilling encountered very fine grained pyrite, pyrrhotite
and magnetite disseminated in the metasediments. Minor sphalerite occurs as inclusions in pyrite.
11. Babinda Area
Old
workings at Babinda appear to have mostly prospected gossanous ironstones with
traces of secondary copper sulphides and copper carbonates. Mineralization in the area extends from within the Devonian volcanics
to as low as the basal conglomerate or breccia. Ironstones also occur in basement rocks, sometimes with noteworthy Ni
and Cu content.
The
deposits in Babinda Area are:
No. Deposit Name
Commodities
240 McManus prospect
Zn (Mn)
241 Babinda copper mine
Cu, Au, Ag
242 Lord Dudley mine
Zn, Pb, Cu
243 Babinda South prospect
Cu (Ag)
The main
mineralization is at Babinda copper mine. This is shear-related base metal mineralization, close to the
Devonian/basement contact, with some high gold and silver values. It is thus comparable with the mineralization setting in the
Burra-Hermitage and South Canbelego mineral areas. Along an interpreted anticlinal axis (Canbelego 8134 geological sheet)
the next outcrop area exposing the Devonian/basement contact is 2 km
southwards; where another extensive network of shafts, pits and costeans has
been developed to prospect narrow steeply dipping veins and quartz-limonite
breccia zones (Cyprus Mines Corp., 1969).
Mineralization
in the Babinda area is not confined to the vicinity of the Devonian/basement
contact and weak base metal mineralization occurs within overlying volcanics
at the Lord Dudley mine. Weak
mineralization within these volcanics is also known to continue intermittently
to the south, into the Nymagee sheet area where a 2 m sulphide zone
assaying 0.2% Cu, 0.2% Pb, 1.9% Zn and 4.5 g/t Ag is developed in the
next known occurrence (Robinson & Edgecombe, 1980). This is similar to the best intersection at Lord Dudley mine.
Babinda Copper Mine
The
Babinda deposit is located in rocks exposed in an anticline core below the
Babinda volcanic pile, close to the Devonian/basement contact. Workings are developed mainly within the Girilambone Group
metasediments but are sometimes collared in and pass through gossanous
Devonian basal conglomerate. The
workings cut narrow veins with up to 7% Cu, 31 g/t Au and 367 g/t
Ag. Copper sulphide float (39.4%
Cu), perhaps from the old workings, has also been found in the vicinity (Mines
Exploration Ltd, 1982). The
mineralization appears to be shear-related and at depth pyrite and
chalcopyrite occur both as disseminations and breccia fill. Several gold assays of mineralized dump samples average 2g/t Au.
Lord Dudley Mine
This
prospect is near Babinda Trig Station, in rhyolitic volcanics in the eastern
limb of the anticline hosting the Babinda deposit. The extent of mineralization is undetermined beyond what is apparently
a very localized dissemination in altered volcanics about a fracture trending
065oT. Chalcopyrite occurs as
disseminations, and within amygdules, in highly choritized and brecciated
volcanics. Minor sphalerite,
galena, bismuthinite and native bismuth are also present. The sphalerite contains abundant exsolved chalcopyrite.
The mineralization is considered to be hydrothermal, perhaps
synchronous with the development of the Babinda volcanic centre. The best diamond drill hole intersection is 1.5 m (true width 31 cm)
of 0.1% Cu, 0.2% Pb and 1.2% Zn.
Discussion of the Canbelego Belt
1. Fault/breccia/silicification gold deposits, with trace base metals,
often in the vicinity of Devonian/basement contacts. Besides common trace lead, zinc and copper, these deposits occasionally
show noteworthy antimony (e.g. 828 ppm Sb at Boppy Boulder mine, 2650 ppm Sb
near Boppy Blocks mine, 2150 ppm Sb at Mt. Boppy mine).
2. Mixed Cu/Pb/Zn deposits hosted close to the Devonian/basement contact,
and known (where discernable) to lie in a basal interval of the Devonian
sequence which is noteworthy for conglomeratic and calcareous facies. Trace gold is present.
3. Low grade Cu/Pb/Zn/Au deposits higher in the Devonian sequence.
These have the mineralization hosted mostly in pyroclastics, and
disseminated pyrite is common.
There has been little agreement as to the likely genesis of the
deposits. As for the Cobar belt,
suggestions have been made which range from epigenetic to syngenetic, and
include volcanic exhalative.
Andrews (1913) regarded the Mount Boppy lode as having formed
by hydrothermal replacement, with solutions following an overall synclinal
structure of the sediments. He
regarded the solutions as also being responsible for silicification of
sediments. As with the Cobar belt,
metahydrothermal activity could be envisaged.
The minor chalcopyrite of the Mt. Boppy lode occurs as rare discrete
grains and as minute exsolution belbs in sphalerite.
The latter form indicates a minimum temperature of 350o-400oC (Buerger
1934). Rare cubanite grains,
associated with chalcopyrite, confirm ore-forming temperatures in excess of
250oC (Ramdohr 1969).
Early workers (Jaquet 1905, Andrews 1915) described the Mt
Boppy lode as occurring within tightly folded micaceous sandstones and slates,
then regarded as Girilambone Group beds. Subsequent
mapping by the New South Wales Geological Survey (e.g. Gilligan 1977)
interpreted the host strata to be Early Devonian marine sediments (Baledmund
Formation) immediately overlying Girilambone Group rocks.
Repetitions of this apparent stratigraphic control have since been
sought at many other mineralization sites along the Canbelego belt.
In the 1970s, syn-sedimentary interpretations were developed
for most major ore deposits in the Cobar-Canbelego-Nymagee region, including
the Mount Boppy mine (e.g. Gilligan 1977, Gilligan & Suppel 1978).
Gilligan (1975, 1977, 1978, 1982) regarded the Mount Boppy
mineralization as a syn-sedimentary chert buildup that became enriched in gold
and metallic sulphides, the source of the metals being volcanic exhalations.
Brecciation of this chert horizon then occurred during subsequent
deformation of the rocks. Gilligan
accepted that much of the gold mineralization in the Canbelego area is
fault-controlled. However, he
suggested that the mineralization in faults could have been remobilized
downwards into them from the basal Baledmund Formation wherein he believed the
metals to be of largely volcanic exhalative origin.
Following Gilligan, many other writers have remarked that one
of the main features of the Canbelego belt is the concentration of
mineralization in proximity to observed Devonian/basement contacts.
Some have suggested that deposits are preferentially associated with
conglomeratic or calcareous strata close above basement.
As noted for the Mt. Boppy lode, some caution is needed in the
interpretation of basal conglomerate or breccia.
The distinction between tectonic breccias, Devonian basal sedimentary
breccias, and sheared conlomerates would be difficult at some prospect areas
(e.g. Mt Boppy mine, Kopyje prospect). Occasionally
a body mapped as "basal breccia" has been subsequently more
carefully traced and found to transgress the Devonian/basement contract, which
reveals the "sedimentary" breccia as having a clearly tectonic
origin. It can be difficult in the
absence of diamond drill core to decide whether poorly exposed breccias along
the contact zone are sedimentary or tectonic.
Fault breccia in basement has more than once been misidentified as
basal Baledmund Formation (Gilligan 1982).
Such misidentification at North Boppy mine resulted in fruitless
developmental work in supposed basal conglomerate.
Although it is of minor volumetric importance, special
attention has been focussed on calcareous facies.
Explorationists have postulated that carbonate-rich beds may have acted
as a prime control on mineralization. The
central area of the Kopyje basin is where the largest expanses of well bedded
micritic limestones and calcarenite outcrop.
Elsewhere, calcareous facies are quite sparse yet appear to be
widesperad towards the base of the Devonian sequence around Canbelego.
Mapping by Penarroya (Aust.) Pty Ltd (1978) distinguished a basal
calcareous or dolomitic siltstone sequence in the Canbelego basin.
Subsequent explorers have attemped to further delineate a basal
carbonate-bearing facies which appears to be characterized by sparse shelly
fossils and fragmental limestone. Known
occurrences are in drill core from West Boppy mine, in drill core from the
Burra mine area, interpreted silicified limestone and bands of calcareous
sediment with brachiopods and corals in outcrop at Burra mine, sheared breccia
of coral and siltstone fragments in Freeport DDH 5 at Hoffnung shaft,
calcareous rock in drill core and interpreted silicified calcarenties at
Hardwick propect, carbonate rocks in DDH C5-1 at Boppy Mount, and drill cores
at C2A prospect. Outcrops
containing the highest gold values (up to 10 g/t) in the Hardwick prospect
area were interpreted by Freeport of Australia (1986) as silicified
calcarenite. The calcareous
facies, possibly forming a continous sub-volcanics interval in the Canbelego
basin, contains scattered skeletal detritus from crinoids, corals,
brachiopods, gastropods, trilobites, bryozoans and tentaculitids, suggesting
an open marine fauna. Also locally
common are small nodular structures (1-4 cm) which are thought to be carbonate
concretions (now leached wherever seen). These
are known from drill core (e.g. DDH DCB1 from West Boppy mine) and are also
known from outcrop. Drill hole DDH
DCB1 interesected about 20 m of formerly calcareous siltstone, with lenses of
fragmental limestone, before passing into the Girilambone Group (GS1981/154).
The core shows numerous elongate ironstained cavities which appear to
be after ramose or phaceloid corals. Crinoidal
limestone reported from the Hoffnung workings is also probably fragmental, as
Freeport's drill hole DDH5 at the Hoffnung shaft intersected a 3 m interval
rich in limestone rubble. Although
such limestone may appear allothchonous, the concentration of possible
phaceloid coral fragments at West Boppy mine suggests that no great distance
of transport need be involved.
Deposits thought to lie close to Devonian/basement contacts,
and likely to be in basal conglomeratic and/or clacareous strata, include the
Mount Boppy gold mine, various copper mines (including Burra and Block 51
mines), Priest's shaft, Canbelego Peak mine, Silver-Lead shaft, Mouramba shaft
and the C2A prospect. Of these the
best data is from the Burra mine area and the C2A prospect.
At both of these, there is present a thin interval of sediments beneath
the Florida Volcanics, with conglomerate, siltstone, mudstone and limestone
evident. The contained ore
minerals are pyrite, pyrrhotite, sphalerite, galena and magnetite.
The sphalerite and galena show some slight preference for the
limestone. Gilligan (1982)
regarded the mineralization as stratabound and syngenetic, with the proximity
of volcanics suggesting an exhalative origin.
Ramsden and Ryall (1979) also interpreted the C2A prospect as a
stratiform volcanic exhalative deposit.
Whereas some authors have inferred a horizon of widespread
syn-sedimenary ore deposition, Rayner (1969) merely considered that the base
of the Devonian sequence around Canbelego formed a favourable locus for ore
deposition because of contrast in lithological competency during deformation.
Likewise ore deposition in sediments close beneath more competent
volcanics could be considered a structural corollary, without any necessary
genetic relationship to the volcanics. Rayner
drew analogy with the concentration of ore deposits along the slate side of
the slate-sandstone discordant contact (Great Cobar Slate - Chesney Formation)
at Cobar. Moreover, the various
mineral deposits found in Devonian rocks at Devonian/basement contacts are not
necessarily at the same stratigraphic level.
It is to be expected that any concentration of faulting in proximity of
the cover rock fold limbs will often wedge out the actual unconformity.
Some Devonian/basement contacts may be major faults along which
synclinorial structures are depressed into basement.
Another quite different line of explanation could involve early
mineralization along a relatively low-angle detachment, which has since been
folded and locally re-sheared.
Neither a plain hydrothermal origin such as Andrews (1913)
envisaged nor the more complex origins such as elaborated by Gilligan (1982)
can be discounted on present evidence. However
there is little to recommend acceptance of a syngenetic origin of the Mt Boppy
lode unless one also regards the Canbelego area base metal deposits which
occur close above basement as syngenetic.
Of special interest for future study of this question is the C2A
prospect, since the basal Baledmund Formation there is reported to be
relatively free of cleavage and shear effects.
Also likely to be obtained in future, but currently unavilable, will be
drill core of the Mt Boppy lode. This
is important to determine the nature of the Mt Boppy lode channel, as
primarily a fault or a key stratigraphic interval.
It has been idealized that basal Baledmund Formation in the lode
syncline consists of breccia and conglomerate grading up into siltstone and
sandstone. Andrews (1915) noted
the eastern leg to occur above conglomerate.
The crenulated phyllite of the basement was thought to be separated
from the siliceous lode by up to 3 m of breccia or polymictic conglomerate.
The lode has typically been considered to have a hangingwall of
silicified stilstone and a conglomeratic footwall.
In the mine, the footwall was regarded as better defined than the
hangingwall (Andrews 1915). Gilligan
(1982) considered the typical sequence to be a breccia or conglomerate zone
grading up into the net-veined lode interval which in turn grades up into
unfractured micaceous siltstone. The
siltstone may contain sandy interbeds 5-20m thick, and may be silicified for
up to 2 m above the quartz-veined interval.
Where silicified it may contain numerous very narrow quartz veinlets
parallel to cleavage. This
detailed picutre was unfortunately not confirmed in the Epoch Mining NL
excavation along the main lode. The
conglomerate is perhaps locally absent near surface but diamond drill core is
desirable to confirm or refute the lode's reported conglomeratic footwall
deeper in the mine.
BALLAST
'SUB-BELT'
The
Ballast mineral belt is an uncertain entity or grouping of prospects.
On the basis of hostrocks it could be placed in affinity perhaps with the Girilambone Group. However on geographic basis it is clearly more akin to the Canbelego, especially if it is imagined that if the northern 'continuation' of the Canbelego age mineralisation belt (Devonian) had the Devonian rocks removed then what might be left could be the lower roots of this set of mineralisation events, in older underlying Ordovician strata. That is the current favoured thinking, and hence the reason for placement here of the Ballast belt mineral areas as a sub-entity of the Canbelego Belt..
Unlike the three major mineral belts described for the Cobar sheet
(Cobar, Canbelego and Girilambone belts) the Ballast belt of sub-belt seems
quite speculative rather than empirical. Compared to the main belt
groupings it is based on very few actual
indications of mineralization (and the belief that more are likely to be
discovered, i.e. that it appears to be a 'favourable' tract).
Its
interpretation rests heavily on the rough co-incidence of a discontinuous zone
of high magnetic intensity and the earlier defined belt of "Ballast
Beds" which trends prominently across the centre of the Cobar sheet (e.g.
Rayner 1961, Brunker 1967). Intermediate
to mafic volcanics inferred from the magnetic pattern may be an extension of
those known in the Bourke 1:250,000 sheet area at Bald Hills.
The
Ballast belt is roughly co-linear with the eastern (type area) belt of
"Ballast Beds" (Andrews 1913, 1915;
Brunker, 1969). The
relationship between the "Ballast Beds" and rocks further east,
placed within the Girilambone Group, remains unclear. The
"Ballast Beds" are now regarded as part of the Girilambone Group
(Felton, 1981). Others who regard
the "Ballast Beds" as separate have usually viewed them as a younger
sequence. Rare grits are reported
from the "Ballast Beds" in the Ballast belt, which is thought to
provide a further contrast with the Girilambone Group (e.g. Paull, 1986).
Rayner (1969) noted that the "Ballast Beds" can be traced
almost the whole length of the Cobar sheet, from the Panjee-Bobadah area in
the south to at least as far north as Drouin Trig. Station (Mount Drouin).
The line from Panjee Trig. through Drouin Trig. Station is the trend of
the high magnetic intensity zone, and of the Ballast belt.
Between the Highway, at Florida railway siding, and Mount Drouin a
discontinuous line of quartzite, conglomerate and pebbly sandstone rises
occurs. Magnetic anomalies
suggestive of ultramafic rocks occur east of this arenaceous sequence to the
north of Florida siding. The
eastern boundary of the Ballast Belt is taken to lie between the coarse
arenaceous sequence and the zone of high magnetic intersity.
The conglomeratic facies rocks have sometimes been referred to the
Cobar Supergroup but are currently interpreted as part of the basement.
Pebbles of vein quartz are rare to absent in these conglomeratic beds,
which greatly contrasts with the quartz pebble conglomerate of the Cobar
Supergroup at Mount Boppy. The
zone of high magnetic intensity trends north-northwest (340oT) across the
sheet area, from 146o37'E in the south to 146o13'E in the north.
In the Bourke 1:250,000 sheet area, the magnetic zone about the Bald
Hills-Mount Dijou gold mining centre is a suspected continuation, offset to
the west.
The
assumption that the high magnetic intensity zone in the Ballast belt reflects
magnetite-bearing volcanics is based on knowledge from three areas:
Bald Hills-Mt Dijou where deformed and metamorphosed basalts crop out;
Pooraka anomaly south of Ballast quarry, where drilling intersected
altered intermediate to basic volcanics; and
the Harts Tank area where acid volcanics and intermediate to basic volcanics
are reported from drilling.
There is
relatively little available to give an overview of the Ballast belt (viz. Pan
Australian Mining Ltd, 1985-1988; Union Corporation Aust. Pty Ltd, 1974-1975).
The nature of the separation of Ballast belt rocks from Canbelego belt
rocks has for the most part been little considered.
Where it has been mapped, in the southern sheet area, the boundary is a
major fault (the Coonara Fault) suspected to truncate the more northwesterly
synclinorial trends of the Devonian strata lying to the west of it.
The Ballast mineral belt comprises two mineral areas.
These are:
1.
Cochranes-Pooraka Area
2.
Harts Tank Area
1. Cochranes-Pooraka Area
The
prospects within the Cochranes-Pooraka area are:
No. Deposit Name
Commodities
25 Cochranes Find
Fe (Zn)
79 North Pole prospect
Pb, Au
80 Chert Ridge prospect
Pb (Zn, Cu)
81 Florida Trig. prospect
Pb
194 Pooraka anomaly
Cu (Zn)
This is a
poorly known area. Cochranes Find
comprises a 1.5 km long zone of weak ironstone development and abundant
quartz veining located a little south of the Bourke-Cobar sheet boundary.
The zone is about 50 m wide, with about 20% ironstone development
across the interval. The ironstone
is ferruginized breccia in places but mostly appears to be replaced phyllite
and siltstone. The ironstaining is
probably the weathering product of a disseminated pyrite zone, as siltstone
shows up to 10% volume of leached pyrite. Base metal content from limited sampling is quite low e.g. 50 ppm
Cu, 100 ppm Pb, 360 ppm Zn. A strong NNW magnetic tend and inferred faulting (Eagle Fault) occurs
in the area, passing just west of Mount Drouin. Carpentaria Exploration Co. (GS1971/357) noted that a low ridge,
paralleling the trace of the Eagle Fault and immediately to the east of it,
extends north from Mount Drouin to just south of Noel's Tank. Mildly compressed quartzite conglomerate outcropping at the north end
of this ridge is very similar to the conglomerate outcropping at Mount Drouin.
Intervening outcrop along the ridge consists of more typical
"Ballast beds" sediments. Several
shallow pits have been dug into quartz reefs which obliquely transect this
ridge. One shallow shaft (ca. 9m),
approximately 9 m, is
located on the eastern side of this ridge about 1.5 km north of Mount Drouin,
and another is said to exist in the same vicinity. Other old workings are reported to occur about 1 km south of Mount
Drouin. Thin ferruginous quartz
veins along the eastern fall close to Drouin Trig station have also been
prospected.
veined and ironstained host rocks.
These include chert, magnetite-bearing chert, phyllite and intermediate
tuff. Only thin gossans are
developed but some outcrops are quite jasperous.
Minor pyrite, pyrrhotite, galena, sphalerite, arsenopyrite and
chalcopyrite are present (e.g. North Pole prospect).
Quartz veining is not a dominant feature at these prospects and it has
been suggested that the mineralization is of disseminated stratabound pyrite
type (Plumridge 1978, Dampier Mining Co. Ltd. 1980).
Drilling
at the Pooraka anomaly (a 900 gamma aeromagnetic anomaly coincident with a 0.9
milligal gravity residual) intersected a suite of altered intermediate to
basic volcanics, including scoriaceous and pyroclastic rocks.
Iron content is high, up to 36% Fe, and the chief magnetic mineral is
magnetite. Sulphides comprise rare
very fine-grained pyrite, chalcopyrite and sphalerite.
Trace metal values are up to 270 ppm Cu, 36 ppm Pb, 250 ppm
Zn, 280 ppm Ni.
2. Harts
Tank Area
The Harts
Tank area contains the major cluster of prospects known along the Ballast
belt. The prospects in the Harts
tank area are:
No.
Deposit Name
Commodities
487
Bill's Retirement
Au(?)
486
Upton prospect
Zn, Pb (Cu)
488
Henry's Hill prospect
Cu (Zn)
413
Harts Tank bismuth prospect
Bi
517
Henry's Hotel prospect
Au
485
Colin's prospect
Au (Bi)
489
Road shaft prospect
Pb (Zn, Cu)
411
Kypros snake prospect
Pb (Ag)
412
Rainbow Ridge prospect
Pb (Au, Zn, Cu)
484
Charlie's Hope
Au (Pb)
518
West Hudson prospect
Pb (Au)
On the basis of aeromagnetic linears, the Harts Tank area could be regarded as lying at the southwestern end of a strong fracture system extending from the Nyngan area (R. Spencer 1988, pers. comm.). Where the interpreted fracture system cuts the Ballast Belt the presence of volcanic rifting and epithermal activity has been postulated (Pan Australian Mining Ltd. 1985, 1986).
The
Harts Tank area contains widespread minor old prospect workings, mostly very
shallow, about which little is recorded. It
is locally stated that the area was prospected for gold after it was
discovered by a Mr Harts (Hartz?) who resided locally.
The area has since then had considerable percussion and diamond
drilling carried out (Pan Australian Mining Ltd. 1985, 1986).
No significant mineralized interval has been discovered but the
widespread nature of high trace values and rich spot sample assay results
remains encouraging.
Throughout
the area outcrops of resistant siliceous rock high in iron, and often high in
manganese, have attracted prospecting. The
nature of the original rock is frequently obscure.
The gossanous zones generally seem conformable and some have been
traced intermittently for long distances.
Occasionally the ferruginous zones are cross-cutting.
Quartz veins may be parallel to cleavage or cross-cutting.
The most important cross-cutting trend is northerly but minor
northeast-northwest quartz veining also occurs.
Widespread geochemical anomalism is associated with the
ferruginous siliceous outcrops and other rock types.
Lead appears to be considerably enriched near surface.
At Rainbow Ridge prospect a 550 x 20 m lead anomaly contains
gossanous outcrops with up to 12% Pb and 9.1 g/t Au.
Elsewhere surface and shallow bedrock samples, including siliceous
gossan, have yielded values up to 2300 ppm Cu, 3.8% Pb, 2% Bi, 4800 ppm
Zn, 1600 ppm W, 155 ppm Sb and 300 ppm Mo.
Gold is mostly below 1 ppm. The
highest tungsten values are from gossan but up to 145 ppm W occurs in
rhyodacite dykes. Bismuth is a
prominent trace element along a small fault zone passing through the Harts
Tank bismuth prospect. This
quartz-filled fault zone has a trend of 015oT and cross-cuts a sequence of
phyllite, and quartzite trending 340oT. Bismuth
values (271-1050 ppm) are present over much of the length of this fault,
accompanied by up to 1 m thickness of intense silicification.
Bismuth is also noted elsewhere in the area (North of Upton prospect,
280 ppm Bi; Colin's prospect,
1750 ppm Bi; Charlie's Hope, 210
ppm Bi).
The
mineralisation in the Harts Tank area is associated with a volcanogenic-sedimentary
pile. A centre of acid volcanism
has been suggested for the Harts Tank-Panjee Trig. Station area by Pan
Australian Mining Ltd (1985, 1986). Rock
types present include radiolarian chert, phyllite, black graphitic pelite,
siltstone, sandstone, quartzite and volcanics.
Some of the siliceous radiolarian sediments are carbonaceous, and
framboidal pyrite is widespread. Pan
Australian Mining Ltd (1985, 1986) has described a sequence cut by rhyodacite
dykes and sills, in addition to differentiated mafic intrusives.
Quartz-veined altered andesite and interbedded sediments overlie
spilites in the eastern part of the area.
Andesite, however, is uncommon and the igneous suite is essentially
bimodal. The volcanics are largely
metamorphosed basic tuffs and volcanicalstics.
However, rhyodacitic and rhyolitic crystal tuffs are also widespread.
Tuff preserving flattened vitroclastic texture occurs at Henry's Hill
prospect. The rhyodacitic rocks
include prominent felsic intrusives at the Road Shaft prospect.
The weak
to trace mineralization of the area includes pyrite, pyrrhotite, magnetite,
sphalerite, chalcopyrite, galena, tennentite, gold, scheelite and a bismuth
mineral. The mineralization occurs
in disseminated form in narrow shears with quartz-ironstone breccia, or in
quartz-filled fault zones. Considerable
quartz-sulphide-carbonate veinleting may be present and carbonate is also
common in breccias. In some cases
carbonate has been substantially replaced by silica.
At Rainbow Ridge prospect carbonate is confined to veins, with none
detected in the host rocks. At
Upton prospect brecciated rock is healed with quartz-calcite, within a
mineralized zone containing intense silicification and minor semi-massive to
massive pyrite. Alteration at the
Harts Tank area prospects includes chloritization, sericitization and
argillization. At the Road Shaft
prospect, extensive argillization and sericitization of the volcanics is
interpreted as potassic hydrothermal alteration (Pan Australian Mining Ltd,
1985, 1986). Rare tourmaline
occurs as a member of the alteration assemblage.
Pan Australian Mining Ltd suggested, from the nature of the alteration
that the Harts Tank area may contain an epithermal system.
In the disseminated mineralization there may be up to 10% fine
sulphides over short intervals. Pyrite
occasionally forms semi-massive to massive bands a few centimetres thick and
clots of fine-grained galena may reach 4 mm size.
No signficant intersections of sulphide mineralization have been made.
However, secondary zone lead values as high as 3% may persist over
intervals of 2 m or more.
REFERENCES
Click
here for list of Cobar 1:250,000 sheet references
RELATED FILES
Cobar index = http://dl.dropbox.com/u/5737284/cobar-index.htm
Cobar's mining history = http://dl.dropbox.com/u/5737284/cobar-mining-history.htm
Cobar 1:250K sheet references = http://dl.dropbox.com/u/5737284/cobar-refs.htm
The Cobar Belt of mineral deposits = http://dl.dropbox.com/u/5737284/cobar-belt.htm
The Canbelego Belt of mineral deposits = http://dl.dropbox.com/u/5737284/canbelego-belt.htm
The Girilambone Belt of mineral deposits = http://dl.dropbox.com/u/5737284/girilambone-belt.htm