The Cobar Mineral Field lies within the Lachlan Fold Belt and is located in central New South Wales, approximately 550 km WNW of Sydney in Australia.
It comprises a north-south cluster of structurally controlled gold, copper and lead-zinc deposits distributed along a linear, largely shear controlled contact between two early Devonian units, the sandstones and siltstones of the Chesney Formation and the overlying siltstones and mudstones of the Great Cobar Slate. This cluster of occurrences is distributed in a corridor extending over an interval of some 11 km, and includes deposits at The Peak (Au) near the southern end, CSA (Cu-Pb-Zn), Great Cobar, Chesney, Occidental and Coronation-Queen Bee (all Cu-Au). This shear zone is close to the eastern margin of the Devonian rift into which these sediments were deposited. The Elura Zn-Pb-Ag deposit within the same general setting is some 40 km to the NW (see the Elura record). The Peak and CSA deposits are 8 km SSE and 11 km NNW of the town of Cobar, respectively.
Within this broad corridor, the lodes lie along three separate lines which are zones of increased deformation and shearing within slates and sandy slates, and are reflected by persistant lines of high magnetic intensity. The parallelism with rock units suggests a stratigraphic control, with the lines being within the CSA Siltstone (the CSA Line, containing the CSA and Spotted Leopard lodes), the middle section of the Great Cobar Slate (the Western Line, carrying the Great Cobar, Dapville and Gladstone lodes) and in the Great Cobar Slate at its contact with the Chesney Greywacke (the Eastern Line which hosts the New Cobar, Chesney, New Occidental, The Peak and Queen Bee lodes).
Within the district and along these lines there is a zonation of metals, with CSA in the north carrying Cu with abundant Pb and Zn. Further to the south, Great Cobar has Cu, a little Au and minor Pb-Zn, passing south again to New Cobar and Chesney which are Cu-Au deposits. Continuing this trend southwards, New Occidental is an Au depost, and then to The Peak which carries Au-Ag, while on the southern limit the Queen Bee reverts to Cu with Pb-Zn.
The zones of deformation vary from 15 to 180 m in width and up to 500 m in length and are strongly cleaved, persisting with depth, with mineralisation throughout, hosted by dark green to black greenschist facies chloritic slate, in contast to the enclosing light, grey-blue slates. All of these deposits and their constituent lodes are discordant to the hosts which are folded, cleaved, early Devonian thin bedded turbidites, and lie at an oblique angle to bedding. The Cu-Au and Cu-Zn deposits are structurally controlled and lie in zones of silicification on, or adjacent to, the controlling faults, occurring in veins (generally quartz) and as disseminations in silicified host rock or as structurally controlled massive sulphide lodes. All of the orebodies in the main Cobar field are tabular to rod like zones of veining and mineralisation up to 300 m in length, 6 to 20 m thick, with dips that are mostly vertical, with pitches averaging around 80°N.
Three primary ore types have been recognised, namely: i). Siliceous ore, in which disseminated gold and chalcopyrite occur in brecciated slate which are characteristic of the Eastern Line, with the exception of Queen Bee, and include pyrrhotite, pyrite and minor magnetite, galena, sphalerite, arsenopyrite, native bismuth, bismuthinite and cubanite. At New Cobar and Chesney chalcopyrite exceeds pyrrhotite, while at The Peak and Ne Occidental the reverse is the case. ii). Siliceous pyritic ore, which comprise chalcopyrite and pyrite in a siliceous gangue as at Gladstone, Queen Bee and some of the CSA lodes, with subordinate arsenopyrite and marcasite with only traces of gold, magnetite and pyrite. iii). Massive sulphide ore, mainly composed of pyrrhotite-chalcopyrite-magnetite at Great Cobar, passing downwards into siliceous magnetite-pyrite-chalcopyrite at depth. At Dapville the ore is principally massive pyrrhotite-magnetite-chalcopyrite with minor arsenopyrite, cubanite and tetrahedrite. At CSA there is a variation from massive pyrite-pyrrhotite-chalcopyrite in the pyritic Western Lode while adjacent Pb-Zn ore is massive, fine grained galena and sphalerite accompanied by pyrite and pyrrhotite. In contrast the Eastern Lode at CSA, is siliceous, with more pyrite and pyrrhotite associatec with chalcopyrite.
The main deposits in the field are:
The CSA deposit comprises four main ore systems which all strike approximately north and dip steeply to the east, grossly sub-parallel to the cleavage orientation. Mineralisation is present as a number of vein complexes or as sub-massive to massive sulphide bodies, known as lenses, defined by a sufficient density of veining to attain ore grades. The massive ore has 70 to 90% sulphides. Most lenses have a steep northerly plunge parallel to the dominant lineation with strike extents rarely exceeding 80 m and thicknesses of 6 to 20 m, but persisting down plunge for several hundred metres. A broad halo of chloritisation surrounds the individual lenses with a lateral extent of up to 50 m, commonly with associated silicification.
The four systems are the:
Western System - comprising relative small lenses with average strike lengths of 45 m, widths of 7 m, which are developed over a northerly strike length of around 300 m and comprise high grade pods of Cu rich veins of chalcopyrite, pyrrhotite, pyrite and quartz, and oftern banded Pb-Zn rich sulphide lenses of galena, sphalerite, pyrrhotite, pyrite and chalcopyrite. The average grade of the lenses was 6% Zn, 3% Pb, 2% Cu, increasing in Cu with depth. The local hosts are chloritised (Mg rich), silicified and quartz veined, with pervasive silicification.
Eastern System - which consists of more than 4 lenses, each of 50 to 80 m in length and variable widths averaging 10 m. Each comprises a number of vein systems which average 3% Cu. Mineralisation is predominantly chalcopyrite, pyrrhotite in quartz and/or pyrite within intensely cleaved chloritic siltstone. The top of the ore zone is below 250 m depth. A subsidiary CZ zone between the Eastern and Western Systems averages 10% Zn, 1% Pb, <1% Cu.
QTS North - which is a blind system which occurs approximately 100 m east of the Eastern System. It comprises a series of lenses that are <100 m in length and average 10 m in width, with a northerly strike and steep east dip. All of the lenses are Cu rich, averaging 5% Cu, with no significant Pb or Zn. The mineralogy is predominantly chalcopyrite and pyrrhotite. Cu garde increase to the east, with the westernmost being similar in grade to those of the Eastern System.
QTS South - is also a blind system, approximately 500 m south of the other systems and 100 m east of QTS North. It comprises several sub-parallel, north-striking, up to 200 m long and 8 to 20 m wide lenses that dip steeply east. It is Cu rich with subsidiary pyrrhotite and isolated pods of galena and sphalerite on the extremities of the system
Production from the CSA mine from 1961 to 1995 was 20 Mt @ 2.11% Cu, 0.62% Pb, 1.98% Zn, 22 g/t Ag
- resources at 1996 were 27.8 Mt @ 3.75% Cu.
Proved reserves in 2002 were - 0.71 Mt @ 6.01% Cu, plus an inferred resource of 4.97 Mt @ 3.90% Cu.
The Peak - see the separate "The Peak" record.
The total production + reserves in 1995 was 4.6 Mt @ 7.6 g/t Au, 0.9% Cu, 1% Pb, 0.9% Zn, 8 g/t Ag.
Resource and reserve figures published by Peak Gold Ltd (2008), as at 31 December, 2006, were:
Measured + indicated resources - 3.52 Mt @ 4.05 g/t Au, 1.12% Cu
Inferred resources - 1.99 Mt @ 6.5 g/t Au, 0.53% Cu
Proved + probable reserves - 1.68 Mt @ 6.75 g/t Au, 0.74% Cu.
The resources are exclusive of the reserves.
Historic production from the larger of these deposits, prior to the modern development of the CSA and The Peak, include:
Great Cobar - 4.186 Mt @ 2.75% Cu, 2.2 g/t Au, for 9 t Au, between 1871 and 1947,
New Occidental - 2.098 Mt @ 9.8 g/t Au, for 20 t Au, between 1889 and 1952,
New Cobar - 0.985 Mt @ 8.7 g/t Au, for 8.6 t Au, between 1890 and 1948,
Chesney - 0.723 Mt @ 1.2 g/t Au, for 0.88 t Au, between 1887 and 1952.
(Source: Porter GeoConsultancy, www.portergeo.com.au, 2008)