Mining commenced in Cobar in the later 1800’s with the first shipment of copper ore from the Great Cobar Mine in 1871. The area has since become a major NSW mining centre with approximate pre-mining resources of >4.7 million tonnes of zinc, >2.8 million tonnes of lead,>2.2 million tonnes copper, 145 million ounces of silver and >7.0 million ounces of gold.
Current operating mines –
|CSA||Glencore||Cu-Pb-Zn-Ag||December 2012 (Peel Mining website)
Reserves: 6Mt @ 4.6% Cu, 18g/t Ag
M&I Resources: 5.4Mt @ 6% Cu, 24g/t Ag
December 2015 (Coeur Mining website)
May 2006 (Peel Mining website)
|Hera||Aurelia Metals||Au-Cu-Pb-Zn-(Ag)||June 2016 (Aurelia Metals website)
Reserves: 0.98Mt @ 5.11 g/t Au, 16.3 g/t Ag, 2.84% Pb, 3.85% Zn
M&I Resources: 2.7Mt @ 4.12 g/t Au, 34.0 g/t Ag, 3.67% Pb, 4.86% Zn
|Peak Mines||Newgold||Perseverance Au-Cu-Pb-Zn
New Occidental Au-(Pb-Zn)
New Cobar Cu-Au
|December 2015 (Newgold website)
Reserves: 2.87Mt @ 2.89 g/t Au, 6.9 g/t Ag, 1.27% Cu
M&I Resources: 4.1Mt @ 3.37 g/t Au, 7.5 g/t Ag, 1.04% Cu
|Tritton||Aeris Resources||Cu||June 2016 (Aeris Resources website)
Reserves: 11.2Mt @ 1.5% Cu
M&I Resources: 23.89Mt @ 1.5% Cu
Historical mines &/or recent discoveries
|Mallee Bull||Peel Mining||Cu-Ag-Au||May 2014 (Peel Mining website)
I&I Resources: 3.9Mt @ 2.3% Cu, 32 g/t Ag, 0.3 g/t Au
|May Day||Peel Mining||Au-Pb-Cu-Zn||(Peel Mining website)
Reserves: 193,000t @ 2.92g/t Au, 11g/t Ag
|McKinnons||Au-As-Sb (Ag-Zn- Pb-Cu-Hg)||May 2006 (Peel Mining website)
Reserves: 1.8Mt @ 2.0g/t Au
Resources: 4.5Mt @ 1.2g/t Au
|Mineral Hill||KBL under Administration||Cu-Au-Ag-Pb-Zn-Ag||June 2013 (Peel Mining website)
M&I Resources: 3.9Mt @ 1.2% Cu, 1.6% Pb, 0.8% Zn, 38g/t Ag, 1.28g/t Au
|Mt Boppy||Black Oak Minerals in Administration||Au||September 2014
(56905 Statement of Environmental Effects March 2015 )
Reserves: 0.549Mt @ 4.3 g/t Au
Resources: 0.766Mt @ 4.1 g/t Au
Historic production: ~0.5Moz Au
|Nymagee||Aurelia Metals||Cu-Zn-Pb-Ag||December 2011 (Aurelia Metals website)
M&I Resources: 8.1Mt @ 1.2% Cu, 0.3% Pb, 0.7% Zn, 9g/t Ag
The Cobar Basin, located in the central portion of the Lachlan Fold Belt, developed during back-arc extension in Siluro-Devonian times. The basin opening was caused by NE-SW extension and influenced by the late Silurian emplacement of granites. Sedimentation continued until the end of the Devonian with the northern portion dominated by silica-rich sediments with locally intruded felsic volcanics and the southern portion dominated by bi-modal volcanic rocks interbedded with sediments.
During the Middle Devonian, the tectonics inverted with compression oriented in a NW direction. This resulted in open folding and axial plane cleavage development and low angle thrusting initially followed by the formation of complex fault arrays and tight folding particularly in the east.
Mineralisation occurred soon after basin inversion and can be classified into early and late types. The early types include the carbonate-hosted Zn-Pb-Ag at Endeavour and Manuka; epithermal Au at McKinnons Tank, Mineral Hill and Mt Boppy; and VMS Au-Pb-Zn-Cu-Ag styles found at Mt Hope.
Endeavour was once the region’s largest zinc, lead and silver producer but lower metal grades commodity prices forced a reduction in production. The mineralisation is sediment hosted and occurs as near-vertical massive sulphide lenses over a 700m strike length and extending 800m deep. The mineralisation occurs as a massive sulphide+pyrrhotite+pyrite core is surrounded by veins with pyrite, sphalerite and galena.
The main late mineralisation types are the “Cobar style” deposits. These are structurally controlled, have short strike lengths (<300m) and narrow widths (<30m) but extend vertically for several hundred metres to over a kilometre. The lenses are comprised of multiple overprinting sets of veins which range from largely massive sulphide to sulphides in quartz. The sulphides consist of chalcopyrite, sphalerite, galena, pyrite and pyrrhotite and magnetite is present in many of the gold rich deposits. Examples of these “Cobar style” deposits are the Peak, CSA and Hera mines.
Elsewhere the late mineralisation types are lithologically controlled and stratabound but retain similar characteristics to Cobar types such as short strike lengths, narrow widths but generally extend less than a kilometre deep. They are characteristically associated with felsic volcaniclastics and magnetite is rare. Examples of these include the Mallee Bull, Shuttleton and Wirlong occurrences.
The Cobar type deposits are epigenetic although no links to igneous rocks have been observed to date. There is a regional metal zonation trend from Au +/- base metals in the south to base metals-Ag then Ag-base metals northwards. Within lodes vertical and lateral zonation is common.
Although not strictly in the Cobar Basin the Tritton copper deposit 100km east of Cobar is a significant deposit. The mine sits in a band of mines and mineral occurrences from Girilambone in the north to Tottenham in the south with Besshi-style volcanic-associated massive sulphide (VAMS) affinities. The mineralisation is currently believed to be coeval with the deposition of lower Ordovician turbidite sediments and is intimately associated with silica-iron rocks. This recent work provides a framework for a new interpretation of the economic potential in this area.
The early mineralisation styles of deposits such as Endeavour are worthy targets given the grades found there and the fact that it is a recent discovery, early 1970s, using airborne geophysical techniques. The early mineralisation gold deposits are predominantly vein deposits and generally associated with sulphides. Geophysical techniques such as Induced Polarisation and Electo-Magnetic are useful tools for their exploration.
Cobar style deposits remain an attractive exploration target due to the long vertical extent of the mineralisation and the relatively high grades. The relatively small surface footprint of these deposits makes them a challenging target. However, a common feature of Cobar deposits is the associated topographic high due to the presence of silica alteration. And the occurrence of pyrrhotite and magnetite in many deposits makes geophysical techniques such as aeromagnetics an effective tool.
Tritton style mineralisation is an attractive exploration target in an area that has had little exploration activity away from the known resources. The presence of massive sulphides and iron-rich rocks makes aeromagnetics and electromagnetic geophysical techniques useful tools when used in conjunction with the new thinking of these deposits.