Cowal, Lake Cowal, Endeavour 42

New South Wales, NSW, Australia

Main commodities: Au
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The Cowal (previously Lake Cowal and Endeavour 42) epithermal gold deposit is located on the western edge of Lake Cowal, approximately 35 km NNE of West Wyalong in central New South Wales, Australia. The deposit falls within the Macquarie volcanic arc of the Palaeozoic Lachlan Fold Belt of southeastern Australia.

Gold mineralisation at Cowal is hosted by the Ordovician Lake Cowal Volcanic Complex, exposed as a north-south elongated 40 x 15 km window of intermediate calc-alkaline intrusives, volcanics and volcaniclastics surrounded by unconformably overlying Siluro-Devonian sediments and volcanics. Immediately to the west of this window a highly deformed north-south zone within the Siluro-Devonian defines the broad Booveri Fault Zone.

The deposit is covered by 30 m of lake sediments and an underlying Tertiary laterite profile with no outcrop of the host volcanics, apart from some minor gossanous float.

The hosts to mineralisation comprise volcaniclastics and coeval extrusives and intrusive magmatic rocks which dip NW at 40 to 45°. This sequence has been subdivided into three conformable units, namely: i). The Great Flood unit of 250 m of monotonous beds, to 72 m of vitric volcaniclastic debris with mass flow structures. The coarser beds are intercalated with less than 3 m thick intervals of laminated shale, siltstone and mudstone, with a 20 m bed of polymictic volcanogenic conglomerate; ii). The Golden Lava unit, comprising 60 to 110 m of trachyandesite interbedded with monomictic sand to breccia with clasts of plagioclase porphyritic fragments - believed to represent a submarine lava with associated hyaloclasite and autobreccia; iii). The Cowal Conglomerate, which is the oldest unit and is around 100 m thick with massive to graded beds of well rounded to very angular, clast supported polymict volcanic debris (andesitic) with interbedded laminated siltstone and mudstone, and evidence of mass flow.

This succession is cut by the 456 ±5 Ma holocrystalline to euqigranular to porphyritic Muddy Lake Diorite (in places a gabbro) and by porphyritic to aphanitic mafic dykes. The latter post date the diorite, are 0.2 to 20 m thick (averaging 1 to 3 m) and were emplaced in active faults.

The gold mineralisation is primarily in narrow dilatant veins of quartz-carbonate-sulphide (with common adularia) and carbonate±quartz-sulphide and narrow, healed faults with a similar mineralogy. Gold occurs to a lesser extent in pyrite stringers and as disseminations, shear chlorite-carbonate veins and chlorite-carbonate-sulphide veins. Approximately 20% of the orebody is in the oxide zone where gold is more erratic, reflecting leaching and dispersion. The veins generally strike at 305° and dip 35°SW and are at their highest density in the Golden Lava, although those in the Muddy Lake Diorite are usually thicker. The veins are typically parallel sided and from <1 to 100 mm thick. Vein alteration haloes are rare and the main sulphides are quartz, sphalerite, chalcopyrite, galena and pyrrhotite, with minor associated visible gold. The best gold accompanies sphalerite and to a lesser extent adularia.

Four alteration styles are recognised, namely i). Propylitic, in the surrounding hosts; ii). Quartz-sericite-carbonate, associated with fault zones and gold mineralisation, and is best developed in the Golden Lava and Great Flood units and grades outward from the faulz zones and associated veining to the propylitic zone - this style usually embraces veins of ankerite-quartz-pyrite-sphalerite-chalcopyrite-galena; iii). Quartz-potassium feldspar - restricted to the Golden Lava as irregular patches and zones usually associated with later chloritisation and occassionally surrounds sulphide bearing dilational veining; iv). Chlorite-carbonate-pyrite, usually associated withe the previous type of alteration and surrounding zones of quartz, K-feldspar, pyrite, sphalerite and chalcopyrite veins.

At December 31, 2003, the pre-mining reserve - resource estimates were:
  Proven + probable reserves - 63.6 Mt @ 1.19 g/t Au, for 76 t of contained Au
  Mineral resources - 47.53 Mt @ 1.04 g/t Au, for 49 t of contained Au.

Mining commenced in mid 2006.   Total production to the end of 2010 was ~34.75 t Au.

At December 31, 2010, the reserve - resource estimates were (Barrick Annual Report):
  Proven + probable reserves - 72.5 Mt @ 1.07 g/t Au, for 77 t of contained Au,
  Mineral resources - 48.3 Mt @ 0.98 g/t Au, for 47 t of contained Au (in addition to reserves).
Production during 2010 totalled 9.27 t of recovered Au.

The most recent source geological information used to prepare this summary was dated: 2004.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information
   Selected References:
Heithersay P, Jones G and Love M,  1996 - The Goonumbla Lake Cowal porphyry copper belt: in   Porphyry Related Copper and Gold Deposits of the Asia Pacific Region, Conf Proc, Cairns, 12-13 Aug, 1996, AMF, Adelaide,    pp 15.1 - 15.10
Henry A D, McInnes P and Tosdal R M,  2014 - Structural Evolution of Auriferous Veins at the Endeavour 42 Gold Deposit, Cowal Mining District, NSW, Australia : in    Econ. Geol.   v.109 pp. 1051-1077
McInnes P and Freer L,  2007 - The Cowal Gold Corridor Opening other doors: in   Mines and Wines, 2007, Mineral Exploration in the Tasmanides AIG Bull.   v. 46 pp. 95-100
McInnes P, Miles I, Radclyffe D and Brooker M  1998 - Endeavour 42 (E42) gold deposit, Lake Cowal: in Berkman D A, Mackenzie D H (Ed.s), 1998 Geology of Australian & Papua New Guinean Mineral Deposits The AusIMM, Melbourne   Mono 22 pp 581-586
Tucker D  2003 - Cowal Gold - discovery, disappointment, recovery: in   NewGenGold 2003, Conference Proceedings, Perth WA,  Louthean Media, Perth    pp 185-199
Zukowski W, Cooke D R, Deyell C L, McInnes P and Simpson K,  2014 - Genesis and Exploration Implications of Epithermal Gold Mineralization and Porphyry-Style Alteration at the Endeavour 41 Prospect, Cowal District, New South Wales, Australia: in    Econ. Geol.   v.109 pp. 1079-1115

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