Cobar Deposits – Structural control

Abstract

The Cobar Superbasin is located in the Central Subprovince of the Lachlan Orogen in the central part of New South Wales, about 700km northwest of Sydney. The Cobar Superbasin is the most mineralised Palaeozoic sedimentary basin in Lachlan Orogen. It has an estimated pre‐mining resource of 202 t of Au, 4.597 t Ag, 2.5 Mt Cu, 4.8 Mt Zn and 2.8 Mt Pb metals. The term ‘Cobar Superbasin’ is introduced to refer to a series of deep-water troughs/basins, inferred to have formed as half graben and shallow water shelfs. Its northern portion is dominated with siliciclastic sedimentary sequences whilst the southern portion comprises sediments, volcaniclastics, volcanic rocks, granites and minor limestone. The basin formed in the Early Devonian by NE-SW transtension and was closed by NW transpression in Late and Middle Carboniferous. The overall inversion structural style is NW-SE folding, overprinted by NE-SW trending and NNW-trending eastwards oblique left-lateral reverse faulting, in a combined thick- and thin-skinned tectonic environment. The Cobar Style mineralisation is a common name for mineral deposits hosted in the Cobar Superbasin and includes massive sulphides (VMS), clastic hosted Pb-Zn mineralisation and epithermal gold. In the Cobar Superbasin, the primary location of mineral deposits is controlled by basement architecture and then overprinted and modified with secondary controlling factors of inversion tectonics. Primary control is related to the location of mineral deposits within basin architecture and directly depends om major basement structures such as: - basin marginal growth fault; - intersection of growth and transform/transfer faults; and - intersection transform/transfer faults. Secondary control is related to deposit geometry and is direct consequence of invasion tectonic such as: - intersection termination and deflection of strike-slip faults; - overlap of en-echelon strike-slip; and - junction of major faults. Cobar Style mineralisation occurs in form of sheeted veins characterised by narrow width (5m - 10m), short strike (50m – 10m) and a significant depth extension (> 2000m). The deposits occur as a group of lenses in an en-echelon array with a steep plunge