Gold metallogeny and the copper-gold association of the Australian Proterozoic

Australian Proterozoic gold-producing deposits, emplaced mainly at 1.55–2.00 Ga, are divided into the following categories: (1) iron oxide-dominated, brecciahosted, Cu-U±Au replacement deposits spatially associated with felsic intrusions (273t Au); (2) stratabound Au±Cu-bearing iron formations (152.4t Au); (3) unconformity-style U ±Cu/PGM/Au deposits (53t Au); (4) Iron oxide-dominated Au±Cu mineralisation hosted within elements of ductile deformation (146.7t Au); (5) Broken Hill and volcanic-hosted massive sulphides (150t Au); (6) iron-sulphide-dominated veins and replacement zones spatially related to felsic intrusions (150.7t Au), and (7) iron-sulphide-dominated veins and replacement zones spatially related to elements of regional deformation (159.9t Au). Categories (1) to (4) are mainly confined to Proterozoic rocks, constituting an association in which Au and Cu are commonly present together, with variable amounts of U, Bi, Co, W, Se, Te and REE. Most examples in categories 1–4 fall into either of two groups: Cu-Aumagnetite ±hematite types formed at relatively high temperature (300–450 °C), and Cu-U±Au-hematite types formed at 150–300 °C. We postulate that these ores formed from a common high salinity (15–35 wt. % NaCl equiv.), low total sulphur (a_S = 10^–3 to 10^–2), high fO₂ fluid-type, in which metal transport was dominated by chloride-complexing. The most effective method of metal deposition was fluid mixing, achieving a synchronous decrease in fO₂ and temperature. This unusual oxidised fluid association was favoured in high heat-flow extensional settings containing oxidised and/or oxidised-evaporitic sedimentary sequences. The intrusion of oxidised fractionated granites, which are commonly temporally associated with metal emplacement, acted in some places to heat and focus basinal fluids, and in others was the ultimate source of metals.