Numerical models of gold‐deposit formation in the Bendigo‐Ballarat Zone, Victoria

Abstract

We present coupled mechanical, fluid‐flow and chemical numerical models that place constraints on some of the processes that formed gold deposits in the Bendigo‐Ballarat Zone of Victoria, Australia, and specifically the Bendigo goldfield. Although many goldfields in this region are located close to major intrazone faults, these major faults are rarely mineralised, and such goldfields are commonly located in the hangingwall of the faults, 5–10 km from the fault plane. Individual gold deposits are hosted within quartz veins associated with reverse faults and chevron folds. Regional‐scale models are aimed at determining if the intrazone faults were more or less permeable than the surrounding host rocks, if these faults acted as conduits to supply fluids to goldfields, if the presence of a blind fault below the Bendigo goldfield was necessary to deliver fluids to the area, and if the upper units of the Castlemaine Supergroup (Darriwilian and Yapeenian) acted as a low permeability cap to facilitate gold and quartz precipitation in the lower units (Castlemainian to Lancefieldian).The models indicate that the combination of permeable intrazone faults, and relatively low permeability units at the top of the Castlemaine Supergroup, allows the greatest fluid flux to occur in the area of the Bendigo goldfield. The presence of a blind fault beneath the Bendigo goldfield is not critical to the supply of fluids to this area. Small‐scale models of a chevron fold with a permeable fault simulate fluid pumping and suggest that fluids transported along the fault mixed with those in the host rocks. Fluids are expelled from, and drawn back into, the fault in a cyclical manner and are controlled by deformation‐induced changes in volume. Intermittent high fluid pressures in the host rocks relative to the fault cause the rocks to yield in tension, simulating hydrofracturing and the formation of quartz veins associated with gold‐bearing reverse faults. Models coupling deformation, fluid flow and chemical reactions demonstrate that gold is precipitated within the fault region when CH4 and H2S are sourced at depth and CO2 is transported along the fault. In the models presented, gold precipitation is strongly controlled by H2S, although the effects of fluids interacting with graphite have not been modelled. Rates of gold mineralisation of 0.09–0.023 g/t over 1 million years suggest that the permeability of the fault was at least two orders of magnitude higher than the permeability modelled here.