Rock-Buffering of Auriferous Fluids in Altered Rocks Associated with the Golden Mile-Style Mineralization, Kalgoorlie Gold Field, Western Australia

Abstract

The Kalgoorlie gold field contains structurally controlled, epigenetic gold deposits hosted by mafic rocks in the Archean Yilgarn craton of Western Australia. Its giant size has prompted much interest in the processes that led to its formation, particularly of the Golden Mile mineralization, which hosts over 70 percent of the gold in the Kalgoorlie gold field. It is generally agreed that the widespread presence of hematite and the moderately negative sulfur isotope composition of some of the pyrite (δ34S of −10 to −2‰) in the Golden Mile lodes and associated alteration indicate the presence of a relatively oxidizing ([SO2−4 ] ~ [HS−] + [H2S], with hematite stable) fluid during gold deposition and wall-rock alteration, but the origin and evolution of this fluid are not well constrained. A piece of evidence that has not been fully integrated into interpretations is the low variance of mineral assemblages in the alteration haloes of the Golden Mile lodes (e.g., coexisting magnetite-hematite-siderite-pyrite-ankerite-albite-muscovite-ilmenite ± rutile-quartz ± chlorite). Thermodynamic modeling, using HCh and a purpose-built code that facilitates investigation of systems that involve complex mineral solid solutions, CO2-rich fluids, and open-system chemical behavior was used to investigate the constraints that the low variance assemblages place on the source and evolution of mineralizing fluids. Results of the modeling show that fluid-rock reaction with decreasing temperature can drive pyrrhotite-magnetite assemblages, in equilibrium with a fluid that contains aqueous sulfide, to hematite-pyrite-magnetite assemblages in equilibrium with a fluid that contains aqueous sulfate. This modeled shift arises from cooling-driven oxidation of sulfides and reduced sulfur-bearing aqueous species by ferric iron in magnetite and formation of hematite and siderite from magnetite and CO2; there is no requirement for electron acceptors other than those provided by the rock. The implication of the model results for the Golden Mile mineralization is that hematite growth and sulfate-bearing fluids could have resulted from fluid-wall rock interaction without involvement of an externally derived oxidizing fluid. The change from aqueous-sulfide dominated to aqueous-sulfate–rich solutions would destabilize gold sulfide complexes in solution and lead to gold precipitation. Formation of aqueous sulfate species would also result in the precipitation of pyrite with negative δ34S. Mass balance calculations show that production of hematite in the carbonate zone of the Golden Mile mineralization could have occurred without any requirement for addition of fluid-derived electron acceptors although open-system behavior is not precluded. Overall, the characteristics of the carbonate zone alteration are consistent with electron redistribution caused by interaction between a reduced auriferous fluid and the host dolerite.