Trace element and sulfur isotopic evidence for redox changes during formation of the Wallaby Gold Deposit, Western Australia

Abstract

Wallaby is a major gold deposit of the St Ives Gold field of Western Australia, with an estimated resource of 8 million ounces of gold. It has a well-established paragenesis across five vein sets that displays macroscopic evidence of changing redox through time; from hematite to magnetite. Micro-analysis of pyrite from each vein generation shows a progressive and gradual change in redox conditions. The sulfur isotope composition has a δ34S range of −7.7 to +9.8‰ using 3μm spots on the Sensitive High Resolution Iron Micro Probe-Stable Isotope (SHRIMP-SI). Negative values indicative of an oxidized sulfur signature are found in the earliest generation of pyrite (which coexists with hematite) that also contains high concentrations of As, Ni, Zn, Ag, Sb, Cu and Pb. Conversely, positive values representative of reduced sulfur are found in later generations of pyrite, with lower concentrations of As, Ni, Sb, Cu, Zn and Pb. These later pyrite crystals display higher As/Ni, As/Sb, and As/Bi, and lower Cu/Te. These geochemical trends are the result of redox controlled transport and partitioning into pyrite of minor and trace elements now within the pyrite structure. Previous studies suggested a single orogenic event formed the Wallaby Deposit. This is not supported by the present study. Trace element ratios such as As/Ni clearly delineate the high Au generations and could be used as an exploration tool. It is suggested that pyrite from the Wallaby Gold Deposit formed via desulfidation and a gradual change in redox conditions within an evolving fluid and did not result from mixing of two separate fluids as previously advocated. Utilizing pyrite to link the entire fluid history of Wallaby demonstrates the general use of pyrite as a valuable mineral tracer in gold-bearing fluid systems.