Gilt by association? Origins of pyritic gold ores in the Victory mesothermal gold deposit, Western Australia

Sulfur isotope data and calculated fluid-rock reaction paths constrain the origin of pyritic gold ores in the Victory mesothermal gold deposit of West ern Australia. Pyrite delta S-34 values range from -4.4 to +5.1 and are neg atively correlated with gold tenor. in mafic host rocks, indicating a signi ficant increase in fluid oxidation state accompanied mineralization. Geolog ic evidence argues strongly that mineralization occurred with wall-rock sul fidation, but this process alone cannot produce the degree of fluid oxidati on necessary to explain the sulfur isotope range. In contrast, either carbo nation of ferric iron-bearing minerals or separation of immiscible fluid ph ases, both of which are consistent with the available geologic data, can dr ive fluid oxidation to tile point of aqueous sulfate dominance. Coupled dec reases in gold solubility and sulfide sulfur isotope values that are predie ted by such a scenario provide an explanation for tile trend between delta S-34 and gold tenor observed at Victory. Because both wall-rock carbonation and fluid phase separation involve loss of aqueous sulfide but do not requ ire precipitation of iron sulfides, these processes may explain the poor co rrelation between Sold tenor and pyrite abundance in many mesothermal depos its and the ocurrence of some gold ores in iron-poor rocks.