Sulfur isotope data and calculated fluid-rock reaction paths constrain the origin of pyritic gold ores in the Victory mesothermal gold deposit of Western Australia. Pyrite δ34S values range from –4.4 to +5.1 and are negatively correlated with gold tenor in mafic host rocks, indicating a significant increase in fluid oxidation state accompanied mineralization. Geologic evidence argues strongly that mineralization occurred with wall-rock sulfidation, but this process alone cannot produce the degree of fluid oxidation necessary to explain the sulfur isotope range. In contrast, either carbonation of ferric iron-bearing minerals or separation of immiscible fluid phases, both of which are consistent with the available geologic data, can drive fluid oxidation to the point of aqueous sulfate dominance. Coupled decreases in gold solubility and sulfide sulfur isotope values that are predicted by such a scenario provide an explanation for the trend between δ34S and gold tenor observed at Victory. Because both wall-rock carbonation and fluid phase separation involve loss of aqueous sulfide but do not require precipitation of iron sulfides, these processes may explain the poor correlation between gold tenor and pyrite abundance in many mesothermal deposits and the occurrence of some gold ores in iron-poor rocks.