Timing of Gold Mineralization Relative to the Peak of Metamorphism at Bronzewing, Western Australia

Abstract

The Bronzewing gold deposit is hosted within the middle to upper greenschist facies, tholeiitic metabasalt of the Yandal greenstone belt within the Archean Yilgarn craton. Gold mineralization at Bronzewing is surrounded by an alteration halo hundreds of meters in thickness, composed of distal chlorite, intermediate chlorite-calcite, and proximal carbonate-K-mica zones. Chlorite (in the distal zone) and muscovite and biotite (in the proximal zone) define a strong foliation that is locally crenulated, indicating that alteration minerals and K2O addition were produced prior to the crenulation event. Within the proximal alteration zone, biotite, ankerite, and calcite cut the crenulated foliation-forming potassic minerals, implying changes in metamorphic conditions subsequent to mineralization. Mineral equilibria modelling indicates that the stable mineral assemblage of biotite-muscovite-calcite-ankerite-albite-quartz can only be produced at a temperature of about 440°C (at 2.5 kbars) with a fluid composition of about XCO2 = 0.5–0.6. However, the textural relationships cannot be explained, and the observed alteration zones surrounding mineralization cannot be reproduced by infiltration of fluid (XCO2 = 0.5–0.6) into a slightly carbonated actinolite-bearing mineral assemblage at this temperature. A fluid of XCO2 = 0.15–0.3, between 330° and 375°C (at 2.5 kbars), can reproduce the observed pre-peak metamorphic alteration assemblages at Bronzewing. Calculated internal buffering paths, which model the mineral assemblage and evolution of fluid composition upon further heating, show that the postmineralization assemblage can be explained by further heating to the peak of metamorphism at about 440°C with fluid composition evolving to XCO2 = 0.5–0.6. The implication of the proposed timing relationship is that synmineralization alteration at Bronzewing occurred at temperatures significantly lower than that of peak metamorphism (60°–120°C lower).