The Bronzewing lode-gold deposit, Western Australia: P–T–X evidence for fluid immiscibility caused by cyclic decompression in gold-bearing quartz-veins

The Bronzewing lode-gold deposit is located in the Yandal greenstone belt in the Yilgarn Craton of Western Australia. Gold mineralization is hosted in tholeiitic basalt that is metamorphosed to greenschist facies. Individual ore bodies are controlled by a complex, gold-bearing quartz vein system that comprises shear and extension veins formed during a progressive D2 deformation event. Gold is localized in quartz along fractures and deformed grain boundaries, and is interpreted to have formed late in the formation of the veins. ed petrography and microthermometry on primary, pseudosecondary and secondary fluid inclusions trapped in gold-bearing shear and extension veins revealed five types of fluid inclusions: (1) CO2±CH4–H2O–NaCl inclusions of variable salinity (0.1 to 17.5 eq. wt.% NaCl) containing between 10 and 99 mol% CO2 and molar volumes that range from 22 to 76 cm3; (2) H2O–NaCl inclusions of variable salinity (0.4 to 22.1 eq. wt.% NaCl); (3) CO2±CH4 inclusions with up to 58 mol% CH4 and molar volumes between 54 and 73 cm3; (4) CH4–H2O inclusions with CH4 ranging from 80 to 90 mol%; and (5) CH4 inclusions with low molar volumes of 19 to 23 cm3. 1, 2 and 3 constitute a fluid inclusion assemblage that occurs consistently in primary, pseudosecondary and secondary trails and clusters within the gold-bearing quartz vein system. These co-existing fluids are interpreted to have formed by fluid immiscibility of a low-salinity, homogeneous parent fluid at about 300°C and 1500 bars. Locally, Type 2 fluid inclusions exhibit total homogenization temperatures that are on average 100°C less than the co-genetically trapped Type 1 aqueous-carbonic inclusions. This discrepancy is interpreted to have involved CO2 effervescence in response to fluid pressure fluctuations. Types 4 and 5 fluid inclusions are rare, and are only present locally in secondary trails and clusters in extension veins. icant pressure fluctuations, but relatively constant homogenization temperatures for Types 1, 2 and 3 fluid inclusions, suggest that cyclic decompression of the hydrothermal fluids, due to seismic activity along the shear zones that host the gold-bearing veins, triggered fluid immiscibility. The process of fluid immiscibility and subsequent lowering of gold solubility is interpreted to be the most efficient precipitation mechanism for gold in the D2 shear zone hosted vein system at Bronzewing.