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

Abstract

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 D 2 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. Detailed 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) CO 2±CH 4–H 2O–NaCl inclusions of variable salinity (0.1 to 17.5 eq. wt.% NaCl) containing between 10 and 99 mol% CO 2 and molar volumes that range from 22 to 76 cm 3; (2) H 2O–NaCl inclusions of variable salinity (0.4 to 22.1 eq. wt.% NaCl); (3) CO 2±CH 4 inclusions with up to 58 mol% CH 4 and molar volumes between 54 and 73 cm 3; (4) CH 4–H 2O inclusions with CH 4 ranging from 80 to 90 mol%; and (5) CH 4 inclusions with low molar volumes of 19 to 23 cm 3. Types 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 CO 2 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. Significant 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 D 2 shear zone hosted vein system at Bronzewing.