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 i n the Yilgarn Craton of Western Australia. Cold 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 tha t comprises shear and extension veins formed during a progressive D-2 defor mation event. Gold is localized in quartz along fractures and deformed grai n boundaries, and is interpreted to have formed late in the formation of th e veins. Detailed petrography and microthermometry on primary, pseudosecondary and s econdary 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 cm(3); (2) H2O-NaCl inclusions of variable salinity (0.4 to 22.1 eq, wt.% NaCl); (3) CO2 +/- C H4 inclusions with up to 58 mol% CH4 and molar volumes between 54 and 73 cm (3); (4) CH4-H2O inclusions with CH4 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 consis tently in primary, pseudosecondary and secondary trails and clusters within the gold-bearing quartz vein system. These co-existing fluids are interpre ted to have formed by fluid immiscibility of a low-salinity, homogeneous pa rent fluid at about 300 degreesC and 1500 bars. Locally, Type 2 fluid inclu sions exhibit total homogenization temperatures that are on average 100 deg reesC less than the co-genetically trapped Type 1 aqueous-carbonic inclusio ns. This discrepancy is interpreted to have involved CO2 effervescence in r esponse to fluid pressure fluctuations. Types 4 and 5 fluid inclusions are rare, and are only present locally in secondary trails and clusters in exte nsion veins. Significant pressure fluctuations, but relatively constant homogenization t emperatures for Types 1, 2 and 3 fluid inclusions, suggest that cyclic deco mpression of the hydrothermal fluids, due to seismic activity along the she ar zones that host the gold-bearing veins, triggered fluid immiscibility. T he process of fluid immiscibility and subsequent lowering of gold solubilit y is interpreted to be the most efficient precipitation mechanism for gold in the D-2 shear zone hosted vein system at Bronzewing. (C) 2001 Elsevier S cience B.V. All rights reserved.