Baker, T., Bertelli, M., Blenkinsop, T., Cleverley, J.S., McLellan, J., Nugus, M., and Gillen, D. (2010) P-T-X conditions of fluids in the Sunrise Dam gold deposit, Western Australia, and implications for the interplay between deformation and fluids. Economic Geology, 105 (5). pp. 873-894.

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Abstract

The Late Archean Sunrise Dam gold deposit (~10 Moz) is hosted within greenschist-facies rocks and is characterized by extreme structural complexity, resulting from a protracted deformation history with evidence of structural reactivation and multiple phases of gold mineralization. Early Group I orebodies are hosted within shallow- to moderately dipping northwest-trending shear zones and occur in foliation parallel veins within a strong penetrative fabric. Group II orebodies occur within steeply dipping shear zones and are characterized by veins and breccias, and Group III and IV orebodies comprise dominantly stockwork veins. The orebodies formed in response to multiple episodes of deformation. Many structures were created during D1 and D2 shortening whereas the bulk of the gold was deposited during D3 and D4 that marks the transition from a dominantly lithostatic pore fluid regime to hydrostatic conditions. Two main fluid inclusion types were recognized at Sunrise Dam: low-salinity CO2-H2O inclusions and CO2 inclusions. CO2-H2O inclusions are more common in the early Group I orebodies. These fluids were trapped under high P-T conditions during D3 (minimum estimates suggest conditions were likely >~300°C and >1 to 3 kbars predominantly within the one-phase field). CO2-H2O inclusions that occur in later Group II and III orebodies were trapped predominantly within the two-phase field at temperatures < 300°C and pressures of ~1 kbar. CO2 inclusions crosscut the early CO2-H2O inclusions and are the dominant inclusion type in Group II, III, and IV structures. They likely represent a major late influx of separately sourced fluid (magmatic or mantle?) during D4 and were trapped at lower pressure conditions than the earlier CO2-H2O fluids. The combined structural and fluid history for the deposit suggests that the early CO2-H2O fluid ponded beneath moderately dipping shear zones during late D2, early D3, and that fluid pressures increased to near- or supralithostatic conditions (>1–3 kbars) at a temperature >300°C. Shear failure along these structures resulted in widespread precipitation of moderate-grade gold mineralization from the CO2-H2O fluid. Continued deformation and exhumation modified P-T-X conditions, and cooling of the host rocks below 300°C resulted in the CO2-H2O fluid entering the two-phase field. A combination of temperature decrease, a transition from lithostatic to suprahydrostatic and/or hydrostatic pressure conditions, fluid immiscibility, and the influx of a second CO2-rich fluid, resulted in fracture during D4 and precipitation of high-grade gold mineralization in steeply dipping structures forming veins, breccias, and stockworks.

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