P-T-X Conditions of fluids in the Sunrise Dam gold deposit, Western Australia, and implications for the interplay between deformation and fluids

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 D 1 and D 2 shortening whereas the bulk of the gold was deposited during D 3 and D 4 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 CO 2-H 2O inclusions and CO 2 inclusions. CO 2-H 2O inclusions are more common in the early Group I orebodies. These fluids were trapped under high P-T conditions during D 3 (minimum estimates suggest conditions were likely >∼300°C and >1 to 3 kbars predominantly within the one-phase field). CO 2-H 2O 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. CO 2 inclusions crosscut the early CO 2-H 2O 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 D 4 and were trapped at lower pressure conditions than the earlier CO 2-H 2O fluids. The combined structural and fluid history for the deposit suggests that the early CO 2-H 2O fluid ponded beneath moderately dipping shear zones during late D 2, early D 3, 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 CO 2-H 2O fluid. Continued deformation and exhumation modified P-T-X conditions, and cooling of the host rocks below 300°C resulted in the CO 2-H 2O 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 CO 2-rich fluid, resulted in fracture during D 4 and precipitation of high-grade gold mineralization in steeply dipping structures forming veins, breccias, and stockworks. © 2010 by Economic Geology.