The origin and evolution of mineralizing fluids in a sediment-hosted orogenic-gold deposit, Ballarat East, Southeastern Australia

The hydrothermal fluids responsible for gold mineralization at the Ballarat East gold deposit (the second largest orogenic gold deposit in the western Lachlan orogen) are thought to have links to a variety of processes, including metamorphism, sedimentation, and/or magmatism. In the current study, noble gases and halogens have been used as fluid tracers to reevaluate the origin and evolution of the gold-related fluids at the Ballarat East deposit. Gold-bearing quartz and carbonate veins from the Ballarat East contain low salinity (∼4 wt % NaCl equiv) aqueous (H2O) and mixed H2O-CO2 fluid inclusions. These fluid inclusions have variable molar Br/Cl values of between 1.2 × 10-3 and 2.9 × 10-3 and I/Cl values between 150 × 10-6 and 500 × 10-6, and Br is strongly correlated with I, defining a mixing line with a Br/I ratio of 5.6. The fluid inclusions have40Ar/36Ar ratios ranging from 322 (close to the atmospheric40Ar/36Ar ratio of ∼296) up to a maximum of 4503.40Ar is strongly correlated with Cl and defines a mixing line with a 40ArE/Cl ratio of 4.6 × 10-4 (40ArE denotes excess 40Ar). The fluid inclusions contain 5.1 to 32 ppm40ArE (by mass) and exhibit minimum 36Ar concentrations ranging from 3.1 to 11 ppb, which exceed air-saturated water (ASW) levels by several parts per billion (ASW = 1.3-2.7 ppb). Fluid inclusion84Kr/36Ar and130Xe/36Ar values are uniformly enriched in Kr and Xe relative to air, but exhibit limited variation. These data provide strong evidence for the involvement of two noble gas and halogen reservoirs. This data is compatible with a deeply sourced fluid, possibly originating by devolatilization of altered volcanic rocks (e.g., basalts) that acquired additional noble gases and organic Br plus I by interaction with sedimentary rocks, including organic-rich shales that are found beneath and surrounding the deposit. The data are also consistent with mixing deeply sourced metamorphic fluids with sedimentary formation waters; however, both interpretations favor the involvement of metamorphic fluids and sedimentary components and highlight the significance of fluid-rock interaction as controls on fluid compositions in Victorian deposits. The data are compatible with genetic models for orogenic gold in which gold mineralization was initiated by metamorphic devolatilization in the lower crust, and was linked to Lachlan orogenesis at ca. 440 Ma