Mineral equilibria and zircon, garnet and titanite U-Pb ages constraining the PTt path of granite-related hydrothermal systems at the Big Bell gold deposit, Western Australia

Abstract

The Big Bell deposit (75 t gold) is located in a narrow spur of the Meekatharra greenstone belt, Yilgarn Craton, Western Australia. Two ore bodies are located in a calcic-potassic contact alteration zone overprinting lineated granodiorite dykes and amphibolite: almandine-cummingtonite-hornblende skarn (1–3 g/t Au, 1700 g/t As, 330 g/t W) and the muscovite-microcline gneiss (3–5 g/t Au, 580 g/t Sb, 620 g/t W) of the Main Lode. Genetic models vary from pre- to post-metamorphic replacement. Hornblende-plagioclase pairs in amphibolite constrain peak metamorphic temperature to 670 ± 50 °C. In contrast, garnet-biotite thermometry provides estimates of 578 ± 50 and 608 ± 50 °C for garnet-cordierite-biotite schist bordering the skarn and enveloping the Main Lode. Garnet-cordierite and garnet-hornblende pairs extend the range of fluid temperature to 540 ± 65 °C, well below peak metamorphic temperature. At 540–600 °C, the alteration assemblage andalusite + sillimanite constrains pressure to 300–400 MPa corresponding to 11–14 km crustal depth. Published U-Pb ages indicate that metamorphism took place in the aureole of the southeast granodiorite-tonalite batholith (2740–2700 Ma), followed by gold mineralization at 2662 ± 5 Ma and by the emplacement of biotite granite and Sn–Ta–Nb granite-pegmatite dykes at 2625–2610 Ma. Amphibolite xenoliths in granite northwest of the deposit record the lowest temperature (628 ± 50 °C), suggesting it lacks a metamorphic aureole. The rare metal dykes are spatially associated with epidote-albite and andradite-diopside skarns (≤1.5 g/t Au), mined where enriched in the weathered zone. We analysed hydrothermal zircon intergrown with andradite. Concordant U-Pb ages of 2612 ± 7 and 2609 ± 10 Ma confirm the presence of a second granite-related system. The zircons display oscillatory zoning and have low Th/U ratios (0.05–0.08). Low-Th titanite from an albite granite dyke has a concordant but reset U-Pb age of 2577 ± 7 Ma.