U-Pb and 40Ar/39Ar chronology of hydrothermal zircon, garnet, rutile and muscovite, Big Bell gold deposit, Yilgarn Craton, Western Australia

The Hemlo-type Big Bell deposit (75 t Au) occurs in a narrow mafic-ultramafic greenstone belt bounded to the southeast by granodiorite-tonalite (2740-2700 Ma) and to the northwest by granite batholiths (2625 Ma). The steeply dipping greenstones are intruded parallel to bedding by granodiorite porphyry, both lineated and metamorphosed at 670±50°C during emplacement of the southeast border pluton at 2700±7 Ma. The calcic-potassic Big Bell alteration zone, centred on amphibolite-granodiorite dyke contacts, contains two scheelite-bearing ore bodies: almandine-cummingtonite-hornblende skarn (1-3 g/t Au, 1700 g/t As) dated at 2662±5 Ma (almandine U-Pb), and the muscovite-microcline gneiss of the Main Lode (3-5 g/t Au, 580 g/t Sb). Garnet-cordierite-biotite schist borders the lode and the skarn. Fe-Mg thermometry constrains P-T conditions during mineralization to 540-600°C and 300-400 MPa. The northwest granite and late Sn-Ta-Nb granite-pegmatite dykes intruded at 2625-2610 Ma. They are associated with auriferous andradite-diopside and barren epidote skarn veins. Zircons intergrown with andradite are dated at 2612±7 and 2609±10 Ma. Granite-pegmatite magmatic and hydrothermal activity did not cause recrystallization in the Main Lode but the thermal anomaly (about 450°C) reset rutile and muscovite in the microcline gneiss to 2614±4 Ma and 2603±8 Ma, respectively.