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 degrees 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 degrees 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 degrees C) reset rutile and muscovite in the microcline gneiss to 2614 +/- 4 Ma and 2603 +/- 8 Ma, respectively.