Constraints on the age of granitoid emplacement, metamorphism, gold mineralization, and subsequent cooling of the Archean greenstone terrane at Big Bell, Western Australia

The Big Bell gold deposit (40 t Au produced) is located in a narrow terrane of the Meekatharra-Wydgee greenstone belt in the Murchison Province of the Yilgarn craton, Western Australia. At the mine site, the greenstones consist of amphibolites and metakomatiites of the 3.0 Ga old Gabanintha Formation, rotated into a subvertical position and foliated parallel to stratigraphic contacts. The amphibolite-metakomatiite sequence is intruded parallel to foliation by swarms of granodiorite-tonalite dikes. The largest dike, exposed in the Big Bell open pit and dated at 2737±4 Ma (U-Pb zircon), is surrounded by a zoned contact-metamorphic aureole (> 1 km wide) of amphibolite-facies grade. The dikes represent the oldest and outermost intrusive phase of the batholith to the southeast, where the border granodiorite pluton has been dated at 2700±7 Ma (U-Pb zircon). The gold-bearing metasomatic rocks at Big Bell are subdivided into central muscovite-microcline and biotite-plagioclase gneisses, and into outer calcic skarns. Both gneisses and skarns form steeply dipping zones bound to the contacts of altered granodiorite dikes. The timing of main gold-sulfide-scheelite mineralization is constrained by a concordant U-Pb almandine age of 2662±5 Ma from a low-grade cummingtonite-hornblende contact skarn. Main mineralization in the mine area thus postdates amphibolite-facies metamorphism by about 80 Ma. Muscovite from the Main Lode microcline gneiss yielded an Ar-Ar plateau age of 2639±16 Ma, interpreted as reset by the thermal effect of the granite pluton at the northwestern margin of the greenstone terrane. The granite U-Pb zircon age (2627±8 Ma) agrees within error with the Ar-Ar muscovite age. Metamorphic recrystallization related to granite intrusion was of very limited extent (<0.1 km), and did not extend to the gold orebodies. A second metasomatic event, about 50 Ma younger than the main mineralization, is indicated by a concordant U-Pb titanite age of 2614±2 Ma from a scheelite- and sulfide-bearing andradite-diopside replacement vein, located in amphibolites southeast of the main gold orebodies. This vein is cut by granite and pegmatite dikes, which must be younger still, provided that the titanite closed to diffusion at the time of skarn crystallization. Proterozoic ages of circa 2.1-2.3 Ga are recorded by the Pb, U-Pb and Rb-Sr isotope systems in a variety of metasomatic and magmatic minerals. They may be related to the intrusion of dolerite dikes, the youngest cross-cutting structures in the mine area. The isotopic resetting of older minerals by the thermal effects of younger intrusions took place in a mid-crustal environment (10 km depth) of elevated ambient temperature (about 300°C) in the greenstone terrane.