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

Abstract

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 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 + or - 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 + or - 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 + or - 5 Ma from a low-grade cummingtonite-hornblende contact skarn. Main mineralization in the mine area thus postdates amphibolite facies metamorphism by about 80 m.y. Muscovite from the Main lode microcline gneiss yielded an Ar-Ar plateau age of 2639 + or - 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 + or - 8 Ma) agrees within error with the Ar-Ar muscovite age. Metamorphic recrystallization related to granite intrusion was of very limited extent (