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 te rrane of the Meekatharra-Wydgee greenstone belt in the Murchison provi nce of die Yilgarn craton, Western Australia. At the mine site, the gr eenstones consist of amphibolites and metakomatiites of the 3.0 Ca Gab anintha Formation, rotated into a subvertical position and foliated pa rallel to stratigraphic contacts. The amphibolite-metakomstiite sequen ce is intruded parallel to foliation by swarms of granodiorite-tonalit e 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-metam orphic aureole (>1 km wide) of amphibolite facies grade. The dikes rep resent the oldest and outermost intrusive phase of the batholith to th e southeast, where the border granodiorite pluton has been dated at 27 00 +/- 7 Ma (U-Pb zircon). The gold-bearing metasomatic rocks at Big B ell are subdivided into central muscovite-microcline and biotite-plagi oclase gneisses, and into outer calcic skarns. Both gneisses and skarn s form steeply dipping zones bound to the contacts of altered granodio rite dikes. The timing of main gold-sulfide-scheelite mineralization i s constrained by a concordant U-Pb almandine age of 2662 +/- 5 Ma from a low-grade cummingtonite-hornblende contact skarn. Main mineralizati on in the mine area thus postdates amphibolite facies metamorphism by about 80 m.y. Muscovite from the Main lode microcline gneiss yielded a n Ar-Ar plateau age of 2639 +/- 16 Ma, interpreted as reset by the the rmal effect of the granite pluton at the northwestern margin of the gr eenstone terrane. The granite U-Pb zircon age (2627 +/- 8 Ma) agrees w ithin error with the Ar-Ar muscovite age. Metamorphic recrystallizatio n related to granite intrusion was of very limited extent (<0.1 km) an d did not extend to the gold orebodies. A second metasomatic event, ab out 50 m.y. younger than the main mineralization, is indicated by a co ncordant U-Pb titanite age of 2614 +/- 2 Ma from a scheelite- and sulf ide-bearing andradite-diopside replacement vein, located in amphibolit es southeast of the main gold orebodies. This vein is cut by granite a nd pegmatite dikes, which must be younger still, provided that the tit anite closed to diffusion at the time of skarn crystallization. Proter ozoic ages of circa 2.1 to 2.3 Ga are recorded by die 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 crosscutting structures in the mine area. The isotopic resetting of ol der minerals by the thermal effects of younger intrusions took place i n a midcrustal environment (10 km depth) of elevated ambient temperatu re (about 300 degrees C) in the greenstone terrane.