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 theMurchison 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 _ 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 cummingtonitehornblende 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 _ 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 m,y. 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 to 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 crosscutting structures in the mine area. The isotopic resetting of older minerals by the thermal effects of younger intrusions took place in a midcrustal environment (10 km depth) of elevated ambient temperature (about 300øC) in the greenstone terrane.

Transpressional zones in ophiolitic mélange terranes: Potential exploration targets for gold in the South Eastern Desert, Egypt

“…However, occurrence of the ore bodies and related sulfidebearing alteration zones along the transpression zone along with boudinaged granitic bodies may indicate a possible hydrothermal activity concomitant with emplacement of the granitic offshoots (e.g., Mueller et al, 1996). This assumption is verified by geochemical analyses that indicate anomalous concentrations of gold (ppb levels) in zones of altered host rocks far-removed (meters to tens of meters) from the quartz lodes.…”

Orogenic gold: Common or evolving fluid and metal sources through time

“…Big Bell and Renco illustrate the particular difficulty of textural interpretation. Whereas Phillips and de Nooy (1988) interpreted Big Bell to be a metamorphosed orogenic gold deposit, Mueller et al (1996) suggested it post-dated amphibolite-facies metamorphism by tens of millions of years. Kisters et al (1998) originally interpreted the Renco deposit to have formed near the time of peak…”

Gold Deposits in Metamorphic Belts: Overview of Current Understanding,Outstanding Problems, Future Research, and Exploration Significance

“…Some syenite-associated deposits in the Abitibi belt (e.g., Robert, 2001) may be a definable subgroup within the intrusion-related group. Other workers (Mueller, 1991;Mueller et al, 1996;Mueller and McNaughton, 2000) have suggested that certain calc-silicate-bearing gold deposits in amphibolite facies settings of well documented orogenic gold provinces, particularly in the Yilgarn craton of Western Australia, are gold skarns. In summary, these intrusion-related gold deposits are suggested by some workers to be representative of a distinct group of deposits, because they all formed as a proximal part of a magmatic-hydrothermal system, rather than from a fluid of more regional extent, favored by many authors to be of a metamorphic origin, which circulated throughout much of an orogen.…”

“…The Hemlo deposit, in the Wawa subprovince of Canada, containing approximately 600 t Au at a grade of >7 g/t Au (Harris, 1989) in three main deposits (Williams, Golden Giant, and David Bell), is an even more contentious deposit, mainly because of the complex magmatic, structural, and metamorphic history of the district (e.g., Muir, 2002). Similar to more poorly understood gold deposits (e.g., Big Bell, Western Australia; Mueller et al, 1996), Hemlo is located in a highgrade metamorphic domain, in this case of mid-amphibolite facies (e.g., Corfu and Muir, 1989b). The metal association of Au-Mo-Sb-As-Hg-V-Tl-Ba, the high Hg and Ag contents of some native gold (e.g., Harris, 1989), and the unusual mineralogy, together with the moderate to high salinities of ore fluids (Pan and Fleet, 1992) and consistently negative δ 34 S of ore minerals (Cameron and Hattori, 1985), clearly distinguish this deposit from most orogenic gold deposits.…”