Abstract

The base metal-rich Mount Gibson gold deposits are hosted within the ductile Mount Gibson shear zone in the Retaliation greenstone belt of the Murchison province of the Archean Yilgarn craton. The deposits are unique in the context of the Yilgarn, as they exhibit two main styles of hydrothermal alteration and sulfide mineralization, consistent with overprinting of synvolcanic, volcanic-hosted massive sulfide-style mineralization by synorogenic, shear-hosted lode gold mineralization. On the basis of model ages based on sulfide Pb isotope ratios, these two events are separated by approximately 300 m.y.Subvolcanic intrusive rocks from within the host sequence to mineralization at the Orion Two and Hornet deposits have SHRIMP II zircon U-Pb ages of crystallization of 2929 + or - 3 and 2934 + or - 6 Ma, respectively. These ages are within error and provide a minimum age for the host sequence, and consequently, the volcanic-hosted massive sulfide mineralizing event, at Mount Gibson. On the basis of geochemical evidence, the intrusions are interpreted to have been derived from the same parent magma as the rocks they intrude, and consequently ca. 2930 Ma is probably the approximate age of the host rocks to mineralization. The regional granitoid, which intrudes the base of the greenstone belt, has a crystallization age of 2935 + or - 3 Ma.A post-tectonic granite porphyry, which crops out approximately 25 km north-northeast of the Mount Gibson mine site, has a SHRIMP II zircon U-Pb crystallization age of 2623 + or - 7 Ma. This granitoid has identical field relationships, similar whole-rock geochemistry, and similar Pb isotope ratios to those of an internal granitoid which cuts gold mineralization in the Deep South pit and is consequently interpreted to be cogenetic and contemporaneous. The 2623 + or - 7 Ma date is therefore interpreted to be a minimum age for synorogenic gold mineralization at Mount Gibson.A morphologically and chemically distinctive population of zircons extracted from the sample of 2934 Ma altered felsic schist from the Hornet deposit gives a SHRIMP II zircon U-Pb age of 2627 + or - 13 Ma. These zircons have characteristics which are consistent with a hydrothermal origin and the age is interpreted to represent the timing of synorogenic gold mineralization at Mount Gibson. The interpreted mineralization age is within error of the inferred age of the postmineralization, post-tectonic granitoid. The age is also within error of the majority of absolute ages for gold mineralization from across the Yilgarn craton, which generally fall in the range 2630 + or - 10 Ma.SHRIMP zircon U-Pb geochronology thus provides strong support for the two-stage model proposed for mineralization at Mount Gibson. Synvolcanic volcanic-hosted massive sulfide-style mineralization occurred at ca. 2930 Ma and was overprinted by lode gold mineralization some 300 m.y. later, at ca. 2630 Ma.

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