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Abstract

Archean mesothermal gold mineralization is commonly located in brittle-ductile structures active during the late deformation events of greenstone belt evolution, and in several cases, reactivation of earlier, mechanically weak structures was a controlling factor. Intensely mineralized greenstone belts in both Western Australia and southern Africa are characterized by an early compressional to oblique-compressional event. Within these belts, zones of low-strain greenstones are bounded by narrow high-strain zones of up to hundreds of kilometers in strike length, which are in turn linked to the brittle-ductile structures hosting gold mineralization. This structural pattern ensured that there was strongly focused fluid flux and probably accounts for the generally high productivity of Archean greenstone belts. In Western Australia, the most mineralized greenstone belt (the Norseman-Wiluna belt in the Yilgarn block) is one in which deformation followed shortly after volcanism and sedimentation and probably resulted from the closure of an extensional basin. Other Yilgarn greenstone belts do, however, host large gold deposits, probably in structures synchronous with and related to those in the Norseman-Wiluna belt. In southern Africa, heterogeneously deformed greenstone belts with high-strain zones of reverse fault-thrust regimes related to externally imposed tectonics are highly prospective. Other important sites are reverse shear zones close to or at granitoid dome margins, which may be due to jostling of these rigid granitoids and differential movement of greenstones during externally imposed deformation.

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