Abstract

The Dumagami Au–Ag–Cu deposits are hosted by strongly deformed and altered Archean felsic metavolcanites of the Blake River Group (BRG), southern Abitibi greenstone belt, Canada. Textural and structural features recorded within the lithologies of the BRG at Dumagami indicate that two stages of hydrothermal alteration, separated by a dynamometamorphic event, have affected the volcanic protoliths in the deposit area. Advanced argillic and sericitic alteration zones, massive pyrite bodies, and massive sphalerite–galena bodies resulted from the first stage of hydrothermal activity. Sericitic shells surround peraluminous cores, which host the massive pyrite bodies and massive sphalerite–galena bodies within the altered zones.This early-stage alteration was followed by a dynamometamorphic event that reached the greenschist–amphibolite grade and almost completely recrystallized both fresh and altered rocks and the enclosed massive sulphide bodies. White-mica schists and andalusite–kyanite schists represent the dynamometamorphic equivalents of the earlier sericitic and advanced argillic zones. Mesoscopic and microscopic structures and textures attest to the ductile behaviour of the massive pyrite bodies during this deformation and accompanying metamorphism.Portions of the deformed and metamorphosed altered zones are characterized by a late cataclastic deformation and by the development of fractures postdating the ductile deformation. The late hydrothermal alteration is concentrated within these cataclastic rocks and is characterized by the retrogression of the greenschist–amphibolite assemblages. Andalusite and kyanite are replaced by diaspore, kaolinite, and pyrophyllite assemblages, and pyrite is replaced by chalcopyrite–gold, chalcopyrite–bornite–gold, and bornite–stromeyerite assemblages. The concentration of the pre-dynamometamorphic alteration and sulphide mineralization within a narrow band along the southern BRG could indicate that this part of the BRG was the locus of a major Archean synvolcanic fault zone.

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