Abstract

Pyritite and massive sphalerite-galena bodies are surrounded by two distinct concentric alteration zones within the metafelsites of the late Archean Blake River Group at the Dumagami Au-Ag-Cu deposit, southern Abitibi belt, northwestern Quebec. The inner alteration zone is characterized by andalusite-bearing rocks and is interpreted as a metamorphosed advanced argillic alteration zone; the outer zone is represented by white mica phyllonite and phyllonitic breccia and is interpreted as a metamorphosed sericitic alteration zone. Surrounding least altered rocks, represented by massive quartziferous augen rocks and ribboned micaceous augen schists, are composed of blue quartz and albite phenocrysts, quartz, biotite, ilmenite, and chlorite. The sericite zone is characterized by the metamorphic mineral assemblage quartz-sericite-rutile-pyrite + or - andesine and by the absence of albite phenocrysts, biotite, and ilmenite. The andalusite zone is characterized by the metamorphic mineral assemblage quartz-andalusite-sericite-rutile-pyrite + or - kyanite and by the absence of albite phenocrysts, andesine porphyro-blasts, biotite, and ilmenite.Petrographic and geochemical features of least altered, sericitic, and andalusite-bearing rocks indicate that felsic effusive rocks were the principal protolith. Strong leaching of alkali and alkaline earths (especially Mg) within the inner alteration zone has resulted in a notable absence of chlorite, a feature atypical of the alteration associated with Archean massive sulfide deposits. Chemical and mineralogical changes from least altered to altered rocks suggest that highly acidic hydrothermal fluids were responsible for the premetamorphic formation of alteration assemblages at the Dumagami deposit. However, gold transport in these fluids would be minimal, and gold emplacement is linked instead to a late, structural event.

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