Synvolcanic and syntectonic gold mineralization at the Bousquet Mine, Abitibi greenstone belt, Quebec

Univ. Montreal, Dep. Geol., Montreal, QC, Canada

The Bousquet pyrite gold deposit, consisting of disseminated- and vein-type mineralization in sheared and altered greenschist facies Archean volcanic rocks, is hosted by two vein sets: foliation-parallel veins in ore zone 3 and foliation-oblique veins in zones 1, 2, 4, and 5. Foliation-parallel veins are composed of pyrite and lesser amounts of chalcopyrite, sphalerite, pyrrhotite, arsenopyrite, galena, bornite, rutile, tellurides, and native gold. The foliation-oblique veins of zones 1 and 2 are composed of pyrite, chalcopyrite, and sphalerite, with subordinate amounts of pyrrhotite, arsenopyrite, galena, stannite, gudmundite, rutile, ilmenite, and gold. Quartz and muscovite are the principal gangue minerals in both vein types in ore zones 1, 2, and 3. In zone 4, foliation-oblique veins are composed of pyrite, magnetite, pyrrhotite, and minor amounts of chalcopyrite and gold. The gangue minerals are calcite, ankerite, white mica, chlorite, and tourmaline. The veins of zone 5 contain pyrite, chalcopyrite, sphalerite, bornite, magnetite, rutile, and gold in a gangue of quartz, carbonate, white mica, and chlorite. Disseminated auriferous pyrite occurs in zones 3 and 5.Mineralogical and textural features indicate that gold emplacement and associated alteration occurred during two events, one predating peak regional metamorphism and another postdating peak metamorphism. Early, premetamorphic mineralization and related synvolcanic (?) hy-drothermal alteration produced initial auriferous sulfides and an advanced argillic alteration. Subsequent deformation, recrystallization, and metamorphic remobilization of early sulfides resulted in the currently observed, structurally controlled vein-type gold deposits. The vol-canogenic event is suggested by highly aluminous rocks closely associated spatially with the mineralized zones, by textures preserved through the overprint of metamorphic textures, and by the occurrence of stannite in ore zones 1 and 2.The late, postmetamorphic gold mineralization is considered to be contemporaneous with retrograde alteration of metamorphic minerals formed during late deformation. This alteration produced a pervasive kaolinite-pyrophyllite-diaspore gangue mineral assemblage involving the replacement of syntectonic andalusite porphyroblasts in felsic rocks. In mafic host rocks, retrograde alteration is characterized by carbonate, chlorite, muscovite, pyrite, and gold. The formation of a white mica-diaspore-carbonate-chlorite assemblage and the emplacement of associated gold are interpreted as epigenetic metamorphic events because these late retrograde minerals overprint the greenschist facies mineral assemblage.

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