Abstract

The Archean Bousquet 2-Dumagami deposit is an Au-rich volcanogenic massive sulfide deposit (VMS) with a total production of 3.87 Moz Au, 2.77 Moz Ag, 80,000 metric tons (t) Cu, and 5,000 t Zn. The deposit is located within the Doyon-Bousquet-LaRonde mining camp in northwestern Quebec and hosted by the 2704 to 2695 Ma Blake River Group, the world’s most productive volcanic assemblage for Au-rich VMS deposits.

The Bousquet 2-Dumagami deposit consists of stacked, deformed, and transposed semimassive to massive pyrite-rich lenses, breccia zones, and associated sulfide veins and stringer zones hosted by the upper member of the Bousquet Formation, ~50 to 100 m stratigraphically below the <2687 Ma Cadillac Group sedimentary rocks. The main ore zone is known as the Massive Hangingwall zone at the Bousquet 2 mine and Zone 5 at the Dumagami mine. Another semimassive to disseminated pyrite-rich auriferous zone with coarsely recrystallized massive pyrite is present in the footwall (Massive Footwall zone). The Massive Hangingwall zone is an Au-Ag-Cu-Zn sheet-like, semimassive to massive, pyrite-rich sulfide lens intermixed with vein and breccia zones. The dominant ore type consists of Au-Cu mineralization, but its upper and eastern parts are enriched in Zn. The ore consists of a complex assemblage of sulfides, sulfosalts, and native gold, including abundant pyrite, sphalerite, a few percent of chalcopyrite, bornite, and galena, with some visible gold. The Massive Hangingwall zone was formed by subsea-floor replacement of footwall calc-alkaline dacitic volcaniclastic rocks and hanging-wall blue quartz-phyric rhyolite.

Despite significant north-south shortening and metamorphism, which was responsible for transposition, flattening, folding, and recrystallization, mineralogical gradients related to alteration-induced compositional variations can still be identified. A number of different metamorphic mineral assemblages can be mapped over several tens of meters from distal to proximal to the ore: (1) quartz-muscovite ± Mn-garnet ± biotite ± chlorite; (2) quartz-muscovite ± pyrite; (3) quartz-muscovite-andalusite-pyrophyllite-pyrite with topaz and diaspore; and (4) massive quartz-pyrite. A quartz-carbonate-biotite assemblage occurs in the hanging wall of the Zn-rich Massive Hangingwall zone and is hosted by andesitic sills. The thickness of each of these assemblages varies from a few meters to tens of meters. All metamorphosed alteration assemblages are characterized by strong progressive Na2O depletion. Gains in MnO, Fe2O3(total), MgO, and CaO are recorded in the quartz-muscovite ± Mn-garnet ± biotite ± chlorite assemblage, whereas gains in K2O and losses in CaO occur in the quartz muscovite ± pyrite assemblage. In the quartz-muscovite-andalusite-pyrophyllite-pyrite and the proximal massive quartz-pyrite assemblages all oxides, except SiO2, Fe2O3(total), and TiO2, were strongly to almost entirely leached. The andalusite-kyanite-pyrophyllite–bearing aluminous assemblages are interpreted to represent metamorphosed equivalents of synvolcanic alteration produced by acidic and oxidizing hydrothermal fluids (i.e., metamorphosed advanced argillic-style alteration), whereas the massive quartz-pyrite assemblage is similar to the massive silicic alteration commonly associated with advanced argillic alteration. The timing of Au mineralization is considered to be close to the age of the host rhyolite (2697.8 ± 1 Ma) and the age of the overlying felsic volcanic rocks (2697.5 ± 1.1 Ma).

The major Au endowment of the Doyon-Bousquet-LaRonde mining camp may be related to favorable source rock or Au reservoirs specific to the lower crust or upper mantle beneath the eastern Archean Blake River Group. Exploration for additional Au-rich VMS in this environment should focus on distal quartz- and Mn-rich garnet-biotite and proximal aluminous assemblages with anomalously high Au and/or Cu and Zn in intermediate to felsic transitional to calc-alkaline volcanic or volcaniclastic rocks located underneath a younger sedimentary cover.

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