The Bousquet 2-Dumagami World-Class Archean Au-Rich Volcanogenic Massive Sulfide Deposit, Abitibi, Quebec: Metamorphosed Submarine Advanced Argillic Alteration Footprint and Genesis

Abstract

The Archean Bousquet 2-Dumagami deposit is an Au-rich volcanogenic massive sulfide deposit (VMS) with a
\ntotal 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
\nwithin the Doyon-Bousquet-LaRonde mining camp in northwestern Quebec and hosted by the 2704 to 2695
\nMa Blake River Group, the world’s most productive volcanic assemblage for Au-rich VMS deposits.
\nThe Bousquet 2-Dumagami deposit consists of stacked, deformed, and transposed semimassive to massive
\npyrite-rich lenses, breccia zones, and associated sulfide veins and stringer zones hosted by the upper member
\nof the Bousquet Formation, ~50 to 100 m stratigraphically below the <2687 Ma Cadillac Group sedimentary
\nrocks. The main ore zone is known as the Massive Hangingwall zone at the Bousquet 2 mine and Zone 5 at the
\nDumagami mine. Another semimassive to disseminated pyrite-rich auriferous zone with coarsely recrystallized
\nmassive pyrite is present in the footwall (Massive Footwall zone). The Massive Hangingwall zone is an Au-Ag-
\nCu-Zn sheet-like, semimassive to massive, pyrite-rich sulfide lens intermixed with vein and breccia zones. The
\ndominant ore type consists of Au-Cu mineralization, but its upper and eastern parts are enriched in Zn. The ore
\nconsists of a complex assemblage of sulfides, sulfosalts, and native gold, including abundant pyrite, sphalerite,
\na few percent of chalcopyrite, bornite, and galena, with some visible gold. The Massive Hangingwall zone was
\nformed by subsea-floor replacement of footwall calc-alkaline dacitic volcaniclastic rocks and hanging-wall blue
\nquartz-phyric rhyolite.
\nDespite significant north-south shortening and metamorphism, which was responsible for transposition,
\nflattening, folding, and recrystallization, mineralogical gradients related to alteration-induced compositional
\nvariations can still be identified. A number of different metamorphic mineral assemblages can be mapped over
\nseveral tens of meters from distal to proximal to the ore: (1) quartz-muscovite ± Mn-garnet ± biotite ± chlorite;
\n(2) quartz-muscovite ± pyrite; (3) quartz-muscovite-andalusite-pyrophyllite-pyrite with topaz and diaspore; and
\n(4) massive quartz-pyrite. A quartz-carbonate-biotite assemblage occurs in the hanging wall of the Zn-rich Massive
\nHangingwall zone and is hosted by andesitic sills. The thickness of each of these assemblages varies from a
\nfew meters to tens of meters. All metamorphosed alteration assemblages are characterized by strong progressive
\nNa2O depletion. Gains in MnO, Fe2O3(total), MgO, and CaO are recorded in the quartz-muscovite ± Mngarnet
\n± biotite ± chlorite assemblage, whereas gains in K2O and losses in CaO occur in the quartz muscovite
\n± pyrite assemblage. In the quartz-muscovite-andalusite-pyrophyllite-pyrite and the proximal massive quartzpyrite
\nassemblages all oxides, except SiO2, Fe2O3(total), and TiO2, were strongly to almost entirely leached. The
\nandalusite-kyanite-pyrophyllite–bearing aluminous assemblages are interpreted to represent metamorphosed
\nequivalents of synvolcanic alteration produced by acidic and oxidizing hydrothermal fluids (i.e., metamorphosed
\nadvanced argillic-style alteration), whereas the massive quartz-pyrite assemblage is similar to the massive silicic
\nalteration commonly associated with advanced argillic alteration. The timing of Au mineralization is considered
\nto be close to the age of the host rhyolite (2697.8 ± 1 Ma) and the age of the overlying felsic volcanic rocks
\n(2697.5 ± 1.1 Ma).
\nThe major Au endowment of the Doyon-Bousquet-LaRonde mining camp may be related to favorable
\nsource rock or Au reservoirs specific to the lower crust or upper mantle beneath the eastern Archean Blake
\nRiver Group. Exploration for additional Au-rich VMS in this environment should focus on distal quartz- and
\nMn-rich garnet-biotite and proximal aluminous assemblages with anomalously high Au and/or Cu and Zn in
\nintermediate to felsic transitional to calc-alkaline volcanic or volcaniclastic rocks located underneath a younger
\nsedimentary cover.
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