The B26 Cu-Zn-Ag-Au project, Brouillan volcanic complex, Abitibi greenstone belt, part 1: geological setting and geochronology

Archean synvolcanic base metal deposits vary greatly in style and metal budget through time and space. This variability is due to contrasting geological and magmatic settings, the influence of which remains a research topic with potential exploration implications. The B26 Cu-Zn-Ag-Au project, located in the Harricana-Turgeon belt of the northern Abitibi greenstone belt, is interpreted as a volcanogenic massive sulphide deposit formed in an evolving calc-alkaline (FII-type) intermediate to felsic centre (Brouillan volcanic complex). The B26 deposit is enriched in Ag and is part of a group of precious metal-enriched Archean volcanogenic massive sulphide deposits that are associated with calc-alkaline volcanic rocks, indicating that such successions are highly prospective. The B26 host succession, informally divided into a lower intermediate member and an upper felsic member, is composed of an andesitic base intercalated with dacitic domes, onto which the felsic member was emplaced. The felsic member comprises three geochemically and texturally distinct rhyolite units that were emplaced as sills, domes, and/or flows with associated relatively voluminous fragmental facies. This felsic succession, dated at 2728 Ma, is coeval both with the host rhyolite of the nearby Selbaie polymetallic deposit and the Brouillan pluton. It is overlain by thin sedimentary strata and cut by andesitic sills and andesitic transposed dykes. This volcanic pile, undergoing extension and containing thick permeable strata, was favourable for the development of a high-temperature hydrothermal system characterized by extensive zones of Cu-rich stringers and sub-seafloor Zn-Ag-(Pb)-rich sulphide lenses in an evolving, possibly shallow-water, active volcanic centre.