Geology of the shear-hosted Brookbank gold prospect in the Beardmore-Geraldton belt, Wabigoon subprovince, Ontario

The Beardmore-Geraldton belt consists of steeply dipping, intercalated panels of metavolcanic and metasedimentary rocks along the southern margin of the granite-greenstone Wabigoon subprovince in the Archean Superior Province, Ontario. It is an important past-producing gold belt that includes classic epigenetic iron formation-hosted deposits near Geraldton and turbidite-hosted deposits, north of Beardmore. The deposits formed during post-2690 Ma regional dextral transpression across the belt. The Brookbank gold prospect belongs to a third group of related gold deposits, which formed during transpression along dextral shear zones localized at contacts between panel of metasedimentary and metavolcanic rocks. The Brookbank prospect occurs along a steeply dipping shear zone at the contact between footwall polymictic conglomerate and hanging wall calc-alkaline arc-basalt. Early during shearing, the basalt acted as a structural and chemical trap that localized brittle deformation, veining and gold deposition, ankerite-sericite-chlorite-epidote-pyrite alteration, and the replacement of metamorphic magnetite and ilmenite by gold-bearing pyrite. This produced a low grade (≤ 5 gr/t Au) ankerite-rich alteration zone that extends up to 20 m into the hanging wall basalt. Later during shearing, gold was deposited within higher grade (≤ 20 gr/t Au) quartz-orthoclase-pyrite alteration zones superimposed on the wider ankerite-rich alteration zone. Auriferous quartz-carbonate veins oriented clockwise and anticlockwise to the shear zone walls are folded and boudinaged, respectively, consistent with dextral slip along the shear zone. The Brookbank gold prospect provides an interesting new exploration model for gold deposits in the dextral transpressive Beardmore-Geraldton belt.