The geology of the Greenstone orogenic gold deposit, Geraldton, Ontario, Canada: Structural controls, mineralogy, geochemistry, and geochronology

Abstract

The Greenstone orogenic gold deposit is located in the Beardmore-Geraldton belt (BGB) along the boundary between the granite-greenstone Wabigoon subprovince and the metasedimentary Quetico subprovince of the Archean Superior craton, Canada. The deposit is hosted by ca. 2700–2694 Ma turbiditic sandstone, banded iron formation, and ca. 2694 Ma feldspar-quartz porphyry, which underwent strong deformation within the 1 km-wide Bankfield-Tombill deformation zone along the southern margin of the BGB. The deformation zone includes folds and cleavage that formed during early D₁ thrust imbrication of the BGB, S-shaped folds and fabrics that formed during D₂ sinistral transpression, and Z-shaped folds, fabrics, and localized shear zones that formed during D₃ dextral reactivation of the deformation zone. Gold mineralization is associated with folded, early-D₁, quartz-carbonate veins (V₁) and with NE- to E-striking, syn-D₂, tourmaline-quartz veins (V₂) as well as quartz-carbonate veins (V₃). The V₁ and V₃ veins are surrounded by sericite-carbonate-pyrite ± albite–rutile alteration halos, and the V₂ veins are surrounded by carbonate-tourmaline-pyrite ± pyrrhotite-chalcopyrite alteration halos. Gold was deposited during fluid-rock sulfidation reactions that resulted in the formation of inclusion-poor pyrite with Ni-Co-As primary crystallographic zoning and inclusion-rich pyrite enriched in Au and other metals (Ag-As-Bi-Co-Ni-Pb-Sb-Te). Hydrothermal alteration associated with the deposition of the veins produced a broad, up to 250 m wide, sericite-carbonate alteration envelope, with S, Te, As, W, and Bi as the best pathfinder indicators to gold mineralization. Contrary to previous studies, which attributed the formation of gold deposits in the BGB to late-D₃, our results suggest that gold was emplaced during early-D₁ and D₂ and involved multiple hydrothermal fluid pulses during several deformation events, as suggested for other major Archean orogenic gold camps associated with major fault zones such as the Timmins and Kirkland Lake camps in the Abitibi subprovince of the Superior craton.