Geologic and Geochemical Features of the World-Class Archean Windfall Intrusion-Related Au Deposit, Abitibi Subprovince, Canada

Abstract

The Neoarchean Windfall gold deposit, hosted in the Urban-Barry greenstone belt of the Abitibi subprovince (Quebec, Canada), represents an emerging and significant Au deposit with a resource of 7.4 Moz of Au. It is hosted in 2717 Ma bimodal volcanic rocks that are cut by several generations of calc-alkaline quartz-feldspar porphyry dikes separated into (1) a 2697.6 ± 2.6 Ma group spatially related to Au mineralization and (2) a 2697.6 ± 0.4 Ma group that truncates the earlier dikes and the Au mineralization. The Au zones are structurally controlled and localized to faults and fractures proximal to the contacts of the early quartz-feldspar porphyry dikes; these zones form thin, subvertical, and elongate lenses plunging 35° east-northeast. Gold mineralization, present as both free gold and inclusions in pyrite, occurs (1) in gray quartz veins and stockworks with pyrite and subordinate carbonate and tourmaline and (2) in pervasive to patchy sericite-silica-pyrite-carbonate ± tourmaline ± fuchsite alteration zones. The Au mineralization and associated hydrothermal alteration, along with all the host rocks that include postmineralization intrusions, are overprinted by D2 deformational features that include a penetrative fabric, shear zones, and associated folds. The spatial and temporal association of the quartz-feldspar porphyry intrusions with the Au mineralizing event at the Windfall gold deposit, along with its elemental association (Ag, As, Sb, S, Se, Bi, Te, ± Zn, Cu, Pb, Mo, W), suggests an intrusion-related model and contrasts with the more abundant orogenic gold deposits in the Abitibi greenstone belt. This interpretation has important implications both locally and regionally for Au exploration in Archean greenstone terranes.