The Westwood Deposit, Southern Abitibi Greenstone Belt, Canada: An Archean Au-Rich Polymetallic Magmatic-Hydrothermal System—Part I. Volcanic Architecture, Deformation, and Metamorphism

Abstract The Westwood deposit (4.5 Moz Au) is hosted in the 2699–2695 Ma Bousquet Formation volcanic and intrusive rocks, in the eastern part of the Blake River Group, southern Abitibi greenstone belt. The Bousquet Formation is divided in two geochemically distinct members: a mafic to intermediate, tholeiitic to transitional lower member and an intermediate to felsic, transitional to calc-alkaline upper member. The Bousquet Formation is cut by the synvolcanic (2699–2696 Ma) polyphase Mooshla Intrusive Complex, which is cogenetic with the Bousquet Formation. The deposit contains three strongly deformed (D2 flattening and stretching), steeply S-dipping mineralized corridors that are stacked from north to south: Zone 2 Extension, North Corridor, and Westwood Corridor. The North and Westwood corridors are composed of Au-rich polymetallic sulfide veins and stratabound to stratiform disseminated to massive sulfide ore zones that are spatially and genetically associated with the calcalkaline, intermediate to felsic volcanic rocks of the upper Bousquet Formation. The formation of the disseminated to semimassive ore zones is interpreted as strongly controlled by the replacement of porous volcaniclastic rocks at the contact with more impermeable massive cap rocks that helped confine the upflow of mineralizing fluids. The massive sulfide lenses are spatially associated with dacitic to rhyolitic domes and are interpreted as being formed, at least in part, on the paleoseafloor. The epizonal, sulfide-quartz vein-type ore zones of the Zone 2 Extension are associated with the injection of subvolcanic, calc-alkaline felsic sills and dikes within the lower Bousquet Formation. These subvolcanic intrusive rocks, previously interpreted as lava flows, are cogenetic and coeval with the intermediate to felsic lava flows and domes of the upper Bousquet Formation. The change from fractional crystallization to assimilation- and fractional crystallization-dominated processes and transitional to calc-alkaline magmatism is interpreted to be responsible for the development of the auriferous ore-forming system. The Westwood deposit is similar to some Phanerozoic Au ± base metal-rich magmatic-hydrothermal systems, both in terms of local volcano-plutonic architecture and inferred petrogenetic context. The complex volcanic evolution of the host sequence at Westwood, combined with its proximity to a polyphase synvolcanic intrusive complex, led to the development of one of the few known large Archean subaqueous Au-rich magmatic-hydrothermal systems.