The Archean Hammond Reef Deposit (Canada): Formation of an Orogenic Gold Deposit in a Contractional Step-Over Zone Along a Major Strike-Slip Fault System

The Hammond Reef deposit is located within the south-central region of the Wabigoon subprovince of the Archean Superior province, Canada. The deposit is hosted by the NNE-trending Marmion shear system, which straddles the contact between the 2890.2 ± 1 Ma Diversion stock and ca. 3000 Ma Marmion batholith. The shear system consists of localized fault-controlled zones of white mica (± carbonate) alteration and shearing, superposed on a wider zone of chlorite-carbonate (± white mica) alteration. Multiple shear sense indicators, including the deflection of mylonitic foliations in shear zones, asymmetrical folds, and shear bands, suggest that the Marmion shear system is a major sinistral transcurrent fault system. The deposit occurs along an ENE-trending segment of the Marmion shear system, where it is associated with hydrothermal quartz breccias and quartz-chlorite-carbonate veins, which were emplaced during the white mica (± carbonate) alteration. Gold is present as disseminated grains in the veins and breccias and as microinclusions and fracture fill in pyrite grains within the white mica–altered wall rocks. New monazite U-Pb ages suggest that the deposit formed at ~2700 Ma during the main orogenic event in the Wabigoon subprovince. NNW-directed shortening during this event resulted in sinistral transcurrent displacement along NNE-trending faults of the Marmion shear system and contraction across ENE-trending segments. Increased fracturing and brecciation due to contraction across these segments channelized the migration of hydrothermal fluids and the precipitation and concentration of gold. The ENE-trending segments represent step-over zones at the terminations of en échelon, NNE-trending, transcurrent faults, which became linked by the formation of throughgoing, ENE-trending faults. Thus, Hammond Reef is an excellent example of an orogenic gold deposit that formed in a contractional step-over zone between two right-stepping sinistral transcurrent faults.