Neoarchean supra-subduction gold in Mesoarchean tonalite-granodiorite: Two separate mineralization events at Hammond Reef defined by disseminated and channelized fluid flow

Declining gold discoveries underscore the importance of informed geological models for future exploration. The genesis of many gold deposits remains ambiguous, but integrated structural and chronological studies shed light on key questions of source, timing and geodynamic setting. The Hammond Reef gold deposit is unusual in that it is hosted in Mesoarchean tonalite of the Wabigoon superterrane, rather than in Neoarchean greenstone belt terranes, as is common elsewhere in the Superior Province. Gold deposition at Hammond Reef occurred in two stages: The main stage of disseminated low-grade mineralization was followed by discrete, channelized veins, both tied to regional tectonic events. Disseminated mineralization is associated with localized sulfidation and chlorite breakdown within a sericitized deformation zone adjacent to the locked greenstone-gneiss boundary. Chlorite-leached halos around small fractures at the deposit margins indicate that fluid penetration into the tonalite was guided by fractures concentrated in the broad deformation corridor. Pervasive alteration was enhanced by grain-scale flow. Hammond Reef may have been particularly susceptible to sulfidation and associated gold precipitation due to the greater local abundance of mafic dykes and chlorite-rich enclaves in the tonalite. The secondary gold mineralization phase is associated with younger and discrete auriferous quartz-carbonate veins that were episodically emplaced along pre-existing structures. Cross-cutting relationships show that both mineralization phases post-date the emplacement of the 2.889 Ga Diversion Stock. The kinematics of mineralized structures relate Hammond Reef to the 2.7 Ga northward subduction of the Wawa terrane beneath the Wabigoon superterrane. This places Hammond Reef in a supra-subduction setting, where an older dormant fault zone with enhanced permeability was infiltrated by auriferous fluids during the an orogeny up to 200 million years later. The two phases of gold enrichment present disseminated and channelized fluid flow through the available permeable pathways within the same regional tectonic setting.