The Archean Hammond Reef deposit: the formation of an orogenic gold deposit in a contractional step-over-zone along a major strike-slip fault system

Abstract

Hammond Reef is an orogenic gold deposit with a measured and indicated resource estimate of 3.3 Moz at an average grade of 0.84 g/t gold. It is located within the south-central region of the Wabigoon Subprovince in northwest Ontario. It is hosted within a system of north- to northeasttrending anastomosing shear zones, called the Marmion Shear System (MSS), which straddles the contact between the Mesoarchean Diversion stock to the west and Marmion Batholith to the east. Multiple shear sense indicators, including the deflection of mylonitic foliations in shear zones, drag folds, and shear bands, suggests that the MSS is a major sinistral transcurrent fault system. The bulk of gold mineralization is concentrated in an ENE-trending bend along the MSS, characterized by intense sericite and carbonate alteration. Mineralization formed between 2700- 2690 Ma and is associated with syn-tectonic hydrothermal quartz breccias and shallowly dipping quartz-carbonate veins with down-dip striations, which formed during bulk NNW-directed shortening across the bend. This suggests that the Hammond Reef deposit formed along a contractional step-over-zone between two regional sinistral transcurrent faults. Compression across the bend resulted in more fracturing that localized the migration of hydrothermal fluids and the precipitation and concentration of gold.