Emplacement of sharp-walled sulphide veins during reactivation of impact-related structures at the Broken Hammer Mine, Sudbury, Ontario

Abstract

Broken Hammer is a hybrid, Cu-Ni-Platinum Group Element (PGE) footwall deposit in the North Range of the ca. 1.85 Ga Sudbury impact structure. The sulphide vein system and associated low sulphide PGE mineralization were mined as an open pit operation over a 15-month period, providing a unique opportunity to study a complete 90-meter vertical section across a footwall deposit. The deposit is hosted within Archean basement rocks and impact-induced Sudbury breccia, 1.5 km north of the Sudbury Igneous Complex (SIC) – basement contact. The low sulphide mineralization consists of disseminated to blebby chalcopyrite (<5%), minor pyrite, chalcocite, galena, sphalerite and platinum group minerals, associated with Ni-bearing chlorite overprinting alteration patches of epidote, actinolite and quartz. The veins comprise massive chalcopyrite and minor magnetite, chalcocite, millerite, and rare sperrylite, surrounded by thin epidote, actinolite and quartz selvedges. They are grouped into five, steeply-dipping, NE-, SW-, SE-, S- and EWstriking sets, which intersect in a common line controlling the plunge (60°) and trend (220°) of ore shoots. The veins were emplaced along syn-impact fractures that were reactivated multiple times during stabilization of the impact crater floor. Early reactivation of the fractures created pathways for the migration of hydrothermal fluids from which quartz and chlorite precipitated sealing the fractures. Renewed slip and reactivation shattered the quartz-chlorite veins into fragments that were incorporated in massive sulphide veins that crystallized from strongly fractionated sulphide melts or high temperature (400°C-500°C) hydrothermal fluids which migrated outward into the basement rocks from a cooling and crystallizing impact melt sheet represented by the SIC. Hydrothermal fluids syn-genetic with the epidote-actinolite-quartz alteration distributed the PGE into the footwall rocks, or late hydrothermal fluids associated with the Ni-bearing chlorite leached Ni and PGM’s from the sulphide veins and re-distributed them within the footwall rocks, forming the low-sulphide high-PGE mineralization. During post-impact tectonic events, reactivation and slip at temperatures below the brittle-ductile transition for chalcopyrite (<200°C-250°C) produced striations along the vein margins. The Broken Hammer deposit exemplifies how Cu-Ni-PGE footwall deposits formed by the reactivation of syn-impact fractures that provided conduits for the migration of melts and hydrothermal fluids.