Emplacement of sharp-walled sulphide veins during reactivation of impact-related structures at the Broken Hammer Mine, Sudbury, Ontario
Date
2019-08-26
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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.
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Keywords
Sudbury footwall deposit, Broken Hammer, impact cratering, crater modification, crater floor fractures, reactivated syn-impact fractures, Cu-Ni-PGE deposits