Revising PGE deposition and remobilization mechanisms using new data from the historic Vermilion and Crean Hill mines, Sudbury, Canada

Abstract

Platinum-group element (PGE) mineralization associated with the Sudbury Igneous Complex (SIC), Canada, generally occurs within brecciated footwall rocks. At the Crean Hill and Vermilion deposits, variations in the ore mineralogy, textures, and whole rock geochemical signatures suggest that PGE deposition involved three stages. During the magmatic stage, sulfide melts were segregated at the base of the SIC and infiltrated the footwall rocks to form sulfide(-PGE) breccia and disseminated PGE mineralization (Crean Hill), and sulfide-PGE veins (Vermilion). Sulfide fractionation is suggested by the disappearance of Ru-bearing michenerite, a decrease in Ru, Rh and Ir tenors, and an increase in Pt, Pd and Au tenors and Cu/Ni away from the SIC contact. The syn-tectonic remobilization stage occurred between ~ 480–550 °C, as suggested by the composition of shear-hosted gersdorffite. At Crean Hill, Pd and Au were decoupled from Pt and remobilized via fluids into the footwall rocks, resulting in Pd and Au enrichment as disseminated michenerite and argentian gold along shear zones. At Vermilion, higher fluid-rock ratios and metamorphic semi-metal melts caused extensive remobilization of Pt, Pd and Au, and deposition of complex telluride, antimonide and arsenide grains within shear zones. A late metasomatic stage at < 300 °C (gersdorffite composition) is observed at Vermilion only, where it caused epidote-albite-quartz-calcite alteration of the SIC rocks and deposition of low-temperature sulfides and precious metals in veins crosscutting shear zones. Together, these findings demonstrate how PGE mineralization should be examined relative to its host rock geology and evolution to resolve the distribution of precious metals in modified Ni-Cu-PGE deposits.