Changes in sulfides and platinum-group minerals with the degree of alteration in the Roby, Twilight, and High Grade Zones of the Lac des Iles Complex, Ontario, Canada

Abstract

The Lac des Iles Complex hosts Pd deposits with unusually high Pd/Pt and Pd/Ir ratios. These high ratios have been attributed to the introduction of Pd by late magmatic–hydrothermal fluids. The presence of hydrous silicates and low-temperature sulfide mineral assemblages has been advanced as evidence for fluid overprint. This idea has been investigated by documenting the change in silicate, sulfide, and platinum-group mineralogy and texture in samples from the deposit ranging from fresh gabbronorite to extensively altered chlorite–actinolite schist. In addition, whole-rock analysis combined with laser ablation analysis of the sulfides was used to determine which phase controlled each of the PGE. In fresh gabbronorite the sulfide assemblage is igneous (pyrrhotite–pentlandite–chalcopyrite) and Pd is present mainly in pentlandite and kotulskite (PdTe) inclusions associated with the sulfide minerals. All other PGE (except Pt) are present in solid solution in pyrrhotite and pentlandite. Platinum is present as sperrylite (PtAs2) and montcheite (PtTe2) inclusions associated with the sulfide minerals. These observations suggest that the PGE in the fresh gabbronorite were all collected by magmatic sulfide liquid. In the chlorite–actinolite schist the sulfide assemblage is a low-temperature assemblage consisting of chalcopyrite, pyrite ± millerite. A minor amount of palladium is present in millerite, but most is present as bismuth-rich kotulskite and Pd arsenides both of which are associated with hydrous silicate. All other PGE (except Pt) are hosted by pyrite. A minor amount of Pt is present in pyrite, but most is found as sperrylite and montcheite associated with hydrous silicate minerals. The pyrite could have formed by Fe-loss from pyrrhotite (resulting in the pyrite inheriting Os, Ir, Ru, and Rh) and Fe-loss from pentlandite to form millerite (resulting in the millerite inheriting some Pd). The PGE concentrations of the whole rock show there is no direct correlation between the sulfide assemblage, the degree of alteration of the silicate minerals and ore grade. Thus, although fluids have clearly altered the sulfide-, silicate-, and platinum-group mineralogy and their textures, it is not clear that these fluids have changed the PGE content of the rocks.