Abstract

Field, mineralogical, and geochemical investigations were conducted on previously identified platinum group element and copper-nickel (PGE-Cu-Ni) sulfide prospects in the North Lac des Iles Complex, located in northwestern Ontario, Canada. The results reveal that the North Lac des Iles Complex prospects are hosted by layered websterite, olivine websterite, and gabbronorite that display variable degrees of hydrous alteration and contain 0.5 to 2% disseminated Fe-Cu-Ni sulfide mineralization. Sulfide minerals are generally characterized by pyrrhotite/(chalcopyrite + pyrrhotite) ratios near 2. S/Se ratios overlap with those of mantle-derived magmas. The prospects have moderate Pd tenors (<1,000 ppm) and exhibit an average Pd/Pt ratio of 2.89 that remains constant with increasing Pd grades. Most occurrences contain Pd-rich platinum group minerals that are dominated by tellurides, bismuthotellurides, and arsenides. It is suggested that sulfide saturation and PGE concentration in the North Lac des Iles Complex resulted from a combination of fractional crystallization and dynamic recharge involving two compositionally distinct ultramafic parental magmas. It is now evident that the Lac des Iles Complex hosts two major styles of Pd-rich PGE-Cu-Ni sulfide mineralization, namely magmatic breccia style, typical of the Roby and Offset deposits in the South Lac des Iles Complex, and stratiform style, typical of the layered ultramafic series rocks in the North Lac des Iles Complex.

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