Palladium–copper-rich platinum-group element mineralization in Legris Lake mafic–ultramafic complex, Western Superior Province, Canada

Abstract

The Legris Lake Complex is a northeast-trending, 7.3 km × 3.5 km, mafic-ultramafic intrusive complex located in the western Wabigoon Subprovince of the Archaean Superior Province. It is part of a circular series of mafic-ultramafic complexes, the most notable of which is the Lac des Iles Complex, ca 7 km northwest of the Legris Lake Complex, which hosts the Lac des Iles palladium mine. Four phases of exploration have been conducted on the Legris Lake Complex property since autumn, 1999, defining mineralized zones on surface and at depth, with drill-core intersections of up to 2.04 g/t Pd, 0.41 g/t Pt, 0.71 g/t Au, 0.42% Cu and 0.13% Ni over 9.95 m.The Legris Lake Complex consists of mostly gabbroic rocks but also contains lithologies ranging from anorthosite to wehrlite and a variety of igneous breccias. The gabbroic rocks vary from melanogabbro to porphyritic leucogabbro. Medium-grained, massive, biotite-rich leucogabbro is the predominant exposed variety and probably caps the complex. The north-western margin of the complex (2 km × 600 m), which contains all the known platinum-group element (PGE) mineralization, is characterized by heterolithic breccia with abundant fragments of sedimentary rocks and numerous gabbroic dykes and sills.The PGE mineralization in the Legris Lake Complex has different characteristics from many PGE deposits, where the ore occurs in sulphide-rich bodies at the base of mafic and ultramafic rocks. In the Legris Lake Complex PGE enrichment occurs in sulphide-poor (1-10 vol%, mostly <5 vol%), medium- to coarse-grained, porphyritic leucogabbro (termed 'Main Showing-type') hosted within zones of intense magmatic brecciation. The mineralized leucogabbro is mineralogically highly evolved and exhibits a sill-like form near the stratigraphic top of the complex. The mineralization consists of disseminated to blebby sulphides rimmed by epidote and disseminated magnetite. The mineralized rocks typically contain Cu ranging from 0.2 to 0.4 wt% and Ni from 0.07 to 0.12 wt%, with low Pt/Pd ratios (ca 0.2) and high Cu/Ni ratios (ca 3). The PGE contents display a positive correlation with those of Cu and Ni. The origin of the mineralization is best explained by preferential partitioning of PGE into an immiscible sulphide melt in evolved silicate magma after fractional crystallization of olivine and clinopyroxene. The immiscible separation of sulphide melt may have been aided by incorporation of silica and sulphide from adjacent sedimentary rocks. Formation of magnetite and hydrous minerals in the mineralized zones suggests that the sulphide melt had high oxygen/(oxygen + sulphur) ratios and high contents of volatiles, most probably reflecting a high oxidation state and volatile-rich nature of the parental magmas. This primary magmatic PGE mineralization was followed by minor redistribution of PGE by deuteric hydrothermal fluids released from the parental magma.