Magmatic mineralization and hydrothermal enrichment of the High Grade Zone at the Lac des Iles palladium mine, northern Ontario, Canada

The economic mineralization of Pd at the Lac des Iles mine occurs in the gabbroic rocks of the Mine Block Intrusion in the 2.69 Ga Lac des Iles Intrusive Complex. The complex intruded the tonalitic rocks of the Lac des Iles greenstone belt in the Wabigoon Subprovince of the Superior Province of Canada. We conducted a detailed study on the Pd mineralization in the southern Roby Zone and the Twilight Zone. Sulphide minerals commonly display exsolution textures where pentlandite and chalcopyrite are exsolved from pyrrhotite. Sulphur contents from these zones display positive correlations with the contents of platinum group elements (PGE), Se, and Te, suggesting a magmatic origin of the mineralization where PGE were concentrated in immiscible sulphide melt in the parental magmas. The average ratios of Se/S (703±192×10^–6) and Te/S (192±104×10^–6) in the two zones are higher than the primitive mantle values of ~300×10^–6 and ~48×10^–6, respectively. The high ratios are consistent with the derivation of their parental magmas from a depleted mantle source. The High Grade Zone forms a narrow northwest-trending zone in the margin of the Roby Zone, and is hosted by an intensely altered clinopyroxenite/melanogabbroic unit. It contains two mineral assemblages; millerite + siegenite ± chalcopyrite ± pyrite co-existing with hornblende + plagioclase ± quartz ± carbonate, and pyrite ± chalcopyrite with chlorite + actinolite ± albite ± quartz ± carbonate. The ore is high in Pd (mean Pd/Pt ratio of 16.5; up to 25) compared to the southern Roby Zone and Twilight Zone where the Pd/Pt ratios are ~8. It shows positive correlations between Se and Te and between Se and immobile metals, such as Ni and Co. The data suggest a primary magmatic origin of mineralization of the High Grade Zone, but there is substantial scatter on diagrams involving S, such as the plot between S and Se. The evidence suggests that the primary magmatic mineralization was followed by hydrothermal transport of mobile elements. Using the relationships between Se and metals, the ore most likely had 0.8–2 ppm Pt and 8–21 ppm Pd during the primary mineralization. The subsequent hydrothermal activity resulted in the enrichment of Pd by up to 40 ppm. The lack of fluid pathways in the High Grade Zone and the distribution of the zone are consistent with magmatic-hydrothermal activity by aqueous fluids exsolved from the parental magmas of the Roby Zone and High Grade Zone. Sulphide minerals from the southern Roby Zone, Twilight Zone, and High Grade Zone have similar ³⁴S values, ranging from 0.0 to +1.5. The data are consistent with the derivation of S from the mantle. In individual samples from the southern Roby Zone and High Grade Zone, pyrite shows lower ³⁴S than chalcopyrite, suggesting isotopic disequilibrium of S. This likely reflects the crystallization and re-crystallization of sulphide minerals over a wide range of temperatures.Editorial handling: B. Lehmann