Hydrothermal Redistribution and Local Enrichment of Platinum Group Elements in the Tootoo and Mequillon Magmatic Sulfide Deposits, South Raglan Trend, Cape Smith Belt, New Quebec Orogen

Abstract

The Tootoo and Mequillon Ni-Cu-Pt-Pd deposits in the South Raglan Trend of the New Quebec orogen host significant but as yet unexploited nickel-copper-platinum group element (PGE) mineralization within the mafic-ultramafic rocks of the Expo Intrusive Suite. The olivine melagabbronorite host rocks have been pervasively altered to hydrous mineral assemblages dominated by chlorite, tremolite-actinolite, and relict late-magmatic hornblende, characteristic of greenschist facies metamorphism. In greenschist-altered rocks, the PGE are spatially associated with intercumulus blebs and networks of pyrrhotite, chalcopyrite, and pentlandite after primary magmatic sulfides. Palladium is significantly enriched in the vicinity of veins associated with local replacement of the early hydrous silicate mineral assemblage by talc, calcite, dolomite, and minor serpentine and disseminated magnetite. The varieties of platinum group minerals (PGM) are similar in samples both with and without carbonate alteration, with platinum hosted exclusively in discrete sperrylite grains, and palladium dominantly hosted by the Pd-Sb intermetallic phases of sudburyite, ungavaite, and naldrettite, as well as merenskyite, kotulskite, and michenerite. Palladium minerals commonly form composite grains with hessite, altaite, and other PGM. Most of the PGM are hosted by silicate minerals, but some also occur attached to sulfide minerals, and a few PGM grains are completely enclosed within sulfides. In samples disturbed by carbonate alteration, the PGM are present in greater abundance and larger size. We suggest that palladium was transferred from massive ores to the net-textured ores via an oxidizing, CO2-bearing, neutral to mildly alkaline fluid capable of mobilizing the semimetals such as antimony as hydroxide complexes. Upon reaching zones where oxygen fugacity was buffered to lower values by an assemblage containing pyrrhotite but not magnetite, antimony in the fluid was reduced to its metallic state, causing coprecipitation of palladium in intermetallic phases. This process appears to have been localized at a reaction front near the outer edges of moderately oxidized carbonate-altered domains surrounding carbonate veins. This reaction front has high Pd/Pt whereas lower Pd/Pt ratios are observed in magnetite-bearing, carbonate-altered samples closer to the carbonate veins. PGE grades in the vicinity of carbonate alteration attain levels of potential economic significance, possibly meriting attention as PGE resources distinct from the Ni-Cu-PGE sulfide assemblage extracted by current beneficiation practices in nearby deposits. If the magnetite-free, carbonate-altered lithofacies can be confirmed by further study to show consistently elevated Pd grades, then it may be possible to improve Pd recoveries.