Post-magmatic modification of the sulphide deposits from the Thompson Nickel Belt, Manitoba, Canada

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Liwanag, Janice
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Disseminated to massive sulphides from the Thompson Nickel Belt, Manitoba, show textural and geochemical evidence of being modified by deformation and fluid alteration. Samples of massive sulphide horizons, hosting metasediments, sulphidebearing ultramafic rocks, and sulphide breccias were collected from the Thompson, Birchtree, and William Lake deposits. Sulphide minerals from these samples were examined petrographically and analyzed by electron microprobe analysis (EMPA) and proton-induced X-ray emission (PDG). Representative samples were analyzed by whole-rock geochemical techniques. Compositional zonation occurs in and around massive sulphide horizons hosted by metasediments. Disseminated pyrrhotite grains in metasediments occurring within -20 metres of a massive sulphide horizon are enriched in Ni relative to pyrrhotite grains that are more distal to the massive sulphide horizon. These Ni-enriched disseminated pyrrhotite grains co-exist with Ni-enriched pentlandite. Chalcopyrite tends to be concentrated into stringers at the contacts between massive sulphide horizons and hosting metasediments. During high-grade metamorphism, Cu may have migrated to the contacts. During final cooling of the sulphides, fluid-assisted diffusion of Ni from the massive ores to the metasediments may have occurred. Massive ores from the Thompson, Birchtree, and Pipe deposit are very depleted in Pt relative to other platinum group elements (PGE) The Pt depletion may be the result of 1) the fractionation of Pt from the other PGE under reducing conditions or 2) mobilization of Pt from the massive ores due to metamorphic or metasomatic processes. The most deformed (i.e. remobilized) massive ores in the Thompson Nickel Belt contain higher Ni tenors, Ni/Co ratios, Se/S ratios, and Pd/Ir ratios than less deformed ores. Furthermore, the most remobilized ores contain higher As, Bi, and Pd concentrations than less remobilized ores; these elevated trace element concentrations coincide with the paragenetically late mineralization of gersdorffite (NiAs), As-bearing pyrite, and Pd-bearing bismuthotellurides. Monoclinic pyrrhotite grains comprising extremely deformed massive sulphide horizons contain higher Ni concentrations than less deformed pyrrhotite grains; co-existing pentlandite grains also contain higher Ni concentrations and higher Ni/Co ratios. Nickel may be complexed by As-, Pd-, and Bi-enriched fluids travelling through stress-induced weaknesses in deformed ores, then subsequently re-deposited in massive sulphides distal to ultramafic sills and incorporated into thermally metamorphosed sulphides. Economic concentrations of sulphide ore in the Thompson Nickel Belt are considered to be structurally, stratigraphically, and metamorphically controlled.