Abstract

At the Dundonald Beach South Ni-Cu occurrence in the western Abitibi subprovince, Ontario, Canada, komatiite-hosted magmatic sulfide mineralization (pentlandite-dominated; 0.5–7 ppm Pt + Pd) contains millimeter- to centimeter-size blebs of polycrystalline gersdorffite and nickeline. The gersdorffite-nickeline blebs are concentrated (~6 vol %) in a 10- to 20-cm-thick by 10-m-long lens that occurs at the base of a massive sulfide horizon. The lens is significantly more enriched in Pt and Pd (15–50 ppm Pt + Pd) than typical sulfide horizons in which sulfarsenide and arsenide phases occur only as trace to minor (up to 0.15 vol %) constituents. In the platinum group element (PGE)-rich lens, ~29 and ~74 percent of the bulk rock Pd and Pt, respectively, are dissolved in the sulfarsenide and arsenide minerals. Almost all the remaining Pd and Pt in the lens occur in inclusions of sperrylite, michenerite (PdBiTe), and an unidentified Pd-Bi-Te phase coexisting with inclusions of tellurobismuthite (Bi2Te3), altaite (PbTe), and electrum that fill former open spaces (vugs) in gersdorffite and nickeline. In the surrounding sulfide mineralization, PGE mineral inclusions are 4 to 10 times less abundant, and the bulk PGE grade is an order of magnitude lower. Overall, PGE grade on a deposit scale may be controlled by the abundance of gersdorffite and nickeline. Even in minor abundances (e.g., ~1 wt %), sulfarsenide and arsenide minerals may produce economically favorable bulk PGE grades if dispersed throughout barren (PGE-poor) sulfide horizons.

The PGE-rich lens may represent the accumulation of either crystallized droplets of an immiscible sulfarsenide melt phase or sulfarsenide phenocrysts that separated from a silicate or sulfide liquid. Arsenic-rich melt or mineral phases extracted (partitioned) the trace metals Pt, Pd, Au, Bi, Te, Sb, Pb, Zn, Ag, and Sn from non–As-bearing phases (silicate liquid, Fe-Ni-Cu sulfide liquid, or monosulfide solid solution [mss]) with inferred values of DAs-phase/sulfide on the order of ~10 to 100. Textural, mineralogical, and geochemical observations highlight the importance of As-rich liquids or mineral phases as scavengers of the PGE and other chalcophile metals in komatiite-associated magmatic sulfide environments as well as the potential for high-grade PGE mineralization to occur where komatiites were contaminated by As-rich sediments or metasedimentary rocks.

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