Abstract

Upper greenschist facies metamorphism of the Aphebian Donaldson West ultramafic sill extensively modified the distribution of primary magmatic sulfides and platinum-group elements. Mobilization of elements in net-textured sulfides during thermal metamorphism caused partial to almost complete pseudomorphic replacement of serpentinized cumulate olivine grains (reverse net-textured) and generation of sulfide blebs in disseminated and net-textured sulfides. Carbonate veins, rich in chalcopyrite and platinum-group minerals, developed during dynamic metamorphism primarily at contacts of units with differing sulfide textures where there were competency contrasts.The abundance of platinum-group minerals varies directly with sulfide content in disseminated and net-textured sulfides, but it is anomalously low in the reverse net-textured and massive sulfides. Within the net-textured sulfides the greatest abundance, diversity, and size of platinum-group element-, Au-, and Ag-bearing grains is found in veins as inclusions in sulfides, along sulfide grain boundaries, or at sulfide-silicate grain contacts. In these veins, Pd-, Sb-, and Te-rich platinum-group minerals are accompanied by hessite, electrum, Co-rich pyrite, As-rich pentlandite, and Co-As minerals. Rare platinum-group minerals occurring in silicates are confined to sheared and carbonatized rocks which are often associated with sulfide veins.In comparison with the net-textured sulfide zone, the large zone of reverse net-textured sulfides is moderately depleted in sulfides and Ni, and moderately enriched in Pd; the massive sulfides are strongly depleted in Cu, Au, Pt, and Pd and moderately depleted in Ni. Pd is enriched and Pt is depleted in the veins relative to surrounding sulfides. Ni-rich veins are rare. Cu-rich veins are enriched in Pd, Sb, Te, As, Co, Ag, and Au, a feature which is interpreted to result from their strong partitioning into hydrothermal fluids with resulting precipitation of platinum-group minerals directly from the fluid as grains in silicates, along sulfide grain boundaries, or by exsolution from vein sulfides.

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