Abstract

The Duke Island Ultramafic Complex in southeastern Alaska is a Ural-Alaskan–type complex that hosts sulfide-rich horizons primarily in the olivine clinopyroxenite unit. Although placer deposits hosting platinum group elements (PGEs) have been found in association with Ural-Alaskan–type complexes in several locations around the world, these ultramafic complexes have traditionally been viewed as sulfide-poor systems and therefore unlikely hosts of economic Ni-Cu sulfide deposits. Sulfide mineralization in the Duke Island Complex consists of massive, net-textured, and disseminated assemblages that formed as a result of the interaction between magma and sulfidic-graphitic country rocks. The sulfide mineralization at Duke Island is characterized by strong depletion in Ir, Os, and Ru (IPGEs), and relative enrichment in Pt, Pd, and Cu. The sulfide-bearing olivine clinopyroxenites formed from a derivative magma produced as a result of the fractional crystallization of olivine and chromite. Although IPGEs may be compatible within olivine and chromite, fractional crystallization of up to 20% cannot explain the observed IPGE depletion. We propose that early crystallization of laurite and Ir alloys accompanied the accumulation of olivine and chromite. R-factor modeling indicates that much of the inter-element variation in the Sulfide mineralization can be attributed to variations in the degree of equilibration between Sulfide and silicate liquids. Mixing between monosulfide solid solution and fractionated Sulfide liquid may also have contributed to the variability. The modeling suggests that the pre-sulfide saturation concentrations of Os, Pt, and Pd in the magmas were approximately 0.03, 2, and 3 ppb, respectively. The concentration of Pd is near that of primitive mantle and the low Pt concentration can be explained by the crystallization of Pt alloy with early-formed dunites and chromitites. This is consistent with models for Pt-rich placers in the Urals that have been derived from chromitites and dunites. The relative enrichments in Pd and Cu are the results of fractional crystallization before Sulfide saturation was achieved. Taken together, the Ni-Cu-PGE elemental characteristics at the Duke Island Complex are similar to those shown by flood basalt-related Sulfide deposits and Cu-rich Sulfide veins studied in different locations around the world.

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