Platinum and gold placer from Tugidak Island, Alaska: Platinum-group minerals and their inclusions, gold, and chromite mineralogy

ABSTRACT Black sand beach placers from Kodiak, Sitkinak, and Tugidak Islands, Alaska, have been mined intermittently for gold and minor platinum-group alloys for more than 100 years. High-grade platinum-rich magnetic separate and accompanying black sand from the southern beach placer of Tugidak Island were studied using electron microprobe WDS and scanning electron microscope EDS; mineral classification and identification were based on these techniques. The major platinum mineral is isoferroplatinum, followed by minor tetraferroplatinum and tulameenite, and rare ferronickelplatinum. Two types of alteration were identified in about 3–4% of the alloy grains: rim formation involving Pt loss and increased Fe, Ni, and/or Cu, and fracturing and vein filling by Cu-rich alloy. Ruthenium-Ir-Os-Pt alloys occur as inclusions and veins as well as form part of composite grains. Ten percent of the alloy grains contain a large variety of platinum-group minerals (PGM). Inclusions of cuprorhodsite, malanite, cuproiridsite, laurite, erlichmanite, cooperite, braggite, bowieite, kashinite, miassite, hollingworthite, irarsite, sperrylite, stillwaterite, genkinite, stibiopalladinite, keithconnite, zvyagintsevite, and probable palladodymite and vincentite were identified. Two unidentified inclusion phases also occur. Most of the PGM inclusions are primary and were trapped by a growing crystal from a melt; some inclusions exhibit textures that suggest trapping of an As,Te,S-rich immiscible melt. Secondary inclusions and evidence of deformation were observed in a few alloy grains. Associated with PGM inclusions or as separate inclusions are various base-metal sulfides. Two silicate-melt inclusions in one isoferroplatinum grain have an andesite–shoshonite composition. Minor gold and Ag-rich gold in the high-grade magnetic separate contain magnetite, pyrrhotite, and chromite inclusions. The gold composition suggests that their sources are the numerous quartz veins and apophyses related to granitoids on Kodiak Island. The composition of the placer chromite is similar to chromite from the Border Ranges mélange fault system and suggests that the Uyak Complex ultramafic and mafic rocks are part of a supra-subduction-zone ophiolite and are the source of the platinum-group minerals.