Constituent Minerals of Placer Platinum-group Minerals Occurred from the Onnebetsu Area, Shibetu City, Hokkaido, Japan

This study investigates the mineral diversity in placer platinum-group minerals (placer PGM) purportedly collected from the Onnebetsu area in Shibetsu City, Hokkaido. Placer particles consist primarily of native elements and alloys and can be broadly categorized into two groups, iridium-group and palladium-group. The iridium-group placer is predominantly composed of rutheniridosmine, osmium, ruthenium, and iridium, with each occurring as both primary and secondary phases. In contrast, the palladium-group placer consists primarily of isoferroplatinum as the primary phase, accompanied by tulameenite, tetraferroplatinum, and ferronickelplatinum as secondary phases. Notably, the iridium-group placer may also contain PGM typically associated with the palladium-group placer in subordinate amounts. In addition to the primary and secondary phases, other PGM occur as inclusions, within the outermost layers of particles, and in exsolution textures. A total of 41 PGM species were identified in this study, along with six unnamed species. The Onnebetsu area stands out as the region in Japan with the highest diversity of PGM discovered to date. This remarkable variety is largely attributed to the palladium-group placer, which is originated from cumulate rocks formed by partial melting of a mantle.