Platinum–Group Minerals in Podiform Chromitite from the Kamuikotan Zone, Hokkaido, Northern Japan

Abstract: Ru–Os–Ir alloys have been found in two podiform chromitites located at the Chiroro and Bankei mines in the Sarugawa peridotite complex in the Kamuikotan zone, Hokkaido, Japan. This is the first report on the occurrence of PGM (= platinum-group minerals) from chromitites in Japan. The Ru–Os–Ir alloys most typically form polyhedra associated with other minerals (Ni–Fe alloys and heazlewoodite) in chromian spinel. The PGM are possibly pseudomorphs after some primary PGM such as laurite and are chemically highly inhomogeneous, indicating a low-temperature alteration origin. This is consistent with intense alteration (formation of serpentine, uvarovite and kämmererite) imposed on the Kamuikotan chromitites. High-temperature primary PGE (platinum–group elements)–bearing sulfides were possibly recrystallized at low temperatures into a new assemblage of PGM, Ni-Fe alloys and sulfides. Placer PGM around the peridotite complexes are chemically different from the PGM in dunite and chromitite possibly due to the, as yet, incomplete search for the rock-hosted PGM. The PGE content in chromitites is distinctly higher in those in the Kamuikotan zone than in those in the Sangun zone of Southwest Japan, consistent with the more refractory nature (Cr# of spinel, up to 0.8) of the former than the latter (Cr# of spinel, 0.5).