R7: Petrology, Mineralogy and Economic geology (Joint Session with Society of Resource Geology)

[R7-04] Shiranuiite and placer deposit of platinum group minerals, Haraigawa, Misato machi, Kumamoto Prefecture, Japan

*Daisuke HAMANE¹, Takahiro Tanaka, Tadashi Shinmachi (1. The University of Tokyo)

Keywords:Platinum-group minerals (PGM), Placer PGM, Shiranuiite

We have discovered a placer deposit of platinum-group minerals (PGM) in the clinopyroxene mass, Haraigawa, Misato machi, Kumamoto Prefecture. Two new minerals, minakawaite and michitoshiite-(Cu), were discovered in this deposit, and an unknown mineral of the thiospinel group was also found. It is named shiranuiite and was approved as a new mineral (IMA2023-072a). Most placer grains are isoferroplatinum: Pt₃Fe, and some grains have been altered at the outer edge (several to several hundred μm) to tulameenite: Pt₂CuFe or tetraferroplatinum: PtFe, showing the influence of post-magmatic alteration. Bowieite: Rh₂S₃, the second most abundant inclusions, rarely appear on the surface of grains, while it is altered to often cuprorhodsite: (Cu⁺_0.5Fe³⁺_0.5)Rh³⁺₂S₄ and rarely shiranuiite: Cu⁺(Rh³⁺Rh⁴⁺)S₄. The empirical formula of shiranuiite is (Cu⁺_0.95Fe³⁺_0.04Ni_0.01)(Rh³⁺_1.19Rh⁴⁺_0.77Ir⁴⁺_0.06)S_3.99, and the unit cell parameter is a = 9.757Å on Fd-3m space group.