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

[R7-P-01] Ezochiite and placer deposit of platinum group minerals in northwestern Hokkaido, Japan

*Daisuke HAMANE¹, Katsuyuki Saito (1. The University of Tokyo)

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

Placer deposits of platinum-group minerals (PGM) were once distributed along the central axis of Hokkaido, but due to overhunting during the war, it has become difficult to collect. On the other hand, the several PGM placer deposits have been discovered in northwestern Hokkaido. In this study, we report on the PGM placer deposits in northwestern Hokkaido and the new thiospinel group mineral eezochiite (IMA2022-101) discovered there. Most of placers consist of rutheniridosmine, osmium, iridium, and osmium, while small amount of isoferroplatinum is included. Isoferroplatinum-based grains contain a variety of inclusions, including ezochiite, which is included as a spherical aggregate with other sulfide minerals, suggesting that ezochiite crystallized from melt trapped by isoferroplatinum. The empirical formula of ezochiite is (Cu⁺_0.85Fe³⁺_0.15)(Rh³⁺_1.09Pt⁴⁺_0.78Ir³⁺_0.08Pt²⁺_0.05)S_4.00, ideally Cu⁺(Rh³⁺Pt⁴⁺)S₄. The unit cell parameter is a = 9.8559(14) Å on Fd-3m space group. Ezochiite was confirmed in ophiolites, Ural-Alaskan intrusions, and orthomagmatic deposits, suggesting that ezochiite is a common mineral in PGM deposit.