Platinum–group minerals in the placer deposit in northwestern Hokkaido, Japan: description of a new mineral, tomamaeite

抄録

Mineralogical investigation of platinum–group minerals (PGM) from the placer deposit in northwestern Hokkaido, Japan, was conducted at six rivers and two coasts covering an area of 70 km north–south and 30 km east–west: the Moshosanbetsu River (M), the Shosanbetsu River (S), the Ainusawa River (A), the Tomamae coast (T), the Obira coast (OC), the Kamikinenbetsusawa River (K), the Obirashibe River (OR), and the Numatapon River (N) from north to south. Details of the major PGM grain and the occurrence of small but diverse PGM inclusions were revealed in this study. Among diverse PGM inclusions, Cu₃Pt mineral was discovered in PGM placer from the coast of Tomamae town, and it has been approved as a new mineral, tomamaeite, named after the type locality, by the International Mineralogical Association, the Commission on New Minerals, Nomenclature and Classification (IMA–CNMNC). Later, tomamaeite was also discovered in five localities. Tomamaeite occurs in Pt–Fe(Cu) alloys such as tulameenite, ferronickelplatinum, tetraferroplatinum, and hongshiite as an anhedral particle with a size of less than 20 µm. Tomamaeite is an opaque mineral and has metallic luster with a pale mist white color in reflected light. The hardness of tomamaeite has yet to be determined, although it is estimated to be ~ 3½ from comparison with compositionally related minerals and the calculated density is 12.4 g·cm⁻³ using the empirical formula and powder X–ray diffraction data. The empirical formula of tomamaeite calculated on the basis of 4 apfu is (Cu_3.01Fe_0.06Ni_0.01)_Σ3.08(Pt_0.90Ru_0.01Rh_<0.01Pd_<0.01Os_<0.01Ir_<0.01)_Σ0.93, (Cu_2.92Fe_0.05Ni_0.04)_Σ3.01(Pt_0.97Pd_0.01Rh_0.01)_Σ0.99, (Cu_2.86Sb_0.13Fe_0.03Ni_0.02)_Σ3.03(Pt_0.92Ir_0.02Pd_0.01Os_0.01)_Σ0.97, (Cu_2.71Sb_0.19Fe_0.02Ni_0.02)_Σ2.94(Pt_1.01Ir_0.05)_Σ1.06, (Cu_2.93Fe_0.02)_Σ2.94(Pt_1.04Rh_0.01)_Σ1.06, and (Cu_2.82Fe_0.04Ni_0.04)_Σ2.90(Pt_1.07Ir_0.03Pd_<0.01)_Σ1.10 from the Tomamae coast, the Moshosanbetsu River, the Shosanbetsu River, the Ainusawa River, the Kamikinenbetsusawa River, and the Numatapon River, respectively. Crystal chemistry was investigated using tomamaeite from the Tomamae coast. Tomamaeite is cubic, Pm3m, with lattice parameters a = 3.683(2) Å and V = 49.97(7) ų (Z = 1) of Cu₃Au–type structure, in which Pt occupies the position of origin, and Cu occupies the face–centered positions on a face–centered cubic lattice. PGM from northwestern Hokkaido probably have a mostly common origin and are characterized by depleted ultramafic rocks, and tomamaeite is a non–unique mineral that is formed during the universal post–magmatic process with alteration of such ultramafic rocks to serpentine.