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

Abstract

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, Cu 3 Pt 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 3.01 Fe 0.06 Ni 0.01 ) Σ3.08 (Pt 0.90 Ru 0.01 Rh <0.01 Pd <0.01 Os <0.01 Ir <0.01 ) Σ0.93 , (Cu 2.92 Fe 0.05 Ni 0.04 ) Σ3.01 (Pt 0.97 Pd 0.01 Rh 0.01 ) Σ0.99 , (Cu 2.86 Sb 0.13 Fe 0.03 Ni 0.02 ) Σ3.03 (Pt 0.92 Ir 0.02 Pd 0.01 Os 0.01 ) Σ0.97 , (Cu 2.71 Sb 0.19 Fe 0.02 Ni 0.02 ) Σ2.94 (Pt 1.01 Ir 0.05 ) Σ1.06 , (Cu 2.93 Fe 0.02 ) Σ2.94 (Pt 1.04 Rh 0.01 ) Σ1.06 , and (Cu 2.82 Fe 0.04 Ni 0.04 ) Σ2.90 (Pt 1.07 Ir 0.03 Pd <0.01 ) Σ1.10from 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, Pm 3m, with lattice parameters a = 3.683(2) Å and V = 49.97(7) Å 3 (Z = 1) of Cu 3 Au-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 postmagmatic process with alteration of such ultramafic rocks to serpentine.

Ezochiite, Cu⁺(Rh³⁺Pt⁴⁺)S₄, a new mineral in the thiospinel group from Hokkaido, Japan

“…The Pt-based alloy grains were composed of isoferroplatinum although the rims of these grains had occasionally been transformed into tulameenite or tetraferroplatinum. Nishio-Hamane and Saito (2022) previously demonstrated that such grains can contain a variety of inclusions by SEM observation for a cross section of a sample. The present study also reports the discovery of ezochiite inclusions.…”

“…The sampling sites employed in the present study are in northwestern Hokkaido, representing the same region previously examined by Nishio-Hamane and Saito (2022). This area included the Moshosanbetsu river, the Shosanbetsu river, the Ainusawa river, the Tomamae coast, the Obira coast, the Kamikinenbetsusawa river, the Obirashibe river and the Numatapon river.…”

“…This area included the Moshosanbetsu river, the Shosanbetsu river, the Ainusawa river, the Tomamae coast, the Obira coast, the Kamikinenbetsusawa river, the Obirashibe river and the Numatapon river. The location of each site is reported by Nishio-Hamane and Saito (2022). During the present research, placer platinum-group minerals (PGM) grains were collected using a specific gravity selection process and were found to consist primarily of an Os-Ir-Ru-based alloy together with a small number of Pt-based alloy grains.…”

“…These placer deposits are all secondary deposits that have been re-deposited after falling out from sedimentary rocks. The original source of the PGM is thought to have been ultramafic rocks that were previously distributed throughout the Teshio mountains (e.g., Suzuki, 1950;Hata, 1961;Nishio-Hamane and Saito, 2022).…”

“…Assuming that this mineral has a spinel structure, it would currently be considered to have the formula Cu + (Rh 3+ Pt 4+ )S4 (= RhPt) and so would be a member of the carrollite subgroup. A review of previously reported thiospinels indicates that RhPt compositions have been found in many locations, including the Manampotsy area in Madagascar (Legendre and Augé, 1992), the San Lorenzo area in Ecuador (Weiser and Schmidt-Thome, 1993), the Fenero-Ojén massif in Spain (Garuti et al, 1995), the Aluchinskii massif in Russia (Gornostayev et al, 1999), the Penikat intrusion in Finland (Barkov et al, 2000), the Bulqiza complex in Albania (Çina et al, 2002), the Kytlym complex in Russia (Garuti et al, 2002), the Zolotaya intrusion in Russia (Scheka et al, 2004), the Burgastain area in Mongolia (Oyunchinmeg et al, 2009), the Coldwell complex in Canada (Ames et al, 2017), the Bushveld complex in South Africa (Rudashevsky and Rudashevsky, 2020), the Tugidak island in Alaska (Belkin and Grosz, 2021) and the Tomamae coast of Hokkaido, Japan (Nishio-Hamane and Saito, 2022).…”