Nickeliferous Pyrrhotite Deposits and Ores from the Oshirabetsu Mine, Tokachi Province, Hokkaido

Abstract

Nickeliferous pyrrhotites are found concentrated in various grade in the Oshirabetsu gabbro mass which occupies the southeastern piedmont of the Hidaka mountains, Central Axial Zone of Hokkaido. One of the most characteristic features is that the pyrrhotite is generally accompanied by graphite. The Oshirabetsu gabbro, host rock of the preceding nickel sulfide ores can be classified into several rock facies: a) olivine gabbro, b) norite, c) hornblende gabbro, d) diorite. The nickeliferous pyrrhotite ores are closely related to the former three rock facies. On the other hand, the nickel sulfide ore is classified into four types: 1) fine-disseminated, 2) coarse-disseminated, 3) semi-massive, 4) massive. The latter two types are structurally controled by fracture or shear zones where abundant quartz, chlorite are generally present and scant garnet and tourmaline are rarely associated. Pyrrhotites and pentlandites from the above-classified sulfide ores were chemically as well as mineralogically investigated, and the following interesting results have been obtained: Almost all pyrrhotite belongs to hexagonal and peak type thermomagnetically. From the microscopic mode of occurrence, the pyrrhotite was classified into lamella type, granular type and spindle type. The value of (1+x) in chemical formula, FeS1+x of Oshirabetsu pyrrhotite has been shown to be 1.09 in average, ranging from 1.04 to 1.14. Nickel contents in Oshirabetsu pyrrhotite range from 0.02 to 1.19, and it has been found that pentlandite is apt to appear in the pyrrhotite which contains around 1% Ni. On the other hand, Co contents in pentlandite are rather abundant (9.99-12.31) in semi-massive sulfide ores. Judging from these results, the formation stage of sulfide ores in the Oshirabetsu gabbro can be classified from orthomagmatic to pneumatolytic-hydrothermal phases.