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

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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.

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Petrological features of olivine-norite along the Ougon-douro, southern part of the Hidaka metamorphic belt, Hokkaido Abstract: Small olivine-norite bodies crop out along the Ougon-douro, southern part of the Hidaka metamor-phic belt. This outcrop has been visited by many geological excursions since the s. However, the geological relations with the adjacent igneous rocks were unclear because the intrusive boundaries were covered by talus for many years. These boundaries could, however , be observed from to. The olivine-norite bodies might constitute syn-plutonic dykes in the biotite granodiorite. It mainly consists of orthopyroxene, pla-gioclase, olivine, amphibole, phlogopite, and clinopyrox-ene, in order of abundance. They include small amount of chromite, magnetite, titanite, pyrrhotite, pentlandite, and chalcopyrite. Serpentinite and chlorite occur around mafic minerals. The rocks are modally classified as olivine-melanorite to olivine-melagabbronorite. Geo-chemically, these rocks have negatively sloped chon-drite-normalized REE patterns. The Sr and Nd isotope ratios are enriched relative to typical MORB.
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The Hidaka metamorphic belt is characterized by the presence of a large quantity of basic plutonic and metamorphic rocks. The Oshirabetsu gabbroic mass forming one of the centers of igneous activity in the Hidaka metamorphic belt, consists mainly of olivine gabbro, troctolite, coarse-grained gabbro, norite and diorite. The gabbroic rocks are highly variable in lithology and are accompanied by nickel-bearing iron sulfide deposits with graphite. Among these rock-types, olivine gabbro and a part of troctolite show cumulate texture and their chemical composition indicates that olivine gabbro and a part of troctolite are the cumulus phase of this mass. Analyses of major and trace elements of representative rock-types from the Oshirabetsu gabbroic mass reveal that the above-stated rock-types have been formed during a series of magmatic differentiation. Chemical composition within the gabbroic mass except the cumulate rocks varies smoothly and its trend shows the characteristics of calc-alkali rock series. Nickel-bearing iron sulfide ores with graphite found in the Oshirabetsu gabbroic mass show various grade of concentration, but the ore deposits occur restrictedly in olivine gabbro and in norite. Considering the modes of occurrence of the deposits composed mainly of spotted pyrrhotite ore frequently taking interstitial form, the main ore bodies seem to have been formed through magmatic differentiation of the host gabbroic magma.
Fe-Ti oxide minerals in the Horoman plutonic complex of the Hidaka metamorphic belt are mostly of ilmenite (α-phase: ilmenite-hematite solid solution) and the occurrence of β-phase (magnetite-ulvöspinel solid solution) has not been reported so far. The ilmenite from Horoman are chemically analyzed in order to estimate the crystallization condition of the dioritic host rocks. Although the Mn/Fe2+ ratio of the host rocks remains constant during the differentiation, the Mn/Fe2+ ratio and R2O3 content of the ilmenite increase successively as the differentiation proceeds. This change in composition must be largely attributed to a decrease of temperature and an increase of oxygen activity in the magma from which the ilmenite crystallized. This observation implies that the Horoman diorite was formed under the 'oxidizing' condition, which means that the decreasing rate of fo2 was lower than that of the standard buffer curve and the crystallization might have proceeded along the course intersecting QFM buffer curve. With these points as background, the crystallization condition of the plutonic rocks in the Hidaka metamorphic belt is briefly discussed with respect to fo2, refering to the isotopic data presented by Sasaki and Ishihara (1979) and Shibata and Ishihara (1979).
Nickeliferous pyrrhotite deposits in Hokkaido
  • J Akaoka
  • T Sugimoto
Akaoka, J, and Sugimoto, T., 1941. Nickeliferous pyrrhotite deposits in Hokkaido. Repr. Hokkaido Industrial Res. Ins!., (1): 1-63 (in Japanese)
Nickeliferous pyrrhotite deposits of the Oshirabetsu mine with special reference to the ore
  • T Bamba
  • T Takahashi
Bamba, T. and Takahashi, T., 1980. Nickeliferous pyrrhotite deposits of the Oshirabetsu mine with special reference to the ore. Mining Gea!. 30: 37 (in Japanese)