Journal of the Faculty of Science, Hokkaido University. Series 4, Geology and mineralogy;Vol.XX, Nos.2-3


The Oshirabetsu Gabbroic Mass in the Southeastern Part of the Hidaka Metamorphic Belt, Hokkaido, Japan

Takahashi, Teruyuki

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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.