1958 Volume 8 Issue 27 Pages 18-32
The Inaushi mine, about 10 km west of Engaru town, Kitami Province, is located in one of the epithermal deposits of Northeast Hokkaido. The sedimentary rocks in the vicinity of the mine consist mainly of black shale, sandstone and conglomerate of Mesozoic age, they are overlain unconformably by Neogene Tertiary (Miocene) sediments. The Mesozoic beds are cut by many dykes. The rocks of the dykes are predominantly quartz-porphyrite, andesite and liparite which range widely in composition.
The No.3 ore deposit of the Inaushi mine is worked as a source of copper ore and occurs along a fault zone in quartz-porphyry, sandstone, black shale and conglomerate of Mesozoic (Pre-Cretaceous?) age. The deposit consists of several parallel veins, which occur at various distances from the main vein (N 50-Vein). The No.3 ore deposit seems to be associated with parallel dykes of quartz-porphyrite. The veins are characteristically of the composite "chlorite-quartz-copper-vein" type and strike N 70-80°E, and dip 80-90° south. The veins may attain widths of 0.4-0.8 meters. These deposits have been believed to be of epithermal origin. The structure of the veins is classified as follows: brecciated, banded and networked. The brecciated veins usually contain angular fragments of country rocks cemented by a matrix consisting of variable amounts of sulphide and gangue minerals. In some veins, ring ore is observed.
The predominant sulphides of the ore deposit are pyrite, chalcopyrite, sphalerite and galena. Other minor minerals are hematite, bornite, chalcocite, marcasite and pyrrhotite. The gangue minerals are chlorite, quartz, calcite and adularia. From the mineralogical association of the ore, the mineralization in the deposit is divided into four stages:
3) Chlorite-sphalerite-galena-pyrite-hematite-stage, and 4) Quartz-pyrite-stage.
The ore mainly shows a brecciated and banded appearance. Most of it contains chalcopyrite and pyrite often accompanied by sphalerite and galena, with gangue minerals such as chlorite, quartz and calcite. The brecciated ore usually contains fragments of wall rocks and the banded ore consists mainly of chalcopyrite and pyrite.
Paragenesis and texture of ore minerals were studied in detail. Chalcopyrite has occasionally been enclosed in the sphalerite. Also, chalcopyrite within sphalerite occurs as emulsion blebs, commonly in seriate arrangement, and shows definite preferred orientation relative with any one host grain, indicating control by the crystal structure of the sphalerite. Therefore, from the mineralogy of the vein, the No. 3 ore deposit may be classified as fissure-filling veins which belong to the "xenothermal deposits" type (shallow high-temperature condition) rather than the epithermal type deposit.
The minor elements in chalcopyrite from sulphide ore of several levels in the N50-vein and also from other chlorite-quartz-copper-vein deposits were determined by spectrographic methods. The results show the presence of Bi and Sn in chalcopyrite from the Inaushi mine, Komaki mine, and Miyatamata mine. The chlorite of the gangue was determined to be aphrosiderite by means of both chemical analysis and X-ray studies.
In conclusion, it seems highly probable that the No. 3 ore deposit of the Inaushi mine was formed under high temperature conditions of subvolcanic origin similar to those prevailing in the Inner Zone of Northeast Japan.