Genetical Relationship between Tertiary Granitic Rocks and Ore Deposit of the Nissho Mine, Yamagata Prefecture

Abstract

Fissure patterns of the Nissho Mine, Yamagata Prefecture are radial and concentric around the dome structure of the Middle Miocene formation. The veins mined are in the southern parts of the dome structure, filling the concentric fissures. Ore minerals are sphalerite, galena, chalcopyrite and pyrite, and gangue minerals quartz and calcite. Besides them, a maggrietite-quartz veinlet, in which pyrite and chalcopyrite are observed under the reflected microscope, has been found. Vertical zoning of the main sulfide veins is observed where sphalerite is predominant in the upper part, chalcopyrite has a tendency to increase in the middle part, and pyrite is rich in the lower part. The sphalerite from the lower part has an exsolution texture with chalcopyrite, and etching patterns on the chalcopyrite crystal surfaces suggest that the temperature of formation of the chalcopyrite was rather high in the lower part. And also, of minor elements in sulfide minerals, especially in sphalerite, Bi. In and Sn contents support that the mineralization might have taken place under xenothermal conditions.
The author discussed the close genetical relationship of the stock to formation of the ore deposit.