抄録
The newly discovered Ezuri kuroko deposits are located in the southwestern part of Hokuroku area in Akita Prefecture, Japan. Detailed geological study on the stratigraphy of acidic volcanic activities in the Fukazawa-Ezuri district had indicated possible existence of the kuroko-bearing formation in this particular area. As a result of a few drillings, favourably altered acidic volcanic rocks were encountered and a systematic exploration program started which included geochemical examination of the behavior of alkali and alkaline earth metals in pre-mineralization acidic volcanic lavas, geophysical survey (time domain IP method), magnetic susceptibility measurement, and systematic grid drillings.
In May 1975, the Iwagami deposit and in July 1976 the Ezuri deposit were discovered. The Ezuri mine comprising these deposits, now has possible or reserved ore of about 3 million tons with a grade of 0.89% Cu, 3.3% Pb, 10.1 % Zn. Since October 1979, the mine has been in operation on a scale of 7, 500 tons per mounth.
The geology of the area is characterized by an intensive volcanic activity. Pyroclastic rocks, lavas and intrusive rocks of the Nishikurosawa and Onnagawa stages of the Miocene age are the major constituents. These are subdivided into three units, Yukisawa, Kagoya, and Shigenai formations in the ascending order, which can be correlated with the stratigraphic column in the nearby Fukazawa area. The orebodies occur at the top of dacitic volcanics of Yukisawa formation and is overlain by a sheet like porphyritic dacite and in turn by basic tuff. Each orebody is mainly composed of stratiform black ores accompanied with minor amount of yellow ores, dissemi-nated or stockwork ores and gypsum ores.