Geology and Paragenesis of Ore Minerals in the Okuzu Mine, Akita Prefecture

On the Gold-Copper-Bismuth Ores of the Mitate Vein

Abstract

The Okuzu mine is one of the mines working the beds of the Neogene Tertiary of the Inner Zone of Northeast Japan, and is located immediately to the west of the Osarizawa mine. A thick formation of the Miocene "Green Tuff" is widely distributed as the basement complex of the area. It is covered or intruded by various volcanic rocks.
The ore deposits of the Okuzu mine occur chiefly in the "Green Tuff" and the mushroom-shaped dacite body, intruded along the anticlinal part of the "Green Tuff " formation.
The Mitate vein is a typical chlorite-quartz-copper vein, consisting of pyrite, pyrrhotite, bismuthinite, chalcopyrite, sphalerite, galena, native gold, and silver minerals accompanied by chlorite, quartz and calcite.
The association and texture of ore minerals are as follows;
1) Pyrrhotite and bismuthinite in massive chalcopyrite.
2) Native gold is especially associated with bismuthinite.
3) Emulsion sphalerite is in massive chalcopyrite.
4) Emulsion chalcopyrite is in sphalerite.
Based on the paragenesis, association and texture of ore minerals, it is highly probable that the Mitate vein was formed under high temperature conditions (xenothermal condition) prevailing in the Inner Zone of Northeast Japan.