Geology and Ore Deposits of the Toyoha Mine

Abstract

The Toyoha mine is located about 25 kilometers northwest of Sapporo Hokkaido
The rocks in the vicinity of the mine consist of Toyoha formation of the Miocene. sediments such as tuffaceous sandstone, shale and conglomerate. They are covered by propylite and intruded by acidic and basic dykes. All rocks mentioned above are covered by Quaternary andesitic lava.
The ore deposits occur as epithermal fissure filling veins in the shear zone having EW trend and in the NW-SE trending tension fracture.
These fractures might have been formed by the tectonic movement in late Tertiary related to the intrusion of acidic dykes along the synclinal axis having EW trend in Toyoha formation. Mineral assembrage are classified by the order of mineralization as follows:
1. Silver bearing Galena-Sphalerite-pyrite of 1 st stage mineralization in the shear zone having EW trend.
2. Chalcopyrite-pyrite of 2 nd mineralization in the tension fracture having NW-SE trend.
3. Rhodochrasite-calcite-silver bearing quartz of latest mineralization in the both fractures.