Observations on the Tanosawa Veins at the Osarizawa Mine

Abstract

The Osarizawa Mine is known as one of the largest mines of vein type in Japan. A group of the Tanosawa veins, located in the southern part of the Osarizawa Mine area, has recently been developed. The stratigraphical succession of this area is as follaws:
Upper Miocene : Matsukozawa Formation
-unconformity-
Middle Miocene : Wakiagari Formation
Shishizawa Formation (Upper and Lower Members)
These formations consist of pyroclastic rocks and mudstone. The volcanic rocks as observed are; Akasawa Rhyolite, Shishizawa Propylite, Akasawa Dacite; and Tanosawa Basalt. Mineralzation of the Tanosawa area appears to have taken place some after the Akasawa Dacite intrustion (late Onnagawa stage-early Funagawa stage).
Two prominent veins, Tanosawa veins 3 and 5, are in the group of the Tanosawa veins. These two veins show a pattern of the shear fracture. In other veins, a pattern of the tension fracture is observed.
It is interesting to note that the position of their bonanza has been controled by sedimentary rocks and intrusives. In regard to the zonal arrangement of ore minerals, the veins appear to be a type identical with that of the eastern vein groups of the Osarizawa Mine.
This is noticeable from the top to the bottom, namely, galena-sphalerite zone, chalcopyrite-galena-sphalerite-pyrite-barite zone and chalcopyrite zone. It is extremely difficult to determine the temperature at which the ore deposits were formed, but it is estimated to be about 210°(C) from insufficient fluid inclusion data obtained by heating-stage method.