1976 Volume 26 Issue 135 Pages 21-33
The Kurosawa mine is located in the western part of Fukushima Prefecture, and is well known as one of the "Kuroko" type deposits in this area.
In this mine, there are four ore deposits named as Sanjin, Shirouma, Daiei, and Tadamigawa, all of which occur within the Upper Oshio Formation (correlated to Onnagawa stage of Miocene age) and are closely associated with submarine rhyolitic volcanism. Two major stages of mineralization are responsible for the ore deposits. The earlier mineralization brought forth the former two ore deposits, which lie on the slope of the lava dome or rhyolite-1. The later mineralization is responsible for the latter two ore deposits, which are relatively rich in gypsum ore and lie on the rhyolite-3 lava dome.
The present study provides with the following additional knowledge on the geology of Sanjin deposits;
1. The flow structure of the rhyolite lava dome (rhyolite-1) shows a radial configuration spatially.
2. The ore-bodies of the Sanjin deposit lying on the volcanic breccia beds are present on the slope of the lava dome and also in the troughs located on the foot of the dome.
3. Breccia like ores were formed probably by slumping processes during and/or immediately after the ore deposition.
4. Two distinct alteration facies can be recognized in the footwall volcanic breccia, i.e., argillization zone in the vicinity of lava dome and silicification zone surrounding the above. The author suggests that the silicification is essentially due to the reprecipitation of silica, which might have been leached out from the argillization zone.
