Exploration of the Stockwork Siliceous Ore Deposit at the Kosaka Mine, Akita Prefecture

Abstract

The Kosaka mine is located in the northern part of Akita Prefecture, and is well known for the Kuroko-type deposits, namely the Motoyama, the Uchinotai and the Uwamuki deposits. These deposits occur in the Miocene tuff breccia at the flank of the so-called white rhyolite, and consist of Kuroko(black ore), Oko(yellow ore), Keiko(siliceous ore) and gypsum ore, in descending order. The black ore and yellow ore bodies are generally bedded in form, while the siliceous ore and gypsum ore bodies are massive.
Recently, another type of siliseous ore body was discovered within the white rhyolite. It is a stockwork siliceous ore body having a cylindrical shape and standing vertically. Sulfide minerals occur in the mode of vein, micro-veinlet and dissemination in the ore body. Vertical zonal arrangement, sphalerite-galena-barite, sphalerite-chalcopyrite and pyrite in descending order, has been recognized. This sequence is the same as that found in common Kuroko deposits.
The stockwork siliceous ore deposits have been found in several places about 100 meters apart. The exploration of these deposits in the white rhyolite is now under way at the Kosaka mine.