The Yokota Mine is located in the center of "Nishi-Aizu Kuroko District" in halfway up the Tadami river about 44 kilometers west of Aizu-Wakamatsu city. The stratigraphic sequence of this area in ascending order is Takizawagawa, Oshio and Ogawazawa formations. The bottom-half of the lower mudstone of the Oshio formation is correlated to Nishikurosawa stage and the top-half of the mudstone is considered to be Onnagawa stage, both of which belong to Miocene in age.
There are two ore horizons in the Yokota Mine area, namely called Upper Ore Body and Lower Ore Body. The Upper Ore Body occurs along the boundary between Takizawagawa and Oshio formations about 100 meters below the present surface. The Lower Ore Body is located at much lower horizon, 200-400 meters from the surface, underneath the thick (160-200m) rhyolite (R_I).
In 1966, the prefectural government of Fukushima initiated a mapping project of this area, and the Metallic Minerals Exploration Agency of Japan also sponsored an exploration project based on "Three-steps method" including deep drilling program between 1967 and 1968. These works brought us many data usefull for prospecting purposes. The prospecting criteria so far valid for the deeper levels Kuroko-type mineralization are, 1) to chase the Kuroko horizon of the Nishi-Aizu District, 2) to drill much deeper parts of the known ore bodies, 3) to follow NE-SW trending structural weakness, 4) to analyze envivonment of acidic igneous activities, 5) to find out CO₂-gas reserves, and 6) to examine zonal arrangement of ore minerals.
In following these indicators, we expanded our prospecting lower limit down to 500 meters from the present surface and found a new mineralized horizon under the rhyolite (R_I) in January, 1969 ; then the Lower Ore Body after a few months. During the two years from 1970, about 2, 800, 000 tons (Cu 1.5%) of ore reserves were confirmed.
Studies of these new ore deposits on the basis of mostly drilling-core material are reported in details for the first time in this paper. The foot-wall side of the ore deposits. consists of spherulite rhyolite (R_D), which is considered to be lava dome extruded onto sea bottom at marginal ts of local basin in the Miocene time. This is definitely related to the mineralization. The ore deposits are divided into four different zones, each of which shows zonal arrangement being composed of Kuroko at the top, Oko (yellow ore), Sekko (gypsum ore) and Keiko (siliceous ore) at the bottom. Colloform textures and grading structures are commonly seen in the former two ores. They seem to have been formed at the sea bottom. On the other hand, various characters of the siliceous ore indicate slightly deeper levels of formation than that of the Kuroko and yellow ore. Genesis of anhydrite, which is abundant in the Lower Ore Body as compared to the Upper Ore Body, is still in question.