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.

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The Uwamuki deposits are located in the southeastern parts of the Uchinotai deposits area, and five ore bodies have been discovered. They are called No.1, No.2, No.3, No.4 and No.5 ore body in following the order of their discovery. The No.1 and 2 ore bodies are accompanied with a lava dome of white rhyolite, while the others (Nos.3, 4, 5) are related to individual lava domes. They consist of massive or stratified black ore and siliceous ore of black ore type. The minerals contained are sphalerite, galena, chalcopyrite, pyrite, and barite.
The guides for prospecting are based on the following geologic features.
1) Ore horizon of the known deposits: It has been realized that most of the black ore deposits discovered belong to the late Nishikurosawa stage.
2) Kind of the wall rocks: The hanging-wall rocks consist of tuff breccia and lapilli tuff, while the foot wall of volcanic breccia or white rhyolite. The white rhyolite shows a shape of autobrecciated lava dome and strongly sericitized and silicified.
3) Structural elements: The white rhyolite occurs at margin of Torigoe rhyolite with a definite direction, which is an earlier extrusive lava in the Kosaka mine area.
4) Alteration of the wall rocks: The hanging-wall rocks contain abundant sericite, various chlorite, and very fine-grained pyrite. These rocks as the whole are dark grey in color.
5) Succesion of ores: The emplacement of ores are succesive, with the black ore at the top, yellow ore, gypsum ore, and siliceous ore at the bottom.
6) Complexity due to later deformation : Although most of the ore deposits are stratified, their shapes have been deformed by the later intrusion of Takaderayama rhyolite and Omori andesite.
The analysis of volcanic activity related to the genesis of the ore deposits are profitable method to select exploration targets.

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In many pyrometasomatic ore deposits, potash feldspar and plagioclase occur witlxskarn minerals. The mode of occurrence of these feldspars is classified into three types, i.e., (1) in banded skarn, (2) in breccia-skarn, and (3) along diopside veins. In general, they exist between biotite hornfels and clinopyroxene zones. From the macroscopic and microscopic observations, it is suggested that these feldspars are formed by the followingreaction :
K₂(Mg, Fe)_6-xAI_x(Si_6-xAI_2+x)O₂₀(OH)₄+6CaCO₃+(12+x)SiO₂=2KAISi₃O₈+xCaAI₂Si₂O₈+(6-x)Ca(Mg, Fe)Si₂O₆+6CO₂+2H₂O
On some assumpsions, the phase assemblage of the right side of the reaction is presumed to be stable at temperatures above 500-600°C and at pressures of 500-2, 000 atm. This condition corresponds to the low-temperature part of the pyroxene-hornfels facies.

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The Western Aizu district is one of the important Kurokô (Black ore) metalliferous provinces in the green tuff region of Northeast Japan, and is widely covered by acidic volcanic rocks and its pyroclastics which are correlated to the Nishikurosawa and Onnagawa stages of the Middle Miocene.
The stratigraphic succession of the Miocene formations in this district can be divided into the following five formations in ascending order; Takisawagawa, Ôshio, Ogawazawa, Fuzawa and Matsusakatôge formations. The chief black ore deposits in this district, including some working mines such as Yokota, Tashiro and Kurosawa, occur syngenetically in pyroclastic rocks between the upper part of the Takisawagawa formation and the lower part of the Ôshio formation.
The general geologic structure of this district is characterized by the tectonic zone of uplifting and sinking belts with a trend of NW-SE. The ore deposits mentioned above, are distributed always along the boundaries between the two tectonic, belts. It is considered that the structure of NW-SE direction is closely related to fracture zones of the pre-Tertiary basement rocks and also with the Kurokô mineralization. Some considerations on the relation between altered zones due to the mineralization and the tectonic features were mainly presented in this paper.

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