Prospecting for uranium in the Tônô district has been carried out by the Atomic Fuel Corporation and the Geological Survey of Japan for the last three years. In 1965, A. F. C. established the "Tônô Prospecting Office", in order to carry out geological survey, geochemical and geophysical prospectings and drilling in this promising district.
The first outcrop of the uranium deposit was discovered in the JÔrinji district by carborne radiometric survey of the Geological Survey of Japan in 1962. Since then, there have been found many outcrops of the ore deposits at Toki-city, Mizunami-city, Mitake-chô, Seto-city and Iwamura-chô. The largest ore body clarified up to now is "Tsukiyoshi deposit" at Mizunami-city. The extent of the promising area for uranium occurrence including the above-mentioned districts is approximately 900 km².
The uranium deposits in the district are of the sedimentary type, and are found in the Nakamura and the Mizunami groups of the Miocene age resting on erosion surface of granitic basement rocks. The ore bodies are developed nearly along the bottom of the channel structure of the basement. Uranium is deposited in conglomerate and/or alternation of sandstone and mudstone containing coaly material. So far as we know, coffinite, autunite and uranocircite are identified as uranium minerals by the X-ray powder diffractory analysis.

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The diorite mass, which is a principal country rock of copper veins in the Ani mining district, intruded in Tertiary age. The direction of intrusion of the diorite is assumed after the studies on variation of crystallinity of constituent minerals in the mass. As a rule, ore shoots of the veins are found in places somewhat distant from the surface of the diorite, which seemed to correspond to passage of the most active circulation of the mineralizing solution within the veins. Basalt dykes intruded into the diorite and some of them are related intimately by to ore deposition.

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It has been a common practice to make use of mud water in drilling Tertiary formations. Thus, clay base mud has been used in diamond drilling operations in the Hokuroku District in Akita Prefecture for the last 15 years, but in performing deep hole drillings various difficulties have been encountered with. Since 1963 the writer began studies on finding suitable kinds of mud water for deep drilling and on the adaptability of equipments as well as on improvement of the method of drilling operations at the Shakanai Mine. As a result, improvement has been achieved by the introduction of wireline core barrel, and by using chromnite mud water. At the same time, non-core drilling has been adopted to curtail drilling expenses and to increase the drilling speed in drilling operations. Finally, remarkable improvements have been attained on the following : a) Drilling speed, b) Reduction in drilling cost per metre, c) Core recovery, d) Prevention of caving-in of holes, and e) Protection of drilling equipments from damages.

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The Yokota Mine is located at about 44km west of Aizu-wakamatsu City, Fukushima Prefecture. The Neogene Tertiary formation in this area are divided into four stratigraphic units as follows : 1) Sodeyama Formation, 2) Oshio Formation, 3) Takizawagawa Formation, and 4) Yokota rhyolite. The first three formations are characterized by rocks of the so-called "green tuff" facies. Black ore deposits of the Yokota mine comprise two types from the viewpoint of their internal structure. Banded black ore deposit occurs under black mudstone of the Oshio Formation, and massive disseminated or network deposit is found mainly in the Takizawagawa Formation and Yokota rhyolite. Zonal arrangement of these to types of ore deposits are shown below.
Oshio Formation
Banded black ore deposit
Massive disseminated or network deposit
Takizawagawa Form. and Yokota rhyolite
mudstone
soapstone (roof)
quartz-hematite zone
compact black ore zone
drusy black ore zone
chalcopyrite rich zone
siliceous yellow ore zone
siliceous ore of stockwork type zone
rhyolite and tuff breccia (bottom)
Motoyama mine lot, situated in the southeastern part of the Yokota mine, forms the center of silicified zone and banded black ore deposit developed along an anticlinal axis. Massive disseminated or network ore deposit are associated with many sand dykes in the anticilinal structure. By these facts, the hydrothermal solution might have ascended continuously along these sand dykes during Takizawagawa and earlier Oshio stages.

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The Kamaishi mine is well known as one of the typical contact-pyrometasomatic iron and copper deposits in Japan. Recently, No. 7 ore body was found in the Shinyama deposit by tracing the contact zone of limestone lenses and igneous rocks. The ore body shows the following characteristics:
In the upper part, there occurs a large amount of skarn minerals, and the nature of the ore body is almost the same as that of the other iron-copper deposits in the mine, but a copper-rich zone with green skarn occurs along the limestone in the middle part.
In the lower part, a skarnized zone between limestone and igneous rock is very narrow in width, in which high-grade magnetite ore and chalcopyrite ore are deposited with relatively small amounts of skarn minerals.
Formation of the iron-rich zone is controlled by the form of igneous rocks. Copper-rich zone is on the same horizon as No.4 copper ore body, and it sometimes branches out, forming some small ore bodics along dyke rocks.

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The rocks in the vicinity of the Akagane mine consist predominantly of pyroclastic rocks (tuff, tuff-breccia and lavas), sedimentary rocks (slate and limestone), and igneous rocks (quartz porphyry, porphyrite, gabbro, granite and granodiorite). These pyroclastic and sedimentary rocks, belonging to the Yonesato and Siba formations of Carboniferous System, have suffered thermal metamorphism by Hitokabe granodiorite. These rocks strike NNS, dip about 30-60° eastward, and are strongly folded. The ore bodies are formed by replacement on limestone and volcanic pyroclastic rocks close to and along the contact of quartz porphyry or gabbro. They are associated with skarn minerals such as brown garnet, epidote, amphyboles, diopside, plagioclase, etc. The deposits are composed predominantly of pyrrhotite, magnetite and chalcopyrite, with subordinate amounts of pyrite, sphalerite, arsenopyrite and cubanite. Geologic history of formation of the ore deposits seems to be as follows: 1. sedimentation of rocks of Carboniferous System, 2. folding and faulting of the System, 3. intrusion of dikes such as quartz porphyry and gabbro, 4. skarnization and metallic mineralization accompanied by minor folding, 5. intrusion of granite, and 6. faulting.

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The Jokoku mine, the largest manganese mine in Japan, is in the south western part of Osima peninsula, Hokkaido. The ore deposits are of epithermal veins of Miocene age, consisting chiefly of rhodochrosite with minor amounts of sphalerite and galena. The Fe content of the rhodochrosite is from 4 to 10 percents. The rhodochrosite formed in the earlier stage of the mineralization is relatively rich in Fe and that in the later stage in Ca and Mg. Champion vein occurs in a wide shear zone which forms the boundary between Palaeozoic slate and Tertiary andesitic breccia. Several veins have been found branching out into the andesitic breccia.

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The Hosokura mine, Miyagi prefecture, is one of the largest zinc and lead mines in Japan. The mine area is composed of propylite, rhyolite, rhyolitic rocks, dacitic rocks, green tuff and dacitic tuff of Miocene age. Most of the workable veins of fissure-filling type occur in the Hosokura formation. The formation is classified as follows :
(1) Upper tuff member
(2) Propylite group
(3) Lower tuff member
(4) Rhyclitic rocks
(5) Lowermost tuff member.
Most of the igneous rocks are lava flows within the pyroclastic members, but some propylites are intruding the formation. These rocks are stepped down towards the center by the N-W fault.
Development of ore shoots is controlled mainly by the following factors:
(1) Nature of host rocks
(2) Fissure patterns and features of N-W system faults
(3) Others.
Generally speaking, ore shoots occur within the upper tuff member, or in or near the igneous rocks.

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The Chin-kua-shih mine is the greatest golc-copper deposits in the northeastern part of Taiwan. The ore deposits are of hydrothermal fissure-filling and replacement type occurring in sedimentary rocks and dacite of Miocene age, Tertiary. The main factors controlling localization of the Chin-kua-shih ore deposits are the N-S fault fractures, bedding faults, intrusive dacite, brecciated contacts between dacite and other rocks, lithologic characters, etc. In this report, the srtuctural controls on the formation of the main ore body in the Chin-kua-shih ore deposits are described and discussed especially in view of the relationships between dacite intrusions and fractures.

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