The Hamayokokawa manganese mine which is located at 10km WSW of Tatsuno-machi, is one of the largest bedded manganese mine in Japan, producing annually more than 10, 000 tons of high grade manganese ore.
The geology of this area consists mainly of Palaeozoic chert, slate, sandstone etc. In the mapped area the Palaeozoic strata are classified into the following two formations :
1) Yokokawa Formation: this formation is apparently conformable with the underlying Narai formation and consists mainly of slate and thick chert members.
2) Narai Formation: this distributes at the northwestern side mainly consisting of muddy turbidite. The general trend shows strike of N 50° E and dip to 60° SE.
Judging from graded beddings observed in the Narai formation, the Palaeozoic strata of this area are overturned, so that the massive chert is the hanging wall at present.
The manganese ore deposits are, embedded in the chert at the boundary between the Narai formation and the Yokokawa formation. The ore deposits are arranged, from west to east, in such a way as low grade ore zone, high grade ore zone, broken ore zone and washed-out zone by turbidity current.
The high grade ore zone, which shows complicated folding pattern, is considered to be formed by turbidity current directly after ore deposition. In the northeastern part of the mining area the chert and the ore deposits are washed out by turbidite. The erosion gives the manganese ore an appearance of conglomerate and/or pebbles in turbidite.

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Two sets of epithermal vein fissures of the Toyoha mine are distinguished; the one is E-W shear veins composing of two linking champion veins; and the other NW-SE tension veins cross-cutting obliquely the former.
The early-stage mineralization occurred in the E-W fissures and deposited sphalerite, galena, pyrite, hematite and small amount of chalcopyrite in the abundant gangue quartz. On the other hand, the late-stage mineralization filled the NW-SE fissures with banded ores of galena, sphalerite and pyrite with or without rhodochrocite and manganiferous calcite.
Lattice constants of sphalerite specimens from different localities were measured by X-ray diffraction method. They were correlated with the powder-colors, and with the iron contents as follows.
Sphalerites of the early-stage mineralization have generally lighter color, and the smaller lattice constant, while sphalerites of the later stage mineralization have the opposite characters, similarly different characters were recognized on the sphalerite in or out of the ore shoot.
Although silver and silver minerals are mainly native silver, argentite, and pyrargyrite, silver is concentrated in the banded ore of the sulphides, especially in galena of later mineralization. Hairly native silver was found in druses. The silver content of the crude ore is about 130 g/t in average. It attains, however, up to several thousands g/t in the silver rich parts.

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The main components of 'Rôseki' ore are pyrophyllite and quartz, accompanied by kaolin, boehmite, diaspore, corundum or andalusite. In the typical 'Rôseki' ore deposits such as distributed in the Mitsuishi area, Okayama Pref., pyrophyllite is the most dominant of constituent minerals. Such high. temperature minerals as corundum and andalusite are rarely found. The 'Rôseki' ore bodies form a complex ore bed with porous silica ore bodies. The bed is apparently conformably covered by an impermeable sheet or bed, and rather gradually merges into footwall.
The depth at the time of mineralization is estimated to be from n×10² m. to 2 km., and the H₂O pressure to be about n×10² bars. The boehmite-bearing ore may have been formed at temperatures between 100° and 300°C., and the diaspore-bearing ore, 300°-400°C., At the end of mineralization, the temperature may have decreased to a point on the geothermobar as suggested by the presence of gibbsite in Mitsuishi area.
The typical 'Rôseki' ore deposits are connected with fumaroles through shallow type 'Rôseki' deposits or through Ugusu-type alunitesilica deposits and replacement deposits of sulphur. From the common features of those deposits as well as the behaviour of hydrothermal fluid in the geothermal area suchh as at Larderello, Wairakei and Matsukawa, the mechanism of mineralization of 'Rôseki' deposits is. inferred as follows:
(1) There was a combination of an impermeable cover, a reservoir for hydrothermal fluid and a footwall with cracks at the depth of n×10²-2×10³m.(2) The hydrothermal fluid ascending along the crack met with descending ground water at the entrance of the reservoir or a little below, forming a strongly acidic hydrothermal solution which leached the pre-existing rocks, leaving a porous. silica ore. (3) Owing to the thermal convection in the reservoir, the temperature rose to 200-300°C., and the acidity of solution was weakened by dissolving metallic ions. And these resulted in the crystallization of pyrophyllite, kaolin or boehmite to form 'Rôseki' ore bodies.
In order to prospect a hidden 'Rôseki' ore body, the first step will be core drilling to get informations, about the combination of an impermeable cover, a complex ore bed of 'Rôseki' and silica. This will. be followed by exploration by galleries to ascertain the form, tonnage and quality of 'Rôseki' ore deposits. With those ideas, an prospecting has been performed with success at Imazaki, Okayama Pref. The plan at 40m. level, the vertical section subparallel to the general strike and the dip section of the Imazaki deposits are presented as Fig. 3 (a), (b) and (c), respectively.

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The Atomic Fuel Corporation (AFC) was resolved and was newly incorporated as a new governmental corporation, the Power Reactor and Nuclear Fuel Development Corporation (PNC) in October, 1968. The main purpose of PNC is to develop within ten years the prototype reactors of the Advanced Thermal Reactor (ATR) and the Fast Breeder Reactor (FBR) suitable for Japan. These reactors are expected to solve the energy problem of Japan in around 2000. The uranium is not fully used in the light water reactors now being built in Japan. The uranium will be fully used as fuel resources only when we succeed in building, for our electricity generating system, the ATR with greater. conversion ratio of Pu and the FBR to be operated with plutonium. In other words the development of the ATR and the FBR is the solution of the problem of uranium resources.
If we build only the light water reactors, we need 500, 000t of uranium by 2000. But if the FBR can be introduced in and after 1980, 300, 000t of uranium would be enough.
The result of prospecting carried out by AFC for last ten years proved the uranium deposits of only 6, 000t (U₃O₈) which is not able to meet the domestic demand for power plants. Therefore, most of the uranium must be procured from abroad. Under these circumstance, it is very important to study the world situation of uranium and make a plan not only for the purchase of uranium but also for the prospecting and development of uranium mines abroad.
1. History of Uranium Resources Development
After World War II, many countries have promoted the development of uranium resources as the strategic materials under the government subsidies. The uranium resources in Australia and South Africa have been rapidly developed with the propping-up from the United Kingdom and U.S.A. through Combined Devlopment Agency (CDA). As a result, the uranium production of the Free World reached 43, 000t (U₃O₈) in 1959. Thereafter, the uranium mining activities have been slowed down with the mitigation of cold war and the unexpected slow development of civil uses of nuclear power. However, the uranium prospecting activities have been activatéd again since the economic prospect of nuclear power was proved at the Third International Conference on the Peaceful Uses of Atomic Energy held in Geneva in 1964.
2. Classification of Uranium Deposit
The uranium deposit will be classified in the following manners by genesis and mode of occurrences_a) Sedimentary type
This type includes most of the uranium resources now under development such as the Elliot Lake area in Canada and Witwatersrand in South Africa which of the Proterozoic and Colorado Plateau, Wyoming and Texas which belong to the Mesozic and the Tertiary. The uranium occurs in some cases in black shale such as Chattanooga shale (Tennessee) and Alum shale (South Sweden) and also sometimes in phosphorite rocks as in Florida.
b) Vein type
The uranium occurs in the veins as a main ore mineral in fractured zone in Beaverlodge (Canada), South Alligator (Australia), La Crouzille (France). It is associated with Ni, Co, Au, Ag, Cu in Shinkolobwe (Congo) and Jachymov (Czeckoslovakia).

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