It has been generally believed that the pyrophyllite deposits in Mitsuishi, Japan, were formed during the Cretaceous period, since the enclosing rocks are the Cretaceous quartz-porphyritic tuff breccias. According to our study, however, rhyolite lavas and tuffs of the Oligocene Tenkadaiyama Series cover the erosion surface of the Cretaceous strata which was uplifted to the present altitude of 200 meters. Quartz porphyry dikes and sills branches out often from the rhyolite lavas. These dikes and sills cut the pyrophyllite deposits. In turn, these are cut by pyrophyllite veins containing diaspore, etc.
Though shale beds within the Cretaceous tuff breccia formation are sometimes silicified, it appears that the shale beds were caprocks against pyrophyllite mineralization. On the other hand, especially in the neighborhood of the veins, the quartz porphyritic tuff breccias are progressively altered to pyrophyllite deposits. Similarly, at progressively greater distances from the veins, the development of porcelain clay in the deposits was noticed.
These evidences suggest that the pyrophyllite deposits were formed by the hydrothermal solution which was introduced by the rhyolite during the Oligocene epoch. This conclusion was also supported by the similar chemical compositions of quartz porphyry, rhyolite, and other members of extrusive and intrusive rocks of this extensive area.

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The Higashiyama mine, one of the principal molybdenum mines in Japan, is situated about 7km to the southeast of Daitô-machi, in the eastern part of the Shimane Prefecture, western Japan.
The area is composed of fine-grained quartz diorite, coarse-grained biotite granite, granitic complex and granite porphyry of the Late Mesozoic era. The granitic complex consists of various rocks, such as medium-grained leucogranite, aplitic granite and medium-grained biotite granite which contains xenoliths of fine-grained quartz diorite. These rocks were all cut by the Miocene andesite dikes.
The major molybdenite-bearing veins occur in the granitic complex, as well as in the coarse-grained biotite granite. On the other hand, some small scale veins occur in the fine-grained quartz diorite. The molybdenite-quartz veins dip about 20 degrees southwards. Genetically, the veins can be classified into two types: a quartz vein and a pegmatitic quartz vein. Among the two the quartz vein type distributed over six ore-bearing areas, yields far larger amount of molybdenite.
Nydrothermal alterations of wall rocks are, in order of decreasing intensity, silicification, chloritization, sericitization and carbonatization, respectively. Molybdenum ores consist mainly of molybdenite and quartz, while in the Ônobe No.4 veins they contain certain amounts of chalcopyrite, zincblende and galena.
Fissures of the Higashiyama veins can be roughly divided into two groups; one is very high-angled fissures of the nouth-nouth-east trend, and the other is low-angled. The latter seems to have been formed earlier than the former. Evidently, these fissures have played an important role on the precipitation of molybdenite from the ore solution.

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The Ôe mine in the southwestern part of Hokkaido is producing monthly 8, 500 tons of crude ores from which Au, Ag, Cu, Pb, Zn, Fe-S and Mn are extracted. The ore deposits are of epithermal fissure-filling vein type. The ores are mainly composed of rhodochrosite, chalcopyrite, galena, sphalerite and pyrite with minor amounts of native gold, argentite, pyrargyrite, etc.
The Senzai vein, the largest one of this mine, is about 1, 500m long along the strike and extends to 500m along the dip with an average width of 5m. The gold contents are high at the upper and lower parts of the western ore shoot consisting of rhodochrosite and sulphide minerals. While at the upper parts of the vein, gold is associated with lead as shown by assays, at the lower parts and in the Manzai vein which runs parallel with the Senzai vein it is associated with copper. Originally, the native gold of this mine was discovered in lead concentrates about ten years ago.
At the lower part, the native gold-bearing ores occur as aggragates of fine sulphide minerals. On the other hand, they occur in the banded ores consisting of rhodochrosite, quartz and sulphide minerals at the lower part of the Senzai vein and in the Manzai vein. These minerals were crystallized at the late middle stage of mineralization.
Mineralogically, the native gold is closely associated with galena of 20μ average size at the upper part of the Senzai vein, while in the Manzai vein most native golds are associated with tetrahedrite of 15μ average size. By etching the polished surface of native gold with CrO₃+HCI, a concentric banded texture was developed. Hence, it is confirmed that the gold grains which show this texture are composed of alternating concentric bands which are relatively rich or poor in silver. The heterogeneity of the grains could be also detected by electron probe X-ray micro-analysis and reflectivity using a microphotometer.

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It is one of the important tasks of mining geologists to find a clue of the vertical extent of hydrothermal veins.The writer reviews many descriptions on the vertical extent of epithermal and xenothermal veins of gold-silver, mercury, copper and copper-lead-zinc veins of the Neogene epoch, especially in Japan.
There seems to be many controlling factors either structural that limit the vertical extent or physico-chemical, the zonal arrangement of ores at depth and the depth of veins with respect to the intrusive rocks or magmatic sources may give us some clues.
The writer believes, however, that the most effective factor of limiting the vertical extent is the level of ground water tables where the ascending ore-forming solution meets the circulating ground water. This is suggested by intimate occurrences of hot spring waters with numerous hydrothermal veins.

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The Ningyô-tôge area is one of the richest uranium areas in Japan. Uranium deposits of the eastern extension of the Ningyô-tôge area are called the Nakatsugô-Nambu body, the Nakatsugô-main body, and the Ombarabody, respectively. The average uranium content throughout these ore bodies is about 0.04% U₃O₈ or less, while high grade ore body of the Nakatsugô-Nambu averages over 0.5% U₃O₈. The ore reserve of the largest Nakatsugô main body are estimated to be about 430, 000 tons (0.035% U₃O₈).
The uranium deposits occur in a thin bed (0-90m thick) of river or lake formations consisting of conglomerates, sandstone and shale, successively from bottom to top. They overlie unconformably over various granitic rocks of the Chugoku Batholith, of which K-Ar dating using biotite ages from 33 to 64 million years. These sediments of the Miocene to Pliocene epoch, are covered by Pliocene andesite and/or andesitic pyoclastics.
The uranium deposits are located at the northern edge of the main channel structure of a paleo-current which flew eastward. Since the trend of the major sheared zone and joints in granites are almost identical to that of the main channel, these weak zones of the granites might have favored the formation of the main channel.
The highly minerallized horizon of the Nakatsugo-Nambu body occurs in the basal conglomerates above the granite. The ningyoite, which is the major uranium mineral of the ore body, fills the matrix of the conglomerates. It is usually associated with pyrite. On the other hand, the major uranium minerallization in the Nakatsugo main body are present in the basal conglomerates, and the minor one in alternations of sandstone and shale over the conglomerates although ningyoite is widely distributed, uraninite is present only locally in this ore body. The low grade ore body of the Ombara consists of uraniferous conglomerates, sandstone, and alternations of sandstone and shale. No uranium mineral has been identified at present.
These ore bodies are the Neogene bedded-type uranium deposits. It appears that the important geologic factors for the formation of these uranium deposits are surface and internal structures of the basement granite, amounts and kinds of the host sedimentary rocks, and presence of a cap rock.

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