Mihara mine, Hiroshima Prefecture, had been exploited for dissemination fluorite deposits in skarn and monzonite. Rocks of this area consist of Paleozoic limestone, chert and diabasic rocks intruded by Cretaceous biotite granite. The Paleozoic formation is thermally metamorphosed and metasomatised by the granite which is partly monzonitised. Fluorite ores occur in the monzonite and adjacent skarns.
Recently presence of beryllium mineralization in this ore deposit was discovered with a beryllium detector. The mineralization occurs mostly in the monzonite and partly in the scheelite-feldspars-fluorite-iron-rich biotite skarn. The beryllium mineral contained in the skarn is exclusively phenacite which coexists with iron-rich biotite, plagioclase and potassium feldspar, filling the grain boundaries of fluorite crystals. A small amount of scheelite is also found in association with the minerals. On the other hand, danalite is the main beryllium mineral in the monzonite. It occurs as disseminated grains of euhedral or subhedral form, and is associated with violet fluorite and feldspars. Phenacite is also present in a trace amount. It is conspicuous that these phenacites are always corroded along cleavages and cracks, and also surrounded by danalite grains.
Volumetric analyses of constituent minerals in thin sections by a point counter reveal that (1) danalite and phenacite are found in specific monzonite which contains fluorite and quartz more than 5.6% and less than 2.3%, respectively; (2) the quantity of modal danalite in such rocks ranges from 2.5% to 3.6%, and that of phenacite is less than 0.4%; and (3) the amount of quartz gradually decreases towards the adjacent skarn body, while total feldspars and fluorite increase in contrast. The degree of Al/Si order-disorder and the Or contents in potassium feldspars in biotite granite and monzonite were determined with powder X-ray diffractometry. Potassium feldspars in biotite granite have a tendency to become ordered and rich in Or component with coming close to monzonite, and the maximum ordering and Or contents are encountered where they coexist with danalite in monzonite. These results are correlative with the modal analysis data.
From the results obtained, the genesis of this beryllium ore deposit may be summarized as follows : (1) Paleozoic rocks were partly metasomatised by the intrusion of Cretaceous biotite granite. (2) Migration of silica from the granite to the adjacent skarn body caused relative increases of alkali and alumina contents in peripheries of the granite, converting them to monzonite. (3) Only a small portion of beryllium contained in the granite could have moved to the skarn body to form phenacite at a limited part of the skarn. (4) The main portion of beryllium was precipitated as danalite crystals within the monzonite.

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This study on the gold veins of the Okuchi mine involves statistical analyses and genetic considerations based on various results of the analyses and conventional geological observations. The factors used for the analyses are strike, dip, width, and grade of the veins. Measurements were made at 400 spots 1.5 meters apart in. the B and E veins. The general feature of the vein system is shown by stereographic projection of the data on the Schimidt's net. The figure is called F diagram in this paper.
Some features of mineralization were made clear from the correlations between width and strike, width and dip, gold and silver contents, etc. The attitude of ore shoots was revealed by the distribution of gold contents and richness (gold content × width) of the veins. It may be assumed that ore shoots were formed along the currents of ore solution. Thus, the main currents of gold-bearing solution were deduced. In the B vein the solution passed through the W 17-W 7 block of No.4 level and rose along the fractures running N 64°-69° E with a dip of about 64°SE and reached the W 20-W 14 block of No.3 level. On the other hand, in the E vein gold-bearing solution passed through the E 13-E 22 block of No.6 level and rose along the fractures running N 54°-66° E with a dip of about 66° NW and reached the E 7-E 14 block of No. 4 level. The inferred currents of the ore solution seem to suggest there is a genetic relation between the veins studied and the neighbouring Honzan vein group.

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Tectonic movement in the Green Tuff region of the northern part of Northeast Japan is divided into two types, i. e., subsidence and formation of horst-graben structure at the latest Oligocene to late Miocene, and uplifting accompanying intense folding and thrusting at the latest Miocene to Recent. The former is interpreted to have taken place under tensile stress field, and the latter under compressional stress field.
Accompanied by the subsidence, vigorous volcanic activity and the Kuroko-type base metal mineralization took place in the inner part of Northeast Japan and coal-bearing sediments were deposited at early stages of the subsidence at the eastern margin of Kitakami massif and at some places of the Green Tuff region. On the other hand, volcanic activity and base metal mineralization (vein-type) related to the uplifting movement were rather weak, and deformation of sediments due to the compressional stress was responsible for the formation of oil-traps in the oil-bearing formations in Japan Sea Coast province.
In terms of the plate tectonics, it is suggested that the tensile tectonic movement and subsidence, which initiated at the latest Oligocene, might have been caused by dragging of the crustal layer by the Pacific plate, which changed its motion from NNW (subparallel to the trend of Northeast Japan) to WNW (perpendicular to the trend of Northeast Japan) about 40 m. y. ago. The intense volcanism during Miocene would have resulted from formation of a great amount of magma at the Benioff zone due to frictional heat generated between the plunging Pacific plate and the overlying mantle. The compressional stress field and the volcanism of the latest Miocene to Recent may probably be a result of convection induced in the overlying mantle by the plunging Pacific plate.

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