Stratabound ore deposits of Uchinotai, Kosaka Mine, Akita Prefecture, are of black ore type. They were discovered in 1959 in gently dipping acidic pyroclastic rocks of the Miocene age. Though several ore bodies have been recognized in the Uchinotai area, the most developed Western ore body has been investigated as follows.
The ore body can be divided into the following zones according to the mineral assemblage, from lower to upper, (1) quartz-pyrite-chalcopyrite zone, (2) pyrite-chalcopyrite zone, (3) polymetallic zone (including barite, pyrite, sphalerite, galena, chalcopyrite, tetrahedrite, etc.), (4) barite-apatite zone and (5) quartz-hematite zone. Ores of zone-(1) keep the structure of original pyroclastic rocks, while in those of zones-(3)-(5) colloform texture and sedimentary structure are commonly observed. Besides these zones, a siliceous ore chimney consisting of quartz, minor pyrite and chalcopyrite with sporadical other sulfiides, occurs near the central part of the ore body. Clay minerals in the ores and the altered host rocks are sericite and chlorite.
Judging from the results obtained, the following processes of mineralization are suggested: hydrothermal mineralizing solution in part impregnated into poorly-consolidated pyroclastics and formed ores, of zone-(1) (and probably of zone-(2)), and a part of it came out on the sea bottom, where the ores of zones-(3)-(5) were deposited. The environmental chemical condition at the time of ore deposition is inferred to have been weakly acidic, becoming less reducing by its mixing with the sea water. With the diminution of supply of the mineralizing solution, the environments were rapidly recovered to the normal condition of sea water which is oxdizing and weakly alkaline. It is highly probable that the siliceous ore chimney is the "fossil" path of the mineralizing solution.
The sea where the mineralization took place might have been a rather shallow part of the basin which was restricted by a sill and communicated, to some extent, with the open sea.

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The Hirase molybdenum mine, consisting of many molybdenite quartz veins, some of which are pegmatitic and contain beautiful euhedral crystals of molybdenite, is located at the second largest productive area of molybdenum in the Inner Zone of the Southwestern Japan. The area is composed of late, Cretaceous pyroclastic rocks of largely rhyodacitic composition and small granitic intrusives within the pyroclastics. These granitic bodies, of which the largest dimensions are 3 by 15 km, consist of aplite (Ga, 19%), related quartz diorite (Dq, 10%), hornblende-biotite granodiorite (Gd, 30%), and biotite granite (Gb, 41%). Relatively dominant fine-grained aplitic textures, as well as modal and chemical compositions of the granitic rocks indicate that they were derived from slightly mafic-poor series of granitic magma in shallower horizons than those of granitic rocks of similar ages in the Inner Zone of the Southwestern Japan.
Small aplitic masses of the biotite granite of Hirase and the southern half of Hatogaya bodies seem to be the source of the molybdenite bearing ore-fluid. Modally and chemically, the aplitic granite belongs to one of the most fractionated granitic rocks and occurs at the margin of the biotite granite. The aplitic granite, however, is the most suitable host rock to ore veins, petering out downward at the Hirase mine. According to concept presented by the author (1967) that molybdenum-bearing ore-fluid would have been separated from silicate melts in a place where the aplitic granite intruded, kind of the host rocks could indicate limit of molybdenum deposits. This is shown in lower parts of the Hirase underground.
Pyroclastic rocks adjacent to the aplitic granite are hydrothermally altered. Potash feldspar and quartz are most abundant in the alteration zone. The innermost zone can be called (pyrrhotite) -andalusite-pale brown biotite-muscovite subzone, while most parts of the alteration zone consist of (pyrite)-carbonates (siderite)-sericite. Loss and gain during the alteration are as follows: Drastic increases in S and CO₂, slight increases in K₂O, Fe₂O₃, and Na₂O; decreases in MgO, CaO, TiO₂, FeO, and P₂O₅, and slight decreases in MnO and AL₂O₃. The inceased elements are roughly coincidental in their behavior with vein-forming elements except Si⁴⁺. Distributions of 15 trace elements indicate that only Mo and U which occur also in ore veins increase in the alteration zone. Most of other elements, i. e., Ba, Sr, B, V, Ni, Co, As, Zn, and Pb decrease in the alteration zone because of re crystallization of potash feldspar, together with decomposition of plagioclase and mafic minerals. Detection of andalusite clots and geochemical prospecting using Mo and U should be applicable in molybdenum exploration in this destrict.

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