The Mitsubishi Onuma geothermal power plant, having 10 MW generator, has been running since 1973 in northern Hachimantai geothermal field, and make an offer the low cost power to Mitsubishi Akita refinery. The geology of this field consists of Quaternary volcanic rocks and Neogene sedimentary and volcanic rocks. Many volcanoes are distributed on the up-heaved block built up by fault systems and folding structure. The distribution of geothermal alteration minerals, for example zeolites, sericite and others, shows that two volcanic or geothermal centers were existed along the anticlinal axial belt of folding structure in the graven and on the upheaved block, but according to the isogeotemperature map compiled from many deep wells, the present geotemperature in the latter center is about 100°C lower than the former center. Thus, it is considered that the productive geothermal reservoir was formed during the geo-history, through which the meteoric water has been infiltrated in proportion to the up heaved movement.

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The Akenobe deposits are polymetallic vein type deposits. More than one hundred veins are distributed in sediments and volcanics of Maizuru Group of Permian age. The recent study of geological structure and ore deposits in the mine and to vicinity revealed following facts and idea which lead the discovery of new deposits.
1. Vein fractures were formed under NW-SE trending lateral compressional force and the natures of fractures was strongly controlled by the competency of structural blocks.
2. The analysis of post ore movement of major faults enabled the reconstruction of displaced vein system, which suggested the long extension of major vein system before faulting.
3. NE-SW trending major faults provided the channel way for ore solution that yielded lateral polyascendent mineral zoning, whereas NW-SE trending fault was newly found to have provided the channel way for Chiemon vein group. In latter case, vertical mineral zoning especially downward direction is remarkable suggesting possible descending flow of ore solution from the channel way.

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Hydrothermally altered rocks at the No. 1 Orebody of the Itaya kaolin deposit, one of the largest hydrothermal clay deposits in Japan, have been divided into the following four types of alteration zones on the basis of indicative mineral assemblage and characteristic occurrence: Alteration Type I-kaolinite-sericite-K-feldspar zone, which occurs exclusively within acid tuffs of the Itaya Formation(Funakawa Stage, Late Miocene). Alteration Type II-breccia dike and pyrite-disseminated sheared zone ("Black Fault"); masses of alteration products filling sheared parts of the altered Miocene tuffs (Type I). Alteration Type III-montmorillonite zone, which is limited to mud flow deposit derived from the eruption of the Azuma Volcano in the Latest Pleistocene. Alteration Type IV-native sulfur-opal-alunite-kaolinite zone, which replaces some parts of the altered Itaya Formation (Type I) and the altered Azuma mud flow deposit (Type III).
From the correlation of the stratigraphic range of their distribution to the stratigraphy of the original rocks, these types of alteration appear to have taken place in the order mentioned above. On the basis of radiometric age determination, two major periods of hydrothermal alteration have been recognized at the No. 1 Orebody. The earlier one (Type I) was appeared during the Late Miocene, and may have been associated with the terrestrial acid volcanism which produced the tuffs of the Itaya Formation. The later one (Types II to IV), on the other hand, took place since the Latest Pleistocene to present, as the result of geothermal activity related to the andesitic Azuma volcanism. This study has shown that the general features of the No. 1 Orebody were initially formed by Miocene sericitic alteration, while the northwestern part of the Orebody was later replaced by sericite-free clay during geothermal alteration on or near the surface since the Latest Pleistocene.

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