The Okuaizu geothermal field is located in the western part of the Aizu district, Fukushima prefecture, northeast Japan. A geothermal investigation in the area was commenced by Mitsui, Mining and Smelting Co., Ltd.(MMS) in 1974. The Okuaizu Geothermal Co., Ltd.(OAG), a subsidiary company of MMS, Mitsui Construction Co., Ltd. and Toshiba Corporation, took over this exploration in 1983.
OAG has drilled fourteen production wells and three reinjection wells so far. The production zone of the field is formed by two fault systems, Chinoikezawa fault system and Sarukurazawa fault system. The reinjection zone is in the Oizawa fault system. These three fault systems have NW-SE strikes and are steeply dipping to NE. The Production wells are drilled by positive displacement control drilling to get many feed points in these declined faults. Major feed points are at a depth from 1, 000 to 2, 600m. The formation temperature at these depths is 200 to 350°C. Geothermal reservoirs governed by these fault systems are estimated to have a potencial to continue supplying steam to a 75MWe power station for 30 years. The production wells produce steam and water mixture. The steam is separated and supplied to a 65MWe power station. The commercial operation of power station began in May, 1995 and is operated by Tohoku Electric Power Co., Inc. An operational peculiarity in the Okuaizu field is that separated water containing dissolved silica is diluted with the overflow water from the cooling tower of power station before reinjection to avoid the scaling problem in the reinjection wells.

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In terms of a new tectonic framework on Neogene magmatism, the areal variation of sulfur isotopic composition in Neogene mineral deposits, excluding Kuroko, is systematized, based on data compiled by ISHIHARA et al. (1992). Sulfur isotopic ratio of pyrite ranges from δ³⁴S=-14to+8 ‰. The value becomes heavier across the NE Japan arc from the trench to the back arc side. The polarity within the forearc igneous belt is not so obvious, probably due to the various degree of contamination of sulfide sulfur in sediments. It starts with about δ³⁴S=-5‰ at about 100km far from the trench, is around δ³⁴S=+3‰ at the volcanic front of arc volcanic belt and attains to around δ³⁴S=+6‰ toward the marginal sea side about 400km far from the trench. As the tectonic character changes from forearc environments to arc environments in eastern Hokkaido and Kyushu, ore sulfur in both areas becomes isotopically heavier.

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The paper is devoted to synnyrites-a high alumina-potassic (17-21% K2O, 22-24% Al2O3) silicate raw material, the main resources of which are concentrated in Siberia and territorially confined to the Baikal-Amur railway. Synnyrites consist of K-feldspar (65-75 vol.%), kalsilite (15-25%), nepheline (up to 10%) and ferromagnesian minerals (biotite, sometimes, garnet and pyroxene, 1-5%). Kalsilite and K-feldspar are in maximum potassic. Nepheline contains up to 25-35mol.% of kalsilite mineral; mica is to magnesian (100MgO/MgO+FeO>25 mol.%) low-titanian biotite. Garnet corresponds to lowalumina melanite. Pyroxene is a member of isomorphic series of diopside-hedenbergite-aegirine. Salic minerals are mainly present as symplectic intergrowths (dactilotypic, poikilitic, pseudoleucite texture), seldom as individual idiomorphic grains.
Large synnyritiferous plutons were formed in Paleozoic and Mesozoic ages during tectono-magmatic activity of older consolidated blocks of the Earth's crust within Baikal-Stanovoy rifting system where potassic alkaline-basaltoid magmatism is widespread. The plutons are large layered ethmoliths. They are made up of two differentiated series : lower melano-, mesocratic series composed of mica pyroxenites, shonkinites, pulaskites and upper leucocratic series represented by nepheline and pseudoleucite syenites, synnyrites. Synnyrites are mainly concentrated in apical and upper of the layered plutons. From the bottom to the top, of the plutons the amounts of K₂O, AL₂O₃ and SiO₂ increase and the ferromangnesian mineral contents decrease in the rocks.
The formation of synnyrite-bearing plutons is the result of slow crystallization of an initial alkaline-basaltoid melt at a substantial depth and widely manifested processes of intrachamber differentiation and fractionation. It is supposed that synnyrites were formed due to segregation of leucite in the top of magmatic chamber and further transformation of leucite into kalsilite and K-feldspar.
A few methods of using and processing synnyrites are suggested : 1) after mechanical enrichment, the ore may be used as chlorine-free low-concentrated potassic fertilizer for a long time ; 2) low-fer-riferous K-feldspar and kalsilite concentrates, potassic alum may be obtained from synnyrites by mechanical enrichment, flotation or acidic decomposition; 3) highconcentrated chlorine-free potassic fertilizers, alumina, fine dispersed silica and others may be produced from synnyrites by deep chemical processing with virtually no wastes.

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