スニール地熱地帯の地質は,先第三紀の花岡岩類とそれを不整合に覆う第四紀の安山岩質～流紋岩質火山噴出物からなる。これらの地層はいくつかの断層によって断たれ,NE-SWに伸長する陥没構造が後期更新世以降に形成された。高温地熱流体は,隆起帯の深部からもたらされ,陥没地帯で地表に流出している。地熱レザーバーの地下水面は,標高1950mにあり,それは,地下温度が高くなるにつれて,西方に向って高くなり2050mに到る。高温のアルカリ性塩化物熱水によって形成された地熱変質は,この地下水面以深にのみ認められる。4450年以前に形成された珪質温泉沈澱物が,実測される地下水面より100m以上も高い位置で見い出された。地熱熱水変質は,地下水面が現在より高い位置にあった時代に形成されたものと考えられ,その時の地下温度は,現在実測される地下温度より高温であった可能性がある。

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White colored alteration halos are widely distributed at the surface of studied area. These alteration halos are divided into four types based on their mineral paragenesis. Characteristic minerals are pyrophyllite and dickite (type I), alunite and kaolinite (type II), cristobalite and alunite (type III) and quartz and K-feldspar (type IV). In addition, green colored alteration zone characterized by chlorite and zeolites (type V) are widely distributed under the ground of geothermal areas. Clay minerals can be found X-ray diffraction analysis of samples separated by dispersion procedure from rocks which are generally considered as fresh rock. These data are very useful for the analysis of water flow of regional scale. Distribution patterns of montmorillonite indicate the path of shallow hot water flow and those of halloysite coincide with deep faults which were identified from the analysis of radar images. The distribution of thermal manifestations and alteration halos is fairly controlled by the deep faults which cut the basement rock but has less relations to E-W directional active faults and linearments which are intensely and clearly identified from the analysis of aerial photograph. Based on the types and distribution patterns of alteration minerals, ages of volcanic rocks and history, present model of hydrothermal systems of the area are inferred. An important point of the formation of hydrothermal systems in this area is the presence of Tertiary basin. Within the basin, fine-grained sedi-ments and formations which took place regional scale hydrothermal alteration prevent the intrusion of large amount of cold waters to deep part and preserve the heat source. After that, many geothermal activities have been occurred and formed many kinds of alteration halos in accordance with their physico-chemical conditions of the areas.

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