The Zunil geothermal field contains the pre-Tertiary granitic basement and various kinds of Quaternary rhyolitic and andesitic rocks. A depression zone and upheaved zones elongating NE to SW have been formed since the late Pleistocene time. The geothermal fluid is derived from deep source at the uplifted zone of west side and discharged at the depression zone during the past time. The water table elevation measured in this geothermal aquifer is 1, 950 meters above sea level, and also elevates to 2, 050 meters with increasing of subsurface temperature from east to west. Alteration produced from alkali chloride hot water/rock interaction is observed below this water table in elevation. The siliceous deposits are found at an elevation higher over 100 meters than the measured water table, of which formation age was dated to be 4, 450 years B.P. by carbon-14 method. It is considered that the hydrothermal alteration were took place during the past time when the water table was higher than the present one, and also when the subsurface temperature might be higher than the present one.
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.