The geology and hydrothermal alterations of Otake geothermal area, Kujyo volcano group, Kyushu, Japan

Abstract

The geologic structure of Otake geothermal area and its surroundings, being covered widely with Holocene volcanic ash, has not been clarified for the whole district, and the subsurface data of drillholes and wells in the past have been scanty for the studies. Parallel with the geothermal development, however, much useful information concerning the volcanic stratigraphy, geologic structure and hydrothermal alteration in depth of the area has been acquired from the drill cores yielded by the deep test bores, T-1 and T-2, which are completely preserved, and partly by the productive wells H-1 and H-2 in the Komatsu field. Although these drill cores and cuttings have been almost entirely replaced by new hydrothermal minerals, they serve for a standard study of the area. The results of the detailed studies are as follows: 1. 1) Hohi volcanic complex. developed widely and deeply underneath the area as the basement of the Kujyu volcanic complex, is subdivided, and it is confirmed that the middle of the complex composed mainly of permeable tuff breccias is believed to be the main reservoir of the steam, up till now utilized by the power station. 2. 2) Weak indurated sediments corresponding with the Miocene Kusu group were recently found to lie under the Hohi volcanic complex at the bottom of H-1 and H-2 wells. The sediments may fairly be presumed to be the new expected reservoir, and are the only base rocks clarified so far in the area. 3. 3) From the viewpoint of the stability range of secondary minerals due to hydrothermal alteration, the altered rocks can be classified into six zones: the alunite, the kaolin, the pyrophyllite, the montmorillonite, the zeolite and the chlorite. It was also revealed that geothermal fluids in the area may have mainly been preserved in such specific zones as the zeolite, the kaolin and the pyrophyllite zones. After the previous report on the area (Yamasaki et al. 1967), field work has been extended to its surrounding areas and a part of the work on the Hibiradai plateau containing the Takenoyu and Haganoyu hot springs, or steam fumaroles, is shown on the north of the map, (Figure 1). It is noteworthy that the Hohi volcanic complex forming the Hibiradai plateau is distributed much higher, about 400 ∼ 1000 m above sea level than that of the Otake area developed deep down to and below sea level. This is interesting as it may indicate that the depression of the geothermal area had occurred during, or in the later period, of the Hohi volcanic activities. The E-W fault, with the downthrown side towards the south (geothermal area), observed along the south front of the Hibiradai plateau may probably be evidence of such a depression. Furthermore, the prominent N-W fault passing the west side of Sujiyu Spa may possibly be the main breeding fault for the geothermal fluids in the area, because the marked altered rocks, hot springs and many fumaroles occur along or near the fault. The fact that the other steam fumaroles of Takenoyu and Haganoyu hot springs are located, as shown in Figure 1, on the extensions of the fault line should be noticed.