Geothermal area

The formation of epidote, K-feldspar, prehnite, wairakite and calcite in the geothermal area is considered to be due to the loss of CO2 gas and rapid precipitation from the solution supersaturated with respect to quartz (Browne, 1978). The widespread occurrence of these minerals in the Seigoshi district seems to be consistent with the above-mentioned consideration, namely that these minerals usually occur as veinlets rather than the replacements of original minerals and filling amygdule. In particular, many veinlets of epidote, prehnite and wairakite are found near the Au-Ag-quartz veins. 

Shikazono (1984) summarized analytical data of the epidote from geothermal areas to consider the relationship between the composition of epidote and that of the original fresh rocks and to inspect the other factors controlling the compositional variations in epidote. The discussion on the epidote composition by Shikazono (1984) is described below. 

Hayashi, M. (1973) Hydrothermal alteration in the Otake geothermal area, Kyushu. J. Jpn. Geotherm. Ener. Assoc., 10, 9-46. 

Iwao, S. (1962) Silica and alunite deposits of the Ugusu mine a geochemical consideration on an extinct geothermal area in Japan. Japan. J. Geol. Geogr., 33, 131-144. 

Morishita, Y. and Takeno, N. (1993) Gold mineralization in the Noya geothermal area, Ohita-ken, Kyushu, Japan. Resource Geology, 43, 173-186. 

Shikazono, N. (1984) Compositional variations in epidote from geothermal areas. Geochem. J., 18, 181-187. 

Sumi, K. (1968a) Hydrothermal rock alteration of the Matsukawa geothermal area, northeast Japan. Rep. Geol. Surv. Jpn., 225, 1-42. 

Sumi, K. (1968b) Structural control and time sequence of rock alteration in the Matsukawa geothermal area, with special reference to comparison with those of Wairakei. J. Jpn. Geotherm. Energy Assoc., 17, 80-92. (in Japanese with English abst.). 

The concentrations of Rb and Cs in feldspars in geothermal areas have not been studied. However, if the concentration ranges are similar to those observed for feldspars in common igneous rocks, we can construct curves to show the relationship of the concentration of an alkali element X+ and the Cl concentration of geothermal waters (Fig. 2.4 and Fig. 2.5). It is assumed that the concentrations of Cs and Rb in feldspars range from lO -" to 10 wt% and I0 to 10 wt%, respectively. 

These results indicate that the chemical composition of geothermal water at 250°C is largely controlled by such minerals commonly occurring in geothermal area as albite, K-feldspar, sericite, calcite, wairakite and quartz. 

Present-day mineralization in subaerial geothermal areas in Japan... 

Base-metal (Pb, Zn, Cu) mineralizations were reported from Okuaizu, Nigorikawa, Arima and Ibusuki geothermal areas as scale products and precipitates from hot springs. Gold precipitations from hot springs are known from the Osorezan volcano and Beppu. 

The gold content of the precipitates is anomalously high, compared with those from the other geothermal areas (Table 2.3). 

The Okuaizu geothermal area is located in Northeast Japan (Fig. 2.18). Pyroclastic rocks are the oldest ones of early Miocene (18-16 Ma). These Miocene and Pliocene... 

The Fushime geothermal area is located at the southeastern part of the Satsuma Peninsula, Kyushu, Japan. 

Opaque minerals identified from active geothermal areas are pyrite, sphalerite, galena, chalcopyrite, and tetrahedrite from Okuaizu, Fushime, and Nigorikawa (Japan), Salton Sea (U.S.A.) and Broadlands (New Zealand). 

Fig. 2.25. Generalized zonal. sequence of alteration minerals in active geothermal areas (Henley and Ellis, 1983 Shikazono, I985b).

During the last three decades, subaerial geothermal areas in the Japanese Islands have been explored considerably and geothermal energy plants were developed. It was recognized that some active geothermal systems are accompanied by present-day base-metal and precious-metal mineralizations. 

Bates MN, Garrett N, Graham B, et al. 1997. Air pollution and mortality in the Rotorua geothermal area. Aust N Z J Public Health 21 581-586. 

Pondweeds, Potamogeton spp. Whole Near geothermal area (11-436) DW 1... 

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