The four major geothermal power plants - Otake, Hatchobaru, Takigami and Sugawara -, are located in the Kuju geothermal district of Oita Prefecture and have a total capacity of 157 MW. Geothermal reservoirs have formed in the margins of uplifted zones and, after the Middle Pleistocene, have been associated with volcanism. The Japan, Oil, Gas and Metals National Corporation (JOGMEC) conducted helicopter-borne time domain electromagnetic survey (HELITEM) and picked up low resistivity anomalies due to argillic alteration zones which indicated impermeable cap rock over geothermal reservoir in 2014. An alteration survey was performed of the low resistivity anomalies for the purpose of investigating argillic alteration zones and evaluating the geothermal potential. Promising alteration zones were selected in the Nanbu-Mizuwake Pass, Otake-Hatchobaru, Sugawara, Nanbu-Waitasan, Makinoto Pass and others. The alteration minerals are composed of acid to neutral argillic minerals such as pyrophyllite, dickite, kaolinite, halloysite and smectite, which are zonally arranged. The alteration zones were formed by hydrothermal solution, ascending from deep underground to shallow levels along faults. The faults predominantly trend WNW-ESE, ENE-WSW and NW-SE. They tectonically formed under a horizontal and extensional stress field trending N-S. Acid alteration zones, which consist of kaolinite, dickite, alunite and so on, are particularly important from the standpoint of geothermal exploration. That is because they have formed as impermeable clay layer over geothermal reservoirs where hydrothermal solution of high temperature flows convectively along faults. Alteration minerals were identified by FieldSpec4 spectrometer. The probability of visual identification of the absorption spectra was 78.1%, verified by X-ray powder diffraction (XRD) of a part of samples. Some samples, which were composed of acid alteration minerals, were incorrectly identified as smectite or zeolite. The incorrect identification is due to the absorption spectrum (1,920 nm) of water contained in the sample. After this, a method for drying samples should be investigated in order to improve the probability.
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