"FINGERPRINT" Geochemical Survey Applied to Onikobe Okunoin Geothermal Area

Abstract

Onikobe geothermal area, situated in northeastern Japan, is characterized by the large hydrothermal alteration zones named Katayama, Okunoin and Arayu."Fingerprint" geochemical survey) at 134 stations in two survey lines was conducted in Okunoin area, where faults are detected by other studies (there are papers concerning to the faults, one of which is Abe and Harada(1985)). In this survey the soil-gas absorbed by activated charcoal was analyzed by mass spectrometry. Thus obtained geochemical data show that there are two types of soil-gas anomalies indicating fault/fracture, one of which is high gas ~mission and the other is low gas emission. Several anomalous zones of the latter type are sandwiched by anomalous zones of the former type, forming the sequence of anomalous zones as high gas emission-low gas emission-high gas emission. Considering all geological and geophysical information, the soil-gas anomaly of the latter type, which has not been found in the other fields, was interpreted to indicate highly altered zone corresponding to top part of fracture zone sealed by clay of hydrothermal origin and the sequence of the anomalous zones was inferred to indicate migration of the passage for geothermal fluid from the center of sealed fractured zone to the outside.