“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 was conducted in Okunoin area, where faults had been detected by other previous studies. 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 emission and the other is low gas emission. Among of two types of soil-gas anomalies, anomaly of the former type is found commonly in other fields. On the other hand, anomaly of the latter type has not been found in previous studies. Moreover distribution map of the classified types of soil-gas anomalies shows that several anomalous zones of the latter type are sandwiched by anomalous zones of the former type, forming a sequence of anomalous zones as high gas emission-low gas emission-high gas emission. Considering all geological and geophysical information, the two types of soil-gas anomalies and the sequence of anomalous zones were interpreted as bellow. (1) Anomaly of high gas emission indicates fracture zone which plays as a passage for geothermal fluid. (2) Anomaly of low gas emission indicates highly altered zone which corresponds to the top of fracture zone sealed with clay, minerals formed by hydrothermal alteration. (3) The sequence of the anomalous zones indicates migration of the passage for geothermal fluid from the center of alteration zone to the outside.