Sr isotope geochemical study of geothermal water and rocks from a newly drilled well for geothermal power generation and hot spring waters in the Okuhida Hot Spring, Gifu, Japan

Geothermal waters from a 1,100-m-deep geothermal well (No. 7) in the vicinity of Yakedake volcano in Gifu Prefecture, central Japan, were monitored for 1 year for chemical and isotope (D, O, and Sr) composition to examine their source. Drill cuttings from the well and surrounding hot spring waters were also analyzed for ⁸⁷Sr/⁸⁶Sr ratios and chemical composition. Combining the calculated reservoir temperatures for geothermal waters from well No. 7 and the ⁸⁷Sr/⁸⁶Sr ratios obtained, we suggest a shallow source for the geothermal waters, such as the Cretaceous to Paleogene Kasagatake Rhyolites, and propose downward-flowing meteoric waters as dominant at depth where the geothermal waters are enriched in Ca²⁺, K⁺, and Sr²⁺ by the interaction with reservoir rock. The ⁸⁷Sr/⁸⁶Sr ratios of hot spring waters in the Shinhirayu area, west of well No.7, are lower than those in three adjacent areas (Nakao, Gamada, and Hirayu), reflecting differences between the geologic formations comprising the reservoir rocks. The geothermal water from well No. 7 and hot spring waters in the study area are not in chemical equilibrium with the reservoir rocks because the reservoirs are young and not strongly altered.