北海道濁川盆地および周辺の温泉に関する化学的考察

概要

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Chemical and isotopic compositions of hot spring waters and well discharges of the Nigorikawa geothermal area were examined. On the basis of SO_4/Cl ratios, geothermal waters can be classified into four groups from high to low value as follows ; (1)Hot springs located at outside the Basin, (2)Hot water discharged from wells drilled into the pre-Tertiary formation of outside the Nigorikawa caldera, (3)Hot water discharged from wells drilled into a fracture zone of caldera wall, (4)Hot springs in the Nigorikawa Basin. Such a classification coincides with the geological conditions of reservoir. Although four types of hot water exist, almost all SO_4 ion are supplied from anhydrite contained in formations, and the SO_4 concentration is controlled by an anhydrite solubility. Anhydrite, Cl and Br are derived from sea water. Geothermal water is basically mixture of sea water and meteoric water. But, hot waters show very high enrichment of ^<18>O (δ^<18>O= 23‰ at δD=0‰ extrapolation), and the relation between Cl and δD of hot waters are different from that of simple mixture of surface water and sea water. It suggests that seawater is altered in the penetrating process.
日本地球化学会の論文
1990-09-29