Chemical Characteristics of Hot Spring Water and Geological Environment in the Northernmost Area of the Itoigawa Shizuoka Tectonic Line
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概要
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Hot springs around the northernmost part of the Itoigawa Shizuoka Tectonic Line (ISTL) are investigated. The Otari and the Hakuba villages, Nagano Prefecture are the study area. The area is divided into two areas by the ISTL, the Hida Gaien belt area (western part) and the Fossa Magna area (eastern part): the former is consists of weakly metamorphosed formations containing melange belt with ultramafic rocks and the latter is of sedimentary rocks of mainly sandy mudstone. Physical and chemical measurements for the hot spring waters are expected to provide a relation between geological structure and characteristics of hot spring water, and also to provide information about correct trace of the ISTL in the area covered with sediments. The percentage of (Mg2＋＋Ca2＋) in total cation (in eq/l) is found to be a good indicator for drawing a line between the Hida Gaien belt area and the Fossa Magna area. In the case of the study area, the percentage for the drawing is 15％: lager values for the Hida Gaien belt area and smaller ones for the Fossa Magna area. Therefore, hot springs in the boundary zone are classified by the percentage. Furthermore, an unusual concentration in SO42－ ion in the Fossa Magna area is ob-served. That is, the concentrations of SO42－ ion in the hot spring waters are smaller than those of the river waters. The unusual concentration reveals that the origin of SO42－ ion in the hot spring water is pyrite (FeS2) in the formation. In the Renge hot spring group, very high concentrations of SO42－ and Mg2＋ are observed, which are explained in terms of interaction among SO2 gas from a volcano, ultra mafic rocks and underground water. That is, underground water reacting with high temperature SO2 becomes strong acid hot water with large amount of SO42－. The hot water reacts with ultra mafic rocks around the hot water reservoir, and the hot water has high concentration of Mg2＋.
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