Geochemical characteristics of Na-Ca-Cl-HCO₃ type waters in Arima and its vicinity in the western Kinki district, Japan

Na-Ca-Cl(HCO₃) type waters of Cl content ranging from a few hundred mg/1 to more than twice that of seawater widely distribute in the studied area. From the chemical and isotopic compositions and reservoir rock types, these waters are classified into the Arima brines (Type I, Arima-type brine of Matsubaya et al., 1973), dilute carbonated waters from Paleozoic sedimentary rocks (Type II), and those from Cretaceous to Paleogene acidic igenous rocks (Type III). Arima brines are highly saline (Cl = 1,000 ~ 36,000mg/l) and are considered to be mixtures of a unique deep brine of δD = -32‰, δ¹⁸ 0 = +10‰ and Cl = 54g/l and bicarbonate-rich dilute waters of meteoric origin. From the chemical and isotopic data presented in this paper and in the light of the high ³ He/⁴ He ratios observed in gases of Arima brines (Sano and Wakita, 1985; Nagao et al., 1981), it is likely that the deep brine originates from deep-lying magma underneath the Arima Spa or from sedimentary rocks during metamorphism induced by the magma. Most waters of Types II and III are often carbonated waters of meteoric origin. Chemical compositions of these waters suggest that both of them are formed by interaction between CO₂-bearing meteoric water and wall rocks. Among the waters of these groups studied, only the Tojo water (Cl = 6,380mg/l) exhibited clear evidence of contribution from Arima brines. Other waters have no such sign or are too dilute to find any relation with Arima brines.