GEOCHEMICAL EVOLUTION OF MAGUMSAN AND BUGOK GEOTHERMAL WATERS IN SOUTH KOREA: INTERRELATIONSHIP AMONG TEMPERATURE, STABLE ISOTOPES AND NOBLE GAS ISOTOPES

The studies are aimed to elucidate the geochemical evolution of geothermal waters and the relationship among temperature, chemical composition, stable isotopes (d¹⁸O, dD, d³⁴S) and isotopic ratios of noble gases of geothermal waters in the Bugok-Makumsan area, South Korea, and to obtain a comprehensive view about degassing and circulation of volatile elements in the southern part of Korean peninsula which is tectonically transitional zone between island arc and continent. The chemical composition of geothermal waters varies according to their locations with pH 7.4-8.7 and electrical conductivity of 53-1,489 mS/cm. Bugok geothermal waters emitting sulfur gas in-situ show reducing potential value. The temperature of geothermal waters show the range of 21.2-78.7 °C. The geothermal waters have been evolved from shallow groundwater of Ca-HCO₃ type into Na-HCO₃(SO₄) or Na-Cl chemical types. ³He/⁴He ratios are in a wide range from 0.04 to 3.3 in the unit of 10⁶, though most of the geothermal waters and gases have ³He/⁴He ratios lower then the atmospheric value of 1.4x10⁶, suggesting dominated radiogenic ⁴He contribution from crustal rocks to the geothermal waters. In ³He/⁴He versus ⁴He/²⁰Ne diagram, these geothermal waters are widely plotted along one air-crust mixing line. It indicates that noble gases in geothermal water are originated from one source. The ³He/⁴He and ⁴He/²⁰Ne ratios approach to the atmospheric values owing to the increasing contribution of atmospheric noble gases to the lower temperature geothermal waters compared with the higher temperature ones. The d³⁴S, d¹⁸O and d²H values of geothermal waters have a close relationship with temperature and pH. The d³⁴S values ranging from -3 to +15 ‰ indicates that the sulfate of geothermal waters is originated from pyrite and gypsum in sedimentary rocks. The high Cl content of the Magumsan geothermal waters was contributed from prehistoric sea level change.