Geochemical and Isotopic Compositions and Geothermometry of Thermal Waters in the Magumsan Area, South Korea

Abstract

The Magumsan thermal waters of the southeastern Korean Peninsula are pumped out of six deep wells (average depth, 300 m) at temperatures of 30.8-49 degrees C. The thermal waters are chemically classified into two groups: NaHCO3 type (<31 degrees C) and NaCl (HCO3, SO4) type (>40 degrees C), both of which have chemical compositions that are distinct from local groundwater (Ca-HCO3 type). delta O-18 and delta D values suggest that the thermal waters originate from meteoric water and they are isotopically fractionated by silicate hydration or H2S exchange. delta S-34 values (+7.0 to +15%) of dissolved sulfate in the thermal waters reflect enrichment in S-34 through kinetically controlled oxidation of magmatic pyrite in the thermal aquifer and mixing with paleo-seawater. On the He-3/He-4 vs. He-4/Ne-20 diagram, the thermal waters plot along a single air mixing line of dominant crustal He, which indicates that the heat source for the thermal waters is non-volcanogenic thermal energy that is generated from the decay of radioactive elements in crustal rocks. Chalcedony geothermometry and thermodynamic equilibrium calculations using the PHREEQC program indicate a reservoir temperature for the immature thermal waters of 54-86 degrees C and 55-83 degrees C, respectively.