Multiple fluid-mineral equilibria approach to constrain the evolution of thermal waters in the Hisaralan geothermal field, Simav Graben, western Turkey

Abstract

In this study seasonal changes in the geochemical and stable isotope compositions of Hisaralan thermal waters in Simav Graben, western Turkey, were investigated with regards to a variety of mineral-water interactions and mixing processes. The Hisaralan and Emendere geothermal waters, with temperatures of up to 99 °C, were mostly of Na-HCO3 and Ca-HCO3 types. The δ18O and δ2H values of the Hisaralan waters ranged from -9.32‰ to -8.73‰ and -65.02‰ to -61.10‰, with maximum seasonal differences of 0.3‰ and 1.8‰. The Emendere waters were represented by a more positive range of δ2H values (-54.95‰ to -54.61‰), while their δ18O compositions (-9.04 to -8.41‰) were very similar to those of the Hisaralan waters. The stable isotope compositions of the Hisaralan thermal waters were consistent with those of the global meteoric water line, whereas the Emendere waters closely resembled those of the Marmara meteoric water line. The δ13C of the dissolved inorganic carbon varied from -4.33‰ to -2.77‰ for the thermal waters and from -13.84‰ to -12.51‰ for the cold waters. These values indicated a marine carbonate origin for the former and an organic source for the latter. Sulfur isotope systematics of dissolved sulfate in the Hisaralan geothermal waters indicated that the sulfate was most likely derived from the dissolution of marine carbonates and terrestrial evaporites. Chemical geothermometers applied to the Hisaralan thermal waters yielded average reservoir temperatures of 123 to 152 °C, which were rather consistent with those estimated using the silica-enthalpy (146 to 154 °C) and chloride-enthalpy (142 to 178 °C) mixing models. The recharge elevations of the thermal waters that were computed from the δ2H compositions were between 1060 and 1330 m.