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 degrees C, were mostly of Na-HCO3 and Ca-HCO3 types. The delta O-18 and delta H-2 values of the Hisaralan waters ranged from -9.32 parts per thousand to -8.73 parts per thousand and -65.02 parts per thousand to -61.10 parts per thousand, with maximum seasonal differences of 0.3 parts per thousand and 1.8 parts per thousand. The Emendere waters were represented by a more positive range of delta H-2 values (-54.95 parts per thousand to -54.61 parts per thousand), while their delta O-18 compositions (-9.04 to -8.41 parts per thousand) 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 delta C-13 of the dissolved inorganic carbon varied from -4.33 parts per thousand to -2.77 parts per thousand for the thermal waters and from -13.84 parts per thousand to -12.51 parts per thousand 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 degrees C, which were rather consistent with those estimated using the silica-enthalpy (146 to 154 degrees C) and chloride-enthalpy (142 to 178 degrees C) mixing models. The recharge elevations of the thermal waters that were computed from the delta H-2 compositions were between 1060 and 1330 m.