Hydrochemical characteristics and geothermometry applications of thermal waters in the Çürüksu Graben, western Turkey

Authors: ALİ GÖKGÖZ, HALİM MUTLU, MEHMET ALİ AKMAN

Abstract: In this study, we investigate chemical and isotopic characteristics of low-temperature geothermal waters issuing from carbonate reservoirs in the Çürüksu Graben within the eastern termination of the Büyük Menderes Graben in western Turkey. Temperatures and pH values of geothermal waters vary from 20.1 to 24.6 °C and 6.62 to 7.11 and those of cold waters are 17.1 to 19.9 °C and 6.85 to 7.72, respectively. Geothermal waters are of Ca-HCO₃ and Ca-SO₄ types whereas cold waters are characterized by these two types and Mg-HCO₃ as well. δ¹⁸O and δD values of samples vary from ?9.27‰ to -7.69‰ (VSMOW) and -58.06‰ to -52.2‰ and indicate a meteoric origin with local recharge. Tritium contents are from 0.12 to 2.17 TU for thermal waters and 0.28 to 4.85 TU for the cold waters implying relatively longer residence time for the hot waters. Thermal water samples mostly have positive δ¹³C values (varying from ?0.32‰ to +1.99‰) and carbon in these waters is likely derived from marine limestone or metamorphic CO₂. δ³⁴S and δ¹⁸O values of dissolved sulfate in the waters indicate that sulfur originates from dissolution of marine evaporite deposits (e.g., gypsum). Çürüksu waters are generally oversaturated with respect to calcite, dolomite and quartz but undersaturated with respect to gypsum. Common ion effect exerted a strong control for the formation of travertine deposits in the area. Chemical and isotopic evaluations indicated that the diversity in the water chemistry of samples is attributed to a combination of processes including water-rock interaction, ion exchange and mixing of various types of waters. Çürüksu thermal waters are immature and not in chemical equilibrium with the reservoir rock. Among the various geothermometers applied to Çürüksu thermal waters, temperatures computed by chalcedony, quartz and Ca- Mg geothermometers are 21-52 °C, 49-83 °C and 73-96 °C, respectively. HCO₃-SO₄-F and anhydrite-chalcedony (quartz) diagrams estimated a temperature range of 63?86 °C and δ¹⁸O(SO₄-H₂O) isotope geothermometer yielded 67-78 °C.

Keywords: Water chemistry, isotope, geothermometry, mineral saturation, Çürüksu thermal waters

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