Stable isotope geochemistry evidences from fossil carbonate and sulfur minerals on the origin of geothermal water, Kızıldere Geothermal Field, Western Turkey

The Kızıldere geothermal field, located at the eastern part of the Büyük Menderes graben in Western Turkey, is the most important geothermal reservoir suitable for electricity generation. Current and fossil fumaroles and alteration zones are directly related to the tectonic zones influenced by N-S directional extension since Miocene period. Associated to fossil geothermal activities carbonate (calcite, dolomite) and sulfate (gypsum, anhydrite) minerals were occurred in the form of void/crack fill and bands/lenses parallel to bedding of Neogene clastic and carbonate rocks. The carbon (δ¹³C_PDB ‰) and oxygen (δ¹⁸O_PDB ‰) isotope compositions of hydrothermal calcites and dolomites and sulfur (δ³⁴S_CDT) and oxygen (δ¹⁸O_SMOW) isotope compositions of gypsum and anhydrites are analyzed first time and correlated current geothermal water composition. The carbon and oxygen isotope data of calcites and dolomites have similar carbon but different oxygen isotope composition which increases in the direction of surface calcite – drill cuttings calcite – drill cuttings dolomite – surface dolomite. The isotope compositions of calcite and dolomite minerals range between limestone and marble host rock compositions and indicate the carbonate mineral-forming fluids originated from dissolution of carbonate rocks during the circulation of hot geothermal waters. According to the calcite-CO₂ and dolomite-CO₂ isotopic fractionation data for the 0–300 °C temperature range, the fossil isotope composition is higher than the composition of current CO₂ and reflects relatively lower temperature conditions. The isotope compositions of gypsum and anhydrite minerals indicate that hot thermal waters dissolved terrestrial evaporites and formed a sulfur-rich geothermal solution, and hydrothermal gypsum and anhydrite precipitated from this solution. The δ³⁴S_CDT compositions of hydrothermal gypsum and anhydrites are similar to the current geothermal water compositions. Stable isotope geochemistry data of hydrothermal carbonate and sulfate minerals in the Kızıldere geothermal field have shown that the fluids forming these minerals were originated from host rocks instead of magmatic volatiles.