Origin and evolution of the thermal waters from the Pamukkale Geothermal Field (Denizli Basin, SW Anatolia, Turkey): Insights from hydrogeochemistry and geothermometry

Abstract

The Pamukkale Geothermal Field (PGF) of the Denizli Basin (SW Turkey) is one of the most distinguished active geothermal fields known in the World. The thermal waters from the PGF are subdivided into two groups (Group 1A: Ca-HCO3 and Group 1B: Ca-SO4 types). The cold waters are also classified into two groups (Group 2A: Ca-HCO3 and Group 2B: Mg-HCO3 types). The hydrogeochemical properties of the PGF thermal waters suggest interactions with the basal metamorphic rocks of the Menderes Massif, tectonically overriding by the carbonate rocks of the Lycian Nappes and sedimentary rocks of the Neogene formations. The bedrock units are stratigraphically overlain by the Neogene sedimentary units of the Denizli Group. The thermal springs of the PGF are affected by several hydrothermal processes including mixing, water-rock interaction and input of mantle volatiles.The thermal water plots of the PGF deviate from the full equilibrium line indicating a low water maturity and/or mixing processes. Mixing models and Na-K-Mg ternary diagram suggest mixing between meteoric and thermal waters during the uprising of the deep-originated fluids. The water-rock interaction leads saturation of calcite and induces travertine precipitation in the discharge area.Low tritium (<1 TU) values of the PGF thermal waters confirm a deep circulation. The δ18O (−9.23 to −8.32‰) and δD (−61.00 to −57.00‰) isotopic values of the PGF indicate that the meteoric waters serve as the source of the thermal waters. Based on the δ18O and δD data, the PGF thermal waters are produced from precipitation in the southern side of Yenice Horst with elevation of 850–880 m.a.s.l. The δ13CDIC values (+6.26 to +8.07‰) of the PGF thermal waters indicate that CO2 in thermal waters is mainly of a metamorphic origin. In order to identify the reservoir temperature of the PGF, various geothermometric methods have been applied. Accordingly, theoretical reservoir temperature ranges from 60 to 125 °C. The total hydrothermal CO2 flux in the PGF is calculated to be approximately 1.4 × 108 mol/year (excluding free CO2 flux) as similar to those of other geothermal fields in the World (e.g., central-southern Italy).This study proposes a conceptual hydrogeological-hydrogeochemical-tectonic model for the PGF: the thermal waters are derived from the infiltration of rainwater through fractures and faults in high recharge areas to the deep hot reservoir. Heating produced by the high geothermal gradient is interpreted in relation with the regional crustal thinning. Thermal waters upwell along WNW-ESE-trending deep faults and fractures (the Pamukkale Fault System) that act as hydrothermal pathways favouring migration of a huge amount of CO2-rich gas from deep geothermal reservoir. The geochemical and isotopic data of the thermal waters and gas indicate that the degassing of both metamorphic CO2 and mantle derived CO2 occurs in the PGF. Helium isotope data indicate that mantle volatiles produce possibly an upwelling into asthenosphere and ascent along these tectonic discontinuities. In this context, the PGF is one of the best example of geothermal fields as an interaction of active magmatism in the actively extending crustal setting.