Hydrogeochemistry, stable isotope composition and geothermometry of CO2-bearing hydrothermal springs from Western Iran: Evidence for their origin, evolution and spatio-temporal variations

This study aims to hydrochemically characterise three CO2-bearing springs representing distinct hydrofacies in NW Gorveh (western Iran) and interpret them in the light of their geological setting. The results of laboratory measurements of elemental concentrations, stable oxygen, carbon and hydrogen isotopes, dissolved and particulate organic and inorganic carbon (DIC, DOC, POC, PIC) and alkalinity are combined with in situ measurements of pH and temperature. Parameters such as alkalinity, DIC, Ca2+ and pCO2 concentration display strong, positively correlated values, with systematic decrease from the spring vent in down-flow direction for the three spring systems. The inverse correlation of pH and δ13CDIC is caused by CO2 degassing. The δ18O and δD values show no significant variation, related to minor or no evaporation due to normal ambient temperatures. The low concentration of POC, PIC and DOC compared to that of DIC and the lack of correlation between them reflect predominant inorganic carbon in these fluids. Spring I is oversaturated in calcite with additional dissolution of CO2, and despite high concentrations of Na+ and Cl−, undersaturated in halite, indicating a fluid of geothermal origin and/or reflecting steady state dissolution. This is related to water-rock interaction processes with carbonate and evaporitic rocks, that affected the isotopic signature of δ18O, which is shifted to the right of the global meteoric water line. Spring I also represents a partially equilibrated and mature (deep) chloride type water. Spring II and III are less saline and represent a different fluid circulation and/or shorter residence time. The two latter springs are characterized by peripheral (shallow) dilute chloride-bicarbonate type waters. Decreasing key parameters especially in Spring III during the winter suggest that superficial mixing with rain and meteoric water results in high temporal variations. Cation and stable carbon isotope geothermometry applied to the studied springs reveals an average reservoir temperature of ~210, 110 and 90 °C for Spring I and II and III, respectively. The geochemical and isotopic data allowed to depict a conceptual model where the hydrothermal reservoir for Spring I is residing in carbonate and evaporitic rocks (most likely the Qom Formation) situated at a depth of 3–4 km whereas those of Spring II and III are likely sourced from a shallower depth (1–2 km) in correspondence with carbonate and porous and permeable volcanic rocks. Integration of hydrofacies with conceptual hydrological and geological models forms the base towards a proper understanding of water circulation patterns, increasingly important for sustainable water management and geothermal applications.