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 inNW Gorveh (western Iran) and interpret them in the light of their geological setting. The results of laboratorymeasurements of elemental concentrations, stable oxygen, carbon and hydrogen isotopes, dissolved and particulateorganic and inorganic carbon (DIC, DOC, POC, PIC) and alkalinity are combined with in situ measurementsof pH and temperature. Parameters such as alkalinity, DIC, Ca2+ and pCO2 concentration display strong, positivelycorrelated values, with systematic decrease from the spring vent in down-flow direction for the three spring systems.The inverse correlation of pH and delta13CDIC is caused by CO2 degassing. The delta18O and deltaD values show no significantvariation, related to minor or no evaporation due to normal ambient temperatures. The lowconcentration of POC, PIC and DOC compared to that of DIC and the lack of correlation between themreflect predominantinorganic carbon in these fluids.Spring I is oversaturated in calcite with additional dissolution of CO2, and despite high concentrations of Na+ andCl-, undersaturated in halite, indicating a fluid of geothermal origin and/or reflecting steady state dissolution.This is related to water-rock interaction processeswith carbonate and evaporitic rocks, that affected the isotopicsignature of delta18O,which is shifted to the right of the global meteoric water line. Spring I also represents a partiallyequilibrated and mature (deep) chloride typewater. Spring II and III are less saline and represent a different fluidcirculation and/or shorter residence time. The two latter springs are characterized by peripheral (shallow) dilutechloride-bicarbonate type waters. Decreasing key parameters especially in Spring III during the winter suggestthat superficial mixing with rain and meteoric water results in high temporal variations. Cation and stable carbonisotope geothermometry applied to the studied springs reveals an average reservoir temperature of ~210, 110and 90 °C for Spring I and II and III, respectively. The geochemical and isotopic data allowed to depict a conceptualmodelwhere the hydrothermal reservoir for Spring I is residing in carbonate and evaporitic rocks (most likely theQom Formation) situated at a depth of 3-4 km whereas those of Spring II and III are likely sourced from ashallower 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 properunderstanding of water circulation patterns, increasingly important for sustainable water management and geothermalapplications.