LAUSR

Abstract The emerging temperature of thermal springs within the Mouil Graben in the Sabalan volcano, a primary prospect for a geothermal power plant in Iran, varies from about 24 to 80 °C. The investigation was carried out by analyzing hydrogeochemistry (i.e., main cations and anions, trace elements and heavy metals) and environmental isotopes (i.e., δ18O, δ2H, tritium, and δ18C) of water from cold and thermal springs, deep wells and the Khiavchay River over the period from 1979 to 2016. The results were also combined with those from a geo–structural survey. The findings reveal that the ascending geothermal fluids around the summit of the Sabalan volcano primarily move into the Mouil Graben fault system where the intersection of the caldera rim faults and the main oblique-slip faults occur at the lowest topographic elevation around the volcano as an emergent dome. The waters in the graben can be categorized into four clusters G1 to G4. The thermal springs which are categorized into groups G1 and G2 result from the contribution of geothermal waters and/or the condensation of the geothermal steam into near–surface waters. The spring waters of group G1 are characterized by being of the chloride water type with the highest values for temperature and EC. The waters of this group predominantly result from the contribution of geothermal water and are then more appropriate for geothermometry. The spring waters of group G2 are characterized by acidic–sulfate water type with lower values for temperature and EC. The waters of this group which predominantly arise from the condensation of geothermal steam, are mainly cold and enriched by H2S into near–surface waters or steam heated waters and are not appropriate for thermometry. Notably, δ18O of the G1 water samples has a significant positive shift while in the G2 water samples there is either no shift or a small positive shift. The cold spring waters with a low δ18O ratio (G3 and G4) originate from precipitation percolated into the surrounding highlands. Moreover, it can be concluded that the controlling silica phase in the Mouil reservoir is quartz and the average temperature calculated by the quartz geothermometer (maximum steam loss) for the water samples from the deep wells is about 225–230 °C which is near the range of values determined in the deep wells (230–242 °C). The temperatures calculated for the water samples from the deep wells by the cation geothermometers of Na–K (169–349 °C), Na–K–Ca 174 to higher than 300 °C), Na–K–Ca–Mg (268 to higher than 350 °C), Mg–Li (167–266 °C), and Na–Li (226–258 °C) are higher than the temperature range determined in the deep wells (230–242 °C). The average temperature calculated for the water samples from the springs of group G1 using all the silica geothermometers (