Hydrogeochemical characteristics of cold and warm (hot) springs in the Mahallat geothermal region, Iran

This study presents a hydrogeochemical evaluation of warm (hot) and cold springs in the Mahallat hydrothermal region of Iran. Sampling and hydrochemical analyses were conducted, and aquifer temperatures were estimated using various geothermometers. Results indicate that most of the warm (hot) waters in the study area fall within the steam-heated water field of Giggenbach, formed through the interaction of warm (hot) water with sulfate minerals and mixing with shallow groundwater. They are classified as Ca–SO 4 type, with higher sulfate concentrations compared with chloride and bicarbonate. Travertine layers, primarily composed of calcium carbonate, promote geochemical processes such as gypsum dissolution, increasing sulfate levels. In cold springs, water types are influenced by the leaching of Ca + , K + , Mg 2+ and Na + , with Na + concentration affected by Na-feldspar dissolution. Geothermometry revealed that in the Mahallat geothermal massif, amorphous silica controls solubility, with aquifer temperatures ranging between 95 and 100°C. The region exhibits high lithium concentrations in thermal waters, primarily sourced from lithium silicate minerals in granite and granodiorite, influenced by biotite chloritization and feldspar sericitization. Overall, Mahallat represents a developing geothermal system with significant geochemical interactions between groundwater and thermal resources.