Source determination and formation mechanisms of sulfur and thermal springs of Siah-Kuh anticline, south-west of Iran, using hydrogeochemistry and isotope characteristics

Dehloran and Doiraj Sulfur and thermal springs having different physicochemical characteristics are both discharged from the Siahkouh anticline located north of Dehloran city, southwest of Ilam province. Geological, hydrogeological, hydrogeochemical and isotopic studies were performed to determine the properties of the springs. Monitoring of discharge, temperature, pH, amount of hydrogen sulfide and dissolved oxygen in situ, and analysis of major ion chemistry in the laboratory were arranged for one year in a monthly intervals. Deuterium, oxygen 18, and sulfur 34 stable isotopes were measured in two dry and wet periods to determine the source of water. Hydrochemical composite diagrams were used to identify the dominant hydrochemical processes. Hydrochemical-based geothermometry and relationships between stable isotopes and comparing with local and global meteoric water lines were performed to determine the source of springs water. Finally, by using the findings of geology, hydrogeology and hydrogeochemistry, a conceptual model is presented for springs formation. Geological and hydrogeological studies showed that the deep faults in Siahkouh anticline have caused hydraulic connection between Ilam and Sarvak karstic formations with old and deep suited evaporite formation. The high temperature of the springs is due to the deep circulation of meteoric waters along fractures and faults. The origin of hydrogen sulfide (H2S) in the springs’ water is the activity of sulfate reducing bacteria. Despite the presence of brines in the oil and gas fields in the study area, the Na/Cl, SO4/Cl and TDS/Br ratios showed that the salinity of Doiraj and Dehloran sulfur springs is the dissolution of evaporite formations. Analysis of sulfur stable isotope (34S) also revealed the source of dissolved sulfate in spring water is evaporite formations. Analysis of Deuterium, oxygen 18 stable isotopes in the springs determined the origin of these springs is the local precipitation.