Heating mechanism of the Abgarm–Avaj geothermal system observed with hydrochemistry, geothermometry, and stable isotopes of thermal spring waters, Iran

In this study, the Abgarm–Avaj geothermal system in Iran is investigated by analyzing hydrochemistry and stable environmental isotopes of water samples collected from cold and thermal water springs and the Khare-Rud River together with tectonic settings. The findings reveal that the geothermal system is associated with the deep fault zone of Hasanabad and can be categorized into a convection dominated and non-magmatic geothermal system in accordance with the catalog of geothermal play types presented by Moeck (Renew Sustain Energy Rev 37:867–882, 2014). In fact, local rainfall that is occurred over the Kuhe-Bozorg limestone highlands percolates into a high depth along the main active fault of Hasanabad and then heats and emerges finally at the lowest topographic elevation of the fault in the form of thermal springs characterized by temperature ranges from 30 to 52 °C. The water samples from the thermal springs are of a high electrical conductivity value (ranges from 6585 to 11265 µS/cm) with the chloride water type. The lower circulation depth of meteoric water in the geothermal system is estimated to be about 3000 m by considering the possible maximum geothermal gradient of about 46 °C/km. The stable isotopes ratios analysis suggests that thermal water originates predominantly from rainfall occurring over the higher elevations, since the oxygen-18 ratios of the thermal spring waters are depleted than that of the cold spring waters. The equilibrium temperatures of the geothermal system are estimated via using the Na–K (Truesdell, Summary of section III: geochemical techniques in exploration. In: Proceedings of the 2nd U.N. symposium on the development and use of geothermal resources, vol 1. U.S. Government Printing Office, Washington, DC, pp liii–lxxx, 1976) and Na–K (Tonani, Some remarks on the application of geochemical techniques in geothermal exploration. In: Proceedings of advances in European geothermal research, 2nd symposium, Strasbourg, pp 428–443, 1980) geothermometers are 142–148 and 146–153 °C, respectively, which fall within the temperature range suggested by the mineral saturation indices (137–160 °C) and by the warm spring mixing model (135–164 °C) for the thermal spring waters. Furthermore, the results show that geothermal hot water mixes predominantly with shallow cold groundwater during ascending, where the portion of the cold shallow and deep-hot waters is about 70 and 30%, respectively.