Geothermal energy has a wide range of uses in houses, farming, industry, infrastructure, and electricity generation. It is very important to characterize the temperature and the depth of geothermal reservoirs. The aim of this paper is the determination of type, origin source of water temperature and depth of circulation of water in the Ayub- Peighambar and Shafa (AP and SH) hot spring reservoir, located in NE-Iran, using geothermometers and environmental 2H and 18O isotopes. AP hot spring has elevated temperature (36-40 oC) and as such is very important for balneotherapy and tourism industry purposes. The average values of δ18O and δ2H for the AP hot spring (-10‰ and -73‰, respectively) are analogous to that of geothermal and meteoric waters. This indicates that the heat source cannot be related to volcanic activities (with average δ18O value of about 5‰) and it is most probably associated with geothermal gradient with deep circulation of groundwater through faults. Importantly, this is also confirmed by the lack of volcanic activities and intrusive rocks in this region. Based on cation geothermometers (Na-K) coupled with isotopic (18O and 2H) geochemistry the temperature of the AP geothermal reservoir was estimated to be in the range of 100-150 oC with 3-5 and 4.2 kilometers depth, respectively. Chemically, the AP hot springs samples are a CaSO4 facies with a chemically homogeneous source and steam heated waters type.

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