The surface manifestations of geothermal resources such as thermal springs, steam grounds, fumaroles, mud pools, and geysers in a region along with their locality, geologic control, physical environment, temperature, chemistry, and the rate of fluid discharge are the most important features that should be taken into account for geothermal explorations.
The preliminary activities in the context of geothermal systems in Iran go back to 1975 when an Italian company (ENEL) carried out comprehensive investigations in geothermal fields in this country. Since then, numerous studies concerning geothermal explorations were done in Iran, and many prospect areas were identified. One of these areas considered to be of prime importance is around the volcanic mount Sabalan in Azerbaijan Block, NW of Iran.
In present study, the authors endeavored to assess and discuss the geological characteristics of the geothermal fields around Sabalan volcano with placing special emphasis on geochemical aspects involved in fluids of this field.
Method of investigation
Among several cold and hot spring waters in the study area, 48 samples from high fluid flow springs were collected. For sampling of hot springs, the priority was given to those of maximum discharge rate and the highest discharge temperature at the surface.
The chemical analyses of these samples for determination of major and trace elements as well as stable isotopes were carried out in hydrogeochemistry lab at Bremen University (Germany).
The stable isotope analyses (δD and δ18O) were carried out by using LGR DLT-100 Laser Spectrometer (Los Gatos Research). The analytical precisions for δ18O and δD were ±0.2‰ and ±1‰, respectively.
Results and Discussion
In the study area, the hot water samples depict temperature and pH range of 9-89ºC and 4.5-8.58, respectively. The total dissolved solids (TDS) vary from 249 to 7006 mg/l.
The results obtained from isotopic analyses of 48 cold and hot spring water samples from Sabalan geothermal system were carried out for determination of δ18O and δD, and 3H of these waters (see Table 1). The δ18O values of these samples vary from -13.4‰ to -7.5‰. The δD values range from -77.6‰ to -71.1‰. 3H values vary from 0.1 to 105 TU.
- By noting to the considerable abundance of Cl-, Na+, Ca2+, and HCO3-, the geothermal fluids in this area were chemically categorized as both Na-Cl and Ca-Na-HCO3 systems. The high concentration values of Cl-, Na+, and Ca2+ in this area stems from water-rock chemical reactions, and high HCO3- value may be due either to solution of ascending CO2 gas in underground waters or to dissolution of basement crystalline limestone.
- Consideration of isotopic compositions of Oxygen (δ18O) and hydrogen (δD) show that the analyzed samples were slightly enriched in δ18O which may be related to the water-rock isotopic exchange reactions. The water-rock interactions caused slight increase in δD as well.
The measurement of 3H in the Sabalan geothermal fluids indicated that the circulating geothermal fluids can be temporally divided in three groups. The first group has 3H values >10 TU and belongs to modern waters. The second group so called sub-modern waters has 3H values within the range of 1-10 TU. The third group which has 3H values <1 TU is regarded to be as old waters.