Fluid Geochemistry at the Nir Geothermal Field, NW-Iran

Abstract

The most southerly of the Mt. Sabalan thermal features is the thermal springs at the Nir area. This area hosts many thermal springs include Boshli, Ilanjigh, Doshanjigh, Saqezchi and Qarashiran. The temperatures of these springs range from 13°C to 77 °C. Thermal waters have a high chloride concentration with mixed chemistry of HCO3 and SO4. The values of the Mg concentrations in these waters are also high. This is suggestive that the Nir springs are the product of deep, low Mg-high Cl geothermal waters being cooled and quenched through mixing with groundwater associated with limestone and calcareous sediments. Such karst type waters contain much magnesium and sulphate through dissolution of dolomite and sulphates far beyond their normal solubility. Thermal waters plot as immature waters on the Na-K-Mg triangular diagram, except 4 samples that they plot at the boundary between immature and partially equilibrated waters. These waters occur in an area where Quaternary intrusive domes are mapped and may be more closely associated with these than Mt. Sabalan. Estimation of subsurface temperature with quartz conductive predicts temperature in the range of 115-161°C. The Nir springs have a δ18O shift from meteoric trend line and define a mixing line between meteoric water precipitated and the andesitic water field. Thermal waters have a range of - 4.2 to - 3.0 ‰ which may be these values due to the CO2 being derived from limestones. Geothermal waters have low absorption of B/Cl steam indicating that the fluid migrates from the old hydrothermal system.