Geochemical characterization of thermal fluids from the Khangay area, central Mongolia

Abstract

In the current study, chemical analyses of water samples from 32 hot springs located in the Khangay area in Central Mongolia were used for interpretation. The Cl-SO4-HCO3 and Na-K-Mg ternary diagrams, silica and cation geothermometers, mixing models, saturation index plots, and the relationship between Cl and B and stable isotopes were used to characterize the waters and estimate the subsurface temperature. The results show that most of the waters are in partial equilibrium with the rock. The hot waters in the study area can be classified as bicarbonate-sodium type and sulphate-sodium type waters. Values for the chalcedony geothermometer and the Na/K geothermometer values are quite different. According to the relationship between Cl and B, all the waters indicated have low B/Cl ratios, suggesting that they originate from old hydrothermal systems. The δ18O and δD (δ2H) are used to trace and determine the origin and movement of groundwater. Thermal waters have more negative δ2H values than cold waters. All the waters are close to the local meteoric line, parallel to the world meteoric line, indicating that the hot and cold waters are of local meteoric origin, with no apparent δ18O shift. This may indicate that hydrothermal alteration reactions have long ceased occurring, that aquifer temperatures are not high, or that good permeability exists in the reservoir. The silica-enthalpy mixing model predicts a subsurface reservoir temperature between 143 and 245°C and most of the hot waters have probably mixed with cold water.