Thermal waters in the central part of Baikal Rift Zone: Hydrogeochemistry and geothermometry (Republic of Buryatia, Russia)

Abstract

The thermal waters of the Central Baikal Rift Zone are considered in this manuscript. The measured temperatures ranged from 19.8 to 75.6 °C. It was obtained that the studied waters are fresh (TDS 273–941 mg/L), generally weak alkaline and alkaline, enriched with Na⁺ (77–263 mg/L), SO₄^2- (70–489 mg/L), F^- (3.4–19.9 mg/L) and SiO₂ (54–118 mg/L) and depleted in Ca²⁺ (4.65–44.91) and Mg²⁺ (0.01–0.93 mg/L). The thermal springs mainly belong to SO₄-Na and HCO₃-SO₄-Na chemical types. SO₄-Na chemical type was characterized by dominant N₂ (89.61–92.20 vol. %) in gas composition, HCO₃-SO₄-Na type – with N₂ (10.72–57.23 vol. %) and CH₄ (37.76–71.43 vol. %) predominant gases. The two groups of springs were selected in accordance with TDS, SO₄^2-, HCO₃^-+CO₃^2-, Ca²⁺ and Na⁺. Likewise, the geochemical features of the thermal waters were identified. It was shown that more alkaline waters have lower pH; TDS forms due to SO₄²- and Na⁺ accumulation; Cl^- and HCO₃^-+CO₃^2- increased simultaneously, unlike SO₄^2-. Furthermore, F^- and SiO₂ also increased with pH and temperature, along with a decrease in Ca²⁺. It was assumed that the thermal waters enriched or depleted in chemical elements depending on the water-rock interaction processes (minerals dissolution/precipitation) and the duration of the water residence in the host rocks. This was illustrated by Cl^-/(Cl^- + HCO₃^-) diagram and calculation of ions ratios Na/Cl, HCO₃/Cl, Ca/Na, Ca/Mg, Ca/Cl, K/Cl and Mg/Cl. In addition, the reservoir temperatures were estimated using various geothermometers and geoindicators, including the Si-enthalpy diagram. The silica-enthalpy mixing model indicated mixing process only for 7 thermal springs. The analysis of estimated reservoir temperatures using various methods showed that the Si- and K/Mg- geothermometer are more reliable for the studying waters.