Hydrogeochemical characteristics of hot springs in the Daju Fault Zone of the SE Tibetan Plateau, China

Tests on 31 water samples from 10 hot springs within the Daju Fault Zone were conducted to evaluate thermal hydrogeochemical processes in terms of reservoir temperature and depth, circulation length, and relationships with seismic activity. Major elements, trace elements, hydrogen and oxygen isotopes as well as strontium isotopes were analyzed. Results show that the hot spring waters could be sub-divided into nine distinct hydrogeochemical types, receiving recharge primarily from meteoric water and snow-mountain meltwater at elevations ranging between 2262.4 and 4122.1 m above sea level (a.s.l.). Additionally, these springs display a wide range of cold-water mixing ratios (ranging from 60 % to 96 %), reservoir temperatures (ranging between 54.4 and 132.3 °C), as well as circulation depths spanning across a range of values (from 1950.8 to 5337.1 m). Ca²⁺ /Na⁺, Mg²⁺ /Na⁺ and HCO₃^–/Na⁺ ratios suggest that hydrogeochemical processes governing geothermal water composition are controlled by interactions between CO₂-enriched fluids and surrounding lithologies, along with cation exchange. The relationship between Cl⁻ and K⁺, B, Li⁺ indicates their distinct origins from disparate deep fluids. Other elements in hot spring waters of this region are generally originated from rock weathering, and they vary significantly with different geological conditions. Through years of continuous sampling analysis, it has been observed that TDS, HCO₃^–, and (Ca²⁺+Mg²⁺) in hot springs HTX, TSQ, and XG1 exhibit significant variations before and after earthquakes, rendering them highly suitable for investigating the correlation between seismic activity and hydrogeochemistry, whereas CQG excels as the premier option for observing temperature variations caused by seismic activities.