Hydrogeochemical Characteristics of Spring Water in the Babaozhen Fault Zone, Qilian Mountains

【Objective】The northeastern margin of the Qinghai-Tibet Plateau is characterized by well-developed fault structures, and fault intersection zones are favorable sites for geothermal system formation. However, the sources, evolution mechanisms, and extent of cold water mixing of geothermal fluids in these areas remain unclear. 【Methods】Taking the fault intersection zone in Babaozhen, Qilian County as the research object, this study quantitatively investigated the hydrogeochemical characteristics and cold water mixing ratios of spring water in the fault zone through hydrochemical analysis, hydrogen and oxygen isotope tracing, and silica-enthalpy mixing model. 【Results】The results indicate that deep geothermal water is of Cl-Na·Ca type, while spring water evolves to Cl-Ca·Mg type. The hydrogen and oxygen isotopic characteristics are generally consistent with those of the Qilian Mountains area, with an average deuterium excess of 24.3‰. Combined with the average recharge elevation being 375 m higher than the outlet elevation, this indicates that the recharge source is high-altitude atmospheric precipitation and snowmelt water, and the fluids have experienced medium-deep circulation. For the first time, this study quantitatively reveals that the cold water mixing ratio of spring water in the fault intersection zone reaches as high as 89.1%-93.8%, with a reservoir temperature of approximately 113°C. The intense mixing is the fundamental reason for the low spring water temperatures (7-13°C). 【Conclusion】Based on these findings, a genetic model of "medium-temperature deep circulation, fault-controlled upwelling, and intense shallow mixing" is proposed: high-altitude precipitation infiltrates along the fault system, is heated by regional terrestrial heat flow during medium-deep circulation, then ascends along the fault zone, undergoes intense mixing with shallow cold water within the fault zone, and eventually emerges as low-temperature spring water at the surface. This study reveals the key controlling role of fault intersection structures in the "deep circulation-shallow mixing" process of geothermal systems, providing theoretical reference for geothermal resource exploration in similar tectonic zones on the northeastern margin of the Qinghai-Tibet Plateau.