From Deep Metamorphism to Cold Spring Emergence: Integrated Isotopic and Geochemical Tracing of Mixing Dynamics in a Low‐Temperature Geothermal System

ABSTRACT The total dissolved solids (TDS) concentration of groundwater commonly increases with temperature because higher temperatures enhance mineral solubility and reaction kinetics. However, the Bingling Mountain artesian mineral springs on the northeastern Tibetan Plateau display an anomalous combination of low discharge temperature and high salinity. This study aimed to elucidate the origin of this hydrochemical anomaly and the processes controlling the formation of the HCO 3 ·SO 4 ‐Na·Ca groundwater. Isotopic (δD and δ 18 O) composition indicates that the spring recharge is derived mainly from local precipitation and cryospheric meltwater, while oxygen isotope exchange suggests interaction with deep‐seated CO 2 . Multimineral equilibrium geothermometry reveals the presence of an active hydrothermal reservoir at approximately 121°C (112°C–140°C), with fluid circulation depths estimated at about 4400 m. In this deep reservoir, CO 2 generated by metamorphic decarbonation promotes silicate weathering and carbonate dissolution, producing high‐TDS geothermal fluids. The low‐temperature yet high‐salinity character of the discharged springs is explained by substantial subsurface mixing, whereby deep geothermal fluids are diluted by 68%–79% with cold shallow groundwater along fault‐controlled pathways. This mixing suppresses the thermal signature while preserving much of the inherited salinity. These findings demonstrate a decoupling mechanism between groundwater temperature and salinity and provide a revised conceptual model for the genesis of anomalously cold, saline groundwater. The study highlights the combined roles of deep geothermal processes, CO 2 ‐enhanced water‐rock interaction, and fault‐mediated mixing, offering broader insight into hydrogeochemical evolution in similar high‐altitude, tectonically active geothermal regions.