Genesis of multi–source mixed geothermal fluids in the eastern Tibetan Plateau

Geothermal activity is exceptionally intense in plate convergence zones. The formation of multi-source mixed geothermal fluids (MSGF) is strongly facilitated by the upwelling of high-temperature molten material and complex fault networks. However, the current understanding of the evolution and genesis of the MSGF remains limited. The eastern Tibetan Plateau (ETP) is characterized by intense tectonic activity associated with subduction and hosts abundant hydrothermal activity, making it a natural laboratory for studying MSGF. Utilizing newly collected hydrochemical (major ions and trace elements) and isotopic (δD and δ¹⁸O) data from 38 thermal springs, this study reveals the genesis of the MSGF under geodynamic constraints. The results indicate that the MSGF originate from snow-melt and meteoric waters (3162-5400 m), which infiltrate along faults to depths of 3.1-7.7 km and are stored in granitic reservoirs. During the upward circulation of the MSGF, water-rock interaction (contributing 51.4%-89.9%) dominates the hydrochemical evolution, involving silicate and carbonate dissolution as well as mixing with cold water. Magmatic fluid input (1.2%-29.7%) constitutes the secondary control, driving enrichment of conservative elements (Cl, Li, and B). Additionally, processes such as the dissolution of geothermal gases (CO₂ and H₂S), redox reactions (involving As), and ion exchange also influence the geochemical characteristics of the MSGF. Due to the combined effects of high-temperature magmatic fluid mixing and radiogenic heat from granites, the estimated initial temperature of the MSGF can reach 281 ℃. Notably, plate subduction drives a counterclockwise return flow of deep-sourced melts. The Bangong-Nujiang suture (BNS), which extends into the asthenosphere, is thereby rendered a principal conduit for magmatic fluid ascent. This mechanism not only leads to geothermal anomalies on both sides of the BNS but also triggers more frequent seismic activity. These findings provide crucial insights into the mechanisms underlying MSGF genesis during plate convergence.