<p>In geothermal mountain settings, water-rock interactions critically shape vital water resources. The aim of this study was to characterize water composition, identify the dominant geochemical processes, and constrain mixing relationships between meteoric and geothermal fluids in the Domuyo Geothermal System (Patagonia, Argentina). To achieve this, we present the first integrated hydrogeochemical and mineralogical evaluation of the system, combining water chemistry with the mineralogy of host rocks and hydrothermal precipitates to resolve the processes controlling water composition in this understudied Andean geothermal setting. Water samples from streams, thermal springs, and wet meadows were analyzed together with igneous host rocks and hydrothermal precipitates. Results reveal a marked contrast: upstream waters are cold Ca/Mg-HCO₃ type with low electrical conductivity (~ 366 µS/cm), whereas thermal springs are Na-Cl type, characterized by high electrical conductivity (~ 6,106 µS/cm) and elevated silica (~ 226&#xa0;mg/L) and SO₄²⁻ (~ 73&#xa0;mg/L) concentrations, indicating deep, high-temperature water-rock interaction and magmatic gas input. Downstream samples show intermediate compositions, indicating variable degrees of mixing processes (electrical conductivity ~ 2,538 µS/cm). Mineralogical analyses document extensive hydrothermal alteration of host rocks (clays, zeolites, iron oxides), while carbonate travertines record CO₂-driven precipitation processes. The integration of hydrochemical data with alteration and secondary mineral assemblages allows reconstruction of water types and mixing proportions and clarifies the geochemical controls that govern fluid evolution in the system. These findings demonstrate the direct impact of geothermal processes on surface water and provide a critical baseline for sustainable water resources management in vulnerable regions where meteoric and geothermal systems interact.</p> Graphical Abstract <p></p>