The growing need to utilise geothermal resources for power generation has intensified the exploration of hotspot volcanic islands in recent decades. Thermal springs represent valuable natural laboratories for applying geothermometry to infer reservoir temperatures. Yet, secondary hydrochemical processes during fluid ascent, such as mixing or CO₂ exchange, often limit the applicability of geothermometry and must be addressed. On La Palma (Canary Islands), the Fuente Santa thermal ponds provide a unique discharge in the archipelago for testing these approaches. Geothermometric calculations for Fuente Santa were carried out using classical chemical geothermometers and multicomponent solute geothermometry simulations with PHREEQC. Simulations evaluated the impact of key hydrochemical processes in the system: (i) mixing with seawater and freshwater, (ii) CO2 loss, (iii) mineral re-equilibration, and (iv) steam loss. The multicomponent modeling, which reconstructed the absolute thermal end-member by sensitivity analysis of saturation index convergence and extrapolation of the mixing path, yielded reservoir temperatures of 158–172 °C. The likely equilibrium mineral assemblage included quartz, mordenite, kaolinite, natrolite, and wairakite. This temperature range was narrower and more reliable than those inferred from silica and Na–K geothermometers (128–160 °C), underscoring the importance of accounting for hydrochemical alterations. The study highlights that reframing the ternary mixing problem into a simplified binary mixing, coupled with systematic sensitivity analysis of CO2 and steam loss and secondary mineral equilibration, provides a more robust framework for multicomponent solute geothermometry. Such an integrated approach aims to enhance the accuracy of reservoir temperature estimates in complex geothermal systems in volcanic islands.
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