Disused mines offer a promising opportunity for sustainable underground thermal energy storage (MTES), repurposing legacy infrastructure to balance seasonal heat supply and demand. As part of the BMFTR-funded MineATES project, we implemented a test-scale MTES system in a controlled experimental drift section of the historic Reiche Zeche silver mine (Freiberg, Germany). Three thermal cycles, with peak water temperatures up to 39 °C and cooling down to 2 °C, were conducted in a 20 m³ basin embedded in grey gneiss at 147 m depth. Alongside detailed thermal monitoring, our focus was on hydrogeochemical responses of mine water and their engineering implications. Tracer dilution tests confirmed continuous groundwater inflow, contributing to advective heat loss but also influencing geochemical conditions. Heating triggered pronounced precipitation of minerals such as of iron oxides, accompanied by pH increase and enrichment of major ions (e.g., Ca²⁺, SO₄²⁻). Laboratory batch and column experiments with Freiberg gneiss and mine water supported in-situ observations, showing accelerated metal removal at elevated temperatures and leaching of specific elements from steel components. Fouling tests on stainless-steel heat-exchanger plates revealed up to 45 % thermal performance loss from scaling and biofilm growth, which was reduced by about 60 % using protective coatings. These results highlight that MTES in flooded mine environments must account for complex mine water–rock–infrastructure interactions, where hydrogeochemistry directly affects efficiency, maintenance, and operational stability.