The Carnmenellis granite and its aureole contain the only recorded thermal groundwaters (up to 52 °C) in British granites. They occur as springs in tin mines at depths between 200 and 700 m and most are saline (maximum mineralization 19 310 mg 1⁻¹). Mining activity has disturbed the groundwater circulation pattern developed over a geological time-scale and levels of bomb-produced tritium (> 4 TU) indicate that a significant component (up to 65 %) of the most saline waters are of recent origin. All components of all the mine waters are of meteoric origin. Radiogenic ⁴He contents, ⁴⁰Ar/³⁶Ar ratios, and uranium series geochemistry suggest that the thermal component has a likely residence time of at least 5 × 10⁴ years and probably of order 10⁶ years.

The thermal waters have molar Na⁺/Cl⁻ ratios considerably less than 1 but they are enriched relative to sea water in all major cations except Mg. The groundwater is also particularly enriched in Li with contents ranging up to 125 mg 1⁻¹. The groundwater salinity, which may reach a maximum of 30 000 mg 1⁻¹, is shown to result from weathering reactions of biotite (probably through a chloritization step) and plagioclase feldspar, to kaolinite. On volumetric considerations, fluid inclusions cannot contribute significantly to the groundwater salinity, and stable isotope ratios rule out any contribution from sea water.

Groundwater silica contents and molar Na⁺/K⁺ ratios suggest that the likely equilibration temperature is 54°C, which would imply a depth of circulation of about 1.2 km.