A survey of stable and radioactive environmental isotopes has been carried out in order to investigate the recharge, thermal history, age, and geothermometry of the thermal waters at Mount Meager, British Columbia, a Quaternary volcano that is currently the site of active exploration for geothermal resources. Isotope determinations include 18O, 2H, and 3H in precipitation, thermal and cold groundwaters, and glacier ice; 13C and 14C in dissolved inorganic carbon; 18O and 34S in dissolved sulphate from thermal and cold groundwaters; and 13C and 18O in hydrothermal calcite crystals. Major ion analyses were performed on thermal and cold spring waters.Precipitation data are used to define the local meteoric water line and to document the altitude effect on waters recharging the geothermal system, demonstrating that there are two hydrogeologically separate reservoirs recharged at different altitudes. Both pools of geothermal waters have experienced shifts of between +0.5 and +2.5‰ in δ18O values, indicating a limited degree of 18O exchange with hot silicate minerals.Tritium contents indicate that these waters recharged prior to 1955. 13C contents of dissolved inorganic carbon and hydrothermal calcites from drill core document contamination of the thermal waters with "dead" volcanogenic CO2 plus carbon exchange with fracture calcite, which precludes the possibility of "dating" the thermal waters using 14C.Several chemical and isotopic geothermometers are used to estimate the maximum temperatures experienced by the thermal waters. The fractionation of 18O between SO42− and H2O in these waters gives calculated maximum temperatures of less than 140 °C. The Mg-corrected Na–K–Ca geothermometer shows excellent correlation with the SO4–H2O estimates with maximum temperatures of less than 140 °C. Fractionation of 13C and 18O in the systems CaCO3–CO2 and CaCO3–H2O using hydrothermal calcites and borehole fluids also offers no indications of subsurface temperatures in excess of 140 °C. Silica geothermometer results are not reliable because of equlibrium with amorphous silica phases in the subsurface.It is concluded that these thermal waters are not deeply circulating and have not experienced temperatures in excess of 140 °C.

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