Isotope hydrogeology and geothermometry of the Mount Meager geothermal area, final report, August 1980

A substantial 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, B.C., a Quaternary volcano currently the site of active exploration for geothermal resources. Isotope determinations include 18 0, 2 H and 3H in precipitation, runoff waters, thermal and cold groundwaters and glacier ice, 13c and 14c in dissolved inorganic carbon and 180 and 34s in dissolved sulphate from thermal and cold groundwaters as well as 13c and 180 in hydrothermal calcite crystals. Major ion analyses were performed on thermal and cold spring waters. Precipitation and cold spring data have been used to define the local meteoric waterline and to document the altitude effect on waters recharging the geothermal system, demonstrating there to be two hydrogeologically separate reservoirs which are recharged at different altitudes. Both pools of geothermal waters have experienced shifts of between +0.5 and +1.5 %0 in o1 8 0 values indicating a limited degree of 180 exchange with hot silicate minerals. Tritium contents indicate these waters to have recharged prior to 1955. Carbon-13 contents in dissolved inorganic carbon and in hydrothermal calcites from drill core demonstrate there to be contamination of the thermal wate; s with "dead" volcanogenic co2 plus carbon exchange with ... fracture calcite which precludes the possibility of dating the thermal waters using carbon-14. Several chemical and isotopic geothermometers are used to estimate the maximum temperatures experienced by the thermal waters. The fractionation of 180 between so 4 2- and H 2 o in these waters gives calculated maximum temperatures of less than 140°C. The dissolved sulphate is shown to comprise inputs from oxidized volcanogenic H2s and so2 gases and from dissolution of marine sulphate in the metasedimentary basement rocks. The Mg-corrected Na-K-Ca geothermometer shows excellent correlation with the so 4 -H2o estimates with maximum temperatures less than 120°c. Fractionation of 13c and 180 in the systems CaCO3-co2 and CaCO 3 -H 2 o using hydrothermal calcites and borehole fluids off er no indications of subsurface temperatures in excess of 140°C.Silica geothermometer results are not reliable due to equilibrium with amorphous silica phases in the subsurface. The thermal waters are shown to have mixed with up to 15% cold groundwaters with apparently no substantial effect on the geothermometer estimates. It is concluded that these thermal waters are not deeply circulating and have not experienced temperatures in excess of 140°C.