GROUNDWATER SR-87/SR-86 VALUES IN THE EYE DASHWA LAKES PLUTON, CANADA - EVIDENCE FOR PLAGIOCLASE WATER REACTION

The Eye-Dashwa Lakes pluton is a Sr-rich felsic intrusive of the sanukitoid suite located in NW Ontario in the Superior Province of the Canadian Shield. The Sr-87/Sr-86 values of the Ca-Na-Cl-type saline, deeper groundwaters fall in the narrow range of 0.7057-0.7070, similar to the Sr-87/Sr-86 value in plagioclase but not to that in K-feldspar, biotite, or the whole rock. This suggests that the water isotopic composition is the result of plagioclase-water interaction. The fracture minerals gypsum and calcite have similar Sr-87/Sr-86 ratios, indicating that they have crystallized from waters similar to the present deep groundwaters. The shallow, Ca-HCO3-type, dilute groundwaters show a wider spread in the Sr ratio of 0.7065-0.7278, reflecting their derivation from minerals in the overlying soil. The results for the saline groundwaters are consistent with thermodynamic and kinetic models that demonstrate the relatively rapid dissolution of plagioclase with respect to other rock-forming silicates, and should have application to most brines located in silicate host rocks.