87Sr/86Sr values in groundwaters of the East Bull Lake pluton, Superior Province, Ontario, Canada

The East Bull Lake pluton is a 2.48 Ga-old, layered mafic intrusion located on the southern margin of the Superior Province, Precambrian Shield in Ontario, Canada. The groundwaters have been studied as part of a detailed geological mapping and geochemical study of the pluton for the Canadian Nuclear Fuel Waste Management Program.All groundwater samples are dilute—the most concentrated NaCl type water contains 3.1 g/l total dissolved solids. Present day87Sr/86Sr values of the rock samples varies from 0.705 to 0.714 and the groundwaters from 0.711 to 0.720. The CaHCO3 recharge waters show the greatest range in values, in contrast to the deeper NaHCO3 and deep NaCl waters where a narrow range of 0.712–0.713 is found. Fracture-filling laumontite, calcite and gypsum have the same isotopic signature as the deep water.The CaHCO3 water is the youngest component of the groundwater system and its spread in isotopic values reflects local conditions at the surface. The NaCl water is considered to be the oldest component and its Sr isotopic ratio value indicates equilibration with silicate fracture mineral phases. The isotopic data suggest that laumontite, calcite, and gypsum fracture phases have either crystallized from this water or are currently controlling its isotopic composition by dissolution. No age can be assigned to the deep waters, except that the NaCl component may date from the Early Paleozoic, based on geological arguments or, more likely, be 104–106 years old based on kinetic studies of rock-water interaction and other isotopic data.If seawater was one component of the NaCl water history, there is no record of this in the87Sr/86Sr ratios.