SALINE GROUNDWATERS AND BRINES IN THE CANADIAN SHIELD - GEOCHEMICAL AND ISOTOPIC EVIDENCE FOR A RESIDUAL EVAPORITE BRINE COMPONENT

Saline Ca-Na/Cl type groundwaters and brines sampled in deep mines over an extensive area of the Canadian Precambrian Shield have elevated Br/Cl ratios which may indicate that the chlorinity of these waters was derived from the infiltration of residual evaporitic brines, remnants of the great marine incursions of the Paleozoic era. Boron concentrations in these waters are generally low (i.e.. < approximately 2 mg/L) relative to seawater or Alberta Basin Devonian formation waters. However, the B-11/B-10 ratios of these waters are significantly greater than the average value for continental crustal rocks with the highest values (approximately 4.19) approaching that of present-day seawater (4.20). Moreover, the boron isotopic ratios generally trend to higher values with increasing chlorinity which supports the conclusion from the Br-Cl relationship that most of the chloride in shield brines is of marine origin, rather than a product of water/rock interactions. If this is correct, crystalline rocks must then be sufficiently permeable on a regional scale to have allowed the brine to infiltrate to depths of several kilometers where it now resides.