Strontium isotopic composition of some brines from the Precambrian Shield of Canada.

Groundwater samples (24) from seven operating mines at Sudbury, Yellowknife and Thompson (Ontario, North West Territories and Manitoba, respectively), all from depths >1 km and ranging in total dissolved solids 1900-250 000 mg/l, were measured for their ⁸⁷Sr/⁸⁶Sr values. Each geographic location gives a limited range in values and each location is distinct from the others. This is interpreted as the result of extensive water-rock interaction on a local scale. For most of the time, these brines were isolated and were only recently exposed to surface water as a result of mining operations. The extent of the isolation is shown by the contrasting isotopic values of two 'pockets' of water (0.711 vs 0.716) located on opposite sides of the same fault system on the north range at Sudbury. The exchange at all sites has probably continued until the present day as indicated by the close agreement between water and present day ⁸⁷Sr/⁸⁶Sr whole-rock values. If so, it is suggested that there is no single age for such brines, but it may be possible to date stages in the water's evolution by determining the age of secondary minerals that equilibrated with the water. (Authors' abstract)-J.M.H.