SULFATE IN BRINES IN THE CRYSTALLINE ROCKS OF THE CANADIAN SHIELD

Deep groundwaters in crystalline rocks typically are very saline and a re characterized by a rather unique Ca-Na-Cl-dominated chemistry. Sulf ate is present in variable amounts and may be linked to both the geoch emical evolution of these fluids as well as to recent processes initia ted through mining activities. It is possible to distinguish on the ba sis of isotopic compositions between brine sulfate and secondary sulfa te formed by oxidation of sulfides: The latter is characterized by del ta S-34 values which reflect the local mineral sulfide precursor and d elta O-18 close to or below 0 parts per thousand SMOW. The isotopic co mposition of the brine sulfate is characterized by delta O-18 and delt a S-34 values which resemble marine isotopic compositions at some loca lities, at others they could be explained as being of magmatic/hydroth ermal origin. It is likely that the sulfate participated in the geoche mical evolution of these brines. Thus, its isotopic composition reflec ts geochemical processes rather than a primary origin. No evidence for the influence of bacterial reduction was found.