SULFATE IN BRINES IN THE CRYSTALLINE ROCKS OF THE CANADIAN SHIELD

Deep groundwaters in crystalline rocks typically are very saline and are characterized by a rather unique Ca-Na-Cl-dominated chemistry. Sulfate is present in variable amounts and may be linked to both the geochemical evolution of these fluids as well as to recent processes initiated through mining activities. It is possible to distinguish on the basis of isotopic compositions between brine sulfate and secondary sulfate formed by oxidation of sulfides: The latter is characterized by delta S-34 values which reflect the local mineral sulfide precursor and delta O-18 close to or below 0 parts per thousand SMOW. The isotopic composition of the brine sulfate is characterized by delta O-18 and delta S-34 values which resemble marine isotopic compositions at some localities, at others they could be explained as being of magmatic/hydrothermal origin. It is likely that the sulfate participated in the geochemical evolution of these brines. Thus, its isotopic composition reflects geochemical processes rather than a primary origin. No evidence for the influence of bacterial reduction was found.