Sulfur-isotope geochemistry of fracture-filling gypsum in an Archean granite near Atikokan, Ontario, Canada

Abstract

Gypsum occurs as fracture-filling material in the Eye—Dashwa lakes pluton near Atikokan, Ontario. Sulfur-isotope analyses and mode of occurrence indicate that the gypsum is probably of Precambrian age, even though associated calcite veins indicate a minimum age of 40,000 yr. Petrographic analysis suggests that the gypsum was formed by the interaction of porewater with epidote, plagioclase and pyrite. Gypsum occurs in fractures at depths > 500 m, where it has remained in stable equilibrium since its formation. The significance of its occurrence is examined with reference to implications for the disposal of nuclear-fuel waste in crystalline rocks. Heat generated by nuclear-fuel waste in a storage vault could cause dissolution of gypsum-filling material and reopen the fractures. However, the impermeable nature of the gypsum-filled fractures and their stable equilibrium at depths > 500 m may provide a method of sealing open fractures.