CHEMICAL WEATHERING OF SMECTITIC SULPHIDE-MINERAL-BEARING UNCONSOLIDATED SURFICIAL SEDIMENTS IN SOUTH-CENTRAL ALBERTA, CANADA

As part of the Alberta Geological Survey and Alberta Energy and Sustainable Resources Edmonton-Calgary Corridor groundwater investigation program, I used a mass-balance simulation approach; in conjunction with geochemical, mineralogical, thermodynamic and isotopic constraints; to assess the origins of Na-SO₄(±HCO₃) type groundwater and springwater associated with smectitic sulphide-mineral-bearing unconsolidated surficial sediments and the underlying Paskapoo Formation in south-central Alberta. Results indicate that alteration of albite to kaolinite and alteration of kaolinite to Na-smectite are the primary controls on dissolved Na and SiO₂ concentrations in groundwater and springwater. Concentrations of dissolved Ca and Mg are controlled by reactions involving carbonate minerals and possibly cation exchange. Dissolved SO₄ is generated primarily through oxidation of pyrite. Most H⁺ generated by oxidation of pyrite is consumed in aluminosilicate alteration reactions. The carbon isotopic composition of CO₂ gas required in mass-balance simulations suggests the presence of an isotopically heterogeneous environment with respect to ¹³C. This apparent isotopic heterogeneity may result from the presence of varying fractions of atmospheric and microbially-respired CO₂.