The isotope geochemistry of fracture calcites from the Chalk River area, Ontario, Canada

Abstract

Calcite is a common fracture inflilling mineral in the Grenville gneisses of the Chalk River area, Ontario, Canada. It exhibits a variety of occurrences and textures which suggests calcite has precipitated under different hydrogeochemical conditions that may be identified through a detailed chemical and isotopic investigation of the calcite and associated infilling minerals. The δ 18O of these calcites range over 20%. but the δ 13C varies over a narrow range of 5%.. None of the calcites analyzed is in isotopic equilibrium with both the δ 18O and δ 13C of the present day ground water. The lightest δ 18O calcites (near 0%. SMOW) are present in sealed fractures and are sometimes associated with laumontite. This suggests that these light calcites formed from hydrothermal solutions (at temperatures less than about 300°C) shortly after the period of metamorphism that formed the gneisses. This interpretation is supported by relatively unradiogenic 87Sr/ 86Sr ratios near 0.709 and δ 13C values of −5 to −6%.. Most of the Chalk River calcites, however, are considerably heavier in 18O and lighter in 13C than the hydrothermal end member. This may be the result of low temperature recrystallization of the hydrothermal calcites by meteoric waters under variable water/rock ratios. The 13C contents and 87Sr/ 86Sr ratios of these younger, low temperature calcites appear to be partially buffered by the isotopic composition of the original hydrothermal calcite. Pyrite is often associated with the fracture calcites. These pyrites display a wide range in δ 34S values of about 70%., which suggests that sulphide precipitation occurred under semi-closed conditions. These data indicate that fracture permeability has been a major control on the isotopic composition of fracture minerals since formation of the gneiss.