Radionuclide migration over recent geologic time in a granitic pluton

Measurements of isotope activity ratios in the 238U decay series have been made for core samples obtained from boreholes up to 1-km depth in the Eye-Dashwa Lakes granite pluton, Atikokan, northwest Ontario, Canada. This work was done to determine the significance of recent radionuclide migration in plutonic rocks for the Canadian Nuclear Fuel Waste Management Programme. Most samples displayed secular equilibrium between 238U, 234U, 230Th and 226Ra, indicating that there has been no significant radionuclide migration in these samples over periods as long as the last 1 Ma. Disequilibrium was observed in some altered near-surface and deeper fracture-filling mineral samples with 234U deficiencies of up to 20%. A similar deficiency of 226Ra was found at ∼ 1-km depth in a gypsum-infilled core sample. These results indicate that U and Ra leaching by groundwater has occurred within periods 1 Ma and 8 ka ago, respectively. One instance of 226Ra excess was found at 94-m depth, indicating deposition of 226Ra in the last 8 ka. All cases of disequilibrium involved fracture-infilling minerals or altered granite cores from fracture zones, indicating that radionuclide migration occurred via fractures. Examples were found, however, of altered samples or mineral infillings that showed no detectable disequilibrium. These locations have been closed to migration in the last 1 Ma. Relative age estimates are made for some of the infilling minerals, based on their U-series isotopic characteristics.