PERMEABILITY EFFECTS ON RADIONUCLIDE MIGRATION IN A HIGHLY FRACTURED ZONE IN THE LAC-DU-BONNET-BATHOLITH, CANADA

Uranium-238 disequilibrium measurements have been used to trace the mi gration of naturally-occurring actinides along a highly fractured zone characterized by various permeabilities. Core sections containing fil led fractures were collected from boreholes crosscutting Fracture Zone 2 at the Underground Research Laboratory in various permeability zone s. The infilling materials consisting of illite/smectite interstratifi ed clay, chlorite and hematite mixed with Fe-oxyhydroxides, have been collected from the fracture surfaces and analyzed for U, Th and the ac tivity ratios U-234/U-238, Th-230/U-234 and Ra-226/Th-230. Large incre ases in U concentration were observ ed in all the infilling minerals c ompared to the unaltered rock. In contrast, large increases in Th cont ent were observed only in hematite/ Fe-oxyhydroxides infillings. The m easured U-234/U-238 and Th-230/U-234 activity ratios indicate two diff erent trends with loss of U-234 from the clay-rich samples and deposit ion of U-234 and, to some extent Th-230 in chlorite or hematite/Fe-oxy hydroxides-rich samples. The Ra-226/Th-230 activity ratios indicate an active deposition of Ra-226 in almost all the fracture-infilling samp les. Chlorite and hematite were produced during brecciation of the roc k and occur as a breccia sealing which surrounds residual grains of qu artz and drastically restricts the movement of groundwater. In contras t, clay minerals occur as honeycomb-structured particles coating the s urfaces of open fractures and allow relatively free movement of ground water. Loss of U-234 can be explained by the effects of alpha recoil, causing the daughter isotope to be ejected from the clay-particle surf ace from where U-238 had been originally deposited. After ejection, wa ter movement carries the radionuclides away from the vicinity of the c lay surfaces.