Ly. Griffault et Ta. Shewchuk, PERMEABILITY EFFECTS ON RADIONUCLIDE MIGRATION IN A HIGHLY FRACTURED ZONE IN THE LAC-DU-BONNET-BATHOLITH, CANADA, Radiochimica Acta, 66-7, 1994, pp. 495-503
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.