Glaciation-induced radionuclide mobility from the Kiggavik uranium deposits: natural analogues for geological disposal of nuclear waste

Safe and effective disposal of Used Nuclear Fuel (UNF) within a Deep Geological Repository (DGR) must isolate and contain UNF from the biosphere for ~1 Ma. This time period is long enough for several glacial cycles to elapse; it is therefore important to understand how glaciation-related processes such as erosion and subsurface fluid infiltration may impact a DGR. Studying the history of uranium (U) minerals from U deposits, which have been impacted by Pleistocene glaciation, provides a natural analogue to investigate the potential impacts of glaciation on a DGR over Ka-Ma timescales.

               Uranium deposits in the Kiggavik region, Nunavut, Canada occur from surface to a depth of ~500m (comparable to depths of proposed DGRs) and have been impacted by multiple post-depositional fluid events and glacial cycles. Uranium minerals comprising uraninite, coffinite, brannerite, and U-Th-Zr silicates are hosted by illite (clay) and hematite altered metasedimentary and granitic rocks. Most U minerals (U1+U2) yield ~1.55-0.3 Ga U-Pb ages indicating they have remained in-situ since before the emergence of dinosaurs despite experiencing multiple fluid infiltration events.

A smaller subset of U minerals (U3) shows stronger evidence of remobilization. U3 minerals are concentrated along redox fronts developed between geothite-bearing oxidized and bleached (clay-dominated) host rocks. These redox boundaries occur within ~5 cm of U1/U2 minerals, and are strongly associated with open fractures and porous veins. U3 minerals have ²³⁵U/²⁰⁷Pb ages of >0.6-65 Ma, providing minimum ages of complete recrystallization and potential large-scale radionuclide release.

Uranium-thorium disequilibrium geochronology indicates widespread leaching of soluble decay-chain isotopes, corresponding to smaller-scale release of radionuclides. This has occurred sporadically between 34-494 Ka, with major episodes correlating with periods of rapid climate change during glaciation. Oxygen and hydrogen stable isotopic values of Illite associated with U3 indicate isotopic exchange with high-latitude meteoric fluids (i.e. snow/glacial melt).  

               The history of U mobility in the Kiggavik region indicates oxidized glacial-derived fluids may infiltrate ≥500m into the subsurface along open fractures and mobilize radionuclides. This mobility occurs cumulatively over multiple glacial cycles and corresponds with ages of climate-induced perturbations to overlying ice sheets. Although longer distance transport from the system cannot be ruled out, the proximity of U3 to U1/U2 mineralization suggests overall transport distances are short (several cm), and geochronology indicates transport timescales are long (10’s-100’s Ka). Interactions with minerals present in both metasedimentary and granitic host rocks such as illite clay, U-oxides, and Ti-oxides have effectively restricted radionuclide mobility to rates millions of times slower than glacial movement over timescales comparable to human evolution.