History of actinide and minor element mobility in an Archean granitic batholith in Manitoba, Canada

Abstract

Surface and borehole core samples from the Lac du Bonnet granite, Manitoba, Canada, have been analysed for major element concentrations,Fe 3+/Fe (total) ratios, rare earth element (REE) content and actinide isotopic abundances. This work forms part of the geological investigations of the Canadian Nuclear Fuel Waste Management Program, performed by Atomic Energy of Canada Limited (AECL). The study attempts to understand the history of, and processes governing, mobilisation of elements and naturally occurring radionuclides during high- and low-temperature alteration events in fluid-bearing fractures in the granite. One surface sample and two core samples (from ∼ 150 m and 730 m) are each in contact with fractures in the granite and show evidence of alteration events that penetrated the rock matrix over distances of at least 3 cm. Loss of Ca and Na is seen in cores from a depth of ∼ 150 m from the highly altered, hematite-rich rock adjacent to sub-horizontal fracture zones at the Underground Research Laboratory (URL) of AECL, near Lac du Bonnet. In contrast, K, Fe,Fe 3+/Fe and U concentrations increase towards the fracture surface due to formation of illite and association of U with hematite and the illite. At the fracture surface, U continues to increase, but Fe and theFe 3+/Fe ratio decrease indicating Fe removal by reduction. The REE also show some enrichment in more altered rock at intermediate depths, but the total REE concentration is lower than in the surface and deep core samples. No clear trends are visible for parent and fracture-surface REE in surface and deep core samples, however. Disequilibrium values of 234U/ 238U and 230Th/ 234U ratios in surface and intermediate depth core samples indicate that U has been mobilised in recent geological time (the last Ma), but Th has remained relatively immobile. High Th/U and 230Th/ 234U ratios in surface samples are indicative of rapid leaching of U but little isotopic fractionation, probably within the last 10 5 a. Apparently unaltered rock, several centimetres distant from the fracture in surface and intermediate- depth samples, has lost appreciable U, but evidence from U-series disequilibrium studies suggests that this process occurred more than one million years ago, perhaps during deuteric or hydrothermal alteration. Core from a fracture at depth in the granite shows little hematite or clay formation and lacks evidence of REE and recent or ancient actinide mobilisation. The U-series results are correlated with the observed concentrations and isotope activity ratios of U in groundwaters sampled from the same or adjacent fractures. Analyses of samples of highly altered rubble recovered from centre portions of fracture zones at the URL show both excesses and deficiencies of 234U and 230Th in neighbouring locations, possibly due to the presence of a redox front whose position is controlled by modern groundwater composition. The implications of these results are discussed for the concept of disposal of nuclear fuel waste at depth in plutonic rock on the Canadian Shield.