Subsurface fracture distribution and its correlation with the shape and thickness of the Lac du Bonnet batholith

Abstract

The Lac du Bonnet batholith (LDBB) is an Archean pluton in the Superior Province in southeastern Manitoba and formerly the site of the Underground Research Laboratory operated by Atomic Energy of Canada Limited. Batholith-scale investigations were conducted there between the late 1970s and 2005 into the Canadian concept of used nuclear fuel disposal in crystalline rocks of the Canadian Shield. A review of legacy data from this period has revealed that the distribution of the fracture network can be related directly to the thickness of the intrusion, as inferred from the model of the base of the batholith. Fractures have preferentially developed in the shallow and thinner sill-like western remnant of the batholith and, in comparison, they are suppressed within and over its deep root. Fracture orientations, frequency and continuity as seen at the surface are not characteristic of the fractures within the batholith interior, even at relatively shallow depths. A substantial volume of unfractured granite has been preserved despite the LDBB’s great age and its exposure to several cycles of loading and unloading, as revealed by the regional stratigraphic record and a unique apatite fission track dating profile. The LDBB is not a one-of-a-kind anomaly, and similar fracture/host rock relationships can be found in the literature. Based on the data reported here, it can be recommended that site models for any continuance of underground nuclear waste storage define the volume distribution of the host rock and its internal structure as these may serve as guides for subsurface exploration and the development and incremental testing of a conceptual fracture model.