Geology and geophysics of the Underground Research Laboratory site, Lac du Bonnet Batholith, Manitoba

Abstract

The lease area of the Atomic Energy of Canada Limited Underground Research Laboratory covers 3.8 km2 and is located 2.5 km north of the south contact of the Lac du Bonnet Batholith. A shaft to 255 m and 130 boreholes up to 1100 m deep expose the third dimension.The underlying granite is largely of two types: (i) pink porphyritic, which may be biotite rich, gneissic, and (or) xenolithic; and (ii) grey homogeneous and equigranular. Composition layering, including xenolith-rich zones, outlines domes along an antiform trending north-northeast through the western part of the lease area. The southeast-dipping flank underlies the eastern half of the site, including the shaft. Axes of folding trend 065 °and 140°. Homogeneous grey granite, being relatively fresh and unfractured, is associated with a magnetic field that is about 100 nT higher and with a resistivity that is up to 5000 Ω∙m higher than those of other units. A pattern of highs in the magnetic field, caused by the high magnetite content of some xenoliths, can be used to map the antiform.Three thrust faults that dip 10–30° east-southeast are partly controlled by the compositional layering. Anomalies in the very low frequency electromagnetic (VLF-EM) field occur at the surface projections of faults. One fault has been mapped at depth by a high-resolution seismic reflection survey. A suite of downhole geophysical methods, including cross-hole seismic, has been used to map discontinuities in boreholes.Subvertical penetrative foliations and pegmatitic dykes are part of the late crystallization fabric, providing (with filled fractures) a continuous deformation history in response to north- to northeast-trending compressive stress.