Natural Shear Stresses in Rock Masses: Example of the Campbell-Red Lake Gold Deposit, Superior Province, Ontario, Canada

Three measurement campaigns were carried out by Natural Resources Canada CANMET Laboratories over the last thirty years within the Red Lake Gold District, Ontario, to determine the magnitude and orientation of in-situ rock stresses on several levels of the Campbell and Red Lake gold mines, at depths ranging from 600 meters down to 2,000 meters below surface. Stress measurements were carried out using standard CSIR triaxial 'hollow-inclusion' and biaxial 'doorstopper-type' measurement cells. In-situ 3D stress tensors were computed. Increasing with the depth of measurement, the major principal stress varies between 23 and 95 MPa, the intermediate stress, between 14 and 68 MPa, and the minor principal stress, between 4 and 38 MPa. The measured vertical stress component also correlates well with the stress level expected within the Canadian Shield at these depths, ranging from 16 MPa at 580 m below surface up to 54 MPa at a depth of 2,000 m. The average stress ratios measured between the principal and vertical stress components are 1.62, 1.01 and 0.55, respectively, confirming the presence of a strong and consistent shear-stress tensor arrangement for all tensors measured below 650 m. This stress arrangement perfectly fits the geological strain-model proposed by Dubé et al. (2002), to explain the formation and the resulting geometry of the Campbell-Red Lake gold deposit, its evolution over time and the development of high grade mineralization zones at depth. The model was namely validated by the latest stress measurement campaign carried out on the deepest levels of the Red Lake Gold Mine during winter 2011. These measurements are key to understand the influence of such high shear stresses on mine design and excavation stability. Sources of variation, e.g. number and complexity of rock layers, presence of faults and folds, dykes and other major geological features, are discussed. ©Copyright reserved, Natural Resources Canada.