Mineralogical and sample selection implications for geomechanical properties of intact heterogeneous and veined rocks from the Legacy skarn deposit

Abstract

Abstract Predicting the geomechanical behaviour of rockmasses is key to the economic success and safety of rock engineering projects including surface and underground excavations. For these projects, unconfined compressive stress (UCS) laboratory testing provides fundamental geomechanical properties of intact rocks. It is currently standard practice for UCS testing to consider only homogeneous specimens that are free of defects and discard heterogeneous specimens such as those that contain intrablock structures (e.g. veins) or extra-granular defects (e.g. phenocrysts). Understanding the complexities of these heterogeneous rocks and rockmasses is critical for excavation projects such as deep cave mines where extensive tunnel networks, caves, and stopes are regularly excavated through heterogeneous rockmasses. This paper presents the test results and analyses of UCS tests conducted on 29 matrix-type and 21 veined drill core specimens from the Legacy Skarn deposit located in northern New Brunswick, Canada. These specimens are sorted into five lithological units: beige, red, and black varieties of quartz-plagioclase granodiorite, calcareous mudstone, and garnet-pyroxene skarn. Mineralogical compositions of these units are determined using field identification techniques and microscopic laboratory methods including petrographic thin section analysis, powdered X-Ray Diffraction, and micro-X-Ray Fluorescence. Significant effects of hydrothermal veining and phenocrysts on the UCS test results are discussed in this study to highlight the significant influence of mineralogy on the variability of geomechanical properties. For example, vein mineralogy dominated by calcite weakened the granodiorites but strengthened the calcareous mudstone. Vein mineralogy dominated by quartz in the skarn unit had a mixed influence on geomechanical properties. Vein thickness primarily influenced geomechanical properties of specimens with single veins, whereas vein density was more useful to characterize specimens that contained vein networks or stockwork. The effects of vein orientation only partially agree with the established Jaeger model for shear failure through critically oriented foliations. The disseminated alteration in the granodiorites (plagioclase phenocrysts) and skarn (garnet crystals) typically increased stiffness and strength as the large grains arrest crack propagation when under load. Lastly, the effects of sample selection on reported UCS test results using four sample selection methods are presented. The results demonstrate the importance of including heterogeneous veined specimens in UCS sample selection and testing programs to capture rock variability and improve the accuracy of numerical geomechanical design. Recommendations are made to improve geotechnical core logging and sample selection protocols when characterizing heterogeneous complex rockmasses.