Geomechanics and Mineralogical Characterization of Intact Heterogeneous Rocks from the Legacy Skarn Deposit

Abstract

Predicting the geomechanical behaviour of rockmasses is key to the economic success and safety of rock engineering projects such as 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 samples and discard heterogeneous samples such as those that are affected by hydrothermal alteration and veining. Understanding the complexities of heterogeneous rocks and rockmasses is critical for excavation projects such as deep block cave mines where extensive tunnel networks are regularly excavated through heterogeneous rockmasses. During this research project, UCS tests were conducted on 29 homogeneous (matrix-type) and 21 heterogeneous (vein-type) core samples from the Legacy Skarn Deposit located in northern New Brunswick, Canada. These samples 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 were determined using hand sample field identification techniques and microscopic laboratory methods including petrographic thin section analysis, X-Ray Diffraction, and micro-X-ray Fluorescence. Significant effects of hydrothermal veining on the UCS test results were observed in the tested samples. Vein mineralogy dominated by calcite weakens the granodiorites but strengthens the calcareous mudstone. Vein mineralogy dominated by quartz in the skarn has a mixed influence on geomechanical properties. Vein thickness primarily influences geomechanical properties of samples with single veins, whereas vein density is more useful to characterize samples that contain vein networks or stockwork. The effects of vein orientation only partially agree with an established model for shear failure through critically oriented foliations. The disseminated alteration in the granodiorites (plagioclase phenocrysts) and skarn (garnet crystals) typically increase stiffness and strength, since the large grains arrest cracks under load. Lastly, the effects of sample selection on reported UCS test results were also investigated using four sample selection methods to the tested core samples. The results demonstrate the importance of including heterogeneous veined samples in a UCS testing program.