Estimating brittleness indexes from mechanical and petrographic characteristics of Norite

Abstract

Norite is a coarse-grained plutonic rock that has been relatively understudied in terms of its mechanical and petrographic properties. This study investigates the brittleness of norite within the Eastern Limb of the Bushveld Igneous Complex (BIC), South Africa. However, there is a scarcity of studies that quantitatively link its petrographic characteristics to establish brittleness indices. The primary aim was to estimate brittleness indexes based on both mechanical and petrographic properties of norite, which is a significant rock type commonly found in the hanging walls of platinum mines. Given the recurring safety incidents, especially falls of ground and rock bursts in underground mining, understanding the brittleness of norite is essential for enhancing geotechnical designs and safety measures. Samples were collected from a 10-meter exposure along Mototolo Road in the Critical Zone of the Eastern Bushveld Complex, near the Anglo-American Platinum Mototolo Mine. Mechanical analysis involved laboratory testing, which includes uniaxial compressive strength (UCS), tensile strength, Young's modulus, and Poisson’s ratio, supported by numerical simulations and multivariate regression models. The results indicate that norite exhibits high compressive strength and low ductility, with brittleness indexes effectively predicted using combinations of strength parameters. Mineralogical investigations were done using thin-section petrography to evaluate grain texture, contact nature, and mineral composition. It was observed that coarse and medium grain textures significantly influence brittleness, whereas grain contact type alone lacks predictive power. The main contribution of this work is the development of integrated predictive models that use both mechanical and mineralogical data. While the use of surface samples presents a limitation, their geological equivalence to underground norite supports the relevance of the findings for subsurface application. The findings enhance the understanding of the structural performance of norite and suggest practical recommendations for underground mine design. This research further contributes to improved and safer mining operations.