Rock-strength and physical properties of Norites of the Merensky and Bastard Units, western Bushveld Complex

Abstract

Platinum mining in the Bushveld Complex has expanded greatly over the last decade with many operations planned for, or already at depths greater than 1000 m. Mine infrastructures, rock fragmentation properties, rock failures and safety issues are important aspects of deeper mining operations and require an understanding of controls and properties which influence rock strength. This study, which includes rock classification of both fresh and altered material, chemical compositions, fabrics and structure, was carried out to investigate mainly the noritic rocks in the vicinity of the Merensky reef in the western Bushveld Complex. Uniaxial compressive strength (UCS) and tensile strength (TS) are shown to be broadly dependent on rock type, and strongly dependent on the degree of structural alteration as reflected by the Rock Quality Designation (RQD). In addition, there is an unexpectedly strong correlation between UCS values and both normative quartz and Zr contents of the whole rock. Both of these compositional parameters relate to the entrapment of a late-stage liquid which when solidified acted as cement and influenced the propagation of fractures under conditions of failure. Micrographic observations indicate that interstitial quartz, even in relatively small amounts, is effective in arresting fracture propagation. Relatively small variations in the quartz content could therefore significantly affect the rock strength of the norites. Other possible controls on rock strength include the relative development of foliation fabric, the distribution of crystal sizes, and the way that the rocks have annealed at high temperature.