Thembelani and Khuseleka Mines in the Bushveld Igneous Complex's Western Limb in South Africa

Abstract

ABSTRACT: The Merensky (MER) orebody in the Bushveld Complex in South Africa has been mined extensively. Regional and local pillars, zones of unmined ground, potholes, and remnants were left in-situ. 15 to 400 m beneath the MER orebody, the UG2 chromitite orebody is currently being exploited. Due to the location of these two orebodies, it is postulated that the unmined ground in the MER above may have an influence on the current UG2 ground conditions. Although many mining variables could have impacted the successful exploitation of the UG2, the in-situ blocks of ground on the MER reef horizon seemed to present an additional risk, which had not been extensively researched before. Of concern is that, this phenomenon is occurring at different depths and different middling between the two orebody packages. In some cases, it was found that relatively shallow UG2 reef mining could encounter conditions like those experienced in deep mining environments (e.g., spalling, scaling pillars, and rockburst hazard). Source mechanisms of significant mining-related seismic events (ML≥1.0) were analyzed and facilitated the comprehension of the dominant modes of failure. Much of the work focused on seismic events triggered by movement along planes of weakness and the closure rate experienced by the stopes in both the MER and UG2. 1. BACKGROUND The Merensky Reef is a regular, persistent tabular orebody with various layers: pegmatoid, chromitite, and pyroxenite (Lougher and Mellowship, 1991). It is generally composed of an upper feldspathic pyroxenite assemblage near the top of the Upper Group of the Upper Critical Zone of the Bushveld Complex. It is overlying a thin basal chromitite stringer, followed by an anorthosite to norite footwall and a thin anorthosite layer (0.1 – 0.2 m thick), which is underlain by norite (Fig. 1). Locally, this is termed a "pyroxenite reef". The UG2 chromitite layer comprises chromitite and pegmatoid units. It is well renowned for its short-distance undulation due to its conformable interaction with the footwall. The UG2 Reef is made up of a coarse plagioclase-orthopyroxene pegmatoid layer that can grade into coarse pyroxenite (Leeb-Du Toit, 1986). The reef's thickness ranges from 0.2 to 2 m, with an average of about 0.7 m. The absence of coarse pyroxenite beneath the UG2 chromitite layer suggests that the stratum is potholed (Lougher and Mellowship, 1991). The UG2 Reef consists of a primary chromitite layer, which contains the majority of the PGM and base metal mineralization, followed by a poorly mineralized pegmatoidal pyroxenite footwall (Fig. 1). The hanging wall to the main chromitite layer is feldspathic pyroxenite with up to four thin, weakly mineralized chromitite layers.