Geoheritage of the Eastern Limb of the Bushveld Igneous Complex, South Africa: a Uniquely Exposed Layered Igneous Intrusion

Abstract

The eastern limb of the Bushveld Igneous Complex outcrops in the Middleveld, a localized physiographic region of northeastern South Africa associated with an erosional embayment into the interior plateau. Superimposed drainage of the Olifants and Steelpoort Rivers has produced a mountainous terrain where the layered ultramafic-mafic rocks (“Rustenburg Layered Suite”) and metamorphic aureole outcrop in regional escarpments and arcuate ridges. Meter-thick igneous layers can be delineated for tens of kilometers. Five principal zones are recognized; each comprised of different groups of cumulates with distinctive marker layers. Orebodies such as chromitites and PGE reefs (Critical Zone) and Ti-magnetite layers (Upper Zone) are constrained to specific zones. Despite the origin of igneous layering being incompletely understood, field evidence suggests that the Rustenburg Layered Suite developed incrementally from injection of thin magma sheets. Each zone is associated with discrete lineages of parental magmas. The Merensky Reef, the most well known of the layered orebodies, has an unusual origin in that it may have crystallized from ultramafic sills that were injected into a pre-existing noritic-anorthositic substrate. The primary layering is transgressed by discordant ultramafic bodies, of which two types, dunite pipes and the unusual iron-rich ultramafic pegmatite bodies, are significant. The eastern limb has a long history of human habitation and the valleys and interior plateau are relatively densely populated. Details of the African Iron Age and history of the Sekhukhune people are preserved at the Tsate cultural center. Three pioneering geologists, Arthur Hall, Hans Merensky, and Percy Wagner, greatly advanced Bushveld geology and were involved in the 1924-9 “platinum rush.” This culminated in discovery of four platiniferous discordant pipes and the Merensky Reef. The mining heritage includes the discovery site of platinum (Mooihoek pipe), the world’s oldest underground platinum mine (Onverwacht pipe), and historical workings on the Merensky Reef (Winnaarshoek). The combination of classic outcrops of layered ultramafic-mafic rocks, together with the unique community and mining heritage, justifies establishing an eastern limb geopark.