The Bushveld Complex -Host to the World’s Largest Platinum, Chromium and Vanadium Resources

Abstract

The geological setting of the Bushveld Complex, the world's largest layered igneous body containing most of the planet's chromium, Platinum Group Elements (PGEs) and vanadium resources, is outlined. The complex is situated in a central position on the ancient Kaapvaal Craton of South Africa, with the mineralisation contained in three large arcuate mafic to ultramafic Limbs, comprised of rocks of the Rustenburg Layered Suite (RLS). The Limbs are overlain by co-magmatic felsic rocks while the floor consists mainly of sediments of the Transvaal Supergroup. The RLS is divided into the Marginal, Lower, Critical, Main and Upper Zones with the chromium and PGE enriched layers occurring in the Critical Zone and the vanadium mineralisation occurring within titaniferous magnetitite layers of the Upper Zone. The chromitite layers are classed into Lower, Middle and Upper Groups and show a general decrease in chromium content upwards from >46% Cr 2 O 3 at the base to 1.5:1 at the base to 5 ppm in the uppermost and economically important PGE bearing chromitite layer (the UG2). The economically important Merensky Reef occurs above the UG2 near the top of the Critical Zone. It typically comprises top and bottom chromitite stringers, separated by up to 14 metres of pyroxenite and/ or pegmatoidal pyroxenite, with the best mineralisation generally being associated with the top chromitite stringer and where the two chromitite stringers are in close proximity to each other. These stringers can eventually merge towards pothole structures in places, with the top stringer transgressing downward to form a range of pothole Reef features. The nature and origin of the Merensky Reef in the Eastern and Western Limbs of the Bushveld Complex is discussed. A synthesis and interpretation of a large amount of platinum mine data from the Western Limb, highlights the great variability in the nature of the Merensky Reef in this segment. Evidence suggests that the top chromitite layer represents a magmatic unconformity resulting from thermo-chemical erosion, reconstitution and mineralisation of the immediate footwall layers that it transgressed. The Platreef, a thick, generally feldspathic pyroxenitic and harzburgitic assemblage, occurs on the eastern flank of the Northern Limb and is broadly equated with the Merensky Reef. About 21 magnetitite layers ranging from massive titaniferous magnetitite, to disseminated and weakly disseminated magnetitite occur within, or are associated with, magnetite gabbro, gabbro and diorite of the Upper Zone. The vanadium content in magnetite is highest in the lowermost magnetitite layers (over 2.2% V 2 O 5 in places) dropping to 1.6% V 2 O 5 in the overlying Main Magnetitite Layer. An important layer (Layer 21 or the Q Layer), towards the top of the Upper Zone, is enriched in titanium (±13% TiO 2 ), at the same time having vanadium values of generally less than 0.2% V 2 O 5 . This layer, which is often over 40 metres thick, contains a huge resource of iron and titanium, as well as vanadium. The mineral apatite appears abruptly at two levels towards the top of the complex, the uppermost occurrence being in a diorite immediately above layer 21 where P 2 O 5 values average 4% over several tens of meters. The use of geophysical and geochemical techniques in exploration and mining in the Bushveld Complex is briefly reviewed and their applicability and effectiveness is illustrated. Given the continuity of layers, including some which extend for well over a hundred kilometres, the resource still present in this unique Complex is vast.