Three-dimensional mapping of the Platreef at the Zwartfontein South mine: implications for the timing of magmatic events in the northern limb of the Bushveld Complex, South Africa

Abstract

The Platreef is a pyroxenitic unit with Ni–Cu–PGE mineralisation that forms the base of the layered igneous succession in the northern limb of the Bushveld Complex. It rests upon sediments of the Transvaal Supergroup and Archaean granite/gneiss basement, and is overlain by norites and gabbronorites assigned to the Main Zone of the Complex. Detailed lithological mapping of a series of bench faces on bench 222 of the Zwartfontein South pit was undertaken to define a rectangular block. This information, coupled with drill chip data obtained during the drilling of the blast grids in the enclosing area, has allowed us to constrain with a high degree of confidence the three-dimensional nature of the lithological relationships on a local scale, not achieved by any previous study. The inter-connectivity of the mapped faces has allowed the first well constrained, three-dimensional representation of the Platreef–hangingwall contact to be generated. It has revealed finger-like intrusions of hangingwall gabbronorite which cut down into the Platreef along zones of low competency such as NE–SW and N–S trending shear zones. This relationship demonstrates that the emplacement of Main Zone type hangingwall magma occurred after a significant period of time that had allowed both crystallisation and deformation of the Platreef to take place.