Too large to be seen: Regional structures in Lower and Middle Group chromitites of the Bushveld Complex, South Africa

Abstract

The Critical Zone of the Bushveld Complex contains thick and laterally extensive Lower, Middle and Upper Group chromitite layers (LG, MG & UG, respectively), which are exposed within many open pit and underground mines. Using field observations from these exposures throughout the Eastern and Western Lobes of the Bushveld Complex, we describe here the morphology and field relationships of the LG and MG chromitites on a regional scale. This is done to gain a new regional perspective on the formation of massive chromitites. Some prominent chromitite layers are traceable over long distances and may occur as one, two or three separate chromitite layers at different locations, with several changes in the number of layers and their thicknesses happening along a distance of a few kilometres. This can be best understood as a structure of regionally bifurcating chromitites, with small-scale bifurcation being visible even at the scale of individual outcrops. We have documented field evidence indicating magmatic erosion of the floor cumulates. Among them are large inclusions of orthopyroxenite in chromitite that are partly attached to the footwall rocks, suggesting that they are erosional remnants of these rocks. Two potholes, roughly circular depressions in the chamber floor, were also documented. They show transgressive relationships of chromitites with their footwall rocks, suggesting that magmatic erosion has occurred prior to chromitite deposition. We propose that the regionally bifurcating chromitites are best explained in the frame of the model involving erosion of the footwall rocks by chromite-only-saturated magma replenishing the evolving chamber. In this process, different locations may have experienced different degrees of magmatic erosion, causing part of the footwall, the entire footwall or even part of the underlying chromitite to have been removed. Upon cooling, the new magma would have deposited chromitite on the erosional surface. Depending on the degree of magmatic erosion, a few separate chromitite layers or a single chromitite layer would form from several influxes of fresh magma into the chamber.