MAGNETITE TRACE ELEMENT COMPOSITIONS FROM MAGNETITE “PIPES” IN THE UPPER ZONE OF THE BUSHVELD COMPLEX, SOUTH AFRICA

The Upper Zone of the Bushveld Complex contains many meter-thick magnetite layers stretching along strike hundreds of kilometers. In the Eastern limb of the Complex there are also isolated magnetite bodies separated in space and stratigraphy from the continuous layers. These small bodies of magnetite were mapped and are commonly referred to as pipes. However, although their surface exposure is circular, their vertical expression is observed only rarely. The trace element composition of magnetite from several exposures was analyzed using LA-ICPMS and allows some insight into the petrology of these enigmatic bodies. Comparing trace element data for magnetite from “pipes” to magnetite from the layers of the Upper Zone shows consistent similarities between some pipes and the layers. Field and textural analysis demonstrates that some pipe-structures may be fragmented, faulted or displaced exposures of (parent) layers.

Meanwhile some occurrences that occur stratigraphically below the Main Magnetite layer have compositions similar to layers much higher in the stratigraphy. Kennedy’s Vale magnetite pipe at the base of the Main Zone has near identical trace element compositions to bodies at the base of the Upper Zone, which is 2-3 km higher in the magmatic stratigraphy. The data indicates a relationship between the isolated magnetite bodies and the layered sequence, as well as a link between bodies in the Main Zone and the Upper Zone. It has been suggested that some of the structures may be failed layers of magnetite that plunged through the underlying layer(s) due to mass and/or structural weakness. On the other hand the circular geometry, if supported by three-dimensional imaging in conjunction with the compositional data, may suggest that these “pipe” structures are remnants of feeders for new magma entering the Upper Zone.