TRACE ELEMENT COMPOSITIONS OF MAGNETITE LAYERS AND PIPES IN THE EASTERN LIMB OF THE BUSHVELD COMPLEX, SOUTH AFRICA

The Upper Zone of the Bushveld Complex contains nearly monomineralic titanomagnetite layers. In the Eastern limb there are at least 26 magnetite layers, one of which is several meters thick and is called the Main Magnetite Layer. There are also many small discrete bodies of magnetite commonly known as ‘pipes.’ Several petrogenetic models have been proposed for the magnetite-rich layers, including fractional crystallization, changes in oxygen fugacity, liquid immiscibility, and chamber rejuvenation/magma mixing.

Magnetite from the layers in the eastern limb, including the Main Magnetite Layer, and the ‘pipe’ structures were analyzed for trace elements by LA-ICPMS. Magnetite from the pipes are enriched in REE, Mn, and Pb compared to the magnetite layers, and have a strong negative Eu anomaly. Magnetite from the layers have lower REE abundances and have no Eu anomaly.

Compatible elements including Mg, Ni, and Co decrease up-section but are replenished periodically. Conversely, incompatible elements including Ga, Ge, Hf, Mn, Mo, Nb, Sc, Sn, Ta, W, Zn, and Zr increase up section, but reverse at points where compatible elements increase. These variations as a function of increasing stratigraphic height are in keeping with previous studies, which were interpreted to reflect input of new magma.

The data suggest the pipe-like structures are not petrogenetically related to the magnetite layers and are not of hydrothermal origin. Additionally, reversals in trace elements up section in magnetite layers suggest fractional crystallization and chamber rejuvenation with a more primitive magma may explain the genesis of the layers. The evidence suggest that the processes of formation of the Main Magnetite Layer may be similar to processes forming the thinner magnetite layers stratigraphically higher.