Magnetite layer formation in the Bushveld Complex of South Africa

Abstract

The great economic significance of layered mafic-ultramafic intrusions like the Bushveld Complex of South Africa results from the existence within them of some layers highly concentrated in valuable elements. Here we address the origins of the Main Magnetite Layer, a globally important resource of Fe-Ti-V-rich magnetite. Previous models of in situ fractional magnetite crystallization require frequent ad hoc adjustments to the boundary conditions. An alternative model incorporating compositional convection near the top of the pile and infiltration of the pile from beneath by migrating intercumulus melt fits observations without any adjustments. Lateral variations in Cr concentration formerly held as indisputable evidence for in situ crystallization can be accommodated better by models of reactive melt infiltration from below. The choice of models has pivotal ramifications for understanding of the fundamental processes by which crystals accumulate and layers form in layered intrusions.

In situ crystallization of non-cotectic and foliated igneous rocks on a magma chamber floor

“…In addition to the arguments presented above, another obstacle for settling models arises from geochemical modeling. Numerous studies have now highlighted the fact that simple fractional crystallization alone cannot explain the variation in Cr along vertical sections across magnetitite layers 7,13,34,41 . The same appears to be true for their upper gradational contacts.…”

“…In this case, the whole-rock D value decreases in a manner that corresponds to the increase in the plagioclase mode, and a much better fit can be obtained (this time using a much thicker liquid layer of 41 m). Of course, there are many other ways in which a better fit with the data may be produced that range from models involving chemical diffusion 7,34 , multiple episodes of magmatic recharge and mixing 13 , or a decrease in the effectivity of crystal-liquid exchange 41 . However, these results suggest that the proportions of magnetite and plagioclase observed in the gradational contact represent the true proportions that are crystallizing from the melt, and that magnetite and plagioclase are not being crystallized in the expected cotectic proportions.…”

“…In addition to the above, domeshaped, high-Cr structures were documented at the bases of these layers that were interpreted as indicating the sites of incipient nucleation and growth of magnetitite, supporting the idea that magnetitite layers crystallize predominantly by in situ growth 13,33 . An alternative explanation for the high-Cr structures have recently been proposed where these structures are interpreted to form by the upward-migration of Cr-rich interstitial melt from the underlying cumulate pile 41 . However, this model is not consistent with field observations.…”