Magnetite layer formation in the Bushveld Complex of South Africa

Zhuosen Yao,James E Mungall

doi:10.1038/s41467-022-28000-9

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.

Highlights

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
We address the effects of melt migration through a porous magnetite cumulate pile driven by density contrasts between residual silicate melt within the pile compared to the melt above it
We examine the consequences of compaction-driven ascent of interstitial melts from the anorthositic footwall in two distinct scenarios, one where the original record of fractional crystallization in the lower part of the profile has been erased by compositional convection (Fig. 1a–c), and another where it has not (Fig. 1d–h)

Summary

Introduction

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. A common approach to attempt to resolve these matters is to produce a forward geochemical or physical model and use it to demonstrate the feasibility of a particular hypothetical process; there are many examples of cases where totally contradictory hypotheses can be framed in forward models that successfully match observations[6,7] The resolution of these questions is of more than purely academic interest because some of the layers in question contain mineral deposits of global economic significance[8]. In the immediate context of mechanisms of layer formation in layered intrusions, we show that purportedly incontrovertible evidence for in situ crystallization of magnetite can better be used to support a role for melt infiltration from below a magnetite crystal mush, but the choice of model remains a matter of preference

Methods

Results

Conclusion