Geochemical Anatomy of the Main Magnetitite Layer, Bushveld Complex, South Africa

Abstract

The mafic layered Bushveld Complex, South Africa, contains numerous examples of monomineralic layers within its succession. The Upper Zone (UZ) contains approximately 24 magnetitite layers, the genesis of which have been extensively debated. The Main Magnetitite Layer (MML) is approximately 2 m thick and is traceable around its edge across >60,000 km2. Its basal contact with underlying anorthosite is planar and sharp, but the top contact grades upward with increasing plagioclase content. Sampling at a vertical spacing of a very few cm from seven profiles in the east over many tens of km, and one other 300 km to the west reveals concentrations of Cr in magnetite that decrease extremely rapidly upward (typically from >8,000 to 1,000 ppm within 30-60 cm from the base), punctuated by sharp concentration reversals and rare decreases. Here, we describe an outcrop where the MML splits into three sub-layers, separated by magnetite-plagioclase rocks. Twelve profiles across this zone of splitting have been similarly analysed. Lateral variations in Cr profiles across a few metres are observed at this locality. We offer the suggestion that magnetite formation may have been induced by shock-wave nucleation on the bottom of the chamber accounting for the abrupt appearance of magnetitite over such a wide area. Bottom growth of magnetite lowered the density and Cr content of the evolving magma, causing turbulence and convective overturn near the base of the chamber that created inhomogeneities in Cr on various scales, preserved in the ensuing magnetite compositions both vertically and laterally. Intermittent and abrupt (on a scale of 1-2 cm) upward increases in the Cr contents of up to 3,000 ppm in magnetite profiles resulted from convective overturn impinging on the floor. The tops of the magnetitite sub-layers grade into magnetite-plagioclase rocks and continue the upward decrease in Cr content in magnetite, typically at 900 ppm Cr, demonstrating upward continuity of fractionation. In contrast, there are reversals in Cr content between the magnetite-plagioclase rocks and the overlying magnetitite sub-layers that we attribute to convective overturn, with an increase in the Cr content. Two profiles through the MML show abrupt upward discontinuities to lower Cr contents that we attribute to physical erosional events. Anorthosite fragments in magnetitite, and magnetitite fragments in anorthosite layers further attest to such processes. The anorthite contents of plagioclase do not change across the MML, suggesting that magma addition was not responsible for the formation of magnetitite layers. Primary topographic variations at the base of the chamber also preclude addition of dense magma for the formation of magnetitite layers. Upward infiltration metasomatism, slightly resetting Cr contents, is limited to <3 cm.