Chromite chemistry of a massive chromitite seam in the northern limb of the Bushveld Igneous Complex, South Africa: correlation with the UG-2 in the eastern and western limbs and evidence of variable assimilation of footwall rocks

Malose M Langa; Pedro J Jugo; Matthew I Leybourne; Danie F Grobler; Jacob Adetunji; Henrik Skogby

doi:10.1007/s00126-020-00964-y

Chromite chemistry of a massive chromitite seam in the northern limb of the Bushveld Igneous Complex, South Africa: correlation with the UG-2 in the eastern and western limbs and evidence of variable assimilation of footwall rocks

Miner Depos. 2021;56(1):31-44. doi: 10.1007/s00126-020-00964-y. Epub 2020 Mar 3.

Authors

Malose M Langa¹, Pedro J Jugo¹, Matthew I Leybourne^{2

3}, Danie F Grobler⁴, Jacob Adetunji⁵, Henrik Skogby⁶

Affiliations

¹ Mineral Exploration Research Centre, Harquail School of Earth Sciences, Laurentian University, Sudbury, Canada.
² Department of Geological Sciences and Geological Engineering, Queen's University, Kingston, Ontario Canada.
³ McDonald Institute, Queen's University, Kingston, Ontario Canada.
⁴ Ivanplats (Pty) Ltd., Mokopane, South Africa.
⁵ Department of Natural Sciences, College of Life and Natural Sciences, University of Derby, Derby, UK.
⁶ Swedish Museum of Natural History, Department of Geosciences, Stockholm, Sweden.

Abstract

The Bushveld Igneous Complex (BIC) is known for its laterally extensive platinum group element-bearing layers, the most famous being the Merensky Reef and the UG-2 chromitite in the eastern and western limbs of the complex. In the northern limb, the Platreef mineralization and a thick chromitite seam below it (referred to as the "UG-2 equivalent" or UG-2E) have been proposed to be the stratigraphic equivalents of the Merensky Reef and the UG-2, respectively. In this study, we compare a suite of UG-2E samples from the Turfspruit project with a UG-2 reference suite from the western limb using petrography, electron probe microanalysis, laser ablation-inductively coupled plasma-mass spectrometry, and Mössbauer spectroscopy. The results show that (a) in Mg# vs. Cr# diagrams, UG-2E chromites have a distinct compositional field; however, when samples of similar chromite modal abundance (≥ 80%) are used, the UG-2E chromites overlap the field that characterizes UG-2 chromites; (b) the UG-2E is more variable in chromite modal abundance than the UG-2; and (c) variations in Mg# and Fe³⁺/ΣFe in the UG-2E indicate contamination of the magma by metasedimentary rocks of the Duitschland Formation (Transvaal Supergroup) during emplacement, followed by partial re-equilibration of chromite grains with a trapped melt. Thus, we conclude that for chromite modes higher than 80%, the chromite composition retains enough information to allow correlation and that the UG-2E in the northern limb is very likely the UG-2 chromitite.

Keywords: Bushveld; Chromite; Platreef; Trapped liquid shift effect; UG-2.