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

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 Fe 3+ /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.

A Process Mineralogical Evaluation of Chromite at the Nkomati Nickel Mine, Uitkomst Complex, South Africa

“…The Mg# (Mg/(Mg + Fe 2+ )) was highest in the chromite found in the massive ore and decreased in the chromite found the disseminated ores (see Figure 4). These trends are consistent with those recorded from the Uitkomst Complex and Northern Limb of the Bushveld Complex [21,36]. The chromite chemistry evolved from being Cr-rich in the massive ores to being more iron-rich in the disseminated ore types.…”

“…In the massive chromite ores, there is minimal interaction with silicates and thus a narrower range in chromite chemistries is recorded. Similar trends were also documented by [21] who characterized the massive chromites as having Mg numbers larger than 0.4 while the disseminated chromites tended to have Mg numbers less than 0.4. In comparison to the Bushveld (i.e., the Northern Limb, [21]) the chromite chemistry of the MCHR at Nkomati is primitive and in agreement with the findings of Yudovskaya et al [36].…”

“…Cousins and Feringa [7] further proposed that the chromitite layers be labelled from the base upwards. Various studies focusing on the chemistry and geological setting of the different chromitite groups of the Bushveld Complex have been carried out ( [8] and references within, [9][10][11][12][13][14][15][16][17][18][19][20][21]). It was shown that the LG chromites have the highest Cr/Fe ratio (i.e., most primitive) which decreases systematically upwards such that the UG chromites have low Cr/Fe ratios and are thus more evolved.…”

“…NKO represents data from[36]. UG2E represents data from[21]. The suffixes M, SM and D are abbreviations for massive, semi-massive and disseminated, respectively.…”

Assessing the extent of local crust assimilation within the Flatreef, northern limb of the Bushveld Igneous Complex, using sulfur isotopes and trace element geochemistry

“…There has been some debate in the literature as to whether or not the Platreef represents the lateral equivalent of the Upper Critical Zone of the eastern and western limbs of the BIC (Langa et al 2020;Grobler et al 2019;Kinnaird 2005;Sharman et al 2013;Mayer et al 2020;Maier et al 2020). Much of this debate has centered around differing interpretations regarding the extent of crustal assimilation in the Platreef magmas, in particular the role of the addition of crustal S to generate the atypical thickness of PGE mineralization in the Platreef compared to the rest of the BIC (Holwell et al 2007;Sharman et al 2013).…”