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

Abstract

The proximity to metasedimentary footwall rocks relative to platinum group element (PGE) mineralized intrusive rocks in the northern limb of the Bushveld Igneous Complex (BIC) has resulted in complex local contamination in the intrusions. To assess the extent of incorporation of non-magmatic material and its effects on PGE mineralization, major element, trace element, and S isotopic data were collected from drill core UMT094 on the Turfspruit farm, where core logging has shown that the mineralized Platreef, forming the Flatreef deposit, is located stratigraphically well above local sedimentary footwall rocks. The S isotopic data combined with whole rock geochemistry data (including CaO/Al2O3, (V/Ti)PM, (Ni/Cr)PM, S/Se, loss on ignition) were used to assess incorporation of a range of local footwall material. The δ34S data show a steady decrease from the footwall assimilation zone (δ34S typically + 8 to + 9‰, maximum 12‰) to near constant δ34S values (δ34S < + 4‰) below the main PGE reef. Similar values have been documented for the Merensky Reef in the eastern and western limbs of the BIC (δ34S ~ 0 to + 3.5‰). Other geochemical parameters, such as S/Se and CaO/Al2O3, also match the ranges documented for the Merensky Reef elsewhere in the BIC. In addition, parameters such as whole rock V/Ti, normalized to primitive mantle (V/Ti)PM, are shown to be useful indicators of contamination and the type of contaminant with 1 < (V/Ti)PM < 2 for uncontaminated magmatic units; [V/Ti]pm > 2 for shale assimilation; and [V/Ti]pm < 1 for carbonate assimilation. The results suggest that the main PGE mineralization in the Flatreef deposit formed without significant in situ contamination and that the primary mechanism of PGE mineralization in the Platreef at Turfspruit was no different than the mechanism that generated the Merensky Reef in the eastern and western limbs of the BIC.

Geological setting and concentration of scandium in the Flatreef and eastern limb chromitites of the Bushveld Complex

“…Although Sc is usually not significantly enriched in layered intrusions (Wang et al, 2021), it is of both scientific and economic interest to investigate this element in the BC, particularly the ore-bearing horizons of the Flatreef and their footwalls, the latter being significantly contaminated with crustal material (Keir-Sage et al, 2021; and are not considered to represent a typical igneous BC cumulate.…”

“…However, PGE mineralization associated with Ni-and Cu-bearing sulphides may extend below the reef for tens of metres into the footwall (Yudovskaya et al, 2017;Grobler et al, 2019). These rocks were emplaced upon the older Transvaal Supergroup sediments, and therefore they have a much greater degree of contamination (Keir-Sage et al, 2021), though the Flatreef is generally considered to be less contaminated than the up-dip Platreef (Grobler et al, 2019;.…”