Microstructural and geochemical characterisation of magnetitite layers from the Khuseleka Mine, Bushveld Complex

The Bushveld Complex is widely known as the world’s largest igneous intrusion, spanning an area of 550km and a depth of 8km (Cawthorn & McCarthy, 2023). The Upper Zone of the Busvheld Complex is characterised with the massive (> 1 m thick) magnetitite layers. The magnetitite layers contain between 70 and 30 vol. % of magnetite (Fe₃O₄) and ilmenite(FeTiO₃). The Upper Zone contains 24 distinct layers of magnetitite with surrounding contacts of anorthosite on the hanging and foot walls of each. The development of these layers is heavily disputed with arguments for fractional crystallization (Reynolds, 1985), in-situ crystallization (Kruger & Latypov, 2020), and crystal mush magmatic emplacement (Vukmanovic, et al., 2019).

In this study we analysed samples from three distinctive cores from Khuseleka Mine, investigating layers 1, 13 and 14 above the Main Magnetitite layer (Reynolds, 1985). Our study relies on quantitative microstructural and geochemical data. Electron backscatter diffraction (EBSD) has been used to investigate rock microstructure such as crystal orientation and intragrain microstructure; and electron probe micro analysis (EPMA) to investigate chemical variations between crystal rims and cores. Orientation analysis revealed an unexpected, but mild crystallographic preferred orientation in both magnetite and ilmenite crystals, exhibiting point maxima at (100) and at [10-10] respectively. Crystallographic preferred orientations in magnetite are rare in oxide phases due to cubic symmetry, however, the CPO exhibited suggests that the CPO is generated from deposition through a flow in the melt, generation through post-depositional deformation and recrystallization (Pilchin, 2011) or conversely, through topotactic reactions between magnetite grains (Barbosa & Lagoeiro, 2010). The hypothesis for post-depositional deformation is further supported by the evidence of recrystallisation and low-angle boundaries in magnetite grains.The investigation of crystallographic relationship between magnetite and ilmenite has epitaxial relationship between the two phases, this is evidenced by grains of ilmenite displaying parallel poles with adjacent grains of magnetite . Ilmenite shows less intragrain microstructure than magnetite, hence the mild CPO in ilmenite (a trigonal phase) could be explained by crystallographic control between magnetite and ilmenite during oxide crystallisation, or ilmenite deposition from a magma flow (Till & Rybacki, 2020).

EPMA data reveals variations in geochemistry between figures for crystal rims and cores and exhibits consistent zoning of TiO₂ in ilmenite samples across separate cores (51.5_core wt% and 52.1_rim wt%). The vertical depletion of Cr, discovered by Kruger & Laytpov, 2020, and Cawthorn & McCarthy, 2023 is challenged by this study as no correlation between vertical displacement and Cr concentrations has been acknowledged, suggesting that the theory of in-situ crystallization at the examined magnetite layers does not apply.

This data acquired from this study suggests that the magnetite layers in RLS have been subjected to post-depositional deformation which has disrupted the primary texture of the oxides. The variations of minor elements in magnetite does not support in-situ crystallisation. However further analyses are needed to decipher the early magmatic history of these enigmatic bodies.