Magnetitites in the Upper Zone of the Bushveld Complex – continuous LIBS and µXRF data of magnetitite layer 17 with a focus on cryptic variation of mineral chemistry

• µXRF and LIBS reveal mineralogical and textural complexity in magnetitites. • LIBS reliably traces Cr and V trends over cores without thin section preparation. • The vanadium trend opposes a simple fractional crystallization model. • Symplectites and hydrous phases suggest late-stage interaction with mafic melt. • Mafic melt interaction is indicated by increased An# and Mg# in associated minerals. The Bushveld Complex in South Africa holds the largest ultramafic–mafic intrusion on earth with world-class resources of chromium, vanadium and platinum-group elements. Vanadium mostly occurs in the magnetitite layers of the Upper Zone. Their formation is still under debate and several models have been published. Cryptic variation is characteristic for the magnetitites with a general trend of upward depletion of the Cr concentrations in magnetite, which is occasionally inverted. This work applies Laser Induced Breakdown Spectroscopy (LIBS) and micro-X-ray Fluorescence (µXRF) mapping to examine the element trends in magnetitites on a µm-scale along a drill core. These methods provide continuous 2D element and mineral distribution maps. The LIBS and µXRF data were validated using thin section microscopy and electron probe microanalysis. The Cr distribution confirms the general trend of upward depletion and shows repeated increases which mostly correlate with plagioclase or sulfide lenses. Vanadium shows an upward increase within the magnetitite layer which rules out simple fractional crystallization models for the magnetitite formation. Furthermore, the contact zones of magnetitite and anorthosite exhibit abundant symplectic intergrowth of olivine and plagioclase with significantly higher An# in the symplectic plagioclase and higher Mg# in olivine accompanied by amphibole and mica, suggesting a flow of a more mafic melt and the interaction with a hydrous melt. In general, the combination of the LIBS and µXRF presents enormous potential for applications where continuous data at both the micro- and the meso-scale are necessary to understand complex processes such as magnetitite layer formation.