Chromite formation in the Lower Zone and Platreef sequences on Uitloop, northern limb of the Bushveld Complex

The Uitloop-II ultramafic body was emplaced into sedimentary rocks of the lower units of the Transvaal Supergroup in the southern sector of the northern limb of the Bushveld Complex. Recent drilling has identified another previously unknown composite pyroxenitic intrusion well below and to the east of the Platreef Unit, significantly expanding the resource potential of the area. The intrusion hosts Ni-Cu-PGE sulphide mineralisation which, together with existing Uitloop II Ni resources, forms one of the largest unexploited large-tonnage disseminated Ni sulphide orebodies globally. Two styles of sulphide mineralisation are observed: (i) reef type associated with discontinuous chromitite seams at the top and ii) contact type within magmatic, hybrid, and metasedimentary lithologies at the base of the intrusion. The Uitloop II massive chromitites, hosted by phlogopite- and quartz-rich feldspathic pyroxenite, contain high-Cr low-Mg chromite (up to 6.4 wt.% MgO and 53.8 wt.% Cr2O3) indicative of a crystallisation pathway from a contaminated Lower Zone (LZ) magma. Chromite from the pyroxenitic intrusion is poorer in Cr (up to 47.1 wt.% Cr2O3), showing a wide range of major oxide variations and diverse textures, which are interpreted to reflect prolonged crystallisation and local interaction with contaminated and hybrid melts. Whole-rock geochemistry, disrupted chromitite morphology, the composition of chromite and associated silicates as well as distribution of sulphide and chromite mineralisation underscore a Platreef affinity of the newly intersected intrusion. Exotic V-rich chromite in basal massive sulphides emphasises contamination and later re-equilibration processes, while a high Cr budget across the Lower Zone (LZ) and Platreef Unit/Critical Zone (CZ) intrusions suggests chromite supply via crystal slurry, dynamically transported with subsequent crystallisation within zones of mixing and hybridisation. This comprehensive characterisation of the intrusions and their chromitites on Uitloop offers critical insights into the integration processes of the earliest conduits and sills into the Rustenburg Layered Suite mafic-ultramafic sequence of the Bushveld Complex (BC).