Stratigraphic variations in chromite chemistry through the UG-2 and UG-2E chromitites, bushveld igneous complex: implications for chromitite petrogenesis

Chromitite petrogenesis in layered intrusions is important because some chromitites host economic Ni-Cu-platinum group element mineralization. Cryptic layering, which could be useful to reconstruct the chromitite origin, has been documented for some chromitites. Here, we document cm-scale stratigraphic profiles of the major and trace element contents in chromite across two chromitite seams from the Bushveld Igneous Complex. One set is from a massive UG-2 seam from the western limb, the other from a chromitite seam in the Turfspruit area of the northern limb, considered stratigraphically equivalent to the UG-2 (henceforth the ‘UG-2 equivalent’ or ‘UG-2E’). Stratigraphic profiles across the massive UG-2 only show a gradual increase in V contents (from bottom to top) and subtle variations in Mg#, Cr#, Mn, Zn, Co, Ti, and Ga contents. In contrast, the UG-2E profiles show significant variations in Mg# but not in Cr#, and distinct shifts in Mn, Zn, Co, Ni, Ti, and Ga contents and Fe3+/∑Fe values. These shifts correlate in part with variations in chromite abundance but also occur in sections as massive as the UG-2 reference. We argue that, in general, the presence of cryptic layering is more consistent with chromitite formation via gravitational settling or in situ crystallization rather than via slurries.