Solving the enigma of UG1 chromitite emplacement in the Bushveld Complex, South Africa

In this work we present remarkable outcrop scale features of the UG-1 chromitite, along with geochemical data (bulk rock and chromite mineral chemistry) of the rocks in UG-1 stratigraphy. The main UG-1 chromitite usually occurs as a 1 metre thick massive layer that is underlain by anorthosite enclosing numerous thin chromitite layers (10's of cm-mm scale). Geochemical data suggests that the top and bottom massive UG-1 layers, separated by an orthopyroxenite layer, formed from individual large pulses of chromite-saturated melt on the chamber floor that experienced closed-system fractionation. Field features of thin chromitite layers in the footwall, such as matching boundaries of anorthosite autoliths enclosed within chromitite, with adjacent anorthosite layer, parallel disposition of bifurcating chromitite branches, and chromitite protrusions into anorthosite that transgress layering in the latter, suggest however, that these were probably emplaced as sills into the anorthosite. Furthermore, undulating contact between anorthositechromitite layers implies magmatic erosion was at play, which may have facilitated the intrusion process. Thus, two modes of emplacement appear to have been involved in the origin of the UG-1 chromitite- the main layer formed from basal flows of melts directly on the chamber floor, whereas the thin ones within anorthosite were produced as sills.