An intrusive origin of some UG-1 chromitite layers in the Bushveld Igneous Complex, South Africa: Insights from field relationships

Abstract

The UG-1 chromitite in the Bushveld Complex is a 1m thick massive layer overlain by orthopyroxenite (4–9m thick) and underlain by anorthosite (1.5–2.0m thick), which is strongly interleaved with numerous thin layers of UG-1 chromitite (mm to 10s of cm thick). The chromitite-bearing anorthosite is further underlain by mottled anorthosite, which is devoid of chromitites. Sometimes, the massive UG-1 chromitite may be hosted entirely within orthopyroxenite; in this study, however, the orthopyroxenite occurs intermediate to the massive UG-1 layer and the footwall interleaved anorthosite unit, which has a 10cm thick chromitite layer at the contact. Though the UG-1 chromitites display complex features in the field outcrops (e.g., presence of numerous anorthosite autoliths, bifurcations, and dimpled contacts with anorthosite), the ones discussed in this study are distinctive and diagnostic of the emplacement mechanism for this chromitite layer. In this study of the UG-1 chromitite at the Impala Platinum mine, the chromitite is seen to form a small pothole (≈1.5m deep; 3.6m wide) that cuts down into the underlying orthopyroxenite (exposed only on the right side of the pothole) and anorthosite layers. A 15cm thick UG-1 chromitite layer (85 modal% chromites) is observed to protrude or branch out of the pothole margin and laterally extend into the anorthosite, with which it shows several crosscutting features. Examples include truncation of layering defined by interstitial pyroxene mottles within the footwall anorthosite against the protruding UG-1 layer, and truncation of several thinner UG-1 layers in the hanging wall anorthosite against erosive outlines of this chromitite layer. These observations strongly attest to the emplacement of the protruding UG-1 chromitite layer as a sill into the anorthosite. The presence of thin anorthosite lenses with high aspect ratios within this UG-1 layer, along with lateral variations in thickness of the layer, its irregular contact with the host anorthosite, and disturbances observed for the UG-1 layers in the footwall anorthosite adjacent to the erosive contact, implies that the superheated melt, parental to the chromitite, chemically dissolved the anorthosite and made space for it to flow as a sill within the latter. The probable mode of emplacement of the laterally protruding UG-1 layer would be through the opening of fractures related to the event of pothole formation where there is ponding of hot, thick and dense melt above the anorthosite. The melt subsequently filled the fractures and flowed laterally into the anorthosite thereby eroding it. An open magmatic system ensured continuous addition of melts in the pothole and their supply into the sill. This study thus emphasizes that some of the UG-1 chromitite layers within anorthosite can be emplaced as sills, and not all need to form from discrete cyclical pulses of chromite-saturated and plagioclase-saturated melts being brought into the chamber.