Abstract
One of the most enigmatic features of the world’s largest layered intrusion, the Paleoproterozoic Rustenburg Layered Suite of the Bushveld Complex, is the bifurcation of chromitite layers associated with the UG1 chromitite. Despite nearly a century of research, no consensus exists regarding the process(es) responsible for the formation of these bifurcated chromitites. Here we report the results of a high-resolution geochemical and petrological investigation conducted on an ∼195 m2 exposure of anorthositic footwall of the UG1 exposed by mining activities at No. 11 Shaft, Impala Platinum Mine, Rustenburg. Samples were taken from four sample cuts across the footwall exhibiting complex chromitite bifurcations in order to study lateral and vertical variations in the mineral chemistry of plagioclase and chromite and the Sr-isotopic composition of plagioclase as determined by LA-MC-ICP-MS. Our investigation supports the idea that several processes were responsible for the formation of the macroscopic features observed in the studied exposure. A sequence of numerous alternating layers of thin chromitite and anorthosite occurring between a thick basal and upper chromitite layer likely formed during a period of intense seismicity related to the emplacement of the Bushveld Complex. Chromite within these chromitite layers is thought to have nucleated and grown in situ on the chamber floor from a basal magma layer with a composition close to the chromite–plagioclase cotectic. This happened in response to the introduction of evolved, H2O-rich interstitial melts from the compacting crystal mush into the basal magma layer, a process aided by repeated shock waves passing through the chamber. Evidence for the involvement of H2O-enriched melts in the genesis of these thin chromitite layers is preserved in plagioclase that is anomalously calcic and poor in K2O compared to the plagioclase found in the intervening anorthosite layers and the relative abundance of phlogopite within chromitite layers. Additional bifurcations formed in response to the injection of chromite-rich slurries from the thick upper chromitite layer into the sequence below.