A mineralogical and geochemical review of cycles V and VI of the Upper Zone of the Bushveld Complex

Abstract

The Upper Zone is situated at the top of the Rustenburg Layered Suite and according to Kruger et al. (1987) represents the final stages of magma recharge. The Upper Zone, which is comprised of gabbro, anorthosite layers and magnetite layers, shows a number of mineralogical and geochemical reversals. Previous studies by Tegner et al. (2006) identified nine cycles (I-IX) that makes up the Upper Zone. Two of the most prominent cycles: Cycle V and Cycle VI were studied to gain a greater understanding of the changes occurring within as well as in between two cycles. The data collected included petrological and electron microprobe data from plagioclase, olivine, pyroxene, apatite, biotite and secondary amphibole. Although the current data showed some similarities to Tegner et al. (2006), this study has provided a number of additional features to the two cycles. Firstly, the cycles vary in terms of lithology. Secondly, the An% and Mg# from Cycle V shows greater fluctuations than previously identified. Lastly, the An% and Mg# trends within each cycle are not as similar as previously thought. The data clearly shows that the two cycles are different and therefore the conditions responsible for the formation of each cycle are different. Using this data, a new position of the boundary between the two cycles is suggested. The genesis of the Upper Zone has been proposed in a number of models however, this study has shown that the model responsible for the formation of the Upper Zone is yet to be established. Despite the theory from Kruger et al. (1987), the possibility of the Upper Zone having formed from more than one magma injection needs to be reconsidered.