Geochemical characteristics and rare earth element trends of the Upper Zone in the western Bushveld Complex

Abstract

The geochemical trends of the Bierkraal 1 drill core are investigated to document fractionated magma, 2) convectional overturning resulting from the disintegration of a stagnant layer, or 3) whether prestratification of the magma in the magma chamber existed. Rare earth element distribution coefficients (calculated by the ratio of rare earth element content of apatite to rare earth element content of host matrix) indicates that the apatite in magnetite and apatite rich zones crystallized under conditions nearer to equilibrium than apatite in any other rock units. This lends credence to the theory that an immiscible Fe-P20 5 rich melt forms during the crystallization of the Upper Zone. Various interwoven geochemical twin trends following in stratigraphic sequence, are identified in some cycles and are found to be similar in geochemical behaviour. These trends are associated with normal fractionation processes. The presence of interwoven twin trends are tentatively ascribed to crystallization being alternatively dominated by adjacent convection cells within the magma. the behaviour of various elements. Special emphasis is placed on the rare earth elements during the final stages of crystallization of the Upper Zone in the western Bushveld. The geochemical characteristics of the western Bushveld Upper Zone are also briefly compared to the Upper Zone of the eastern Bushveld as exposed in the Roossenekal district. The rock types intersected by the Bierkraal 1 borehole consist largely of gabbro, magnetite gabbro, lesser magnetite and anorthosite layers. Plagioclase comprises 50 to 70 volume percent of most samples investigated. Up to 87 volume percent plagioclase is found in anorthosite while magnetitite layers consist of up to 97 volume percent oxide. Unlike the eastern Bushveld, olivine occurs throughout the Bierkraal 1 borehole and, is generally limited to the range of 15 to 30 volume percent. Pyroxene is generally well below 10 volume percent with hornblende becoming a major constituent only in the upper part of the borehole. Apatite, which generally occurs in the percent range, is the mineral with the highest concentration of rare earth elements in the Upper Zone. The accessory minerals are K-feldspar, quartz and zircon are limited to the uppermost parts of the sequence, while biotite occurs throughout. Closely spaced samples indicate that the overall trend in the Bierkraal 1 borehole can be subdivided into 4 cycles. The 4 cycles are identified by means of apatite and olivine composition, whole rock and trace element contents. The individual cycles undergo normal fractionation. The modelling of overall Rb enrichment suggests a trapped liquid content of 1 5 to 37 percent. Modelling each cycle individually, shows that the cycles undergo crystallization in an open system with residual liquid escaping and, probably mixing with overlying fluid. Four cycles, corresponding to the whole rock geochemistry, are evident in the whole rock and apatite rare earth element content. The rare earth element trends are shown to be largely dominated by modal proportions of apatite and plagioclase and, to a lesser extent, by normal fractionation. Alteration and metamorphism are shown to have little effect on the rare earth element patterns. Apatite samples are light rare earth enriched, at approximately 1000 times chondrite. The light rare earth element enrichment is typical of late stage differentiation, while certain light rare earth element ratios imply contamination of the mafic magma by Transvaal sediments. A specific melt type, from which the Upper Zone crystallized, is not clearly indicated by the whole rock rare earth element patterns. The replenishment by a less fractionated magma and the resulting mixing, rather than the introduction of new magma, are thought to be responsible for cyclic trends. It is not clear whether these mixing events are due to, 1) pulses of the same, but less