ABSTRACT

The Bushveld Complex, South Africa, is the world's largest body of rock that crystallized beneath the Earth's surface from molten rock over two billion years ago. It is host to Earth's largest resource of platinum group elements (PGE) and also a significant resource for chromium (Cr) and vanadium (V). The enrichment of the Bushveld Complex in these elements is likely related to a number of factors, including the original source of the melt and processes occurring during its ascent and emplacement into the crust. Studying the Bushveld is important not only for the understanding the formation of PGE ore deposits but also to comprehend the Earth processes that led to such a large and unusual body of rock. The Bushveld melt was rich in magnesium, which suggests it was originally derived from the Earth's mantle, but puzzling crustal isotopic signatures, including heterogeneous isotopic compositions of Sr, Nd, Pb, O and S, suggest that it endured a complicated history of ascent, interactions with its surroundings and possible incorporation of crust. The recent discovery of a thick succession of highly magnesian rocks (the Basal Ultramafic Sequence) below what have generally been accepted as the lowermost layers of the Bushveld provides a unique opportunity to investigate rocks derived from the most pristine melts associated with the Bushveld and to test hypotheses about the sources of these crustal signatures. A novel approach that uses the isotopic composition of the element sulfur (S) will be used to investigate how deep magma interacted with the shallower crust to yield these rocks and constrain which Earth processes that led to the formation of the largest PGE deposit on Earth.

The Basal Ultramafic Sequence provides the opportunity to investigate rocks that have experienced minimal alteration during emplacement and crystallization (Wilson 2012; 2015). Existing chemical and mineralogical data and planned Sr and Nd isotope analyses will be compared to measurements of Del33S on the same samples. Questions to be addressed include (1) whether there is a signal of surface-derived S in these lowermost Bushveld igneous rocks, (2) whether these lowermost units are uniform in Del33S values, and (3) how S isotopic compositions correlate with other indicators of crust such as Sr and Nd isotopes. Several hypotheses for the source of the crustal isotopic signals will be tested including (1) crustal signatures of the Bushveld derived from sub-continental lithospheric mantle, (2) crustal signatures derived from continental crust perhaps in a deep crustal staging chamber, (3) crustal signatures derived from interaction with wall rock upon emplacement, and (4) exchange between silicate melt and immiscible sulfide melt which enriched the sulfide in crustal S. The Sr isotopic composition of the Bushveld Complex varies throughout the igneous stratigraphy, and this variation has been interpreted as the result of incorporation of different amounts of crustal material in different magmas ? with varying opinions on the numbers of different pulses of magma (e.g., Sharpe, 1985; Kruger, 1994). So far, it appears that the Sr and S isotopic systems are not linked in the Bushveld Complex, since there are distinct variations in Sr isotopes, but the S isotopic composition appears to be uniform. However, there are no studies in which both these systems have been measured on the same samples. This project provides the opportunity to investigate these two isotopic systems, both indicators of crust, on the same samples.

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