STRATIGRAPHIC UNIFORMITY OF THE MULTIPLE SULFUR ISOTOPE COMPOSITION OF THE BUSHVELD COMPLEX

The Bushveld Complex (BC), South Africa, has been studied for decades due to its geological uniqueness and economic importance. The mantle-derived magma was emplaced into the crustal rocks of the Transvaal Supergroup in multiple pulses at ~ 2.06 Ga. One line of evidence for multiple magma pulses comes from large variations observed in the ⁸⁷Sr/⁸⁶Sr and ε_Nd values through the stratigraphy of the BC. These isotopic systems also suggest the presence of a crustal component in the magma, which is interpreted as contamination, but the source of the contamination is debated. The oxygen isotopic composition of the BC also suggests contamination (average value of 7.1‰, higher than the mantle value of 5.7‰), but it does not display the same variability through the igneous stratigraphy. Measurements of the sulfur isotopic composition of the BC show evidence for a mass-independent, crustal sulfur signal (Δ³³S≠0) in these rocks, and suggest uniformity of Δ³³S (0.11±0.02‰ (1σ)) in the igneous body. Here we systematically analyze samples from the BC stratigraphy to test the hypothesis that Δ³³S is uniform and to better understand the source of contamination.

Measurements of the sulfur isotope composition of samples from the different stratigraphic levels (from Lower to Upper Zone) of the BC yield values of δ³⁴S that range from -1.18‰ to 3.2‰ and Δ³³S values ranging from 0.06‰ to 0.16‰, with an average of 0.12±0.03‰ (1σ). These results confirm earlier findings of a crustal sulfur signal in the Bushveld. The Δ³³S values are also uniform within the uncertainty of measurement regardless of geographic location (both Eastern and Western limbs) or stratigraphic location. Similar Δ³³S values were obtained for both pristine and altered rocks, suggesting that the measured values represent the primary magmatic sulfur isotope composition. Additionally, lateral compositional homogeneity between the different limbs of the BC, and the lack of a large variation in isotopic values throughout the stratigraphy suggest that questions such as timing, source and amount of contamination of the BC magma needs to be revisited in more detail.