Contrasting sulphide contents of the Bushveld and Sudbury Igneous Complexes

Abstract

The Sudbury Igneous Complex (SIC) contains abundant sulphides, especially near the base, and hosts one of the world’s largest nickel and copper deposits. The Bushveld Complex (BC) contains relatively little sulphide, but hosts the world’s largest platinum-group element deposits. The most recent calculations of the sulphur solubility in magmas that produced the BC are based on the sulphur solubility of mid-ocean ridge basalts that have less SiO2 than Bushveld magmas. Such a difference may lead to an overestimation of sulphur solubility by as much as 25%. The revised sulphur solubility curve presented here for Bushveld magmas may also have relevance to the SIC in view of its siliceous nature. Sulphur solubility curves can be used to determine the proportion of sulphide expected in cumulate rocks once sulphur saturation is attained. These models are tested using observed sulphide contents in both intrusions. The observed decreasing sulphur contents (>0.3–0.05% S) from the base of the SIC upward are broadly consistent with these sulphur solubility curves, and are consistent with sulphide saturation through the entire mafic portion. In contrast, the lower half of the BC contains extremely little sulphur (generally <0.02% S), except for two thin layers, which is not consistent with sustained sulphide saturation at any level. Previous interpretations of the sulphur content of Bushveld rocks have suggested that the Lower and Critical Zones were sulphide saturated, but that they had then lost some of the sulphide due to various processes. The present sulphide content of the cumulates of the BC is so low that, if they had once been saturated, over 90% of all the sulphide must have been removed. Mass balance calculations indicate that these large amounts of displaced sulphur remain unaccounted for in such models. Instead, the observed sulphur contents are in reasonable agreement with that expected in a cumulate sequence forming from a sulphur-undersaturated magma. Whereas the Merensky Reef and Bastard pyroxenite contain minor sulphides, the compositions of the immediate hanging wall rocks indicate sulphide undersaturation. Such an abrupt return to sulphide undersaturation is not consistent with models involving sulphide formation from large volumes of magma. One possible explanation for these two observations is that intermittent sulphur degassing occurred through a fractured roof of the BC, so that the magma was never continuously sulphur-saturated with respect to an immiscible sulphide liquid.