Sulfide mineralogy at the main magnetite layer, upper zone, eastern Bushveld Complex, and the effect of hydrothermal processes on pentlandite composition

Abstract

The sulfide assemblages within a few metres above and below the main magnitete layer (MML) in the upper zone of the eastern Bushveld Complex have been studied including detailed microscopic work and elecron microprobe analyses of pentlandite and pyrrhotite. Rocks at this stratigraphic level, especially below the MML, are enriched in sulfides relative to the concentrations normally encountered in the lower part of the upper zone. The main sulfide minerals, are, in decreasing oder of abundance, pyrrhotite, chalcopyrite, cubanite and pentlandite. Pyrrhotite compositions span the whole compositional range from monoclinic pyrrhotite to troilite. Within pairs of coexisting pyrrhotite and pentlandite, the Fe content in puyrrhotite is related to the Fe/Ni ratio of pentlandite. However, pyrrhotite compositions with and without visible contact to pentlandite can not be distinguished. The same applies to pyrrhotite with and without visible contact to chalcopyrite or cubanite, and for the composition of pentlandite with and without pyrrhotite or chalcopyrite. Above the MM, the composition of pentlandite varies considerably within individual samples (by up to 18 at.% Fe, 22 at.% Ni and 28 at.%Co) which is related to hydrothermal processes. These processes led to an increase in the Co content of all pentlandite grains above the MML. While substitution of Co for Ni is dominant, Co also replaces Fe in Co-rich pentlandite. Furthermore, Ni in Co-poor pentlandite is replaced by Fe. In constrast, the composition of pyrhhotite was only found to be affected if in contact with secondary magnetite or Co-rich pentlandite. Although changing the cation proportions in pentlandite, the hydrothermal processes did not influence the metal/sulfur ratio. it is concluded that hydrothermal activity played an important role in changing the sulfide mineralogy and composition in this part of the upper zone. The absence of minerals indicative of hydrothermal overprinting in a sulfide assemblage can not be taken as evidence that this assemblage has not been affected by hydrothermal processes