Compositional variation of Co-rich pentlandite; relation to the evolution of the upper zone of the western Bushveld Complex, South Africa

The uppermost 12@ m of the upper zone of the western Bushveld Complex at Bierkraal were investigated by wholerock geochemistry and mineral chernistry of olivine and sulfides. The rocks are depleted in Ni, owing to olivine crystallization and separation sf immissills sulfide melt tlroughout most ofthe upper-zone sequence. This resulted in a higb ColNi ratio in the sulfide melt and produced cobalt-rich pentlandite ea sssling. Some of ttre pentlandite is not stoichiometric; no systematic relation between departure from stoichiometry and metal contents or metal ratios is detectable. Variation of Co,/Ni in pentlandite is reflected by ColNi in coexisting pyrrhotite and in wholerock compositions. The upper zone does not represent an unintemrpted differentiation of a homogeneous melt. Replenishment by less differentiated maena is reflected in cyclic changes in whole-rock chemistry, and in pentlandite and olivire compositions. The position of layers of magnetite at the base of geochemical cycles strongly indicates a genetic link between the formation of maguetite layers and magma mixing.