Compositional variation of laurite at Union Section in the Western Bushveld Complex

Abstract

One hundred and forty five grains of laurite in polished sections of samples from one borehole through the major chromitite layers and some chromite-bearing silicate rocks of the Lower and Critical Zones of the western Bushveld Complex at Union Section have been located and analysed by scanning electron microscope. Ninety per cent by number of laurite grains are included within chromite, with the remainder being located on chromite-silicate grain boundaries, and in interstitial silicates and sulphides. The composition of laurite shows considerable variation within individual samples. Furthermore, there is no apparent correlation between whole-rock Ru and Cr contents in our samples, arguing against a model whereby laurite exsolved from the chromite lattice. Based on a well-defined correlation between whole-rock S, PPGE (Rh+Pt+Pd), and IPGE (Os+Ir+Ru) contents, we favour a mechanism whereby laurite crystallized from segregating sulphide melt and was subsequently entrapped by growing chromite grains.