The role of fluid-induced partial melting in magmatic crystal mush in the origin of stratiform chromite deposits of the Bushveld Complex, South Africa

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This project aims at addressing unresolved key questions about the origin of layers chiefly composed of Cr-spinel in mafic-ultramafic layered rocks of the Bushveld intrusive complex in South Africa. In terms of economic geology, the layers are synonymous with stratiform chromite deposits and constitute the greatest world reserve of metallurgical chromium and platinum-group elements. The project aims at testing a working hypothesis that chromitite layers of the Bushveld Complex formed in situ by partial melting and replacement reactions at the floor of the magma chamber. Partial melting and replacement of silicate minerals by chromite at the top of the earlier formed crystal mush is proposed to be triggered by additions of volatile components to the resident mama. It is assumed in the working hypothesis that the volatiles were introduced by crustal fluids generated by dehydration of Transvaal sediments below the Bushveld magma chamber. The research program is a combination of petrographic, geochemical, experimental and structural studies. It includes experimental study of the effects of H2O on the crystallisation of the Bushveld parental magma; experimental modelling of replacement reactions involving Cr-spinel; documentation of disturbances and subsequent chemical and textural equilibration in natural rock samples of the chromite layers and silicate rocks immediately below and above them. Special attention in the study of natural samples will be given to equilibration temperature estimations based on trace elements and oxygen isotope distribution between minerals.