A study of titanium-bearing oxides in heavy mineral deposits along the East Coast of South Africa.

Abstract

Heavy mineral deposits along the east coast of South Africa represent the world's largest demonstrated resource of beach placer ilmenite. This mineral occurs as homogeneous, subrounded grains, with chemical compositions close to pure FeTi03• Concentrates contain between 48 and 52 per cent Ti02, with minor impurities of MnO, MgO, and Cr203. Most coastal ilmenites are unaltered or display only incipient alteration, but the entire spectrum of alteration products from ilmenite to rutile or anatase, is observed. Transmission electron microscopy of weathered ilmenites reveals that ilmenite commonly alters to pseudorutile and then to rutile or anatase, as described by Teufer and Temple (1966) and Grey and Reid (1975). Ilmenite may also alter directly to rutile (or anatase) in a single-stage process. In addition, ilmenite altered by high temperature oxidation and hydrothermal processes is found in the deposits. There is good mineralogical evidence that the alteration of ilmenites found in the coastal sediments is best described by a multi stage model, in which some ilmenite grains were altered prior to final deposition. Other common iron-titanium oxides in the deposits include magnetite, rutile and hematite, which may occur as discrete grains or as composite grains of two or more oxides. Ilmenite and magnetite in the coastal sediments are derived from rocks of both the Karoo Igneous Province and the Natal Basement, while rutile is derived solely from the latter. Ilmenites from certain rock groups may be distinguished on the basis of their chemical composition. However, magnetite chemistry is a better indicator of provenance, and magnetites from the above two sources can be clearly distinguished. The petrography of the iron-titanium oxides may be used as a provenance indicator, but may be misleading, as the proportions of the oxide intergrowths change with transport and weathering. Variations in the proportions and chemical compositions of iron-titanium oxides and other heavy minerals within the coastal sediments are caused by provenance, selective sorting during deposition, age of the deposit, weathering, and the recent geological history of the area. A model is proposed in this study which describes the formation of the heavy mineral deposits in relationship to the above influences.