Characteristics of zircon in placer deposits along the west coast of South Africa
Mining along the west coast of South Africa is dominated by the exploitation of onshore and offshore diamond deposits. The relatively recent discovery of vast resources of heavy minerals in the area has resulted in the establishment of a major related industry. Today, Namakwa Sands is a 10-million-ton-per-year operation and significant producer of ilmenite, zircon and ruffle by world standards. Heavy minerals are widely distributed along the entire west coast and are mainly concentrated in Mesozoic fluvial, Cainozoic marine and Recent aeolian unconsolidated placers. Basement rocks form part of the middle to late Proterozoic Namaqualand Metamorphic Complex, Gariep Group and Palaeozoic Cape Supergroup. Their diverse lithologies are the source of the younger heavy-mineral concentrations. The present study focuses on the characteristics of zircon (ZrSiO4), the mineral with the highest intrinsic value of the entire heavy-mineral suite. Both the heavy-mineral fraction and zircon concentrates of a representative suite of samples along the west coast were investigated using analytical techniques that included: transmitted/reflected light microscopy, electron microprobe mineral analysis, ICP for rare earth element analyses, PIXE for single-grain analyses and cathodoluminescence. Their radiometric characteristics were determined using a hyper-pure germanium detector. All samples studied comprised a heterogeneous population of zircon grains with diverse physico-chemical properties. This was expressed by large differences in colour and trace element chemistry of single grains. The percentage of grains hosting inclusions, such as ilmenite, magnetite, monazite, quartz and fluids, varied for each sample. Zoned, metamict and grains with overgrowths and replacement textures contributed to the diverse characteristics of zircon samples. Concentrations of the various grain types differed among samples and contributed to the unique character of each population. Bulk chemistry and radiometric signature support this observation. This variation depends on the composition of the source rock and stage of sedimentological evolution of the sands. A high degree of heterogeneity of the zircon population will adversely affect beneficiation of these mineral deposits. As a result, its quantification is required to optimize mineral recovery.