The coastal heavy mineral sand deposits of Africa

Abstract

The trailing margin of the stable African continent is the depositional environment of several heavy mineral placer deposits of which seven have developed into viable world class operations producing titanium feedstock (ilmenite, rutile, leucoxene) and zircon. At least 30 other deposits are marginal under the present global economic climate but have the potential to be turned into account in the medium term with an increase in demand and higher commodity prices. Combined, the seven operating mines are the biggest single producer of titanium feedstock in the world and the second largest producer of zircon after Australia. Mineral resources are vast and the extensive African coastline offers significant exploration potential and possibly is the largest depository of valuable heavy minerals (VHM) on the planet. Generally, Pliocene to Holocene unconsolidated, siliciclastic sands of predominantly marine-aeolian, but locally fluvial origin host the heavy mineral suite and deposits are large coast-parallel dune fields with or without minor strandlines. Tonnages of these deposits are large and in several instances exceed a billion tons with grades between 2 and 12 percent total heavy minerals (THM). Deposit characteristics are influenced by continental tectonics, coastal morphology, sea level changes, tides, climate and the altered, eroded nature and composition of the provenance. The formation of an economically viable deposit requires the juxtaposition of most, if not all of these conditions to favour heavy mineral concentration with a high VHM content. Mining is by open cast using either dry mechanized, hydraulic monitoring, or floating dredge and wet concentrator methodologies. Mineral separation uses physical properties such as density, magnetic susceptibility and electrical conductivity to produce high quality concentrates for the international market. Only three of the mines, all located in South Africa, have their own smelters that convert ilmenite into titanium slag and pig iron. Coastal environments are all eco-sensitive and mine site rehabilitation is employed throughout, including effective disposal of radiogenic tailings associated with high U+Th zircon and monazite. The most effective exploration methods for these deposits include identification of trailing continental margins with Neogene to Quaternary coastal strandlines and dune fields spatially related to geomorphological features such as headlands and significant fluvial systems. High-resolution ground and aeromagnetic surveys reflecting ilmenite and traces of magnetite, coupled with radiometric surveys related to monazite and high U+Th zircon content have been the most effective geophysical methods for these deposits. Reconnaissance followed by detailed infill drilling of anomalous areas by truck mounted rotary drill to collect samples for THM and VHM determinations is the industry standard.