Geology of the Cenozoic Namakwa Sands Heavy Mineral Deposit, West Coast of South Africa: A World-Class Resource of Titanium and Zircon

Abstract

Exploration for heavy minerals along the coastal strip of southwest Africa led to the discovery and subsequent delineation of the Namakwa Sands deposit near Brand-se-baai in 1987. Currently, Tronox Limited controls the Namakwa Sands operation, which has developed into a significant, global producer of titanium slag, pig iron, ceramics quality zircon and rutile concentrates. The Namakwa Sands deposit is classified as a megaresource with 1.17 billion tons (Bt) of in situ ore, containing an estimated 92.6 million metric tons (Mt) of total heavy minerals and the equivalent of 23.9 Mt TiO2 units and 9.4 Mt zircon. The siliciclastic, arenaceous deposit is divided into two adjacent orebodies, referred to as Graauwduinen West and Graauwduinen East. The Graauwduinen West orebody consists of three strandline-dune couplets set in a transitional shallow marine-aeolian environment, whereas the Graauwduinen East orebody comprises a largely unrelated dune deposit. Superimposed onto the ore-bearing sequence is a pseudostratigraphy of duricrust that effectively cemented the bulk of the mineralized sands to various degrees of hardness. The two orebodies are profoundly different in many aspects. Spatially, the planar mineralization bears northeasterly in the Graauwduinen West orebody, strikingly perpendicular to the southeasterly trend observed in the Graauwduinen East orebody. Contrasts in their bulk chemistry and mineralogy are statistically significant. The Graauwduinen West orebody features noticeably better total heavy mineral grades, accompanied by conspicuously high proportions of the gangue minerals garnet and pyroxene. The valuable mineral fraction is predominated by ilmenite (FeTiO3), followed in abundance by zircon (ZrSiO4), leucoxene (an alteration product of ilmenite), and rutile (TiO2). Their geochemistry, mineralogy, and petrography are extremely variable, but the bulk of the current valuable mineral fraction is recoverable to premium-grade products. The proximal, Mesoproterozoic Namaqualand Metamorphic Province is considered the main contributor to the current heavy mineral population. The Graauwduinen West orebody received source sediment via established fluvial-marine courses, whereas the source of the Graauwduinen East orebody is considered to originate from a distinct fluvial-aeolian corridor. The genesis of the Namakwa Sands deposit is strongly entrenched in the geomorphic evolution of the west coast of southern Africa since the Cenozoic, which was driven by regional tectonism, fluctuating sea levels, and alternating climatic patterns.