Nearshore marine garnet and magnetite placers in the Erongo and S-Kunene regions, Namibia

Abstract

Reddish to black colored heavy mineral sands occur alongside the several hundred-kilometer long placer district in the Erongo and S-Kunene regions in Namibia and are the focus of this study. The sedimentary packages of these nearshore marine ‘garnet’ and ‘magnetite’ placers consist of millimeter to decimeter thick discontinuous, alternating layers of heavy minerals (ρ > 2.90 g/cm³) set into dominantly feldspatho-quartzose layers. The fine-to medium-grained sands are largely unimodal, well sorted, and have similar bulk mineralogical compositions. A clear trend in grain size distribution is evident with translucent (Al2SiO5 phases, apatite, epidote, garnet, monazite, pyroxene, rutile, schorl, sphene, staurolite, zircon, zoisite) and opaque heavy minerals (Fe–Ti oxides) mostly related to the fine-sand fraction and rock-forming minerals (quartz, feldspars, micas, carbonates) to the medium sand-fraction. Fines are negligible. Grain circularity varies largely between 0.9 and 0.8. Heavy mineral sands between the Omaruru and the Huab Rivers are garnet-dominated, whereas the southernmost heavy mineral sands close to Swakopmund are (titano)magnetite-ilmenite dominated. Overall, the heavy mineral content of sampled layers is ~22–94 vol%. Three placer areas (1) the area around Swakopmund, (2) the region north of the Omaruru River, straddling north beyond Cape Cross and (3) the Northern region, south of Horing bay to Toscanini, can be delineated based on mineralogical and grain parameters. The bulk of the heavy minerals is likely derived from metamorphic rocks within the catchment of the Orange River of South Africa, with minor contributions from the Kuiseb and Swakop Rivers in Namibia. A Damara provenance of some detrital material is indicated. The contribution of ephemeral hinterland rivers is negligible. Naturally occurring tidal actions of the Atlantic Ocean continually reshape, redeposit and replenish the high-grade heavy mineral sands and impinge on the regional sorting trends. Future mining and mineral processing activities may concentrate on the heavy mineral sands close to Swakopmund as they have the highest zircon and monazite potential and bear the most promising Ti resource. However, these coastal environments are eco-sensitive and most of the locations are within national parks belonging to the natural heritage of Namibia.