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

Philander, C.; Rozendaal, A.

doi:10.2113/econgeo.110.6.1577

Cited by 22 publications

(7 citation statements)

References 46 publications

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“…If an attempt is made to approximate the prismatic crystals of the yellow and red zircon grains a shape like that on display in Figure 12d provides the best fit (Figure 12d). Using the data from Pupin [82] the temperature of formation of this elongated zircon crystal stands at a T value of more than 900 • C, a fact in full agreement with the origin of these placer-type minerals sourced by basic magmatic rocks which are also of attraction to gemologists. What does the color of the zircon grains in the placer deposits tell us about the origin of the heavy minerals in question?…”

Section: Zirconium-bearing Gemstone Placers-zirconsupporting

confidence: 67%

“…Their shape has been preserved on release from the basalt and on transport to the proximal placer as can be deduced from a simple comparison of xenocrysts in the basaltic source rock and in the placer deposits, where zircon is associated with ilmenite, spinel, and corundum [81]. The high preservation potential of grain morphology sparked an intensive study whether the crystal morphology of zircon is a function of its physical-chemical conditions of [82][83][84][85][86]. The investigations of crystal morphology of zircon are very encouraging and the results can also be applied to heavy mineral accumulations in order to identify the source rock [87].…”

Section: Zirconium-bearing Gemstone Placers-zirconmentioning

confidence: 99%

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Gems and Placers—A Genetic Relationship Par Excellence

Dill¹

2018

Minerals

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Gemstones form in metamorphic, magmatic, and sedimentary rocks. In sedimentary units, these minerals were emplaced by organic and inorganic chemical processes and also found in clastic deposits as a result of weathering, erosion, transport, and deposition leading to what is called the formation of placer deposits. Of the approximately 150 gemstones, roughly 40 can be recovered from placer deposits for a profit after having passed through the “natural processing plant” encompassing the aforementioned stages in an aquatic and aeolian regime. It is mainly the group of heavy minerals that plays the major part among the placer-type gemstones (almandine, apatite, (chrome) diopside, (chrome) tourmaline, chrysoberyl, demantoid, diamond, enstatite, hessonite, hiddenite, kornerupine, kunzite, kyanite, peridote, pyrope, rhodolite, spessartine, (chrome) titanite, spinel, ruby, sapphire, padparaja, tanzanite, zoisite, topaz, tsavorite, and zircon). Silica and beryl, both light minerals by definition (minerals with a density less than 2.8–2.9 g/cm3, minerals with a density greater than this are called heavy minerals, also sometimes abbreviated to “heavies”. This technical term has no connotation as to the presence or absence of heavy metals), can also appear in some placers and won for a profit (agate, amethyst, citrine, emerald, quartz, rose quartz, smoky quartz, morganite, and aquamarine, beryl). This is also true for the fossilized tree resin, which has a density similar to the light minerals. Going downhill from the source area to the basin means in effect separating the wheat from the chaff, showcase from the jeweler quality, because only the flawless and strongest contenders among the gemstones survive it all. On the other way round, gem minerals can also be used as pathfinder minerals for primary or secondary gemstone deposits of their own together with a series of other non-gemmy material that is genetically linked to these gemstones in magmatic and metamorphic gem deposits. All placer types known to be relevant for the accumulation of non-gemmy material are also found as trap-site of gemstones (residual, eluvial, colluvial, alluvial, deltaic, aeolian, and marine shelf deposits). Running water and wind can separate minerals according to their physical-chemical features, whereas glaciers can only transport minerals and rocks but do not sort and separate placer-type minerals. Nevertheless till (unconsolidated mineral matter transported by the ice without re-deposition of fluvio-glacial processes) exploration is a technique successfully used to delineate ore bodies of, for example, diamonds. The general parameters that matter during accumulation of gemstones in placers are their intrinsic value controlled by the size and hardness and the extrinsic factors controlling the evolution of the landscape through time such as weathering, erosion, and vertical movements and fertility of the hinterland as to the minerals targeted upon. Morphoclimatic processes take particular effect in the humid tropical and mid humid mid-latitude zones (chemical weathering) and in the periglacial/glacial and the high-altitude/mountain zones, where mechanical weathering and the paleogradients are high. Some tectono-geographic elements such as unconformities, hiatuses, and sequence boundaries (often with incised valley fills and karstic landforms) are also known as planar architectural elements in sequence stratigraphy and applied to marine and correlative continental environments where they play a significant role in forward modeling of gemstone accumulation. The present study on gems and gemstone placers is a reference example of fine-tuning the “Chessboard classification scheme of mineral deposits” [1] and a sedimentary supplement to the digital maps that form the core of the overview “Gemstones and geosciences in space and time” [2].

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Section: Zirconium-bearing Gemstone Placers-zirconsupporting

confidence: 67%

Section: Zirconium-bearing Gemstone Placers-zirconmentioning

confidence: 99%

Gems and Placers—A Genetic Relationship Par Excellence

Dill¹

2018

Minerals

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“…Arctotis "chr ysantha" may have arisen as an early adaptation to southwestern redistribution of Kalahari Group red aeolian sediments. The oldest Kalahari sand deposits are of Pliocene to Pleistocene age (Vainer et al, 2018), but the deposits on the Namaqualand coastal plain are of more recent origin (e.g., Philander & Rozendaal, 2015).…”

Section: Photo By Robert Mckenziementioning

confidence: 99%

Monarchs in the veld: The Arctotis Annual Clade provides insight into recent floristic diversification in a semi-arid region of southern Africa

McKenzie¹,

Oberlander²,

Barker³

2022

TICA

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A revision of the main clade of annual species of Arctotis L. recognises eight species. All but one taxa are distributed in the winter and/or year-round rainfall regions of southern Africa, with the highest species diversity in semi-arid areas. Altitude and several temperature and precipitation parameters best explain latitudinal and longitudinal gradients in the distribution of taxa. Cypsela and pappus morphology suggest adaptation for reduced dispersibility in a semi-arid environment during diversification. Phylogenetic analyses of ITS and noncoding cpDNA sequence data resolve strongly suppor ted basal nodes and a haplotype network suggests a degree of taxonomic structure to cpDNA diversity. Fur ther research to resolve a genetically diverse "A. fastuosa species complex" is required. Diversification of the clade is not explainable solely by a simple vicariance model with parapatric speciation. A biogeographic hypothesis for diversification of the clade is formulated for future testing.

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“…Anorthosite-related rocks continue to be valuable sources of Ti (Woodruff et al, 2013). The most economically important Ti resources are heavy mineral sands in which rutile, ilmenite, and leucoxene are concentrated along with other heavy minerals in alluvial, marine, and eolian placer deposits (e.g., Philander and Rozendaal, 2015). Approximately 95 % of Ti ore is refined to produce white-colored TiO 2 pigment, and the remainder is used to produce Ti metal (Bedinger, 2015).…”

Section: History and Usementioning

confidence: 99%