Mineralogy and provenance of the Namakwa Sands heavy mineral satellite deposits

[摘要] ENGLISH ABSTRACT: Five areas proximal to the world class Namakwa Sands heavy mineral deposit have beenstudied and include the farms Houtkraal Remainder Portion 2, Houtkraal Remainder, GeelwalKaroo, Graauwduinen and Rietfontein. These are locally referred to as the satellite depositsand are sub-economic occurrences. The primary objective of the study was to quantify themineralogy and mineral chemistry, determine the provenance of the heavy mineral suite anddraw a comparison between the satellite deposits and the Namakwa Sands deposit from anexploratory point of view. Methodology used to achieve the above objectives included opticalmicroscopy, Scanning Electron Microscope (SEM), Laser Ablation Inductively CoupledPlasma Mass Spectrometry (LA-ICP-MS), Quantitative Evaluation of Minerals by ScanningElectron Microscopy (QEMSCAN), X- Ray Fluorescence (XRF) and Zr-geothermometry ofrutile.The five satellite areas contain the same heavy mineral suite but mineral proportions differ.The total heavy mineral population (THM) are diverse and consist of ilmenite and itsalteration products (hydrated ilmenite, pseudorutile and leucoxene), magnetite, hematite,spinel, rutile, tourmaline, pyroxene, amphibole, garnet, aluminosilicates, staurolite,corundum, epidote, zircon, monazite and sphene. Ilmenite and garnet are the two mostdominant heavy minerals present. The valuable heavy minerals (VHM) suite consists ofilmenite, zircon, rutile and leucoxene.The mineralogy of the satellite areas and chemistry of the ore minerals (rutile, zircon,ilmenite and leucoxene) are similar to the Namakwa Sands deposit.The whole spectrum of ilmenite alteration products (hydrated ilmenite, pseudorutile, andleucoxene) is present and allowed the quantitative use of the alteration index. The indices islow (22-24%) and indicates that the surficial deposits have formed under arid to semi-aridclimatological conditions which preserved the pristine character of most of the minerals.This allowed reliable provenance studies using the characteristics of most of the heavymineral suite, which showed that the minerals were derived from a diversity of source rocks.These included mainly medium to high-grade metamorphites and felsic intrusives of theunderlying Mesoproterozoic Namaqualand Metamorphic Complex and a minor contribution from the Neoproterozoic Gariep Supergroup. This relationship indicates a limited transportdistance from source to depositional basin.Mineral ratios in particular the THM-VHM relationship showed that the deposits locatedclose to the shoreline such as Geelwal Karoo, Graauwduinen and Rietfontein have arelatively low proportion of valuable heavy minerals whereas those more inland such asHoutkraal Remainder Portion 2 and Houtkraal Remainder are close to unity.Heavy mineral concentration as such is low in the satellite areas and the mechanism toincrease the concentration is clearly not only a function of distance from the present shorelinebut is also topographically controlled. Steep sided linear depressions channelled theunconsolidated sediments and heavy minerals were upgraded into economic concentrationsby aeolian processes. The quality of the valuable heavy minerals in the satellite areashowever is similar to those of the adjacent Namakwa Sands deposit. This study hasdemonstrated that Houtkraal Remainder is the northeasterly continuation of the red aeoliansand (RAS) associated East Mine orebody and offers the best exploration potential.

[发布机构] Stellenbosch University