South Namibia Research Articles

Diatreme-hosted fluorite mineralization in S-Namibia – A tale of cryogenic brine formation and fluid mixing below an unconformity in the context of Pangea rifting

The Garub fluorite deposit in south Namibia is hosted by a Neoproterozoic tuffisite diatreme which is cut by a hydrothermal vein with base metal mineralization. Petrographic observations indicate a single hydrothermal event producing a paragenesis of fluorite-baryte with base metal sulfides, quartz and carbonates. Microthermometry data from fluid inclusions show that hydrothermal fluorite and baryte were precipitated from a high-salinity (16.9–22.7 wt% NaCl + CaCl2), moderate-temperature brine (180–210 °C). This brine formed by mixing between a warmer, Ba- and F-rich endmember likely representing a deep-seated basement brine (chemically equilibrated with Namaqua-Natal-Metamorphic Complex basement rocks) and a cooler, SO4- and Ca-rich endmember that likely equilibrated with Nama Group limestone. Chlorine/Br mass ratios between 152 and 612 indicate the mixing of a fluid with a halite dissolution brine signature (endmember 1) and an equally saline brine derived from Nama Group limestone (endmember 2). Rubidium/Cesium (Rb/Cs) ratios between 2.2 and 20.5 provide evidence for significant fluid interaction with clay minerals. Since clay minerals are abundant in the tuffisite of the Garub diatreme, these Rb/Cs ratios indicate that fluids migrated along the diatreme-gneiss contact along a zone of weakness. Late-stage quartz and calcite likely reflect cooling and induced a shutdown of the hydrothermal system. The sulfide phases in this hydrothermal vein are strongly depleted in trace elements compared to other hydrothermal vein districts, which is interpreted as an inherited signature of the fluid source.There is no record of evaporites in the sedimentary rocks of the Nama Group. Based on the fluid chemistry of the mixed fluid, which clearly indicates halite dissolution and water-rock interaction with basement rocks, it is concluded that the basement brine fluid endmember is likely generated through cryogenic brine formation during the large scale Dwyka glaciation event. As a farfield consequence of Pangea breakup fluid pathways were established that acted as host structures for the Garub fluorite deposit. This could be the first evidence for unconformity-related hydrothermal fluorite deposit formation in the context of Dwyka cryogenic brine formation in southern Africa.

Timing and conditions of regional metamorphism and crustal shearing in the granulite facies basement of south Namibia: Implications for the crustal evolution of the Namaqualand metamorphic basement in the Mesoproterozoic

Granulite facies basement gneisses from the Grünau area in the Kakamas Domain of the Namaqua-Natal Metamorphic Province in south Namibia show high-grade mineral assemblages, most commonly consisting of garnet, cordierite, sillimanite, alkali feldspar and quartz. Cordierite + hercynitic spinel, and in some places quartz + hercynitic spinel, indicate granulite facies P-T conditions. The peak assemblage equilibrated at 800–850 °C at 4.0–4.5 kbar. Sillimanite pseudomorphs after kyanite1 and late-stage staurolite and kyanite2 indicate that the metamorphic record started and ended within the stability field of kyanite. Monazite in the metamorphic basement gneisses shows a single-phase growth history dated as 1210-1180 Ma, which we interpret as the most likely age of the regional metamorphic peak. This time coincides with the emplacement of granitic plutons in the Grünau region.The ∼10 km wide, NW-SE striking Grünau shear zone crosscuts the metamorphic basement and overprints high-temperature fabrics. In sheared metapelites, the regional metamorphic peak assemblage is largely obliterated, and is replaced by synkinematic biotite2, quartz, alkali feldspar, sillimanite and cordierite or muscovite. In places, gedrite, staurolite, sillimanite and green biotite3 may have formed late- or post-kinematically. The mylonitic mineral assemblage equilibrated at 590–650 °C at 3.5–5.0 kbar, which is similar to a retrograde metamorphic stage in the basement away from the shear zone. Monazite cores in two mylonite samples are similar in texture and age (∼1200 Ma) to monazite in metapelites away from the shear zone. Chemically distinct monazite rims indicate a second growth episode at ∼1130-1120 Ma. This age is interpreted to date the main deformation episode along the Grünau shear zone and the retrograde metamorphic stage seen in the basement.The main episode of ductile shearing along the Grünau shear zone took place 70–80 million years after the thermal peak metamorphism and granite emplacement, and after substantial isobaric cooling of the basement. Metamorphism and regional shearing in the Grünau area can be correlated with the crustal evolution in the Kakamas Domain in South Africa, but not with the timing of metamorphism in the Aus area, 230 km to the NW of Grünau, which is significantly younger.

Multi- and hyperspectral spaceborne remote sensing of the Aggeneys base metal sulphide mineral deposit sites in the Lower Orange River region, South Africa

New tools and algorithms for geological remote sensing are developed and verified at test sites throughout the world in preparation of the German hyperspectral satellite Mission (EnMAP), which is an Environmental Mapping and Analysis Program. The Aggeneys Cu-Pb-Zn deposit, situated in the arid north western part of South Africa, represents a unique field laboratory for testing these new tools. Here spaceborne hyperspectral data covering the Swartberg, Big Syncline and Gamsberg area were collected by the Hyperion sensor. New synergies between multispectral and hyperspectral spaceborne data can be demonstrated, such as the Iron Feature Depth index (IFD), which has recently been proposed for mine waste mapping in the North West Province of South Africa and for gossan detection at Haib River in South Namibia. The work presented here explores the potential of the IFD for gossan mapping and characterization at Gamsberg and Big Syncline, from EO-1 ALI and Landsat-8 OLI data together with mineral maps from expert systems such as the United States Geological Survey (USGS) Material Identification and Characterization Algorithm (MICA), and first results from EnMAPs EnGeoMAP algorithm. Field spectroscopic measurements and field sampling were carried out to validate and calibrate the results from the expert systems and the IFD. This ground truthing is a necessary complementary step to link the results from the expert systems and the IFD to in-situ field spectroscopy. Future mineral exploration initiatives may benefit from the techniques described here, because they can significantly narrow the expensive, exploration activities such as hyperspectral airborne data, field activities and drilling, by identifying the most promising mineral anomalies in an area from the spaceborne data.

Morphology and long-term landscape evolution of the South African plateau in South Namibia

The South Namibian Plateau, which belongs to the South African Plateau, is situated between 25°‐29°S and 15°‐20°E. This study examines the relationships between this portion of the plateau and the development of planation surfaces and scarps bounding the plateau. Geomorphological analysis through morphometric indices extracted from a DEM (90m) and its derived maps as well as field observations supported by the literature data permit the demonstration that tectonics, climate and base level are the main factors that control the formation and evolution of the planation surfaces and scarps that separate them. From west to east, the South Namibian Plateau consists of four main morphological provinces: the coastal province and the intermediate, upper and Kalahari provinces in the interior plateau. These provinces include five Cenozoic planation surfaces that are differentiated by their elevations, regional slopes, relative reliefs and erosion processes. These surfaces have been formed by etchplanation and pediplanation. The drainage pattern and river length profiles show that the coastal province evolved differently than the interior plateau. The development of the coastal province is controlled by eustasy and tectonics, whereas climate-related processes and tectonics control that of the interior plateau. The presence of several scarps separating these surfaces, the most degraded and older of which are inside the plateau, reflects a polyphase evolution. This evolution can be highlighted by two models: (1) an eastward migration mantle dynamics causing a slight uplift and rejuvenation of the system by destabilizing the coupled weathering/erosion equilibrium and (2) an internal propagation of isostatic rebound induced by the retreat of the western scarp leading to a significant flattening in the central part of the plateau.

The timing of mantle and crustal events in South Namibia, as defined by SHRIMP-dating of zircon domains from a garnet peridotite xenolith of the Gibeon Kimberlite Province

The Gibeon Kimberlite Province, southern Namibia, comprises numerous, 70–80 Ma old kimberlite pipes and dykes commonly containing abundant mantle and crustal xenoliths. We dated by SHRIMP II zircons from a garnet peridotite xenolith (Pipe Hanaus 1). The xenolith consists of garnet, orthopyroxene, clinopyroxene and, almost completely serpentinised, olivine. Crystallisation P– T conditions range between ca. 3.3 GPa, 910 °C (core composition of minerals) and ca. 3.5 GPa, 1000 °C (rim composition). Fifteen ca. 150–40 μm long and 70–10 μm wide zircon crystals show more or less pronounced oscillatory zoning indicating precipitation in a melt or fluid. The SHRIMP results define three age groups: 1013 ± 22 Ma, 751 ± 28 Ma and 627 ± 24 Ma. These ages are clearly correlated with large-scale crustal events in the broad area of Namibia, namely with the granulite-facies metamorphism during the Namaqua collisional event and post-collisional magmatism (ca. 1000–1100 Ma), with the intense magmatic activity and initial rifting stages in the Damara belt (ca. 730–770 Ma) and with the Early Pan-African orogeny, responsible for the formation of the Damara orogen (ca. 650 Ma), respectively. Formation of zircon in the xenoliths is related to the generation and migration of fluids/melts in the mantle via e.g. asthenospheric mantle upwelling or dehydration of subducting lithosphere, associated with crustal large-scale events.

A statistical study of NDVI sensitivity to seasonal and interannual rainfall variations in Southern Africa

The relation between rainfall and the Normalized Difference Vegetation Index (NDVI) in Africa south of 15 S (1983-1988) is studied. For 115 1 by 1 grid-points, the spatial distribution of annual NDVI and rainfall means is highly comparable. Both parameters have overall decreasing values from Mozambique to South-Western Africa. The strongest correlations occur when NDVI monthly values are compared with the bimonthly preceding rainfall amounts, attesting a time response of one to two months. At these time and space scales, NDVI does not appear to be sensitive to the seasonal and interannual rainfall variations in the Namib desert, South Namibia and western Cape Province. Along the Indian Ocean coast, it is weakly sensitive to the seasonal cycle only. It becomes largely sensitive to the seasonal cycle in Zimbabwe, and in South-Western Zambia. A high sensitivity to the interannual rainfall variability is only observed on the Southern African Plateau, around the Kalahari basin. Multivariate analyses show that geographical conditions of seasonal and interannual rainfall-NDVI associations strongly differ. While a sensitivity to seasonal rainfall is observed in areas where mean annual rainfall varies from 300 to 900mm and where the contrast between wet and dry seasons is strong, the sensitivity to interannual rainfall anomalies is observed only for relative dry areas, where mean annual rainfall varies from 300 to 500mm. In both cases, the relation is observed whatever the soil type or vegetation formation.

Influence of Benguela upwelling on the structure of demersal fish populations off Namibia

The effect of bottom-water dissolved oxygen on the structure of demersal fish communities off south Namibia was analized. Collections were made during two cruises of the zone in winter 1987 and summer 1988, i.e., before and after the intensive upwelling season. Multivariate analysis revealed four distinct associations, separated by latitude (ca 27°S) and depth (ca 300 m isobath), indicating that dissolved oxygen is an important factor affecting the distribution of demersal fish communities in the zone. Faunal diversity decreased in areas of low oxygen levels. The seasonal variation in upwelling reflected the extent of fish association, their composition, and the seasonal geographical displacement of their boundaries.