Kalahari Group Research Articles

Interaction of pans and groundwater: A case study in the Khakhea- Bray Transboundary aquifer portion in South Africa

Understanding the groundwater-surface water interaction is important for the sustainable management of water resources and ecology. Most of the studies on groundwater-surface interactions are confined to aquifer-river and aquifer-lake interactions. The interactions of water pans and groundwater have not received much attention. The study aims to provide insights into the interaction of the pans and groundwater at a case study in the Khakhea-Bray dolomite Transboundary Aquifer (TBA) shared by Botswana and South Africa. The study uses field observations, geology, hydrogeophysics, and stable isotopes complimentary tools. Complimentary tools provide several evidence to corroborate the findings. The hydrogeophysics taps into the recently developed Telluric Electric Frequency Selection Method (TEFSM). Groundwater discharge zones/springs were delineated at the geological contact of the boundary of the Khakhea-Bray dolomite transboundary aquifer and the granite of the Kalahari group. 4 of the 33 research pans are in the groundwater discharge zone and are classified as groundwater dependent. The findings highlight the benefits of complimentary approaches to corroborate the findings on groundwater-surface water interactions. The TEFSM hydrogeophysics can be able to delineate groundwater discharge zones, aquifer boundaries, and the mapping of TBAs. More studies are recommended to test the TEFSM hydrogeophysical approach to study groundwater-surface water in different hydrological environments.

Contribution of the time domain electromagnetic method to the study of the Kalahari transboundary multilayered aquifer systems in Southern Angola

AbstractThe Cunene Province (Southern Angola) is facing recurrent and pluriannual droughts. Surface water supply could be reinforced using the groundwater resources of the multilayered aquifer systems (MAS) hosted in the siliciclastic sediments of the Kalahari Group. The MAS were first identified in the early 2000s in Northern Namibia and recently in the Cunene Province, by studies of the PLANAGEO project based on modern processing and reinterpretation of legacy data from the 1960s and 1970s (electrical resistivity data and deep boreholes). This article presents the results of a time domain electromagnetic (TDEM) survey conducted in the Cunene Province to: (i) contribute to the design of the hydrogeological conceptual model of the transboundary MAS, namely their geometry and extension; (ii) validate the reprocessing of the legacy data; and (iii) guide the future location of boreholes. Results depict the geometry of the sedimentary basin and the characterization of the MAS, with particular emphasis on the intermediate and deep aquifers. The borehole siting, based on the interpretation of the new TDEM data and the legacy data (clay markers in borehole logs), was successful, with a good agreement between estimated and observed horizons of the deep aquifers. However, the presence of clayey layers, a clay-rich matrix in the detrital deposits and saline/brackish groundwater led to uncertainties in the interpretation of the electrical transects. As such, recommendations are made to improve future data collection and mapping of the MAS.

The Kalahari sediments and hominins in southern Africa

The temporal coupling of the structural evolution of the Kalahari Basin and the accumulation of the Kalahari Group sediments has been an accepted paradigm leading to the assumption that the Kalahari group sediments have been accumulating gradually since the mid-Cretaceous. Here we review the first actual ages for the Kalahari Group based on cosmogenic ages from six geological localities. These results demonstrate that Kalahari Basin infill was a more dynamic process than previously thought and that the Kalahari Group sediments are mostly Plio-Pleistocene in age (∼4 Ma to 1–2 Ma). The hiatus between the initial structural subsidence of the basin, during the Cretaceous, and the general young age of the investigated sediments, implies a dynamic landscape in which significant phases of erosion occurred during the Mesozoic and Cenozoic. The magnitude of erosion is manifested by the fact that in many locations Kalahari Neogene to Quaternary sediments overlie Precambrian basement.The age of the present infill of the Kalahari thus falls within the temporal range of the genus Homo. In light of this new understanding, we provide a review of the archaeological evidence from the Kalahari Basin and along its southern fringe. Initial hominin presence is found at Wonderwerk Cave during the Olduvai Event and there is subsequent high-density occupation along the southern fringe of the Kalahari Basin during the Acheulean and the Fauresmith. Middle Stone Age occupation is limited to localities of limited size and small artifact counts and it appears that the focus of human occupation, particularly in the later stages of the Middle Stone Age, shifts southward, including along the coastal regions.

ВЛИЯНИЕ ЗАМАЧИВАНИЯ ПРОСАДОЧНЫХ ГРУНТОВ НА НЕСУЩУЮ СПОСОБНОСТЬ ФУНДАМЕНТОВ ПРИ НАРАЩИВАНИИ ХВОСТОХРАНИЛИЩА ГРО "КАТОКА"

It was revealed that light loams of Kalahari group have subsidence properties on the territory at the concentrating mill. A numerical filtration model has been developed to predict the hydrodynamic regime of groundwater under the conditions of tailing dump exploitation during its heightening. An increase of groundwater levels at the area of the pumping station has been established when the flow is backed up by technical waters of the tailing dump during its filling. The calculation of compressible strata depth under buildings was carried out. The calculation of soil subsidence value at the base of pumping station foundations was carried out under the conditions of soil moisture increasing due to its wetting from below during groundwater rising. Reduction to acceptable values of foundations subsidence is provided by measures connected with lowering the groundwater level.

Zn-clays in the Kihabe and Nxuu prospects (Aha Hills, Botswana): A XRD and TEM study

Abstract Zinc clays are commonly found in oxidized Zn deposits and, even though they rarely represent the main target of the ore exploitation, they can be used as a proxy to restore the genetic conditions during ore-forming processes. This work sheds light on the micro- to nano-mineralogy and on the genesis of Zn-clays in the Kihabe and Nxuu prospects (located in the Aha Hills district, Northern Botswana), through an integrated XRD and TEM study of the mineralized facies occurring in the ore system. The Kihabe and Nxuu ores are hosted in a Neoproterozoic metamorphozed quartzwacke unconformably covered by the recent sedimentary rocks, also containing calcretes, of the Kalahari Group. In the analyzed samples, four distinct mineralogical facies have been recognized: (1) vanadate-calcrete facies, poor of Zn-clays; (2) low Zn-clay facies, characterized mostly by clays showing low Zn concentrations; (3) Zn-clay facies, containing proper Zn clay minerals; and (4) sulfide facies, devoid of Zn-clays. In all the facies detrital dioctahedral mica (muscovite and illite) is interstratified with smectite in the form of random (R0) to short-range ordered (R1) I/S, which locally shows significant Zn concentrations. In the sulfide facies kaolinite overgrowing onto mica packets has been detected. The low Zn-clay facies is dominated by Zn-bearing beidellite, with minor kaolinite and fraipontite. The Zn-clay facies consists mostly of a random (R0) interstratified clay between a 7 Å phase corresponding to fraipontite, and a 2:1 swelling clay component identifiable with the dioctahedral smectite, with minor sauconite. The micro- to nano-scale paragenetic study performed by TEM indicates that the above-mentioned clays formed through a multistage process, eventually ending with the genesis of Zn-bearing phyllosilicates assemblages. The main steps were: (1) alteration of detrital mica and dissolution of feldspar clasts, which led to the formation of epitaxial kaolinite and replacive beidellite; (2) fertilization of barren clays and formation of replacive to epitaxial fraipontite/smectite and of Zn-bearing mica, through input of Zn2+ deriving from sphalerite or willemite dissolution by mixed meteoric-hydrothermal fluids; (3) formation of low tetrahedral charge sauconite, either in pores or as replacement of K-feldspars under surficial hypersaline conditions, possibly also linked to the establishment of the arid climate in region. These processes provide insights into genesis of Zn-Pb-V ore deposits in northwest Botswana. Furthermore, the identification of a Zn-smectite species having a stevensite-like stoichiometry is valuable for future studies dealing with the systematics of clay minerals.

A reappraisal of legacy reflection seismic data from the western margin of the Kaapvaal craton, South Africa, with implications for Mesozoic-Cenozoic regional tectonics

The 150 km long, 6 s TWT, 2D seismic profile KBF03A, which was acquired in 1994 and lies near the western edge of the Kaapvaal craton in South Africa, has been reprocessed using standard reflection seismic processing methods. The results exhibit a significant improvement in the imaging quality of the subsurface features and an evident boost in the signal-to-noise ratio. The improved seismic data, combined with application of seismic attributes, integration with surficial geological and geophysical maps, and computation of velocity tomograms, has revealed previously undetected structural features within the supracrustal sequences underlying the profile. In particular, the Phanerozoic sediments found along the profile, comprising exclusively Kalahari Group and Dwyka Group deposits, are disrupted by multiple folds of varying wavelengths (~1–10 km) and variably oriented normal and thrust faults. Additionally, the effect of the Moshaweng fault, previously characterised as a listric fault extending to depths of >10 km, on the Phanerozoic sediments has been further constrained to suggest repeated extensional reactivations and a more recent (<1 Ma) inversion. Considerations of the eastern end of the profile have suggested links between some of the observed structural features and the nearby ~146 Ma Morokweng impact structure, the lateral expanse of which is poorly constrained in the literature. The collection of these newly imaged features is interpreted as evidence for multiple Late Mesozoic to Cenozoic tectonic events, including polyphasic reactivation of basement structures, under both extensional and compressional stress regimes. By analysing these newly detected features, our study not only provides new insight into the neotectonic evolution of the Phanerozoic sediments along the western margin of the Kaapvaal craton, but also demonstrates the utility in reprocessing legacy data and reveals its untapped potential.

Lithostratigraphy, sedimentary petrography and geochemistry of the Upper Karoo Supergroup in the Central Kalahari Karoo Sub-Basin, Botswana

The Kalahari Karoo Basin of Botswana is an intracratonic basin developed in south-central Gondwana and is filled with Upper Carboniferous–Lower Jurassic Karoo Supergroup volcano-sedimentary units. The Karoo Supergroup units are unconformably covered by the Cretaceous to recent Kalahari Group. The Upper Karoo Supergroup in the Central Kalahari Karoo Sub-Basin of Botswana consists of the lacustrine sediments of the Beaufort Group (Tlhabala Formation), overlaid unconformably by the continental sediments of the Lebung Group, and by the continental basalts of the Stormberg Group. The Late Triassic to Early Jurassic Lebung Group is subdivided into the Mosolotsane Formation and the Ntane Formation. This sedimentary sequence is poorly exposed, and formation boundaries are tentatively identified by lithological changes in boreholes. Here, new sedimentological and geochemical data are used for the refinement of the stratigraphy of the Upper Karoo Supergroup in the Central Kalahari Karoo Sub-Basin of Botswana, new criteria for the identification of unit boundaries are set and provenance of the Lebung Group sediments has been defined. The geochemical investigation of the Upper Karoo Supergroup in the Central Kalahari Karoo Sub-Basin of Botswana revealed clear geochemical markers that have allowed the pinpointing of unit boundaries between the Tlhabala Formation (Beaufort Group), Mosolotsane Formation (lowermost Lebung Group). The sediments from the Tlhabala Formation have a geochemical signature typical of wackes, while the sandstones from the overlying Mosolotsane Formation and the Ntane Formation are dominated by arkose to subarkose. The transition from wacke to subarkose pinpoints the boundary between the Beaufort Group and the base of the Lebung Group. The Tlhabala Formation is characterized by increasing SiO2/Al2O3 ratio, with a maximum at the base of the Lebung Group, and by an upward decrease in the abundance of Na, K and transition metals such as Cr, Mn and Fe. The SiO2/Al2O3 ratio decreases upward in the Lebung Group. Similarly, the abundances of Na, K, Cr, Mn and Fe slightly increase upward only to show a negative peak at the Lebung Group and the Kalahari Group boundary. This contact is marked by a clear increase in the total REE content.The chemical composition of the Lebung Group sandstones also indicates a quartzose sedimentary provenance. The CIA (chemical index of alteration) values for these sandstones ranged from 63 to 73, indicating a high degree of chemical weathering. This type of intense weathering should produce high volumes of clay minerals, which are rather scarce in the studied samples. This indicates that the large amount of quartz in the Lebung Group has been recycled from the Paleoproterozoic sedimentary sequences of the Palapye Group, located southeast from the study area.

Landscape responses to intraplate deformation in the Kalahari constrained by sediment provenance and chronology in the Okavango Basin

AbstractThe structural depression that occupies the Okavango Basin in southern Africa comprises a depo‐centre within the intracratonic Kalahari Basin where sediments of the Cenozoic Kalahari Group have accumulated. The Okavango Basin has been formed due to stretching and subsidence at an area of diffused deformation, southwestwards to the main East African Rift System (EARS). Sediments from two full Kalahari Group sequences, located on opposite sides of the Gumare Fault that forms a major fault within the Okavango Basin, were studied to determine their provenance and chronology. Terrestrial Cosmogenic Nuclide (TCN) 26Al/10Be burial dating was used to constrain a chronostratigraphical framework, and Pb, Sr, and Nd isotopic ratios combined with geochemical and sedimentological analyses were applied to track the source areas of the sediments.Results indicate the following sequence of basin filling: (a) Accumulation between ca. 4–3 Ma during which the currently downthrown (southern) block received a mixture of sediments mostly from the Choma‐Kalomo, Ghanzi‐Chobe, and Damara terranes, and possibly from the Lufilian Belt and/or Karoo basalts during earlier stages of deposition. Simultaneously, the upthrown (northern) block received sediments from more distant Archean sources in the Zimbabwe and/or Kasai cratons, (b) Hiatus in sedimentation occurred at both sites between ca. 3–2 Ma, (c) Sediments on both sides of the Gumare Fault share a similar source (Angolan Shield) with minor distinct contributions to the downthrown block from the Kasai Craton and local sources input to the upthrown block, and (d) Regional distribution of aeolian sand since at least 1 Ma. The change in source areas is attributed to rearrangements of the drainage systems that were probably linked to vertical crustal movements on the margins of the Okavango Basin. The tectonically induced morphodynamics controlled the landscape evolution of the endorheic basin where vast lakes, wetlands and salt pans have developed through time.

Reassessing southern African silcrete geochemistry: implications for silcrete origin and sourcing of silcrete artefacts

AbstractA synthesis of the geochemistry of silcretes and their host sediments in the Kalahari Desert and Cape coastal zone, using isocon comparisons, shows that silcretes in the two regions are very different. Kalahari Desert silcretes outcrop along drainage‐lines and within pans, and formed by groundwater silicification of near‐surface Kalahari Group sands. Silicification was approximately isovolumetric. Few elements were lost; silicon (Si) and potassium (K) were gained as microquartz precipitated in the sediment porosity and glauconite formed in the sub‐oxic groundwater conditions. The low titanium (Ti) content reflects the composition of the host sands. Additional elements in the Kalahari Desert silcretes were supplied in river water and derived from weathering of silicates in basement rocks. Evaporation under an arid climate produced high‐pH groundwater that mobilized and precipitated Si; this process is still occurring.In the Cape coastal zone, pedogenic silcretes cap hills and plateaus, overlying deeply weathered argillaceous bedrock. Silicification resulted from intensive weathering that destroyed the bedrock silicates, almost completely removing most elements and causing a substantial volume decrease. Some of the silica released formed a microcrystalline quartz matrix, and most Ti precipitated as anatase, so the Cape silcretes contain relatively high Ti levels. The intense weathering that formed the Cape silcretes could have occurred in the Eocene, during and after the Palaeocene‐Eocene Thermal Maximum, when more acidic rainfall and high temperatures resulted in intensified silicate weathering worldwide. This could have been responsible for widespread formation of pedogenic silcretes elsewhere in Africa and around the globe.Trace element sourcing of silcrete artefacts to particular outcrops has most potential in the Cape, where differences between separate bedrock areas are reflected in the silcrete composition. In the Kalahari Desert, gains of some elements can override compositional differences of the parent material, and sourcing should be based on elements that show the least change during silicification. © 2020 John Wiley &amp; Sons, Ltd.

Eolian indicator mineral dispersion haloes from the Orapa kimberlite cluster, Botswana

This paper presents the results of an investigation into the structure of eolian kimberlite indicator minerals (KIMs) haloes present within Quaternary Kalahari Group sediments (up to 20 m thick) overlying the Late Cretaceous kimberlites in the Orapa field in North-East Botswana. A database of more than 8000 samples shows that kimberlites create a general mineralogical blanket of KIMs of various distances of transportation from primary sources in the Orapa area. Models of the reflection and dispersion patterns of KIMs derived from kimberlite pipes including AK10/ AK22/AK23 have been revealed based on 200 selected heavy mineral samples collected during diamond prospecting activities in Botswana from 2014 to 2017. Short distance eolian haloes situated close to kimberlite bodies cover gentle slopes within plains up to 500 × 1000 m in size. They have regularly have oval or conical shapes and are characterized by the presence mainly of unabraded or only slightly abraded KIMs. A sharp reduction of their concentration from hundreds and thousands of grains / 20 l immediately above kimberlites toto 10 grains/20 l at a distance of only 100–200 m from the pipes is a standard feature of these haloes. The variation of concentration, morphology and abrasion of specific KIMs with increasing distance from the primary sources has been investigated and presented herein. Sample volumes recommended for pipes present within a similar setting as those studied, with different depth of sedimentary cover are as follows: up to 10–20 m cover at 20–50 l, 20–30 m cover at 50–100 l and 30–80 m cover at 250 l. It is important to appreciate that the discovery of even single grains of unabraded or slightly abraded KIMs in eolian haloes are of high prospecting significance in this area. The results of the research can be applied to in diamond prospecting programs in various regions with similar environments.

Provenance and depositional environments of Quaternary sediments in the southern Kalahari Basin

The vast Kalahari Basin is characterized by uplifted margins, terrestrial sedimentation within semi endorheic sub-basins, subdued morphology and tectonic quiescence. This intracratonic basin has been subjected to a prolonged period of subsidence affecting its sedimentary fill by plate motion and climatic cycles. Provenance studies of Kalahari Group sediments mainly focused on the surficial deposits that represent only the last phase of sedimentation, leaving unresolved questions related to the rest of the strata.The southern Kalahari Group succession exposed along the walls of the Mamatwan Mine, Northern Cape, South Africa, reveals three main depositional environments: a bottom low-energy water-body, a middle fluvial, high-energy environment and an upper aeolian sandy unit. The entire section, which was deposited during the early to middle Quaternary, records significant environmental and depositional changes suggesting a highly dynamic landscape.The fully exposed section (55m) of the Kalahari Group at Mamatwan Mine was analysed for its mineralogy, elemental composition, Sr, Nd and Pb isotopic ratios and iron species. Isotopic fingerprints were used to determine the source rocks of the sediments. The basal part of the section was derived from the distant Angolan highlands supporting previous suggestions for the existence of a paleo, trans-Kalahari drainage system. Weathering sensitive indices show that these sediments underwent considerable chemical weathering typical to humid areas. Mineralogical assemblage and iron phases imply that a brackish and alkaline shallow water-body existed during the early-middle Pleistocene contemporaneous with relative dense hominin occupation of the area.During the Early-Middle Pleistocene transition, the lacustrine environment was rapidly filled with volcano-sedimentary Archean clasts that were derived mainly from the surrounding hills and experienced limited degree of source rock chemical weathering, but underwent subsequent groundwater alteration by iron, silica, and calcium-rich solutions and precipitation of calcrete and silcrete duricrusts. These changes in provenance and depositional environments may be related to tectonic adjustments along the southern margins of the Kalahari basin. Ultimately, a shift into an aeolian-transport dominated landscape occurred in the region. This was accompanied by reduced surface water availability and a supply of sediments from Paleoproterozoic western and north-western source areas following the establishment of the modern wind regime.

Groundwater potential of the eastern Kalahari region of South Africa

Abstract An integrated approach involving geological, borehole data, hydrogeochemical and environmental isotope analyses was used to determine the groundwater potential of the eastern Kalahari region of South Africa, an area to the west of Mahikeng that stretches northward from the Orange River into Botswana. The total groundwater resource potential for the eastern Kalahari region of South Africa is estimated at 10127 Mm3/a, with the Kalahari Group aquifer showing the greatest potential, comprising 51% of the total resource. The storage capacity of the Kalahari Group aquifer (7130 Mm3) is also impressive, estimated to be more than twice that of the dolomite aquifer (2728 Mm3). Despite having such great potential, the aquifer is not actively recharged and is often associated with very saline water that is not suitable for human and livestock consumption. The limestone and dolomite aquifers of the Campbell Rand Subgroup, as well as the weathered granitic rocks of the Archaean basement, are considered as the most prospective water bearing formations, with a groundwater resource potential estimate of 1981 Mm3/a and 1845 Mm3/a, respectively. Aquifers with the least potential in the project area comprise the fractured basement rocks of the Kraaipan - Amalia greenstone belt, with a groundwater resource potential of 26 Mm3/a, and the fractured sedimentary rocks of the Asbestos Hills Subgroup, with a groundwater resource potential of 108 Mm3/a. The calculated groundwater storage and resource potential in the eastern Kalahari region of South Africa satisfies a large proportion of the water demand in the region.

Deterioration of teeth and alveolar bone loss due to chronic environmental high-level fluoride and low calcium exposure.

Health risks due to chronic exposure to highly fluoridated groundwater could be underestimated because fluoride might not only influence the teeth in an aesthetic manner but also seems to led to dentoalveolar structure changes. Therefore, we studied the tooth and alveolar bone structures of Dorper sheep chronically exposed to very highly fluoridated and low calcium groundwater in the Kalahari Desert in comparison to controls consuming groundwater with low fluoride and normal calcium levels within the World Health Organization (WHO) recommended range. Two flocks of Dorper ewes in Namibia were studied. Chemical analyses of water, blood and urine were performed. Mineralized tissue investigations included radiography, HR-pQCT analyses, histomorphometry, energy-dispersive X-ray spectroscopy and X-ray diffraction-analyses. Fluoride levels were significantly elevated in water, blood and urine samples in the Kalahari group compared to the low fluoride control samples. In addition to high fluoride, low calcium levels were detected in the Kalahari water. Tooth height and mandibular bone quality were significantly decreased in sheep, exposed to very high levels of fluoride and low levels of calcium in drinking water. Particularly, bone volume and cortical thickness of the mandibular bone were significantly reduced in these sheep. The current study suggests that chronic environmental fluoride exposure with levels above the recommended limits in combination with low calcium uptake can cause significant attrition of teeth and a significant impaired mandibular bone quality. In the presence of high fluoride and low calcium-associated dental changes, deterioration of the mandibular bone and a potential alveolar bone loss needs to be considered regardless whether other signs of systemic skeletal fluorosis are observed or not.

New chronology for the southern Kalahari Group sediments with implications for sediment-cycle dynamics and early hominin occupation

Kalahari Group sediments accumulated in the Kalahari basin, which started forming during the breakup of Gondwana in the early Cretaceous. These sediments cover an extensive part of southern Africa and form a low-relief landscape. Current models assume that the Kalahari Group accumulated throughout the entire Cenozoic. However, chronology has been restricted to early–middle Cenozoic biostratigraphic correlations and to OSL dating of only the past ~300ka. We present a new chronological framework that reveals a dynamic nature of sedimentation in the southern Kalahari. Cosmogenic burial ages obtained from a 55m section of Kalahari Group sediments from the Mamatwan Mine, southern Kalahari, indicate that the majority of deposition at this location occurred rapidly at 1–1.2Ma. This Pleistocene sequence overlies the Archaean basement, forming a significant hiatus that permits the possibility of many Phanerozoic cycles of deposition and erosion no longer preserved in the sedimentary record. Our data also establish the existence of a shallow early–middle Pleistocene water body that persisted for >450ka prior to this rapid period of deposition. Evidence from neighboring archeological excavations in southern Africa suggests an association of high-density hominin occupation with this water body.

Lithological variations of sedimentary succession within a meteorite impact crater - Jwaneng South Structure, Botswana

The Jwaneng South Structure is a meteorite impact crater located in the Kalahari region of Botswana. The structure has the shape of a bowl 1.3 km in diameter and a maximum depth of 275 m in the centre. It was discovered by an airship-mounted full tensor gravity gradiometer and penetrated by nine vertical diamond drill-holes. The crater is underlain by the Gaborone Granite (2785 Ma) and basalts of the Karoo Supergroup (182 Ma). The covering aeolian sediments of the Kalahari Group (Late Cretaceous-Recent) completely obscure the structure. A succession of the following lithofacies overlying authigenic in situ brecciated granite was intersected in the boreholes (from base to top): (i) allogenic heterolithic/oligomictic “fallback” and resedimented breccia (ii) sedimentary breccia and conglomerate with sand matrix; (iii) six intervals of carbonate sediments, with traces of evaporites and mudstone interbeds, which are interlayered with (iv) five intervals of sandstone and sedimentary breccia composed of granules, pebbles and cobbles, mostly of granite, embedded in a matrix of well-sorted medium-grained sand; (v) bioturbated, mostly massive sandstone rich in mud matrix (wacke), with locally preserved interbeds of mudstone and cross-bedded sandstone, and abundant root traces; (v) silcrete and calcrete that occur at the top of the succession. This lithological association suggests that deposition within the Jwaneng South meteorite impact crater took place in a playa lake surrounded by steeply-dipping talus piedmont fans. The depositional cycles were controlled by pronounced climatic oscillations. Wet periods are recorded by lithofacies (iii), which reflects intense supply of sand eroded from the Kalahari dune field surrounding the crater and coarse detritus derived from its rim and steep talus below. Dry intervals of high evaporation and fall of the lake level are reflected by lithofacies (iv). During the youngest wet period (v) the lake filled up with alluvia sands interbedded with muds, abundantly vegetated and homogenised by bioturbation. The silcrete and calcrete layer at the top of the succession is the product of pedogenic processes that affected the Kalahari Desert environment. An asymmetry of lateral distribution of the lithofacies (ii) – (iv) and the presence of sedimentary breccia redeposited into the marginal E (and NE) parts of the crater suggest an asymmetry of the crater depression and its coarse clastic rim, which may imply an oblique trajectory of the impactor approaching from the SW.

U–Pb detrital zircon dates and source provenance analysis of Phanerozoic sequences of the Congo Basin, central Gondwana

The Congo Basin (CB) is the largest sediment sink of central Gondwana, built on a mosaic of Precambrian crustal blocks amalgamated during the mid-Paleoproterozoic (Eburnian; 2.1–1.8Ga), late Mesoproterozoic (Kibaran; 1.4–1.0Ga), and late Neoproterozoic–early Cambrian (Pan African; 750–500Ma). Sporadic uplift, tilting and erosion of these Precambrian terrains form the source regions for the sedimentary sequences that fill the CB. We investigate the Phanerozoic successions in the field and along four historic deep boreholes drilled in the center of the basin, and date detrital zircons from the main stratigraphic groups to characterize their provenance ages and reconstruct the paleogeographic evolution of the CB during amalgamation and break-up of the Gondwana supercontinent. Sedimentological data show that the oldest, upper Neoproterozoic–lower Paleozoic Redbeds (the Inkisi, Aruwimi and Biano Groups) were derived from the north. Zircons from these sequences have two dominant age-populations of 1100–950Ma and 800–600Ma, likely sourced from Kibaran and Pan African terrains within the Oubanguides (e.g. the Central Saharan Belt) and parts of the North African Shield (e.g. Darfour). The overlying Carboniferous–Permian glacial and deglaciation sequences (the Lukuga Group) have similar peaks, as well as abundant zircons of 2.05–1.85Ga and a subordinate number dated at 1.42–1.37Ga. The latter are from Eburnian and Kibaran sources in east-central Africa, consistent with west-facing glacial paleo-valleys preserved along the eastern margin of the CB. The succeeding Triassic (the Haute–Lueki Group) and Jurassic–Cretaceous (the Kwango Group) fluvial and aeolian red sandstones were again derived from the north. Their range of zircon dates has two main peaks at 1000Ma and 600Ma, but also contain small younger grains of 290–240Ma and 200–190Ma. We interpret these younger zircons to be derived from volcanic dust that originated during late Paleozoic–Jurassic magmatism of the Choiyoi and Chon Aike Provinces flanking the Andean subduction margin of Gondwana. By contrast, the uppermost Cenozoic alluviums of the CB (the Kalahari Group) contain diamond concentrates and large zircon fragments dated at 3–2.5Ga, derived from the Kasai and Cuango Cratons, to the south, which host Cretaceous diamondiferous kimberlites.

High fluoride and low calcium levels in drinking water is associated with low bone mass, reduced bone quality and fragility fractures in sheep

SummaryChronic environmental fluoride exposure under calcium stress causes fragility fractures due to osteoporosis and bone quality deterioration, at least in sheep. Proof of skeletal fluorosis, presenting without increased bone density, calls for a review of fracture incidence in areas with fluoridated groundwater, including an analysis of patients with low bone mass.IntroductionUnderstanding the skeletal effects of environmental fluoride exposure especially under calcium stress remains an unmet need of critical importance. Therefore, we studied the skeletal phenotype of sheep chronically exposed to highly fluoridated water in the Kalahari Desert, where livestock is known to present with fragility fractures.MethodsDorper ewes from two flocks in Namibia were studied. Chemical analyses of water, blood and urine were executed for both cohorts. Skeletal phenotyping comprised micro-computer tomography (μCT), histological, histomorphometric, biomechanical, quantitative backscattered electron imaging (qBEI) and energy-dispersive X-ray (EDX) analysis. Analysis was performed in direct comparison with undecalcified human iliac crest bone biopsies of patients with fluoride-induced osteopathy.ResultsThe fluoride content of water, blood and urine was significantly elevated in the Kalahari group compared to the control. Surprisingly, a significant decrease in both cortical and trabecular bones was found in sheep chronically exposed to fluoride. Furthermore, osteoid parameters and the degree and heterogeneity of mineralization were increased. The latter findings are reminiscent of those found in osteoporotic patients with treatment-induced fluorosis. Mechanical testing revealed a significant decrease in the bending strength, concurrent with the clinical observation of fragility fractures in sheep within an area of environmental fluoride exposure.ConclusionsOur data suggest that fluoride exposure with concomitant calcium deficit (i) may aggravate bone loss via reductions in mineralized trabecular and cortical bone mass and (ii) can cause fragility fractures and (iii) that the prevalence of skeletal fluorosis especially due to groundwater exposure should be reviewed in many areas of the world as low bone mass alone does not exclude fluorosis.

GEOPHYSICAL INTERPRETATION OF THE NATURE AND EXTENT OF THE XADE MAFIC COMPLEX, BOTSWANA

The Xade Mafic Complex, situated in central Botswana, was identified during the first regional aeromagnetic survey of the country in 1975–77. It is covered by sedimentary rocks of the Kalahari Group and Karoo Supergroup, including Karoo lavas, with a combined thickness varying from 220 m to 1000 m, extending to greater depths in the north beneath the Meso-proterozoic Passarge Basin. What has generally been viewed as the Xade Complex is evidenced by a high amplitude kidney-shaped zoned magnetic anomaly with two semi-linear anomalies extending to the northwest and northeast in a Y-shaped form. The complex is also marked by a coincident Bouguer gravity anomaly. Historical work has been limited, with only three diamond drill holes having been drilled, separately intersecting gabbroic rocks, weathered basalt grading into dolerite, and amygdaloidal lavas followed by Waterberg Group shales. An U-Pb zircon age of 1109.0 ± 1.3 Ma, which is coeval with the Umkondo Igneous episode, has been published for the gabbroic unit. This study has shown, for the first time, that the Xade Complex comprises two lobes of potentially different magmatic character, i.e. a Southern Lobe, which is the historically identified kidney-shaped zoned magnetic anomaly, as well as a hitherto unrecognized large Northern Lobe. The latter is mostly deeply buried in the north and northwest beneath the Passarge Basin, as evidenced by deep magnetic and gravity sources, but its southern and eastern margins partially suboutcrop beneath Karoo sediments, forming the Y-shaped anomalies north of the Southern Lobe. Forward modelling, of both magnetic and gravity data, indicates that the Southern Lobe is a lopolith with a depth extent of approximately 4 km, whereas the southeastern margin of the Northern Lobe has westerly dips consistent with deepening to the northwest. In addition to the historic boreholes, a recent borehole drilled into the Southern Lobe shows that it comprises a mafic volcanic sequence with subordinate gabbro. A further two boreholes drilled into the Northern Lobe margins indicate that it comprises a texturally heterogeneous and magmatically differentiated sequence of gabbroic rocks, with minor dioritic and monzonitic rocks. The present interpretation has also identified a dyke system associated with the Northern Lobe, which may represent either feeder or exit magmatic conduits. The interpretation further shows that the complex is located in a craton margin setting. The combined extent of both lobes is approximately one-third the size of the Bushveld Complex, making the Xade Mafic Complex the largest Meso-Proterozoic magmatic system in southern Africa. The craton margin setting, size, Meso-Proterozoic age and variability of plutonic and volcanic rocks hallmark the Xade Mafic Complex as being a very large dynamic magmatic system, with potential for Ni-Cu-PGE mineralization.

Geophysical signatures of some recently discovered large (> 40 ha) kimberlite pipes on the Alto Cuilo concession in northeastern Angola

This paper presents a comparison of geophysical responses from several large kimberlite complexes discovered and delineated on the Alto Cuilo concession in the diamond fields of northeastern Angola in the years 2005 to 2008. Several geophysical methods were used in combination with geochemical and mineralogical prioritization techniques to guide exploratory, delineation and bulk sample drilling, in order to rapidly identify and evaluate the kimberlite bodies. The kimberlites were emplaced through Karoo Supergroup sandstones and shales, have eruption ages contemporaneous with the sand-dominated Cretaceous-age Calonda Formation, and are covered by sand-dominated poorly consolidated sediments of the Kalahari Group. Given that sand-dominated non-kimberlite lithologies are magnetically transparent, a low level, high resolution helicopter-borne magnetic gradiometer survey proved to be exceptionally effective in discriminating kimberlite targets, even for low-amplitude anomalies (e.g. 1–2 nT). The helicopter magnetic data outlined approximately 244 probable kimberlite targets and drilling of 103 targets confirmed 80 new kimberlites greater than 5 ha in area. Most kimberlites take the form of well-preserved crater edifices containing a full range of crater-related kimberlite lithologies. Ground gravity and electromagnetic surveys were conducted over all kimberlites prioritized for follow-up investigation. Geophysical responses were ground-truthed against magnetic susceptibility and density measurements, which were routinely collected on all drill cores. The geophysical signatures resolved by the three independent geophysical methods were surprisingly variable and are inferred to be sourced primarily in the crater facies materials, which demonstrate characteristically variable lithologies. Geophysical interpretations guided the drill targeting at all stages of the program at Alto Cuilo, from exploration to evaluation. Combined with geochemical and mineralogical prioritization techniques, the geophysical signatures in magnetic, gravity and electromagnetic data provide a sound basis to guide exploratory, delineation and mini bulk sample drilling of kimberlites buried under 10–70 m of overburden. The significant variability in the geophysical responses from kimberlites that are similar in size, structure and geometry highlights the importance of applying all three independent geophysical methods in order to effectively achieve kimberlite exploration and evaluation goals. The effectiveness of a multidisciplinary approach to kimberlite evaluation is demonstrated in the rapid assessment of a cluster of large kimberlites discovered at Project Alto Cuilo.

Detection of concealed Cu–Zn massive sulfide mineralization below eolian sand and a calcrete cover in the eastern part of the Namaqua Metamorphic Province, South Africa

The regolith studied here is located at the defunct Areachap mine and the newly discovered Kantienpan Cu–Zn volcanic-hosted massive sulfide (VHMS) deposit, located in the Areachap Group of the eastern part of Mesoproterozoic Namaqua Metamorphic Province. This area is highly prospective for further VHMS discoveries. Paleo and recent weathering of the upper most parts of massive sulfide deposits led to the formation of a gossan zone. Due to semi-arid climatic conditions during the late Cretaceous, affecting the African Land surface, the lowermost units of the Kalahari Group and the underlying floor rocks were calcretized. An approximately 6 m thick calcrete layer formed above the gossan zone and this was later covered by eolian Kalahari sand. Samples were collected from the eolian sand cover in the study areas to determine the best analytical method that would enable recognition of the concealed ore deposits and detect the widest secondary dispersion halo. Mobile metal ions from the finest fraction of the eolian sand samples (< 75 μm) were extracted with a NH 4EDTA (EDTA) solution. The solution was analysed for Cu, Zn, Pb and Mn by inductively coupled plasma mass spectrometry (ICP-MS). The same grainsize fraction of the original samples was also analysed for comparison purposes by means of X-ray fluorescence (XRF). Results indicate that the ore zone in both areas may be recognized by both partial and total analyses of the eolian sand samples collected, although the calcrete layer, below the sand cover, acts as a partial geochemical barrier. The recognition of the ore zone depends on the regolith forming processes and the thickness of the eolian sand cover. In the Areachap area, with a relatively thick sand cover (in excess of 1 m) above the calcrete layer, the detectable geochemical halo is related to the distribution of the mobile metal ions, and partial extraction (EDTA solution) results define a larger dispersion halo than that, that could be detected by total analysis (XRF). Whereas, in the Kantienpan area with a very thin sand cover (< 50 cm) dispersion appears to be related more to the secondary redistribution of gossaniferous clasts released by recent weathering out of the calcrete, than to dispersion of mobile metal ions on the surface of sand particles. In this area, the XRF results reveal a wider dispersion of the elements of interest.