De Wit M.C.J.,University of Pretoria |
Jelsma H.A.,De Beers Group Services
Geology and Resource Potential of the Congo Basin | Year: 2015
An overview is provided of the exploration history and geological setting of the kimberlites in the Democratic Republic of Congo (DRC). Exploration for diamonds, in what was then known as Congo Belge, started in 1900 and the first diamonds were found in 1903 in Shaba (now Katanga) Province, in 1907 in Kasai Occidental Province near Tshikapa Town and in 1918 in Kasai Oriental Province near Mbuji Mayi Town. While the Kundelungu kimberlites in Katanga Province were discovered in 1908, other kimberlite fields were discovered much later (Mbuji Mayi 1946; Tshibwe 1956; Bas-Congo 1974; Kasendou and Lukashi 2005), during exploration work by Forminière (Société Internationale Forestière et Minière du Congo—Tshikapa), MIBA (Societé Minière de Bakwanga—Mbuji Mayi), the De Beers Group and Bugeco S.A. Published age constraints on the kimberlites show Late Cretaceous ages for the Mbuji Mayi kimberlites (~70 Ma) and Eocene-Oligocene ages for the Kundelungu kimberlites (~32 Ma). Emplacement of the Late Cretaceous kimberlites (Mbuji Mayi, Tshibwe, Kasendou and Lukashi) was concomitant with the deposition of Cretaceous sedimentary sequences. The majority of the pipes show crater-facies preservation and some of the pipes are flared displaying so-called ‘champagne glass-shaped’ morphologies, suggesting emplacement into unconsolidated sediments overlying basement. The age of the Eocene-Oligocene Kundelungu kimberlites corresponds to lithospheric extension associated with the southward propagation of the East African Rift. © Springer-Verlag Berlin Heidelberg 2015.
Perritt S.,University of Johannesburg |
Perritt S.,De Beers Group Services |
Preston R.,De Beers Group Services |
Viljoen F.,University of Johannesburg |
Van Der Linde G.,De Beers Group Services
South African Journal of Geology | Year: 2015
Garnet megacrysts found in the Cretaceous Group II Monteleo kimberlite (Free State province, South Africa) exhibit an unusual flattened and elongated morphology. Internally, the garnets are characterized by abundant kelyphite veins and microcracks, arranged as preferentially orientated microstructural sets. Numerous rod-like sulphide inclusions are present within the garnets, also characterized by a preferential orientation, with their long axes generally normal to the short axis of the host megacryst. The observed garnet megacryst morphologies and internal micro-structures are consistent with a shear-related deformation event, although the deformation mechanism remains unresolved. The deformation of the garnet megacrysts is inferred to have occurred prior to the entrainment of the garnets in their host kimberlite and is attributed to localized shearing in the lithospheric mantle, possibly associated with overlying crustal shearing that developed parallel to the Agulhas-Falkland Fracture Zone during Gondwana breakup. Variations in major and trace element concentrations indicate that the garnet megacrysts formed as a result of a simple fractional crystallization process, and pressure-temperature modelling indicates that they crystallized from a >1330°C melt at a depth of ∼180 km, within a zone of metasomatism within the lithospheric mantle. A similarity in chemical composition between the megacrysts and garnets from associated lherzolitic lithologies suggests that percolation of the megacryst parental melt through the lithospheric mantle may have contributed to the metasomatism. © 2015 December Geological Society of South Africa.
McCourt S.,University of KwaZulu - Natal |
Armstrong R.A.,Australian National University |
Jelsma H.,De Beers Group Services |
Mapeo R.B.M.,University of Botswana
Journal of the Geological Society | Year: 2013
In an attempt to better understand the tectonic evolution of the continental crust forming SW Angola, zircon grains from the principal Precambrian rock types exposed in the Lubango area have been analysed using the sensitive high-resolution ion microprobe method. U-Pb ages of 2038 ± 28 Ma and 1954 ± 6 Ma were obtained on weakly deformed granite samples from the basement below the Humpata Plateau. The Chela Group on the Humpata Plateau is a relatively undeformed Palaeoproterozoic supracrustal sequence with an eruptive age of 1798 ± 11 Ma on ignimbrite of the Humpata Formation. The age of the northern part of the Kunene Complex is constrained by zircon data from a xenolith of basement gneiss and a mangerite dyke cutting anorthosite, which give an emplacement age of 1385 ± 7 Ma. The c. 2.0 Ga granites below the Chela Group are part of a Palaeoproterozoic granitoid terrane that extends from north of Lubango in Angola into NW Namibia. This terrane is referred to as the Angolan Shield. Regionally, the Angolan Shield is interpreted to be part of a Palaeoproterozoic magmatic arc that extends NE from Angola and Namibia under Phanerozoic cover into NW Zambia. The resultant crustal terrane defined the southern margin of the developing Congo Craton at c. 2.0 ± 0.04 Ga.
Joy S.,De Beers Group Services |
Jelsma H.,Anglo American |
Tappe S.,De Beers Group Services |
Tappe S.,University of Johannesburg |
Armstrong R.,Australian National University
Journal of Asian Earth Sciences | Year: 2015
Proterozoic basins in cratonic India, referred to as "Purana Basins", cover about 20% of the Archean basement of the subcontinent. Although the stratigraphy of most of these basins has been well established, it is only recently that radiometric age constraints have been obtained for some of the sedimentary sequences. This study provides new data for two of the Purana Basins in southern India using SHRIMP U-Pb analysis of detrital zircons. For the Pranhita-Godavari Basin, an age limit of 709. Ma is provided for the age of deposition of the Sullavai Group. This prolongs the duration of Proterozoic sedimentation to approximately one billion years (ca. 1700-709. Ma) and establishes the Venkatpur Sandstones of Sullavai Group as the youngest Purana Basin succession identified in India so far. A major zircon provenance age peak at about 1000. Ma, and zircon ages between 800 and 750. Ma, are correlated with major tectonothermal events in the Eastern Ghats Mobile Belt (EGMB) and intrusion of granites and associated pegmatites at that time. The main provenance area of the Venkatpur Sandstones is interpreted to be the EGMB, with subordinate supply of sediments from the Eastern Dharwar craton. It is suggested that portions of the EGMB must have been an integral part of Peninsular India at the time of deposition of these sediments. For the Cuddapah Basin, new U-Pb detrital zircon age constraints are provided for the Nallamalai Group sediments (Bairenkonda Quartzites within the Nallamalai Fold Belt), the youngest population of which yielded an age of ca. 1550. Ma. This age is similar to the emplacement age of the Vinukonda Granite (1589. ±. 4.4. Ma) in the Nellore Schist Belt. This suggests that the Nellore Schist Belt was a source region for the Bairenkonda Quartzites and that deposition continued until at least 1550. Ma. A high-precision Archean age (ca. 3366. Ma) for a detrital zircon grain indicates the presence of Paleoarchean components within the sediment source region. Provided that Paleoarchean rocks are as yet unknown from the largely juvenile Neoarchean Eastern Dharwar craton, there exists a possibility that the Nallamalai Group was sourced from the Ongole Domain of the EGMB. © 2015 Elsevier Ltd.
Evans R.L.,Woods Hole Oceanographic Institution |
Jones A.G.,Dublin Institute for Advanced Studies |
Garcia X.,Barcelona Center for Subsurface Imaging |
Muller M.,Dublin Institute for Advanced Studies |
And 9 more authors.
Journal of Geophysical Research: Solid Earth | Year: 2011
A regional-scale magnetotelluric (MT) experiment across the southern African Kaapvaal craton and surrounding terranes, called the Southern African Magnetotelluric Experiment (SAMTEX), has revealed complex structure in the lithospheric mantle. Large variations in maximum resistivity at depths to 200-250 km relate directly to age and tectonic provenance of surface structures. Within the central portions of the Kaapvaal craton are regions of resistive lithosphere about 230 km thick, in agreement with estimates from xenolith thermobarometry and seismic surface wave tomography, but thinner than inferred from seismic body wave tomography. The MT data are unable to discriminate between a completely dry or slightly "damp" (a few hundred parts per million of water) structure within the transitional region at the base of the lithosphere. However, the structure of the uppermost ∼150 km of lithosphere is consistent with enhanced, but still low, conductivities reported for hydrous olivine and orthopyroxene at levels of water reported for Kaapvaal xenoliths. The electrical lithosphere around the Kimberley and Premier diamond mines is thinner than the maximum craton thickness found between Kimberley and Johannesburg/Pretoria. The mantle beneath the Bushveld Complex is highly conducting at depths around 60 km. Possible explanations for these high conductivities include graphite or sulphide and/or iron metals associated with the Bushveld magmatic event. We suggest that one of these conductive phases (most likely melt-related sulphides) could electrically connect iron-rich garnets in a garnet-rich eclogitic composition associated with a relict subduction slab. Copyright 2011 by the American Geophysical Union.
Val A.,University of Witwatersrand |
Carlson K.J.,University of Witwatersrand |
Steininger C.,University of Witwatersrand |
Kibii J.M.,University of Witwatersrand |
And 3 more authors.
South African Journal of Science | Year: 2011
Conventional methods for extracting fossilised bones from calcified clastic sediments, using air drills or chemical preparations, can damage specimens to the point of rendering them unidentifiable. As an alternative, we tested an in silico approach that extended preparation and identification possibilities beyond those realisable using physical methods, ultimately proving to be crucial in identifying a fragile fossil. Image data from a matrix-encased hemi-mandible of a micromammal that was collected from the Plio-Pleistocene site of Malapa, Cradle of Humankind, South Africa, were acquired using microtomography. From the resultant images, a 3D rendering of the fossil was digitally segmented. Diagnostic morphologies were evaluated on the rendering for comparison with extant comparative specimens, positively identifying the specimen as an elephant shrew (Elephantulus sp.). This specimen is the first positively identified micromammal in the Malapa faunal assemblage. Cutting-edge in silico preparation technology provides a novel tool for identifying fossils without endangering bone integrity, as is commonly risked with physical preparation. © 2011. The Authors.
Barnett W.P.,De Beers Group Services |
Kurszlaukis S.,De Beers Canada Inc. |
Tait M.,De Beers Group Services |
Dirks P.,James Cook University
Bulletin of Volcanology | Year: 2011
Current kimberlite pipe development models strongly advocate a downward growth process with the pipe cutting down onto its feeder dyke by means of volcanic explosions. Evidence is presented from the K08 kimberlite pipe in Venetia Mine, South Africa, which suggests that some pipes or sub-components of pipes develop upwards. The K08 pipe in pit exposure comprises >90 vol.% chaotic mega-breccia of country rock clasts (gneiss and schist) and <10 vol.% coherent kimberlite. Sub-horizontal breccia layers, tens of metres thick, are defined by lithic clast size variations and contain zones of shearing and secondary fragmentation. Textural studies of the breccias and fractal statistics on clast size distributions are used to characterize sheared and non-sheared breccia zones and to deduce a fragmentation mechanism. Breccia statistics are compared directly with the statistics of fragmented rock produced from mining processes in order to support interpretations. Results are consistent with an initial stage of brecciation formed by upward-moving collapse of an explosively pre-conditioned hanging wall into a sub-terranean volcanic excavation. Our analysis suggests that the pre-conditioning is most likely to have been caused by explosions, either phreatic or phreatomagmatic in nature, with a total energy output of 2.7 × 109 kJ (656 t of TNT). A second stage of fragmentation is interpreted as shearing of the breccia caused by multiple late kimberlite intrusions and possible bulk movement of material in the pipe conduit related to adjacent volcanism in the K02 pipe. © 2011 Springer-Verlag.
Verster A.,University of the Free State |
de Waal D.,University of the Free State |
Schall R.,University of the Free State |
Prins C.,De Beers Group Services
Mathematical Geosciences | Year: 2012
The metallurgical recovery processes in diamond mining may, under certain circumstances, cause an under-recovery of large diamonds. In order to predict high quantiles or tail probabilities we use a Bayesian approach to fit a truncated Generalized Pareto Type distribution to the tail of the data consisting of the weights of individual diamonds. Based on the estimated tail probability, the expected number of diamonds larger than a specified weight can be estimated. The difference between the expected and observed frequencies of diamond weights above an upper threshold provides an estimate of the number of diamonds lost during the recovery process. © 2011 International Association for Mathematical Geosciences.
Mangera R.,De Beers Group Services |
Morrison G.,De Beers Group Services |
Voigt A.,De Beers Group Services
Proceedings of the 2015 Pattern Recognition Association of South Africa and Robotics and Mechatronics International Conference, PRASA-RobMech 2015 | Year: 2015
Ensuring that a conveyor belt runs true on an X-ray transmission (XRT) particle sorting system is important for a number of reasons. It limits the damage to the edge of the belt, reduces wear to the conveyor and minimises material spillage. Knowing where the belt is also defines the region of interest (ROI) for the applied algorithm, thus reducing false positives outside the ROI. In addition, correct belt tracking ensures that particles are all contained within the detectable area, avoiding the loss of material. Therefore, ensuring correct conveyor belt tracking can greatly impact the cost and efficiency of the sorting system. This paper presents a novel method for tracking the edge of a conveyor belt in a dual energy X-ray transmission (DE-XRT) sorting system. Using a classifier to determine where the belt edge is, the position of the belt is estimated with a pixel error of less than 15 pixels across all test images. An error of this order is expected as the belt edge is 18 pixels wide in the image. The current method, which is not affected by particles lying against the belt, is shown to be better that the previous approach using morphology, which exhibited poorer tracking accuracy when particles touch the belt. In addition, no additional sensors need to be installed and the belt does not need to be specially manufactured, reducing the cost of the conveyor system. © 2015 IEEE.