The Okavango giant mafic dyke swarm (NE Botswana): its structural significance within the Karoo Large Igneous Province

The structural organization of a giant mafic dyke swarm, the Okavango complex, in the northern Karoo Large Igneous Province (LIP) of NE Botswana is detailed. This N110°E-oriented dyke swarm extends for 1500 km with a maximum width of 100 km through Archaean basement terranes and Permo-Jurassic sedimentary sequences. The cornerstone of the study is the quantitative analysis of N>170 (exposed) and N>420 (detected by ground magnetics) dykes evidenced on a ca. 80-km-long section lying in crystalline host-rocks, at high-angle to the densest zone of the swarm (Shashe area). Individual dykes are generally sub-vertical and parallel to the entire swarm. Statistical analysis of width data indicates anomalous dyke frequency (few data <5.0 m) and mean dyke thickness (high value of 17 m) with respect to values classically obtained from other giant swarms. Variations of mean dyke thicknesses from 17 (N110°E swarm) to 27 m (adjoining and coeval N70°E giant swarm) are assigned to the conditions hosting fracture networks dilated as either shear or pure extensional structures, respectively, in response to an inferred NNW–SSE extension. Both fracture patterns are regarded as inherited brittle basement fabrics associated with a previous (Proterozoic) dyking event. The Okavango N110°E dyke swarm is thus a polyphase intrusive system in which total dilation caused by Karoo dykes (estimated frequency of 87%) is 12.2% (6315 m of cumulative dyke width) throughout the 52-km-long projected Shashe section. Assuming that Karoo mafic dyke swarms in NE Botswana follow inherited Proterozoic fractures, as similarly applied for most of the nearly synchronous giant dyke complexes converging towards the Nuanetsi area, leads us to consider that the resulting triple junction-like dyke/fracture pattern is not a definitive proof for a deep mantle plume in the Karoo LIP.     

Geochemical characteristics of mineral air particles emitted inside a clay atomisation plant which introduces waste recycling processes

The objective of this work is to assess the concentrations of three factions of air particles (settable particles, TSP and PM10) and the levels of several toxic elements in a clay atomisation industry through aerosol sampling at several points inside an industrial plant. Mechanical activities, which produce diffuse emissions, are the main process of discharge of particles in both indoor and outdoor workplace environments in the atomisation plant. The levels of As, Cd, Pb, Zn, Ba and Ni increase in the zones with higher concentrations of particles and lower ventilation. The concentrations of As and F are not influenced by the recycling processes. The levels of Cd and Pb do not show great enrichment in air particles collected inside the atomisation plant although the content of both elements is associated with ceramic muck recycling. Finally, the content of B in waste water is mainly transferred in gaseous phase to the atmosphere during the process of drying by atomisation.     

Lithostratigraphic position and petrographic characteristics of R.A.T. (“Roches Argilo-Talqueuses”) Subgroup, Neoproterozoic Katangan Belt (Congo)

The Neoproterozoic Katangan R.A.T. (“Roches Argilo-Talqueuses”) Subgroup is a sedimentary sequence composed of red massive to irregularly bedded terrigenous-dolomitic rocks occurring at the base of the Katangan succession in Congo. Red R.A.T. is rarely exposed in a continuous section because it was affected by a major layer-parallel décollement during the Lufilian thrusting. However, in a number of thrust sheets, Red R.A.T. is in conformable sedimentary contact with Grey R.A.T which forms the base of the Mines Subgroup. Apart from the colour difference reflecting distinct depositional redox conditions, lithological, petrographical and geochemical features of Red and Grey R.A.T. are similar. A continuous sedimentary transition between these two lithological units is shown by the occurrence of variegated to yellowish R.A.T. The D. Strat. “Dolomies Stratifiées” formation of the Mines Subgroup conformably overlies the Grey R.A.T. In addition, a transitional gradation between Grey R.A.T. and D. Strat. occurs in most Cu–Co mines in Katanga and is marked by interbedding of Grey R.A.T.-type and D. Strat.-type layers or by a progressive petrographic and lithologic transition from R.A.T. to D. Strat. Thus, there is an unquestionable sedimentary transition between Grey R.A.T. and D. Strat. and between Grey R.A.T. and Red R.A.T.The R.A.T. Subgroup stratigraphically underlies the Mines Subgroup and therefore R.A.T. cannot be comprised of syn-orogenic sediments deposited upon the Kundelungu (formerly “Upper Kundelungu”) Group as suggested by Wendorff (2000). As a consequence, the Grey R.A.T. Cu–Co mineralisation definitely is part of the Mines Subgroup Lower Orebody, and does not represent a distinct generation of stratiform Cu–Co sulphide mineralisation younger than the Roan orebodies.     

Lithostratigraphy, basin development, base metal deposits, and regional correlations of the Neoproterozoic Nguba and Kundelungu rock successions, central African Copperbelt

The Neoproterozoic Katangan Supergroup comprises a thick sedimentary rock succession subdivided into the Roan, Nguba, and Kundelungu Groups, from bottom to top. Deposition of both Nguba and Kundelungu Groups began with diamictites, the Mwale/Grand Conglomérat and Kyandamu/Petit Conglomérat Formations, respectively, correlated with the 750 Ma Sturtian and (supposedly) 620 Ma Marinoan/Varanger glacial events. The Kaponda, Kakontwe, Kipushi and Lusele Formations are interpreted as cap-carbonates overlying the diamictites. Petrographical features of the Nguba and Kundelungu siliciclastic rocks indicate a proximal facies in the northern areas and a basin open to the south. The carbonate deposits increase southward in the Nguba basin. In the southern region, the Kyandamu Formation contains clasts from the underlying rocks, indicating an exhumation and erosion of these rocks to the south of the basin. It is inferred that this formation deposited in a foreland basin, dating the inversion from extensional to compressional tectonics, and the northward thrusting. Sampwe and Biano sedimentary rocks were deposited in the northernmost foreland basin at the end of the thrusting. The Zn–Pb–Cu and Cu–Ag–Au epigenetic, hypogene deposits occurring in Nguba carbonates and Kundelungu clastic rocks probably originate from hydrothermal resetting and remobilization of pre-existing stratiform base metal mineralisations in the Roan Group.     

COMPARISON OF MEASUREMENT METHODS BASED ON A MODEL FOR THE ERROR STRUCTURE

Comparison of methods is a technique often used for investigation of systematic errors of measurementmethods.As concerns the design and analysis of such comparisons,much variety of opinion and practiceexists.In one approach a few specimens are measured several times by different operators in differentlaboratories(reproducibility conditions)and in another approach several specimens are measured on oneor a few occasions by one operator in the same laboratory(repeatability conditions).In this paper amodel for the error structure of measurements is formulated and it is emphasized that one has todistinguish between two types of systematic errors:the first type depends only on the level of themeasured quantity and the second type is specific for the separate specimens.On the basis of this modelthe information which can be obtained from the different designs of method comparisons is discussed.A new approach for the analysis of method comparisons with many specimens is also proposed.     

Sediment-hosted Zn–Pb–Cu deposits in the Central African Copperbelt

Stratabound epigenetic sulphide Zn–Pb–Cu ore deposits of the Central African Copperbelt in the Democratic Republic of Congo and Zambia are mostly hosted in deformed shallow marine platform carbonates and associated sedimentary rocks of the Neoproterozoic Katanga Supergroup. Economic orebodies, that also contain variable amounts of minor Cd, Co, Ge, Ag, Re, As, Mo, Ga, and V, occur mainly as irregular pipe-like bodies associated with collapse breccias and faults as well as lenticular bodies subparallel to bedding. Kipushi and Kabwe in the Democratic Republic of the Congo and Zambia, respectively, are the major examples of carbonate-hosted Zn–Pb–Cu mined deposits with important by-products of Ge, Cd, Ag and V in the Lufilian Arc, a major metallogenic province famous for its world-class sediment-hosted stratiform Cu–Co deposits. The carbonate-hosted deposits range in age from Neoproterozoic to early Palaeozoic (680 to 450 Ma). The formation of the relatively older Neoproterozoic deposits is probably related to early collision events during the Lufilian Orogeny, whereas the younger Palaeozoic deposits may be related to post-collisional processes of ore formation. Fluid inclusion and stable isotope data indicate that hydrothermal metal-bearing fluids evolved from formation brines during basin evolution and later tectonogenesis. Ore fluid migration occurred mainly along major thrust zones and other structural discontinuities such as karsts, breccias and faults within the Katangan cover rocks, resulting in ore deposition within favourable structures and reactive carbonates of the Katangan Supergroup.