We report the results of a reconnaissance study of several small carbonatite and syenite bodies with very low topography (up to 1.5 m), that were recently discovered in the Archean basement in the Amaragh area near the Gleibat Lafhouda (GL) carbonatite, previously interpreted as a primary carbonatite. Two varieties of carbonatite have been identified: 1) brownish to greyish banded dolomite carbonatite, composed of dolomite as the main carbonate phase, and calcite, magnetite, apatite, monazite-(Ce), magnesite and minor bastnaesite-(Ce), 2) calciocarbonatite, mainly composed of calcite and several feldspar aggregates (the latter are interpreted as xenocrysts). The Amaragh syenites consist of small outcrops and show white to dark grey colors. The main outcrop is a nepheline syenite that contains coarse-grained rounded magnetites, rimmed by biotite and epidote within a feldspar and nepheline groundmass, suggesting fluid-mineral reactions. The other syenites consist mainly of alkali feldspar, albite, mica, amphibole and accessory apatite, epidote, and titanite. The host minerals of Nb and REE are pyrochlore and allanite. All syenite samples display a peraluminous composition and strong LREE enrichment with negative EuCN anomalies, except for two samples with a slightly positive EuCN anomaly, suggesting at least two types of differentiated syenites. All samples show strong postmagmatic and hydrothermal overprint, which is revealed by, for example, the occurrence of negative YCN and positive CeCN anomalies and mica-epidote mineral reaction rims between magnetite and feldspar. The close spatial association of the Amaragh carbonatite and syenite bodies with the GL carbonatites along with geochemical and mineralogical similarities and REE-Nb enrichment suggest a close petrogenetic relationship, pointing toward a single carbonatite-syenite alkaline complex that was probably formed in an extensional tectonic setting.

The sediments in two stacked megafans in the Owambo Basin of northern Namibia and southern Angola were made accessible by a c. 400 m long, continuously cored borehole. Previous studies have indicated that the lower buried Paleocene–Eocene Olukonda Megafan was deposited by a palaeo-Kunene River transporting material from the NW (i.e. from the Kunene Intrusive Complex and the adjacent Angola Shield). The morphology of the overlying Eocene–Pliocene Cubango Megafan suggests the input of sediments from the north via the Cubango River. Mineralogical and geochemical data from the upper fan indicate felsic metamorphic and granitoid sources. Previous studies, however, did not provide a unique provenance identifier. Combining detrital zircon U–Pb data from both megafans with previously published and newly obtained mineralogical and geochemical data confirms two distinct provenances. The Olukonda Megafan can now be uniquely attributed by means of its detrital zircon ages to the Kunene Intrusive Complex and the surrounding Epupa Metamorphic Complex. In good agreement with the geochronologically more varied geology of the source region in the north, the Cubango Megafan detrital zircon record shows a wide distribution of Paleoproterozoic to Archean ages, but also a younger age range, probably related to a later Damaran/Pan-African source, which is absent in the Olukonda detrital zircon record. Supplementary material: Data obtained for this study, e.g. geochemical data, heavy mineral composition, heavy mineral morphology, zircon morphology, U-Th-Pb isotopes and ages on zircon, instrument settings, and U-Th-Pb isotope data of analysed reference zircon are available at https://doi.org/10.6084/m9.figshare.c.6280517

Abstract The Neoproterozoic Lofdal alkaline carbonatite complex consists of a swarm of carbonatite dykes and two plugs of calcite carbonatite known as the ‘Main’ and ‘Emanya’ carbonatite intrusions, with associated dykes and plugs of phonolite, syenite, rare gabbro, anorthosite and quartz-feldspar porphyry. In the unaltered Main Intrusion calcite carbonatite the principal rare-earth host is burbankite. As burbankite typically forms in a magmatic environment, close to the carbohydrothermal transition, this has considerable petrogenetic significance. Compositional and textural features of Lofdal calcite carbonatites indicate that burbankite formed syngenetically with the host calcite at the magmatic stage of carbonatite evolution. The early crystallisation of burbankite provides evidence that the carbonatitic magma was enriched in Na, Sr, Ba and light rare earth elements. In common with other carbonatites, the Lofdal burbankite was variably affected by alteration to produce a complex secondary mineral assemblage. Different stages of burbankite alteration are observed, from completely fresh blebs and hexagonal crystals through to complete pseudomorphs, consisting of carbocernaite, ancylite, cordylite, strontianite, celestine, parisite and baryte. Although most research and exploration at Lofdal has focused on xenotime-bearing carbonatite dykes and wall-rock alteration, this complex also contains a more typical calcite carbonatite enriched in light rare earth elements and their alteration products.

The recently discovered REE and Nb mineralization in the Twihinat area in the western part of the Oulad Dlim Massif (Adrar Souttouf) in South Morocco is linked to a Cretaceous calciocarbonatite intrusion which was likely formed in an intracontinental rift setting and crops out locally within a ring structure that mainly consists of massive Fe-oxide mineralization and silica breccia. The carbonatite shows intensively metasomatized zones, which contain bastnaesite and pyrochlore-group minerals as the main REE and Nb ore minerals. They are usually associated with apatite, quartz and Fe-oxides, or trapped in calcite voids, suggesting a secondary ore formation. Within the associated Fe-oxide mineralization, pyrochlore and monazite-(Ce) are the main ore minerals occurring closely associated with quartz and magnetite or hematite. The silica breccia also shows significant subsequent infill of barite, bastnaesite-(Ce) and hydrated ceriopyrochlore, which was identified by EPMA and Raman spectroscopy. Bastnaesite commonly forms prismatic aggregates whereas pyrochlore and ceriopyrochlore usually display subhedral grains along tiny fractures. Structural and textural relationships clearly indicate epigenetic ore formation induced by multiple stages of hydrothermal fluid flow and fracturing. Ore precipitation likely resulted from interaction between low-pH mineralizing hydrothermal fluids and the wall-rock. The latter efficiently buffered the acidity of the fluids and allowed significant amounts of REE and Nb ore minerals to precipitate. Trace element ICP-MS analyses show very high REE and Nb concentrations of up to 0.76 wt% Sigma REE and 0.21 wt% Nb in carbonatite and up to 3 wt% Sigma REE and 1.3 wt% Nb in the associated silica and Fe-oxide mineralization. The results clearly demonstrate that the Twihinat REE-Nb deposits are significant and represent a potential new high-grade resource for these critical metals.

Recent exploration work in South Morocco revealed the occurrence of several carbonatite bodies, including the Paleoproterozoic Gleibat Lafhouda magnesiocarbonatite and its associated iron oxide mineralization, recognized here as iron-oxide-apatite (IOA) deposit type. The Gleibat Lafhouda intrusion is hosted by Archean gneiss and schist and not visibly associated with alkaline rocks. Metasomatized micaceous rocks occur locally at the margins of the carbonatite outcrop and were identified as glimmerite fenite type. Rare earth element (REE) and Nb mineralization is mainly linked to the associated IOA mineralization and is represented by monazite-(Ce) and columbite-(Fe) as major ore minerals. The IOA mineralization mainly consists of magnetite and hematite that usually contain large apatite crystals, quartz and some dolomite. Monazite-(Ce) is closely associated with fluorapatite and occurs as inclusions within the altered parts of apatite and along cracks or as separate phases near apatite. Monazite shows no zonation patterns and very low Th contents (<0.4 wt%), which would be beneficial for commercial extraction of the REE and which indicates monazite formation from apatite as a result of hydrothermal volatile-rich fluids. Similar monazite-apatite mineralization and chemistry also occurs at depth within the carbonatite, although the outcropping carbonatite is barren, suggesting an irregular REE ore distribution within the carbonatite body. The barren carbonatite contains some tiny unidentified secondary Nb-Ta-U phases, synchysite and monazite. Niobium mineralization is commonly represented by anhedral minerals of columbite-(Fe) which occur closely associated with magnetite-hematite and host up to 78 wt% Nb2O5, 7 wt% Ta2O5 and 1.6 wt% Sc2O3. This association may suggest that columbite-(Fe) precipitated by an interaction of Nb-rich fluids with pre-existing Fe-rich minerals or as pseudomorphs after pre-existing Nb minerals like pyrochlore. Our results most strongly suggest that the studied mineralization is economically important and warrants both, further research and exploration with the ultimate goal of mineral extraction. (C) 2019 Published by Elsevier B.V. on behalf of International Association for Gondwana Research.

Heavy minerals are typically rare but important components of siliciclastic sediments and rocks. Their abundance, proportions, and variability carry valuable information on source rocks, climatic, environmental and transport conditions between source to sink, and diagenetic processes. They are important for practical purposes such as prospecting for mineral resources or the correlation and interpretation of geologic reservoirs. Despite the extensive use of heavy mineral analysis in sedimentary petrography and quite diverse methods for quantifying heavy mineral assemblages, there has never been a systematic comparison of results obtained by different methods and/or operators. This study provides the first interlaboratory test of heavy mineral analysis. Two synthetic heavy mineral samples were prepared with considerably contrasting compositions intended to resemble natural samples. The contributors were requested to provide (i) metadata describing methods, measurement conditions and experience of the operators and (ii) results tables with mineral species and grain counts. One hundred thirty analyses of the two samples were performed by 67 contributors, encompassing both classical microscopic analyses and data obtained by emerging automated techniques based on electron-beam chemical analysis or Raman spectroscopy. Because relatively low numbers of mineral counts (N) are typical for optical analyses while automated techniques allow for high N, the results vary considerably with respect to the Poisson uncertainty of the counting statistics. Therefore, standard methods used in evaluation of round robin tests are not feasible. In our case the 'true' compositions of the test samples are not known. Three methods have been applied to determine possible reference values: (i) the initially measured weight percentages, (ii) calculation of grain percentages using estimates of grain volumes and densities, and (iii) the best-match average calculated from the most reliable analyses following multiple, pragmatic and robust criteria. The range of these three values is taken as best approximation of the 'true' composition. The reported grain percentages were evaluated according to (i) their overall scatter relative to the most likely composition, (ii) the number of identified components that were part of the test samples, (iii) the total amount of mistakenly identified mineral grains that were actually not added to the samples, and (iv) the number of major components, which match the reference values with 95% confidence. Results indicate that the overall comparability of the analyses is reasonable. However, there are several issues with respect to methods and/or operators. Optical methods yield the poorest results with respect to the scatter of the data. This, however, is not considered inherent to the method as demonstrated by a significant number of optical analyses fulfilling the criteria for the best-match average. Training of the operators is thus considered paramount for optical analyses. Electron-beam methods yield satisfactory results, but problems in the identification of polymorphs and the discrimination of chain silicates are evident. Labs refining their electron-beam results by optical analysis practically tackle this issue. Raman methods yield the best results as indicated by the highest number of major components correctly quantified with 95% confidence and the fact that all laboratories and operators fulfil the criteria for the best-match average. However, a number of problems must be solved before the full potential of the automated high-throughput techniques in heavy mineral analysis can be achieved.

ABSTRACTThe Cenozoic Kalahari Basin covers large parts of southern Africa. A continuous 400 m core was obtained in northern Namibia and analyzed in detail. Here, we present sedimentological, geochemical, mineralogical, granulometric, and hydraulic data, which were used to derive the sedimentation history and the Cenozoic paleoclimate and paleogeography of SW Africa. The first absolute ages for the Kalahari Basin were obtained by dating of calcretes, which showed that the core covers almost the entire Cenozoic. Two megafans could be distinguished. The older, buried Olukonda Megafan stems from a mafic source rock, potentially the Kunene Intrusive Complex, and was deposited by a paleo–Kunene River towards the southeast and east, under a semiarid climate. The younger Cubango Megafan (Andoni Formation) has a completely different provenance, namely felsic metamorphic and granitoid rocks, transported from the north by the Cubango River. The capture of the Kunene towards the Atlantic during the Eocene resulted in this change in provenance. Despite the distinct differences between the formations, the temporal hiatus between them must have been short. The results are a showcase of the potential of megafans for hosting major deep freshwater aquifers.

Polymetallic nodules are a potential source of industrially demanded metals such as Ni, Co, Cu, and Mo (up to 3 wt %). Even if there is no deep-sea mining of manganese nodules today, a forecasted gap between metal demand and supply as well as continuously high metal prices may make seabed mining economically viable in the future. Up to now, a well-established industrial-scale extraction method for manganese nodules has been missing. Therefore, the aim of this study is to explore how economically interesting metals can be extracted from the nodules in a cost- and energy-efficient way. Polymetallic nodules have a heterogeneous chemical and structural composition without individual metal-rich particles. The economically interesting metals are distributed between different mineral phases (Mn-Fe-(oxy)hydroxides) as well as different growth structures that are intergrown with each other on a nm‒µm scale. Because of that a typical ore processing with the beneficiation of valuable particles is not feasible. The process presented here starts with a pyro-metallurgical pre-treatment of the polymetallic nodules, with the aim of creating artificial metal-rich (Ni, Cu, Co, Mo) particles with enrichment factors up to 10 compared to the original average metal contents. Afterwards, these particles should be beneficiated by conventional mineral processing steps to create a concentrate while reducing the mass stream in the process. The resulting metal particles can be further treated in conventional hydrometallurgical and/or pyro-metallurgical processes.

Pyrite is a common minor constituent of terrestrial freshwater sediments and a sink for trace elements. Different amounts and morphological types (framboids and euhedral crystals) of sedimentary pyrites were found in the heavy mineral fraction of cores obtained from several drillholes located in the Emsland region, NW Germany. Their trace element contents were investigated to assess their potential for groundwater contamination after oxidation, e.g. induced by dewatering or autotrophic denitrification. Nickel, arsenic and cadmium were found in significant concentrations in pyrite. Geochemical modeling showed that elevated trace metal concentrations in groundwater, potentially exceeding drinking water standards, should preferentially occur in a less than 1 m thick zone situated around the depth of the redoxcline, where nitrate is reduced by pyrite. This was confirmed by depth-specific groundwater sampling in the Emsland and by previously published studies. The absolute concentration of released trace metals depends on their content in the pyrite but also strongly on the nitrate load of groundwater. (C) 2016 Elsevier Ltd. All rights reserved.

Columbite-group minerals (CGM) account for the majority of the production of tantalum, an important metal for high-technology applications. Along with other Ta-Nb oxides such as tapiolite, wodginite, ixiolite and pyrochiore supergroup minerals, CGM are recovered from rare-metal granites and granitic rare-element pegmatites. In this paper mineralogical and geochemical data with a focus on CGM, tapiolite, wodginite and ixiolite are presented for rare-element granites and pegmatites from worldwide occurrences except Africa that has been covered in a previous contribution (Melcher et al., 2015). Major and trace element data of the Ta-Nb oxides are presented and compared for a total of 25 granite/pegmatite provinces, and one carbonatite for comparison. Based on CGM compositions, the data allow to distinguish between various subgroups of Li-Cs-Ta (LCT)-family pegmatites, Nb-Y-F (NYF)-family pegmatites, mixed LCT-NYF pegmatites, and rare-element granites. Each period of Ta-ore formation in Earth history is characterised by peculiar mineralogical and geochemical features. Some of the largest and economically most important rare-element pegmatite bodies are located within Archean terrains and intruded ultramafic and mafic host rocks (e.g., Tanco/Canada, Wodgina and Greenbushes/Western Australia, Kolmozero/Kola). They are highly fractionated, of LCT affinity throughout and yield complex mineralogical compositions. The variety of minor and trace elements incorporated attests to a rather insignificant role of the immediate host rocks to their geochemical signature and rather points to the significance of the composition of the underlying crustal protoliths, internal fractionation and the processes of melt generation. Many of the Archean pegmatites carry significant Li mineralization as spodumene, petalite, and amblygonite, and all of them are also characterised by elevated Li in CGM. In addition, Sb and Bi are important trace elements, also reflected by the occasional presence of stibiotantalite and bismutotantalite. REEN patterns of CGM are dominated by the MREE or HREE, and range from very low to high total REE concentrations. Negative Eu anomalies are omnipresent. Scandium contents are also highly variable, from very high (Tanco) to very low concentrations (Wodgina, Kolmozero). A second period of worldwide pegmatite formation was in the Paleoproterozoic. All CGM analysed derive from LCT-family pegmatites except samples from the Amazonas region where Ta is mined from rare -metal granites at Pitinga. Pegmatites intruded highly variable lithologies including metasediments, metabasites, gneiss, granite and quartzite within a variety of structural and paleogeographic settings; however, most of them are syn- to postorogenic with respect to major Paleoproterozoic orogenic events. Minor and trace element signatures are similar to CGM from Archean pegmatites. Some are characterised by considerable REE enrichment (Sao Joao del Rei/ Brazil; Amapa/Brazil; Finnish Lapland/Finland), whereas others have normal to low total REE concentrations (Black Hills/USA, Baster/India). Examples with high REE commonly are enriched in Sc and Y as well, and are often transitional to NYF-family pegmatites. The Mesoproterozoic period is comparatively poor in rare-element pegmatites and rare -metal granites. Mineralogical and chemical attributes of ixiolite-wodginite, tapiolite, CGM and rutile from placer material in Colombia point to an unusual pegmatite source of NYF affinity, yielding high total REE, Sc and Th at low Li and Bi. REE patterns have typical negative Eu and Y anomalies. A third major period of pegmatite formation was the Early Neoproterozoic at around 1 Ga, documented in the Grenvillian (North America), the Sveconorwegian (northern Europe) and the Kibaran in central Africa. CGM are present in numerous, mostly small pegmatites, although larger examples also occur (e.g., Manono in the D.R. Congo; Melcher et al., 2015). Pegmatite fields often display a zonal arrangement of mineralised pegmatites with respect to assumed "fertile" parent granites. They intrude metasediments, metabasites, gneiss and granite of middle to upper crustal levels and display a variety of mineralogical and chemical characteristics. Pegmatites of the Sveconorwegian and Grenville domains are usually of the NYF type and CGM are characterised by elevated Y, REE, Th and Sc. In contrast, the pegmatites of central (Kibara Belt) and southwestern Africa (Orange River Belt) are commonly of LCT affinity carrying spodumene, beryl and cassiterite (Melcher et al., 2015). These CGM have elevated concentrations of Li, Mg, Sn and HE Total REE concentrations are low except for the Sveconorwegian, and exhibit a variety of shapes in normalised diagrams. The fourth major pegmatite-forming event coincides with amalgamation of Gondwana at the Neoproterozoic/ Paleozoic boundary around 550 Ma ago. This event is omnipresent in Africa ("Panafrican") and South America ("Brasiliano event" documented in the Eastern Brazilian pegmatite and Borborema provinces). Pegmatites often intruded high-grade metamorphic terrains composed of metasediments including schist, marble, quartzite, as well as gneiss, amphibolite, ultramafic rocks, and granite. Within the Neoproterozoic, rare-metal granites of NYF affinity are locally abundant. Pegmatites show both LCT and NYF affinities, and mixed types occur in Mozambique. The Alto Ligonha and Madagascar provinces are characterised by abundant REE and Sc both within Ta-Nb-oxides and as separate mineral phases. Notably, some pegmatite provinces are almost devoid of cassiterite, whereas others carry cassiterite in economic amounts. In the Phanerozoic (younger than 542 Ma), pegmatites formed at all times in response to orogenetic processes involving various continents and terranes during the long-time amalgamation of Pangea and the Alpine orogenies. Whereas some activity is related to the Pampean, Acadian and Caledonian orogenies, the Variscan/Hercynian and Alleghanian orogenies are of utmost importance as manifested in pegmatite formation associated with Sn-W mineralised granites in central and western Europe as well as in the Appalachians. Most of the Variscan and Alleghanian pegmatites are of La affinity, although NYF and some mixed types have been described as well. Variscan pegmatite formation culminated at ca. 330 to 300 Ma, whereas Alleghanian pegmatites range in age from about 390 Ma to about 240 Ma. Most are syn-to post-orogenic and were emplaced at different crustal levels and into a variety of host rocks. Degree of fractionation as well as minor and trace element geochemistry of TaNb oxides are rather variable and cover the complete field of CGM compositions. REE patterns are characterised by prominent negative Eu anomalies. Some Mesozoic and Cenozoic pegmatites and rare-metal granites from Southeast Asia and the Russian Far East are induded in the compilation. Rare-metal granites of the Jos Plateau (Nigeria) were previously investigated (Melcher et al., 2015). The proportion of NYF pegmatites and rare-metal granites in the Mesozoic is striking, i.e. illustrated by Jos, Orlovka, Ulug Tanzek as well as the southeast Asian deposits related to tin granites. CGM from these areas are invariably rich in REE, Sc, Y and Th. In all rare-metal granites, Ta-Nb oxides are characterised by high total REE concentrations and both, negative Eu and Y anomalies in chondrite-normalised REE diagrams. Although constituting a vastly different magmatic system compared to rare metal pegmatites and granites, we included the Upper Fir carbonatite from the Canadian Cordillera, for comparison, because it is characterised by unusal high Ta contents. As expected, the CGM differ from the pegmatitic CGM by having high Mg and Th, and low U concentrations in columbite-(Fe) and lack an Eu anomaly. However, they also show similarities to primitive CGM from rare metal pegmatites of the NYF family in terms of the REE pattern and the increase in #Ta and #Mn towards the margins of the CGM. Our findings support recent results presented in Chudy (2014) indicating that the Ta enrichment in some carbonatites might be attributed to magmatic processes and conditions that are similar to the pegmatitic systems. (C) 2016 Elsevier B.V. All rights reserved.

Compositional variation (results of electron microprobe analyses and mass-spectrometry analyses) of columbite-group minerals (CGM) from fully differentiated albite–spodumene pegmatites at Kolmozero in the Kola Peninsula is evaluated. Concentric zoning, typical of rare-metal pegmatites, was not observed in the Kolmozero pegmatites. Columbite-group minerals occur in all main parageneses of the pegmatites and form four generations, reflecting the sequence of pegmatite formation. These minerals demonstrate wide variations in the content of major and trace elements. The composition of CGM ranges from columbite-(Fe) to tantalite-(Mn). Fractionation trends were observed in Mn/(Mn + Fe) versus Ta/(Ta + Nb) diagrams and trace-element abundances plotted versus XTa and XMn. The early CGM paragenesis is characterized by homogeneous, oscillatory and progressive oscillatory zoning and corresponds to a primary magmatic type. Late-generation CGM show patchy irregular internal textures replacing earlier regular patterns of zoning. The irregular zoning points to metasomatic replacement processes. For the first time, it is shown that distributions of rare earth elements (REE) in CGM reflect the evolution of a pegmatite-forming system. At Kolmozero, the main trend of REE variation from early to late generations of CGM involves decreasing total REE contents due to a decrease in heavy REE and Y, decreasing negative Eu anomaly and decreasing magnitude of M-shape tetrad effect between Gd and Ho. These changes are accompanied by gradual flattening of the “bird-like” patterns of chondrite-normalized REE distribution. All these features are typical for late differentiates of granitic volatile-rich magma. Late metasomatic tantalite-(Mn) is characterized by sharp changes in its REE distribution pattern: decreasing total REE contents, changing shape of the REE distribution pattern, the absence of Eu anomaly and tetrad effects, and the appearance of a negative Ce anomaly. The textural characteristics and mineral chemistry of CGM indicate that the pegmatite-forming system underwent several stages of evolution. The earliest magmatic stage can be divided into two sub-stages, involving direct crystallization and collective recrystallization, respectively, and was succeeded by a late hydrothermal–metasomatic post-magmatic stage. Variations in chemical composition among the different generations of CGM are explained by the interplay of several processes: fractional crystallization; competitive crystallization of main rock-forming (feldspar, muscovite, spodumene) and accessory (triphylyte–lithiophilite, spessartine, fluorapatite, zircon, microlite) minerals; and evolution of the mineral-forming environment from a melt to a hydrothermal–metasomatic fluid.

Tantalum, an important metal for high-technology applications, is recovered from oxide minerals that are present as minor constituents in rare-metal granites and granitic rare-element pegmatites. Columbite-group minerals (CGM) account for the majority of the current tantalum production; other Ta-Nb oxides (TNO) such as tapiolite, wodginite, ixiolite, rutile and pyrochlore-supergroup minerals may also be used.In this paper mineralogical and geochemical data with a focus on opaque minerals as well as age determinations on CGM using the U-Pb method are presented for 13 rare-element granite and pegmatite districts in Africa, covering Archean, Paleoproterozoic, Neoproterozoic, Paleozoic and Mesozoic provinces. Geological, economic and geochronological data are reviewed.Each period of Ta-ore formation is characterised by peculiar mineralogical and geochemical features that assist in discriminating these provinces. Compositions of CGM are extremely variable: Fe-rich types predominate in the Man Shield (Sierra Leone), the Congo Craton (Democratic Republic of the Congo), the Kamativi Belt (Zimbabwe) and the Jos Plateau (Nigeria). Mn-rich columbite-tantalite is typical of the Alto Ligonha Province (Mozambique), the Arabian-Nubian Shield and the Tantalite Valley pegmatites (southern Namibia). Large compositional variations through Fe-Mn fractionation, followed by Nb-Ta fractionation are typical for pegmatites of the Kibara Belt of Central Africa, pegmatites associated with the Older Granites of Nigeria and some pegmatites in the Damara Belt of Namibia. CGM, tapiolite, wodginite and ixiolite accommodate minor and trace elements at the sub-ppm to weight-percent level. Trace elements are incorporated in TNO in a systematic fashion, e.g. wodginite and ixiolite carry higher Ti, Zr, Hf, Sn and Li concentrations than CGM and tapiolite. Compared to tapiolite, CGM have higher concentrations of all trace elements except Hf and occasionally Zr, Ti, Sn and Mg. The composition of TNO related to rare-element pegmatites is rather different from rare-metal granites: the latter have high REE and Th concentrations, and low Li and Mg. Pegmatite-hosted TNO are highly variable in composition, with types poor in REE, typical of LCT-family pegmatites, and types rich in REE - showing affinity for NYF-family or mixed LCT-NYF pegmatites. Major and trace elements show regional characteristics that are conspicuous in normalised trace element and REE diagrams. In general, CGM from Ta-ore provinces are characterised by the predominance of one type of REE distribution pattern characterised by ratios between individual groups of REE (light, middle, heavy REE) and the presence and intensity of anomalies (e.g. Eu/Eu*).Despite textural complexities such as complex zoning patterns and multiple mineralisation stages, the chemical compositions of CGM, tapiolite and wodginite-ixiolite from rare-metal granite and rare-element pegmatite provinces indicate that they are cogenetic and reflect specific source characteristics that may be used to discriminate among rocks of different origin.Geochronological data produced for CGM from ore districts are discussed together with the respective ore mineralogy and minor and trace element geochemistry of TNO to reconsider the geodynamics of pegmatite formation. In Africa, formation of rare element-bearing pegmatites and granites is related to syn- to late-orogenic (e.g., West African Craton, Zimbabwe Craton), post-orogenic (Kibara Belt, Damara Belt, Older Granites of Nigeria, Adola Belt of Ethiopia) and anorogenic (Younger Granites of Nigeria) tectonic and magmatic episodes. The late-orogenic TNO mineralisation associated with A-type granites in the Eastern Desert of Egypt shares geochemical features with the anorogenic Younger Granites of Nigeria. (C) 2013 Elsevier B.V. All rights reserved.

The development of microhardpans on mining residues has major influence on the water infiltration. Hardpans on the blocky unsorted heaps at Peña del Hierro, Spain, at the headwaters of Rio Tinto, are 1–2 mm thick and guide water over 100 m horizontally. We investigated some of these microhardpans in detail with SEM, μ-CT, EDXRF-microscope and MLA (mineral liberation analyses). From SEM, it was obvious that the uppermost 100 μm of the hardpan show a reduced porosity. Extremely fine pores could be observed, and porosity was below 15%. On the other hand, μ-CT showed a pronounced coarse pore system, mainly parallel to the hardpan surface below the top 100 μm. We interpret the rapid change in pore size as well as in alteration style as a well-pronounced capillary barrier, which is even emphasized towards the lower hardpan boundary where horizontal megapores developed. By combining EDXRF and MLA of the polished surface with μ-CT, a better attribution of densities to minerals could be performed, allowing one to highlight zones of mineral enrichment due to sedimentary transport in microchannels prior to the formation of microhardpans. A comparison of EDXRF-microscope Fe (a) and Ba (b) element distribution with 2D µ-CT density (c) distribution of the surface layer extraction of the polished section showing the distribution of smaller and larger pores and fragments as well as denser particles of barite (green rings) and Fe-phases (red rings) as well as the Fe-hydroxide surface coating (red arrow) as a false colour image (d); the MLA detail shows the complexity best. The combination of the different techniques provides a better understanding of the process of hardpan formation in such an environment and the potential remediation effects.

The Mesoproterozoic Kibara belt in Central Africa has recently been redefined and subdivided into the Karagwe-Ankole belt (KAB) and the Kibara belt (KIB), separated by Palaeoproterozoic (Rusizian) terranes. The KIB and KAB are characterised by the presence of numerous rare metal mineralised (Nb-Ta-Sn) pegmatites and Sn-W mineralised quartz veins that are related to the youngest granite generation, i.e. the G4-granites in Rwanda, which formed at 986 +/- 10 Ma. The pegmatites of the Gatumba area (western Rwanda) have historically been mined for their columbite-tantalite and cassiterite mineralisation, but contain also beryl, apatite, spodumene, amblygonite, and rare phosphates. Columbite-tantalite formed during the crystallisation of the pegmatites, followed by intense alkali metasomatism, i.e. widespread growth of albite and white mica. The major part of the cassiterite mineralisation is, however, concentrated in zones associated with intense phyllic alteration.U-Pb ages of columbite-tantalite samples vary between similar to 975 Ma and similar to 930 Ma. The oldest ages (975 + 8.2/-8.3 Ma and 966 + 8.7/-8.6 Ma) overlap with previous reported Rb-Sr ages of the emplacement of the pegmatites (similar to 965 Ma) and are interpreted to reflect the crystallisation of the Nb-Ta mineralisation. The youngest ages (951 +/- 15 Ma to 936 +/- 14 Ma) are apparently related to variable degrees of resetting by (metasomatic) post-crystallisation processes. The resetting could either be due to recrystallisation of early Nb-Ta minerals or due to the disturbance of the U-Pb isotopic signature of the Nb-Ta minerals. The Ar-40-Ar-39 spectra of muscovite samples associated with different steps in the paragenesis of the pegmatites show a spread of apparent ages between similar to 940 Ma and similar to 560 Ma that reflect Late Neo-proterozoic tectonothermal events. One plateau age of 592.2 +/- 0.8 Ma is interpreted to reflect far-field effects of the East African orogeny on the Karagwe-Ankole Belt. (C) 2011 Elsevier Ltd. All rights reserved.

Coltan (the African trade name for columbite‐tantalite, a tantalum ore) is one of several raw materials that finance the civil wars in the eastern provinces of the Democratic Republic of the Congo. To improve the transparency along the tantalum trade chain, a ‘certificate of origin’ for so‐called ‘conflict minerals’ has been recommended by the United Nations. Accordingly, the German Federal Institute for Geosciences and Natural Resources (BGR) has developed an analytical fingerprint procedure for coltan. Mineral formation age, modal mineralogy and chemical composition are important fingerprint parameters. The original workflow to obtain these parameters was streamlined and is now based on mineral liberation analysis and LA‐ICP‐MS. The use of an ICP‐MS instrument with a detector system covering an extended linear dynamic range and the application of an internal standard‐independent calibration strategy allowed data for major and trace element determination and mineral formation age estimates to be obtained simultaneously. The analytical results of this new approach were compared with analytical techniques of the original workflow and showed excellent agreement in terms of mineralogical and chemical characterisation and mineral formation age of coltan samples. Within a test, samples of different origin were allocated correctly and simple, binary mixtures were also identified successfully.

Illegal mining of gold, diamonds, copper, cobalt and, in the last decade, “coltan” has fuelled ongoing armed conflicts and civil war in a number of African countries. Following the United Nations initiative to fingerprint the origin of conflict materials and to develop a traceability system, our working group is investigating “coltan” (i.e. columbite-tantalite) mineralization especially in Africa, also within the wider framework of establishing certified trading chains (CTC). Special attention is directed towards samples from the main Ta-Nb-Sn provinces in Africa: DR Congo, Rwanda, Mozambique, Ethiopia, Egypt and Namibia. The following factors are taken into consideration in a methodological approach capable of distinguishing the origin of tantalum ores and concentrates with the utmost probability: (1) Quality and composition of coltan concentrates vary considerably. (2) Mineralogical and chemical compositions of Ta-Nb ores are extremely complex due to the wide range of the columbite-tantalite solid solution series and its ability to incorporate many additional elements. (3) Coltan concentrates may contain a number of other tantalum-bearing minerals besides columbite-tantalite. In our approach, coltan concentrates are analyzed in a step-by-step mode. State-of-the-art analytical tools employed are automated scanning electron microscopy (Mineral Liberation Analysis; MLA), electron microprobe analysis (major and trace elements), laser ablation-ICP-MS (trace elements, isotopes), and TIMS (U-Pb dating).