The Congo Craton (Fig. 1), which is defined here as the central African large landmass that amalgamated at the time of Gondwana assembly at ~ 550 Ma, consists of several Archaean nuclei supposedly welded together around 2.1 Ga and later exhumed around 1.8 Ga as a result of Eburnean-aged collisional orogeny during the “Columbia” (also called “Nuna”) supercontinent amalgamation (Fernandez et al., 2011; Tait et al., 2010). Since the late Paleoproterozoic, the precursor of the Congo Craton (termed proto-Congo Craton by De Waele et al., 2008) has stabilized and remained a united entity throughout the Meso-Neoproterozoic (Tack et al., 2008). It underwent only intra-cratonic tectonic events (e.g., rifting, basin development, sedimentation, magmatism, etc), which never evolved into the formation of juvenile oceanic crust and break-up of the craton. As a result of Rodinia supercontinent fragmentation, several Neoproterozoic sedimentary basins developed in and around the Congo Craton. During Gondwana amalgamation, the Craton became bordered by Pan African collisional high-grade terranes to the N (“Central African Orogenic Belt”) and E (“East African Orogenic Belt”), while the W and SE rim acted as foreland domain for respectively the “Aracuai/West Congo” and “Katanga/Zambezi” (also called Lufilian Arc/Belt and/or Copperbelt) Pan African accretionary belts. In both forelands, Neoproterozoic tabular sedimentary sequences were largely preserved and define respectively the West Congo and Katanga Supergroups.
The Neoproterozoic rocks of the Zambian Copperbelt host world-class sediment-hosted stratiform and stratabound Cu-Co deposits in both fine- and coarse-grained metamorphosed siliciclastic rocks. Ore deposits in the coarse-grained siliciclastics occur below (footwall deposits) and above (hanging-wall deposits) the fine-grained Ore Shale Formation and are termed arenite-hosted deposits. The arenite-hosted deposit at Mufulira is studied and compared with similar deposits in the Central African Copperbelt to propose an integrated ore-forming model. The arenites are characterized by several alteration, dissolution, and cementation phases. Authigenic phases include quartz, albite, and K-feldspar overgrowths and calcite and dolomite cements filling pores. Carbonaceous matter or pyrobitumen postdates compaction and coat detrital grains. The pyrobitumen formed as a residue of oil cracking and/or as a byproduct of sulfate reduction and the oxidation of hydrocarbons. Chalcopyrite, digenite, chalcocite, covellite, carrolite, pyrrhotite, malachite, and iron oxides occur as finely disseminated ores or as replacive blebs in sericitic quartzites. The ore minerals replace the compacted rock, and may enclose all diagenetic phases and occur together with quartz and calcite in veins, indicating they formed late in the paragenesis. Ore minerals precipitated as the result of the mixing of a highly saline, metal-bearing brine with an H2S-rich hydrocarbon reservoir.
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.
1. Introduction The platinum group elements (PGE: Ru, Rh, Pd, Os, Ir & Pt) are considered as critical metals (European Commission, 2014) and are highly valued for their high-tech applications. They are being recycled and intensely mined, but still deficits are experienced and expected in the coming years (European Commission, 2014). Since the large PGE deposits, such as the Bushveld Complex in South Africa and the Noril'sk-Talnakh deposits in Russia, will become depleted with time, new deposits need to be explored for their PGE potential, to sustain future demand. The mafic-ultramafic intrusions in Burundi, which are part of the Kabanga-Musongati alignment, are such potential deposits. They intruded the Mesoproterozoic rocks of the Karagwe-Ankole belt around 1375 Ma and form a SW-NE alignment of nine intrusions in Burundi, with further continuation towards Tanzania (Fig. 1; Fernandez-Alonso et al., 2012). Several drilling campaigns have been executed between 1970 and 1990 to explore the nickel and PGE potential of these intrusions (PNUD-UNDP, 1977; Exploration und Bergbau Gmbh, 1985;Deblond, 1994; Deblond & Tack, 1999). Although some limited data on the concentration of PGE in the boreholes of these campaigns is available (e.g. Klerkx, 1975, 1976), not much is known about the PGE distribution. In addition, the petrogenesis of the intrusions needs further elaboration, expanding on the work of e.g. Bandyayera (1997) and Duchesne et al. (2004). Figure 1. (A) Regional geology o
Over the recent years, several laboratories have tried to solve the matrix-match problem for quantitative analysis of sulphides with Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA ICP-MS) by developing artificial standards. This paper investigates the feasibility of a modified welding technique for the production of artificial sulphide crystals as a matrix-matched external standard for quantitative analysis of the trace element content of natural samples with LA ICP-MS. During this research, focus was on pyrite, since this mineral phase is an ubiquitous mineral in several ore deposits and sedimentary/diagenetic successions. The matrix-matched standard has been produced by resistance heating of a µm-size mixture of pyrite and pure elements in graphite electrodes with a procedure modified after Odegard (1999). During the loss of S during the welding, the formed bead consists of pyrrhotite. Pyrrhotite, however, exhibits similar ablation behaviour as pyrite. For several elements of interest (i.e. Co, Ni, Cu and Ga), the matrix-matched standard shows a homogeneity ≤ 15 % RSD, which is sufficient for a (semi-)quantitative calibration. Some elements (i.e. Zn, Se and Pb) show a rather poor homogeneity (RSD ≥ 15 %), which only allows a qualitative analysis. The concentration of Ge and As is below the detection limit in the produced standards, which can be due to vaporisation during welding or during ablation of the beads. The obtained homogeneity is comparable to most of matrix-matched LA ICP-MS sulphide standards described in the literature. The use of the matrix-matched standard has been illustrated on hydrothermal pyrite from a mesozonal orogenic mineralisation (Marcq area, Belgium), which has been analysed as well with the electron microprobe. Owing to the higher sensitivity many more trace elements can be measured with LA ICP-MS. The LA ICP- MS analyses clearly demonstrate a variation of trace element content and enrichment of the elements (Cu, Pb, Zn, Se) towards the rim of the pyrite crystal.
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