Kibara Belt Research Articles

Magmatic-Hydrothermal Fluid Processes of the Sn-W Granites in the Maniema Province of the Kibara Belt (KIB), Democratic Republic of Congo

The Kibara belt (KIB) in the Maniema province hosts orebodies bearing cassiterite-wolframite, which are associated with equigranular to pegmatitic late Mesoproterozoic (1094–755 Ma) granites and Sn-W bearing quartz veins that cut through metasedimentary country rocks. Alteration assemblages of muscovite-quartz (±topaz-fluorite-tourmaline) occur in the granites, and muscovite-sericite-quartz occurs in Sn-W quartz veins. Petrographic analyses, including cathodoluminescence (SEM-CL) on cassiterite grains, reveal two types of cassiterite: yellow transparent cassiterite (lighter under SEM-CL: type I) and dark translucent cassiterite (darker under SEM-CL: type II). These types are organized in micro-textures as oscillatory (growth) zones and replacement zones (type II replaces type I). Unlike cassiterite, wolframite is texturally homogenous. LA-ICP-MS results reveal that type II cassiterite is relatively enriched in Fe, Al, Ga, In, As, Pb, Zn, and U, whereas type I is enriched in V, Ti, Zr, Ta, Hf, and Nb. Contrasting Ce anomaly values in the cassiterite types suggest a transition of redox potentials during the Sn precipitation. Fluid inclusion assemblages (FIAs) in quartz, fluorite, and cassiterite are dominantly aqueous, liquid- or vapor-rich, and rarely carbonic-bearing aqueous inclusions. These often texturally coexist in a single “boiling” assemblage in granites. Raman spectroscopy on the bubble part of fluid inclusions in quartz and cassiterite shows various gas species, including CO2, CH4, N2, and H2. Boiling assemblages in the granites suggest that fluid phase separation occurred at about 380–610 bars, which is about 1–2 km (lithostatic) or 3–5 km (hydrostatic) in apparent paleodepth. FIAs in the granites show ranges of salinities of 4–23 wt.% (NaCl equivalent) and homogenization temperatures (Th) of 190–550 °C. FIAs hosted in cassiterite displayed distinctively lower and narrower ranges of salinities of 2–10 wt.% and Th of 220–340 °C compared to the FIAs hosted in quartz in the granites (salinity of 4–23 wt.%, Th of 190–550 °C) and the quartz veins (salinity of 1–23 wt.%, Th of 130–350 °C). This suggests a less salinized and cooler fluid during the cassiterite precipitation. We suggest that magmatic-derived Sn-W bearing fluids be mixed with less saline and cooler aqueous fluids, possibly meteoric water, during the major cassiterite and possibly wolframite depositions in the KIB. This is based on (1) temperature and salinities, (2) hydrothermal alterations, (3) cassiterite micro-textures, and (4) trace element distributions.

Cu-Bearing Mokama Granite Prospect of the Kibara Belt in the Maniema Province, DRC: A Preliminary Petrography, Geochemistry, and Fluid Inclusion Study

Cu-Bearing Mokama Granite Prospect of the Kibara Belt in the Maniema Province, DRC: A Preliminary Petrography, Geochemistry, and Fluid Inclusion Study

GEOCHEMISTRY AND SECONDARY ALTERATIONS OF MICROLITE FROM ELUVIAL DEPOSITS IN THE NUMBI MINING AREA, SOUTH KIVU, DEMOCRATIC REPUBLIC OF THE CONGO

Microlite and U-enriched microlite were found in eluvium close to highly weathered granitic pegmatites actively mined as a source of tantalum ore and situated within the ~1000–900 Ma old Kibara belt unit, Numbi mining area, South Kivu province of the Democratic Republic of the Congo. Two examined pegmatites differ in their mineral assemblages and in the degree of fractionation: in the Mungwe pegmatite (lepidolite subtype) cassiterite, microlite-group minerals (microlite, U-enriched microlite), columbite-group minerals (columbite–tantalite), wodginite, ferrotapiolite; at the Misumari II pegmatite (beryl-columbite subtype) microlite, cassiterite, columbite–tantalite, wodginite–ferrowodginite. Both pegmatites underwent deep tropical weathering; however, the degree of hydrothermal alteration of the (Nb,Ta)-oxide minerals is rather low. Fragments of primary microlite and U-enriched microlite (the latter present only at the Mungwe pegmatite) are mostly homogeneous and compositionally uniform; locally U-enriched microlite forms outer zones of the grains. Primary microlite from the lessfractionated Misumari II pegmatite is rather homogeneous and has Ta/(Ta+Nb)=0.74–0.94 and higher Ti=0.07–0.23 apfu at the B-site compared to the Mungwe pegmatite with similar Ta/(Ta+Nb)=0.82–0.88 and low Ti. Microlite from Misumari II has higher contents of almost all trace elements (including REE) except for Li, Sb, and Pb. Primary microlite from both sites has Ca/Na close to 1, low A-site vacancy, and high F. Uranium-enriched microlite shows 0.29 apfu U, moderate A-site vacancy, and lower concentrations of F. Alteration of the U-poor microlite is very limited compared to the U-enriched microlite where radiation damage disrupted the mineral structure prior to the alteration. The altered microlite is depleted in Na and F and has high vacancy at the A-site, but the B-site population is almost identical to that of the primary microlite, as are the U concentrations. Such a composition is typical for secondary low-T alterations related to weathering. We demonstrate that the combined results from Automated Mineralogy, Electron Microprobe Analysis, and LA-ICP-MS techniques (along with the specific alteration features) can be successfully used to prove the provenance of tantalum ore by comparison with other localities where microlite is a major Ta-bearing mineral.

Tantalum-(niobium-tin) mineralisation in pegmatites and rare-metal granites of Africa

Tantalum is an important metal for high-technology applications of the modern world. It is recovered from oxide minerals that are present as minor constituents in rare-metal granites (RMG) and granitic rare-element pegmatites (REP). Significant potential exists as a by-product of spodumene mining from pegmatites, and from Nb-Zr-rare earth element (REE) mining from carbonatites and peralkaline igneous rocks. Columbite-group minerals (CGM) account for the majority of the current Ta production; other Ta-Nb oxides such as tapiolite, wodginite, ixiolite, rutile and pyrochlore-supergroup minerals are of minor importance. The estimated Ta resources of Africa are >50,000 tons of contained Ta 2 O 5 , representing 16% of the world resources. Currently, the economically most important mining districts in Africa are located within the Kibara Belt (mainly Rwanda and the DR Congo), around the Kenticha mine (Ethiopia) and in the Alto Ligonha Province (Mozambique). Production from pegmatites and related placer deposits is mostly from hundreds of artisanal and small-scale workings with only a handful of semi-industrial Ta mines. Collectively, African operations are currently responsible for more than half of the world’s primary Ta. Rare-metal granite and REP districts in Africa are part of Archaean, Palaeoproterozoic, Neoproterozoic, Palaeozoic and Mesozoic provinces. Each period of Ta ore formation is characterised by specific mineralogical and geochemical features. Compositions of CGM are variable: Fe-rich types predominate in the Man Shield (Sierra Leone), the Congo Craton (DR 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 (Egypt, Ethiopia) and the Tantalite Valley pegmatites (southern Namibia) and 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 many pegmatites in the Damara Belt of Namibia. Trace element concentrations in CGM and other Ta-Nb oxides are highly variable, with Ti, W, Sn, Zr, Hf, Y, Sc and REE displaying significant regional differences that may be used to discriminate Ta provinces.

Mineralogical and chemical evolution of tantalum–(niobium–tin) mineralisation in pegmatites and granites. Part 2: Worldwide examples (excluding Africa) and an overview of global metallogenetic patterns

Mineralogical and chemical evolution of tantalum–(niobium–tin) mineralisation in pegmatites and granites. Part 2: Worldwide examples (excluding Africa) and an overview of global metallogenetic patterns

U–Pb detrital zircon and 39Ar–40Ar muscovite ages from the eastern parts of the Karagwe-Ankole Belt: Tracking Paleoproterozoic basin formation and Mesoproterozoic crustal amalgamation along the western margin of the Tanzania Craton

The Karagwe-Ankole belt forms part of a complex system of orogenic belts in Central-East Africa that record the amalgamation of Archaean and/or Paleoproterozoic continental kernels in the Proterozoic. The actual timing of tectonic processes, including initial basin formation and subsequent deformation, is only poorly constrained so that the geodynamic significance of the Karagwe-Ankole belt within the broader system of Proterozoic orogens in Central-East Africa is not well understood. This paper presents U–Pb ages of detrital zircons constraining the age of sediments of the eastern parts of the Karagwe-Ankole fold belt overlying the western Tanzania Craton and 39Ar–40Ar muscovite ages from mylonites and phyllonites associated with the main phase of subsequent shortening and fold-and-thrust tectonics (D2).39Ar–40Ar muscovite ages from the main detachment between Archaean basement gneisses and imbricated cover rocks of the Kagera Supergroup in the eastern, frontal parts of the Karagwe-Ankole Belt constrain the timing of the main fabric forming event and thrusting (D2) to 1326±10Ma. This age is at the young end of the age range for the main phase of granite plutonism and mafic dyke emplacement that have previously been related to crustal extension and associated intracontinental rifting. If correct and representative for the main phase of deformation in the rest of the Karagwe-Ankole belt, the ages may rather point to a collisional event between the Congo and Tanzania Cratons west of the Karagwe-Ankole belt in Mesoproterozoic times, indicating a closer correlation with rocks of the Kibara Belt (sensu stricto) to the immediate south.U–Pb detrital zircon ages for the older Muyaga and overlying Bukoba Group of the Kagera Supergroup confirm the Paleoproterozoic timing of sedimentation from ca. 1780Ma, but before ca. 1568Ma. The presence of a distinct group of ca. 1750–1790Ma zircons in the Bukoba Group indicate uplift, erosion and subsequent reworking of sediments and volcanic units contained in the Lower Muyaga Group to the west. The ages also suggest a correlation between the Bwezigoro Group of SW Uganda and the Bukoba Group indicating the presence of a far more extensive Paleoproterozoic and/or Early Mesoproterozoic foreland basin overlying the western margin of the Archaean Tanzania Craton and Paleoproterozoic Uganda Block.

Geological setting and timing of the world-class Sn, Nb–Ta and Li mineralization of Manono-Kitotolo (Katanga, Democratic Republic of Congo)

The Central African Mesoproterozoic Kibara belt in Katanga (DRCongo) forms a metallogenic province that hosts a variety of granite-related mineralization, rich in cassiterite, columbite–tantalite, wolframite/ferberite, spodumene and beryl. This mineralization is mainly present in pegmatites and quartz veins that are thought to be associated with the youngest granite generation in the Kibara belt (i.e., so-called “E-group” granite generation).Manono-Kitotolo is one of the world's largest Sn, Nb–Ta and Li mineralized pegmatites, with a large resource of spodumene, columbite–tantalite and cassiterite still remaining. Mineralized pegmatites have intruded along the foliation in the Mesoproterozoic metasedimentary rocks and dolerites of the Manono-Kitotolo area. The pegmatites have been emplaced very late during the climax of the Kibaran orogeny, probably during the transition from orogenic collapse to extensional tectonics, based on their structural position. The emplacement of the pegmatites resulted in an intense alteration of the doleritic and metasedimentary host-rocks, resulting in muscovitization, tourmalinization and silicification. A mineralogical and geochemical zonation typical for granitic pegmatites has been identified that was affected by metasomatic/hydrothermal alteration, mainly albitization and greisenization. The latter alteration is associated with the main phase of cassiterite mineralization. A first stage with Nb–Ta, Li and minor Sn, however, already formed pre-alteration, directly associated with pegmatite crystallization. 40Ar–39Ar muscovite dating of unaltered pegmatites resulted in ages of 938.8±5.1Ma and 934.0±5.9Ma. These ages are overlapping with the U–Pb age of the Nb–Ta mineralization (940±5.1Ma), also giving new temporal constraints on the fertile E-group granite generation in the KIB. The 40Ar–39Ar muscovite age of a mineralized greisen is 923.3±8.3Ma, which is younger but still partly within error of the 40Ar–39Ar ages of the unaltered pegmatites. Based on the partial overlap in age, but also on field and paragenetic relationships and the stable isotope composition of the fluids, this greisenization and associated cassiterite mineralization can still be linked to the pegmatite crystallization.

Regional geodynamic context for the Mesoproterozoic Kibara Belt (KIB) and the Karagwe-Ankole Belt: Evidence from geochemistry and isotopes in the KIB

The Mesoproterozoic Kibara Belt and Karagwe Ankole Belt formed between the Congo Craton and the Tanzania-Bangweulu Block in central Africa. The subduction model proposed for the Kibara region is difficult to reconcile with the simultaneous intracratonic transtension in Karagwe Ankole at c. 1380Ma. Nonetheless, our whole rock petrochemical and Sr–Nd isotope data reinforces earlier evidence for a subduction setting for the Kibara Belt. Model ages for the mafic samples associated with the 1380Ma event indicate crustal residence times of c. 2.7Ga, suggesting derivation from evolved subcontinental lithospheric mantle with minor juvenile input. The latter is analogous to an active continental margin. In combination with the petrochemical evidence, this favors a subduction setting, the bimodal nature of the magmatism notwithstanding.The Kibara Belt granites are weakly metaluminous to strongly peraluminous, with low Ba – high Rb contents and restitic micaceous enclaves indicating an origin by metapelite dehydration melting. Two previously recognized granite suites around 1380Ma and 1000Ma in the Kibara Belt are, to an extent, distinguishable geochemically. The age-corrected Nd isotope ratios from Kibara Belt metapelites and granites are similar, confirming that the strongly peraluminous Kibara Belt granites are mostly derived from similar metapelites. The Kibara Belt granites show depleted mantle Nd model ages (TDM) of at least 2.4Ga. Some granites have model ages in excess of 3.0Ga, which can be attributed to multi-stage Nd evolution.Finally, a comparison with the contemporaneous magmatism in the adjacent Mesoproterozoic Karagwe-Ankole Belt indicates similar geochemistry of subordinate mafic dykes, yet reveals distinct differences with the 1370±40Ma Lake Victoria Dyke Swarm and the 1403±14Ma–1374±14Ma layered (ultra)mafic intrusives of the Kabanga-Musongati lineament. We propose that this corresponds to a regional transition toward a collisional setting, related to amalgamation of the proto-Congo Craton. In the Karagwe-Ankole Belt, convergence is superimposed on asthenospheric upwelling, as evidenced by the Lake Victoria Dyke Swarm. Magmatism along the Kabanga-Musongati lineament initiates at 1403±14Ma, and these layered (ultra)mafic intrusives, and is here attributed to preferential upwelling along a transcrustal fault zone. Subordinate mafic to intermediate units interlayer the sediments and could relate to mantle delamination or (back) arc magmatism. Subsequent collision then accounts for the strongly peraluminous granites in the western part of this belt. At c. 1000Ma, strongly peraluminous granites intruded in both belts. Together with similar deformation styles in both adjacent belts, this suggests a regional convergent setting between the Congo Craton and the Tanzania-Bangweulu Block, possibly related to the pan-Rodinian Orogeny.

Tantalum–(niobium–tin) mineralisation in African pegmatites and rare metal granites: Constraints from Ta–Nb oxide mineralogy, geochemistry and U–Pb geochronology

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.

Early Neoproterozoic rare metal (Sn, Ta, W) and gold metallogeny of the Central Africa Region: a review

The four metals of economic significance in the Central Africa or Great Lakes region, i.e. gold, tin, tantalum and tungsten, are part of one composite metallogenic system that operated about 980±20 Ma. The main driving agent was peraluminous ilmenite-series granite magmatism, synchronous with intracratonic compression and associated with the final amalgamation of the supercontinent Rodinia. The granitic melts were emplaced at intrusive levels of ≧2 kbar (≧8 km); the intrusions display a variable and often advanced degree of fractionation, including abundant Sn–Ta–Li–Be–Rb–Cs pegmatites, and are associated with hydrothermal systems enriched in tin, tungsten and/or gold. Based on cumulative past production and present metal prices, gold in hydrothermal quartz veins is the major commodity, followed by tin either in rare metal pegmatites or in sheeted, hydrothermal quartz veins. Many deposits in the province occur in siliciclastic metasedimentary, or metabasaltic roof rocks above parental granites; mainly in its western part, the zone of mineralisation retracts into the granite roof. Typically in the first case, antiformal sites acted as fluid escape zones, with carbonaceous or metabasaltic rocks as chemical traps for tungsten and gold. Examples of pegmatitic and magmatic–hydrothermal deposits are presented in some detail in order to illustrate characteristics and genetic controls, and to support the metallogenic hypothesis here advanced. Impeding strategic exploration, published elements of understanding the evolution and mineralisation of the Kibara belt are contradictory and essential links are missing, foremost an understanding of the 1 Ga flare up of fertile granites. Towards solving this conundrum we suggest that the key is delamination of the mantle lithosphere and dense mafic lower crust, residual after extraction of voluminous 1·38 Ga granitic melts. During pan-Rodinian orogenic events, the Tanganyika spur of the Tanzania craton acted as an indenter whose impact caused foundering of the early Kibaran lithosphere. Consequent influx of asthenospheric heat triggered large-scale crustal melting that resulted in the tin granites. The stress state was largely compressive but possibly punctuated by short or local extensional events. The correlation of geological evolution and mineralisation substantiates the formal recognition of a Kibara Metallogenic Domain, which is composed of two units: The Mesoproterozoic (1·4 Ga) Kabanga-Musongati nickel (±copper, cobalt, platinum) province; and the early Neoproterozoic (1 Ga) Kibara rare metal and gold province that is the main subject of this paper. The present understanding of the operating metallogenic systems remains limited. Regarding the application of modern concepts and technologies, this province is drastically underexplored.

The Mesoproterozoic Karagwe-Ankole Belt (formerly the NE Kibara Belt): The result of prolonged extensional intracratonic basin development punctuated by two short-lived far-field compressional events

The Mesoproterozoic Kibara Belt (also Kibaran Belt or Kibarides in some references) of Central Africa was often portrayed as a continuous, c. 1500km long orogenic belt, trending NE to NNE from Katanga, Democratic Republic of Congo (DRC) in the south, up into SW Uganda in the north. Recently however, the Karagwe-Ankole Belt (KAB; formerly the NE Kibara Belt) has been redefined as the part north of a NW oriented Palaeoproterozoic basement high of the Ubende-Rusizi Belts, while the Kibara Belt (KIB) is now limited to the part south of this rise.We present a lithostratigraphy for the KAB that takes into account two rheologically contrasting structural domains (Western and Eastern Domain); each of them being characterised by independent sedimentary sub basin(s) and depositional conditions: the ED with Archaean basement versus the WD with Palaeoproterozoic basement. We document new volcanic and detrital U–Pb SHRIMP zircon data which provide new constraints on the timing of deposition and on the detrital provenance of the sedimentary sequences in the KAB. We discuss the evolution of the KAB in a wider regional context, comparing it to other Mesoproterozoic units and with reference to the general geodynamic history of this part of the African continent in Proterozoic times.The lithostratigraphic successions of the KAB are only valid respectively in the ED (Kagera Supergroup) or in the WD (Akanyaru Supergroup), with no correlations between them. Deposition of the Kagera Supergroup in the ED is bracketed between 1.78Ga and 1.37Ga and the deposits have to be considered an Eburnean-age “molasse”. Detrital components comprise material only of Archaean and Palaeoproterozoic age, consistent with derivation from nearby source regions. In the WD, deposition of the two lowermost groups of the Akanyaru Supergroup is bracketed between 1.42Ga and 1.37Ga. The large contribution of detrital Palaeoproterozoic components in the WD strengthens the view that this domain is underlain by Palaeoproterozoic basement and supports the concept that part of the Akanyaru Supergroup sediments consists of reworked Eburnean-aged molasse. In the WD of the Kivu-Maniema area (DRC), later sedimentation periods are documented at respectively 1222Ma and 710Ma. The KAB documents a long-lived period of intracratonic intermittent depositional activity (with periods of interruption of deposition, erosion and magmatism) showing a recurrent subsidence trend controlled by structural activity moving with time from E to W.On a regional scale, we postulate that since 1.8Ga, following the amalgamation of Archaean and Palaeoproterozoic landmasses into a single coherent ‘proto-Congo Craton’, various long-lived shallow-water intracratonic basins (aulacogenes) developed. These basins underwent a comparable Mesoproterozoic geodynamic evolution, as shown not only in the sequences of the KAB and of the relatively close Kibara (KIB), Bangweulu Block and Northern Irumide Belts, but even in more distant sequences located in SW Angola and E Brazil.The long-lived aulacogene history of the KAB within the proto-Congo Craton is interrupted only twice by short-lived compressional deformation reflecting far-field effects of global orogenic events, external to the proto-Congo Craton. The first event at 1.0Ga is related to Rodinia amalgamation. The second event at 550Ma results from Gondwana amalgamation and develops a N–S Pan African overprint in the KAB which has previously been underestimated or even overlooked. Three mineralisation provinces occurring in the KAB, respectively the Bushveld-type, the tin-coltan-wolfram and the gold province, can be ascribed successively to the 1375Ma Kibaran magmatic event, the 1.0Ga Rodinia and the 550Ma Gondwana amalgamation events.Our results give additional weight to the recent redefinition of the KAB and the KIB, forming two distinct Belts respectively north and south of the Palaeoproterozoic basement high of the Ubende-Rusizi Belts, the more that within this basement rise local Mesoproterozoic strike-slip basins, with their own unique lithostratigraphic and geodynamic characteristics (e.g. Itiaso Group) are documented, which differ from those of the KAB or the KIB.

Late Neoproterozoic overprinting of the cassiterite and columbite-tantalite bearing pegmatites of the Gatumba area, Rwanda (Central Africa)

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 ∼975 Ma and ∼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 (∼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 40Ar– 39Ar spectra of muscovite samples associated with different steps in the paragenesis of the pegmatites show a spread of apparent ages between ∼940 Ma and ∼560 Ma that reflect Late Neoproterozoic 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.

The 1375 Ma “Kibaran event” in Central Africa: Prominent emplacement of bimodal magmatism under extensional regime

In previous literature, the “Kibara belt” has often been portrayed as a Mesoproterozoic belt trending NE over 1300 km across the Central African Congo craton, from the Angola-Zambia-D.R.Congo border triple-junction in the SW, through Katanga and Kivu-Maniema (DRC), Rwanda and Burundi, up to SW Uganda and NW Tanzania in the NE. However, north of Katanga in the DRC, there is a clear break in continuity of the thus-defined “Kibara belt”, cross-cut by Palaeoproterozoic (Rusizian) terranes, in structural continuity with the NW-SE trending Ubende shear belt further south in Tanzania. In this paper, we redefine the use of the term “Kibara belt” (“KIB”), restricting it henceforward to the belt occurring SW of the Ubende-Rusizian terranes, i.e. in the Kibara Mountains type area of Katanga (DRC). The other belt situated NE of the Ubende-Rusizian terranes and east of the Western Rift, previously referred to as the “Northeastern Kibaran Belt” (NKB), is henceforward and for clarity reasons re-named “Karagwe-Ankole belt” (“KAB”). In our re-definitions, we do not take into account the Kivu-Maniema (DRC), because of its geological complexity apparent from satellite imagery and the lack of recent field data, although “some continuity” of the KAB in Kivu-Maniema is obvious. For the KAB, we document 10 new SHRIMP U–Pb zircon ages, in addition to new 40Ar/ 39Ar and laser-ablation zircon Hf data, all of them obtained from previously already isotopically “dated” rock specimens. Contrary to previous belief, magmatism in the KAB (and the KIB) is punctuated by the profuse emplacement of bimodal intrusions between 1380 and 1370 Ma. Moreover, the occurrence of Palaeoproterozoic basement within the KAB is confirmed. The prominent c. 1375 Ma bimodal magmatism in the KAB consists of (1) the 350 km long Kabanga-Musongati (KM) alignment of mafic and ultramafic, Bushveld-type, layered complexes, originating from an enriched lithospheric mantle source and (2) voluminous S-type granitoid rocks with accompanying subordinate mafic intrusive rocks. Both coeval magmatic suites are interpreted to have been emplaced under extensional regime in a regional-scale intra-cratonic setting. During ascent the mantle-derived magmas have taken advantage of the regionally occurring crustal-scale zone of weakness in the KAB, i.e. the rheological boundary between the Archaean craton of Tanzania, to the east, and the adjacent Palaeoproterozoic basement (2.1 Ga mobile belt), to the west, both overlain by Mesoproterozoic (meta)sedimentary rocks. The mantle-derived magmas initiated concomittantly and under extension, large-scale crustal melting preferentially of the Palaeoproterozoic basement, and characterised by the absence of a thick lithospheric profile in contrast to the nearby Archaean craton. Such petrogenetic processes have intra-plate characteristics and are thus not associated with normal plate boundary processes nor with their typical magmatism. On the contrary, they may include rift-related packages, characteristically associated with successful or attempted, though unsuccessful, continental break-up as was the case here. In the KAB, later magmatic events occurred respectively at c. 1205 (A-type granitoids) and c. 986 Ma (“tin-granites”). They represent minor additions to the crust. For decades the term “Kibaran” has been used to name the orogenic cycle and/or orogeny occurring in (Central) Africa in “late” Mesoproterozoic times (1.4–1.0 Ga), which was considered to have a protracted character. Here, we propose to restrict henceforward the term “Kibaran” only to the prominent tectono-magmatic “event”, giving rise to the coeval c. 1375 Ma bimodal magmatism emplaced under extensional regime. This “Kibaran event” pre-dates compressional deformation, reflecting far-field effects of global orogenic events, external to the craton and possibly related to Rodinia amalgamation.

Persistent fault controlled basin formation since the Proterozoic along the Western Branch of the East African Rift

The Western Branch of the East African Rift System is outlined by elongate sedimentary basins, frequently occupied by Cenozoic rift lakes. The role of the inheritance of the leading rift faults from pre-existing basement structures has often been invoked. Recent studies in western Tanzania confirm the extent of the northwest orientated Palaeoproterozoic Ubende Belt contribution to the Phanerozoic Rift. Attention is drawn here on the occurrence of different Meso- and Neoproterozoic sedimentary basins that developed along the ductile shear belt as a result of repeated sinistral wrench fault reactivation. These basins partly overlap each other and typically bear shallow and weakly evolved sediments. North of the Ubende Belt, the Mesoproterozoic Kibara Belt is inferred to have originated as a basin controlled by the complex termination of the Ubende wrench fault. Phanerozoic rift basins also develop along the northwest orientated Ubende Belt structure. They display the same elongate shape as the Proterozoic basins. In Late Palaeozoic-Early Mesozoic the Karoo rift basins formed from a dextral lateral shear reactivation of the inherited Proterozoic shear faults. During the first phase of development the Lake Tanganyika Basin is belived to bear the same characteristics as all previous basins along the Ubende Shear Belt, mainly controlled by strike-slip movements along pre-existing shear faults. The present Lake Tanganyika Basin is subdivided in two sub-basins, separated by the transverse Mahali Shoal, which is an active structure located on the Ubende Shear. The deep lake basin mainly developed outside the Ubende Belt. The northern sub-basin appears to be structurally controlled by the reactivation of the Mesoproterozoic sinistral wrench fault termination of the Ubende shear faults. Structural control of the Palaeoproterozoic basement is however unclear for the southern sub-basin of Lake Tanganyika: this part of the rift segment is flanked by Palaeoproterozoic basement which has not been affected by the Ubende Shear.

Metallogeny of the northeastern Kibara belt, Central Africa—Recent perspectives

The Kibara belt is an intracontinental mobile belt formed between 1400 and 900 Ma within a craton of Lower Proterozoic age. The belt's evolution started by early rifting at about 1400 Ma, and continued by transition into a marine basin filled by clastic sediments > 10 km thick, with minor basic and acidic volcanic rocks. At about 1300 Ma, the pile was deformed by thrusting and folding of the main Kibaran Orogeny and intruded by numerous large syn-orogenic granite bodies. Wide areas of the Kibaran experienced only very low-grade metamorphism, but halos of high-temperature/low-pressure metamorphism surround intrusive bodies. A post-orogenic phase of rifting followed at about 1275 Ma and was accompanied by the intrusion of alkaline granites and layered mafic intrusions with Ni, Cu, Co, Ti, V and possibly also PGM mineralization. Also at about this time, molasse sedimentation set in, both within the belt and in foreland troughs. The western molasse and the whole belt were again orogenically deformed at around 950 Ma by the Lomamian Orogeny; concurrently, small metallogenetically specialized granite bodies were emplaced. Fertile members (“tin granites”) of the latter produced numerous pegmatitic and quartz vein deposits with Sn, W, Nb Ta , Li, Be and Au. Other auriferous and locally Pt-bearing quartz veins and breccias are not spatially related to tin granites, but appear near large fault zones that may be associated with deep structures separating basement blocks. Both the tin granites, and the fluids which formed gold ores not related to granites, are suggested to have been derived from metamorphic devolatilization and partial melting of older (Archean and Lower Proterozoic) basement underneath the Kibaran Orogen.