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2013

Mineralogical characterization of cassiterite concentrates from quartz vein and pegmatite mineralization of the Karagwe-Ankole and Kibara Belts, Central Africa.

Dewaele S.,Goethals H., Thys T.

Geologica Belgica, volume 16 (1-2): 66-75, 2013

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Abstract: The Mesoproterozoic Kibara belt (KIB) and the Karagwe-Ankole Belt (KAB) in Central Africa are characterized by the presence of numerous rare metal mineralized Sn-(Nb-Ta) pegmatites and Sn-W mineralised quartz veins that are related to a S-type granite generation formed at 986 ± 10 Ma. Cassiterite concentrates have been studied by different petrographic and mineralogical techniques. The concentrates have been collected from the rock and mineral collection of the Royal Museum for Central Africa (RMCA) and originate from historical exploitations of eluvial and alluvial cassiterite deposits. No quantitative study of the concentrates has been envisaged since no information is available about the history of the samples prior to sampling. Microscopic investigation revealed the presence of cassiterite crystals with metallic and non-metallic luster, of which the latter show growth zoning. The color from the cassiterite crystals can vary from transparent colorless to black non-transparent. The variation in color in a single grain can be as varied as the color variation between grains for an entire concentrate. The mineralogical composition of the cassiterite concentrates contains minerals that are typical gangue and accessory minerals in the primary mineralization and that were liberated during weathering. In addition, minerals can be found that result from the weathering from the metasedimentary and doleritic host-rocks of the primary mineralization. Except the occasional presence of a certain mineral or a special color for a certain cassiterite, no systematic variation can be observed between the concentrates from the different locations. Often, the mineral and color variation in one concentrate can be as large as for concentrates found from different locations.

Land cover fragmentation using multi-temporal remote sensing on major mine sites in southern Katanga (Democratic Republic of Congo). Advances in Remote Sensing

Dupin L., Nkono C., Muhashi F., Burlet C., Vanbrabant Y.

Advances in Remote Sensing, Vol.2 No.2, pp. 127-139, 2013

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Abstract: The study areas are located in the South Eastern part of the Democratic Republic of Congo (DRC), in the Katanga province. It focuses on the Kolwezi and Tenke-Fungurume mining centres, located in the buffer zone, and the Basse-Kando reserve. The Katanga province is a high value ecosystem in the Democratic Republic of Congo because of the natural reserves and resources it provides. The focused areas have faced large scale human induced land use and land cover changes (LUCC). A combination of ancillary data and satellite imageries were interpreted to construct LUCC dynamics over the last 30 years. This study is an initial step towards assessing the impact of LUCC on sustainable land use in the Katanga. The results show that large trends of LUCC differently occurred over the last 30 years (1980’s to 2010’s) in the three focused areas. The most dominant LUCC processes were gained in barren soil and cities surface and in a sharp reduction of burned areas. In Kolwezi globally there is a relation between growth and regression of barren soil and cities with vegetation. The Tenke-Fungurume site shows a growth during the first decade and regression of vegetation during last two decades. The Basse-Kando site analyse brings out the growth of vegetation and the regression of burned area due to vegetation conservation efforts. The variation rates of LUCC observed are higher, and are related to the socio-economical activities. This is one of the few studies in Katanga around mining activities that combine multi-source spatio-temporal data on land cover to enable long-term quantification of land cover changes. In the discussion we address future investigation needs for the area based on the results of this analyse.

Mineralogical Characterization of Cobaltic Oxides from the Democratic Republic of Congo, in Ni-Co 2013

Vanbrabant, Y., Burlet, C. and Louis, P.

Ni-Co 2013, John Wiley & Sons, Inc., Hoboken, NJ, USA., Pages: 241–254, 2013

Abstract: he largest cobalt ore reserves are located in DRC, the Democratic Republic of Congo. Most of the cobalt is observed as black cobaltic oxide minerals: heterogenite [HCoO2] and asbolane [(Ni,Co)2-xMn(O,OH)4.nH2O] which are hardly differentiable since they exhibit similar macroscopic habit and textures. These minerals are frequently observed in similar environment (oxidized horizon of ore deposits) and they are commonly poorly-crystallized limiting their study with XRD. Their chemical composition is also not very well-constrained since they exhibit significant chemical substitutions with cations as Cu, Co, Ni and Mn. The difference in mineralogy and chemical composition of the different cobalt minerals will have an impact on their hydrometallurgical treatment. We compared the natural chemical variability of heterogenite, with that of asbolane samples from DRC. These minerals are compared by EDS and Raman microspectrometry techniques. We show that Raman microspectrometry is a quick and reliable tool to discriminate asbolane from heterogenite.

2012

Remobilisation features and structural control on ore grade distribution at the Konkola stratiform Cu-Co ore deposit, Zambia. Journal of African Earth Sciences

Torremans, K., Muchez, Ph., Gauquie, J., Boyce, A., Barrie, C., Dewaele, S., Sikazwe, O.N.

Accepted, in Press.

Abstract: The Konkola deposit is a high grade stratiform Cu-Co ore deposit in the Central African Copperbelt in Zambia. Economic mineralisation is confined to the Ore Shale Formation, part of the 10 km thick Neoproterozoic metasediments of the Katanga Supergroup. Petrographic study reveals that the copper-cobalt ore minerals are disseminated within the host rock, sometimes concentrated along bedding planes, often associated with dolomitic bands or clustered in cemented lenses and in layerparallel and irregular veins. The hypogene sulphide mineralogy consists predominantly of chalcopyrite, bornite and chalcocite. Based upon relationships with metamorphic biotite, vein sulphides and most of the sulphides in cemented lenses were precipitated during or after biotite zone greenschist facies metamorphism. Deep supergene enrichment and leaching occurs up to a km in depth, predominantly in the form of secondary chalcocite, goethite and malachite and is often associated with zones of high permeability. Detailed distribution maps of total copper and total cobalt content of the Ore Shale Formation show a close relationship between structural features and higher copper and lower cobalt contents, relative to other areas of the mine. Structural features include the Kirilabombwe anticline and fault zones along the axial plane and two fault zones in the southern limb of the anticline. It is interpreted that these structures played a significant role in (re)mobilisation and concentration of the metals, in agreement with observations made elsewhere in the Zambian Copperbelt. Cobalt and copper behave differently in relation to these structural features. New data on δ34S values for the Konkola deposit is presented. The sulphur isotope values range from -8.7‰ to +1.4‰ V-CDT for chalcopyrite from all mineralising phases and from -4.4‰ to +2.0‰ V-CDT for secondary chalcocite. Similarities in δ34S for vein generations, earlier sulphides and secondary chalcocite can be explained by (re)mobilisation of S from earlier formed sulphide phases.

Heterogenite (HCoO2) as a delafossite structure mineral: a Raman micro-spectroscopic study on 2H - 3R polytypes

Burlet, C., Vanbrabant, Y., Goethals, H.

In review

Heterogenite is commonly referred in mineralogy literature as a cobalt oxy-hydroxyde (CoOOH). However, detailed analysis of Raman spectra acquired on particularly well-crystallized natural samples of heterogenite suggests that the mineral can be better characterized by a delafossite-type structure, with a general chemical formula ABO2. Indeed, the Raman spectrum of heterogenite, along with grimaldiite (HCrO2), lacks visible free OH-group vibrational modes while the IR spectrum shows a strong hydrogen bond absorption band. HCoO2 is thus a better formulation of heterogenite that describes more clearly its vibrational behavior and avoid the confusion in literature. A band assignment of the Raman active modes of heterogenite is made in correlation with EBSD to map the 2H and 3R heterogenite natural polytypes. As a supplementary Raman active mode, observed at 1200 cm-1, distincts the 2H polytype, Raman spectroscopy is proved to be a very efficient tool to distinguish the two varieties.

2011

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

Dewaele, S., Henjes-Kunst, F., Melcher, F., Sitnikova, M., Burgess, R., Gerdes, A., Fernandez-Alonso, M., De Clercq, F., Muchez, P., Lehmann, B.

Journal of African Earth Sciences 61, 10-26.

Abstract: 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 (Ruzizian) 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 mineralization 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.

Supergene copper deposits and minerals in the world-class SHSC deposits of the Central African Copperbelt (Katanga, DRC).

De Putter, T., Mees, F., Decrée, S., Dewaele, S. 2011.

Proceedings of the 11th Biennal SGA Meeting: Let’s talk ore deposits, Antofagasta, chili, 26-29 september 2011..

Abstract: Central Africa is a region with an exceptional wealth in mineral deposits. The Copperbelt, a world-class copper and cobalt deposit, located in northern Zambia and in the south-eastern part of the Democratic Republic of Congo (Katanga), has received considerable attention from ore geologists. However, only few of these studies have focused on the supergene Cu- and Co- ore deposits capping the primary ores. This secondary ore “bonanza” results from a combination of geodynamic and geochemical processes, from the beginning of the Cenozoic era onward. These secondary deposits deserve attention, as features providing key information about recent regional geodynamics, as Cu ores with a great diversity in mineralogical composition, and, especially, as deposits of great economic importance, in particular for artisanal mining.

SIMS U–Pb dating of uranium mineralization in the Katanga Copperbelt: Constraints for the geodynamic context.

Decrée, S., Deloule, E., De Putter, T., Dewaele, S., Mees, F., Yans, J., Marignac, C.

Proceedings of the 11th Biennal SGA Meeting: Let’s talk ore deposits, Antofagasta, chili, 26-29 september 2011..

Abstract: The Katanga Copperbelt, in the southeastern part of the Democratic Republic of the Congo (DRC), occurs within the Pan-African Lufilian fold-and-thrust belt and hosts numerous uraniferous mineral occurrences in addition to the world-class sediment-hosted copper and cobalt ore deposits. In the 1950s and 1960s, Cahen and co-workers gave a relatively wide range of U mineralization ages, using U-Pb dating on whole-rock samples. Although these ages nowadays appear as inaccurate because of large error ranges, they were frequently used to discuss the regional geodynamic events. This paper offers a reappraisal of the age range previously published, using in-situ SIMS U-Pb analyses. Based on these analyses, two ages are regarded as analytically reliable and significant (652.3 ± 7.3 Ma and 530.1 ± 5.9 Ma), and hence as having implications for the understanding of geological and geodynamic context in the study area. In the Katanga Copperbelt, the siliciclastic rocks of the Lower Roan Group are slightly enriched in uranium. A major U mineralization/(re-)mobilization episode took place at ~652 Ma, during proto-oceanic rift basin development. This phase took place in an extensional tectonic regime and led to the (re-)mobilization of U within fractures and voids. Such deposits are observed along the ~ 250 km-long Kalongwe-Luishia tectonic lineament. This metal concentration was reworked at ~530 Ma, at the occasion of the climax of Lufilian deformation and metamorphism. This event gave rise to the U deposits in the southern border of the Katanga Copperbelt (and in Northern Zambia). The Kolwezi deposit is a deposit located further to the North. Notwithstanding its initial position further to the south, the deposit is heavily faulted and the thrusts and faults could have focused the fluids associated with the deformation, at ~530 Ma.

Raman spectroscopy as a tool to characterise heterogenite (CoO.OH) (Katanga Province, Democratic Republic of Congo)

Burlet, C. , Vanbrabant, Y., Goethals, H., Thys, T., Dupin, L.

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 80, 138-147.

Abstract: Natural heterogenite (CoO·OH) samples were studied by Raman microspectroscopy, electronic microprobe and Electronic BackScattered Diffraction (EBSD). Raw samples and polished sections were made from 10 mines covering the Katanga copperbelt (Katanga Province, Democratic Republic of Congo). Four typical Raman responses have been obtained leading to investigate the laser-induced dehydroxylation of heterogenite into a Co-spinel structure. The results are also compared with EBSD patterns from oven heated heterogenite samples. A close relationship was established between the chemical substitutions of Co by mainly Cu, Ni, Mn and Al and their impact on the mineral Raman response.

2010

Geology of the cassiterite mineralisation in the Rutongo Area, Rwanda (Central Africa): current state of knowledge

Dewaele, S., De Clercq, F., Muchez, P., Schneider, J., Burgess, R., Boyce, A. & Fernandez-Alonso

Geologica Belgica 13: 91-112.

The Mesoproterozoic Kibara orogen in Central Africa hosts different granite-related rare element deposits that contain cassiterite, columbite-tantalite (“coltan”), wolframite, beryl, spodumene, etc. as typical minerals. The primary deposits of these minerals are formed by pegmatites and quartz veins that have historically been related to the youngest, most evolved G4-granite generation in the northern part of the Kibara orogen. This study focuses on quartz vein-type cassiterite mineralisation in the Rutongo area in Rwanda. The Rutongo area consists of a large anticline that is characterised by the presence of cassiterite-mineralised quartz vein sets that dominantly occur in quartzites. The emplacement of the quartz veins has been related to a later phase in the deformation history of the Kibara orogeny. The mineralised quartz veins are associated with intense alteration, comprising silicification, tourmalinisation, sericitisation and muscovitisation. Cassiterite itself is associated with muscovite in fractures in and along the margins of the quartz veins. Cassiterite crystallisation is followed by the precipitation of different sulphides, such as arsenopyrite, pyrite, chalcopyrite and galena. Cassiterite mineralisation resulted from the circulation of high-temperature and moderate-salinity fluids with a H2O-CO2-(CH4-N2)-NaCl composition. The stable isotopic composition of the cassiterite mineralising fluids indicates precipitation during metamorphic hydrothermal conditions, during which the metamorphic fluids where in isotopic equilibrium with granitic rocks. The circulation of these fluids probably resulted in the remobilisation of the Sn from these magmatic rocks, as indicated by the relative low Sn concentration of the specialised G4-granites. 40Ar-39Ar age dating of muscovite associated with the mineralisation gives an integrated age of 869  7 Ma, which is clearly younger than the age of the G4-granites (~986 Ma) and the pegmatites with associated columbite-tantalite mineralisation (~965 Ma) in the area. Based on this large time gap, the 40Ar-39Ar age is interpreted to reflect a hydrothermal event post-dating the emplacement of the Kigali granite, only indicating a possible minimum age for the formation of the cassiterite mineralisation. Based on the structural setting, petrographical observations, the geochemistry of the G4-granites, stable isotope geochemistry, we therefore propose a model in which Sn was mobilised from primary magmatic rocks by a metamorphic hydrothermal fluid system that was generated after crystallisation of the granites and pegmatites. Cassiterite was precipitated in structurally controlled locations, together with the alteration of the host-rocks.

Geodynamic and climate controls in the formation of Mio-Pliocene world-class oxidized cobalt and manganese ores in the Katanga Province, DR Congo.

Decrée, S., Deloule, E., Ruffet, G., Dewaele, S., Mees, F., Marignac, C., Yans, J. & De Putter, T.

Mineralium Deposita 45, 7, 621-629.

The Katanga province, Democratic Republic of Congo, hosts world-class cobalt deposits accounting for ~50% of the world reserves. They originated from sediment-hosted stratiform copper and cobalt sulfide deposits within Neoproterozoic metasedimentary rocks. Heterogenite, the main oxidized cobalt mineral, is concentrated as “cobalt caps” along the top of silicified dolomite inselbergs. The supergene cobalt enrichment process is part of a regional process of residual ore formation that also forms world-class “manganese cap” deposits in western Katanga, i.e. the “black earths” that are exploited by both industrial and artisanal mining. Here we provide constraints on the genesis and the timing of these deposits. Ar-Ar analyses of oxidized Mn ore and in-situ U-Pb SIMS measurements of heterogenite yield Mio-Pliocene ages. The Ar-Ar ages suggest a multi-phase process, starting in the Late Miocene (10-5 Ma), when the metal-rich substratum was exposed to the action of meteoric fluids, due to major regional uplift. Further oxidation took place in the Pliocene (3.7-2.3 Ma) and formed most of the observed deposits under humid conditions: Co- and Mn-caps on metal-rich substrata, and coeval Fe laterites on barren areas. These deposits formed prior to the regional shift toward more arid conditions in Central Africa. Arid conditions still prevailed during the Quaternary and resulted in erosion and valley incision, which dismantled the metal-bearing caps and led to ore accumulation in valleys and along foot slopes.

Malachite, an indicator of major Pliocene Cu remobilization in a karstic environment (Katanga, Democratic Republic of Congo)

De Putter, T., Mees, F., Decrée , S. & Dewaele

Ore Geology Reviews 38, 90-100.

Katanga malachite [Cu3(OH)2(CO3)] is a common mineral in the Central African Copperbelt, where it constitutes a sought after high-grade (~57% Cu) copper ore. It is exploited industrially and by artisanal miners. The genesis of this ore in Katanga has yet received little attention. Here we focus on the genetic processes giving rise to this regionally important mineral. Malachite is secondary to primary copper sulfides (as heterogenite [CoO(OH)] is secondary to cobalt sulfides), which indicates that it formed by oxidation of the latter. It precipitates in very distinctive habits, such as big nodules, thick laminated crusts and speleothemes (up to several tens of centimetre in length), within open voids. REE geochemistry suggests that malachite formed in groundwater environments, through in situ recombination of carbonate ions originating from dissolving host rock carbonates and downward percolating Cu2+ (leached from copper sulfides). Thermodynamic modeling confirms this genetic environment, and shows that chrysocolla [(Cu,Al)2H2Si2O5(OH)4.nH2O] coexists with malachite when Si content increases in the groundwater fluids. The importance of karstic features in the Katanga is also re-evaluated. Karstic features appear to be rather frequent and to constitute ideal site/traps for the formation of secondary base metal ores, as malachite in shallow groundwater environment.

Genesis of sediment-hosted stratiform copper–cobalt mineralization at Luiswishi and Kamoto

El Desouky, H., Muchez, Ph., Boyce, A., Schneider, J., Cailteux, J., Dewaele, S. & von Quadt, A

Mineralium Deposita 45, 8, 735-763.

The sediment-hosted stratiform Cu–Co mineralization of the Luiswishi and Kamoto deposits in the Katangan Copperbelt is hosted by the Neoproterozoic Mines Subgroup. Two main hypogene Cu–Co sulfide mineralization stages and associated gangue minerals (dolomite and quartz) are distinguished. The first is an early diagenetic, typical stratiform mineralization with finegrained minerals, whereas the second is a multistage synorogenic stratiform to stratabound mineralization with coarse-grained minerals. For both stages, the main hypogene Cu–Co sulfide minerals are chalcopyrite, bornite, carrollite, and chalcocite. These minerals are in many places replaced by supergene sulfides (e.g., digenite and covellite), especially near the surface, and are completely oxidized in the weathered superficial zone and in surface outcrops, with malachite, heterogenite, chrysocolla, and azurite as the main oxidation products. The hypogene sulfides of the first Cu–Co stage display δ34S values (−10.3‰ to +3.1‰ Vienna Canyon Diablo Troilite (VCDT)), which partly overlap with the δ34S signature of framboidal pyrites (−28.7‰ to 4.2‰ V-CDT) and have Δ34SSO4-Sulfides in the range of 14.4‰ to 27.8‰. This fractionation is consistent with bacterial sulfate reduction (BSR). The hypogene sulfides of the second Cu–Co stage display δ34S signatures that are either similar (−13.1‰ to +5.2‰ V-CDT) to the δ34S values of the sulfides of the first Cu–Co stage or comparable (+18.6‰ to +21.0‰ VCDT) to the δ34S of Neoproterozoic seawater. This indicates that the sulfides of the second stage obtained their sulfur by both remobilization from early diagenetic sulfides and from thermochemical sulfate reduction (TSR). The carbon (−9.9‰to −1.4‰Vienna Pee Dee Belemnite (VPDB)) and oxygen (−14.3‰ to −7.7‰ V-PDB) isotope signatures of dolomites associated with the first Cu–Co stage are in agreement with the interpretation that these dolomites are by-products of BSR. The carbon (−8.6‰ to +0.3‰ VPDB) and oxygen (−24.0‰ to −10.3‰ V-PDB) isotope signatures of dolomites associated with the second Cu–Co stage are mostly similar to the δ13C (−7.1‰ to +1.3‰ VPDB) and δ18O (−14.5‰ to −7.2‰ V-PDB) of the host rock and of the dolomites of the first Cu–Co stage. This indicates that the dolomites of the second Cu–Co stage precipitated from a high-temperature, host rock-buffered fluid, possibly under the influence of TSR. The dolomites associated with the first Cu–Co stage are characterized by significantly radiogenic Sr isotope signatures (0.70987 to 0.73576) that show a good correspondence with the Sr isotope signatures of the granitic basement rocks at an age of ca. 816 Ma. This indicates that the mineralizing fluid of the first Cu–Co stage has most likely leached radiogenic Sr and Cu–Co metals by interaction with the underlying basement rocks and/orwith arenitic sedimentary rocks derived from such a basement. In contrast, the Sr isotope signatures (0.70883 to 0.71215) of the dolomites associated with the second stage show a good correspondence with the 87Sr/86Sr ratios (0.70723 to 0.70927) of poorly mineralized/barren host rocks at ca. 590 Ma. This indicates that the fluid of the second Cu–Co stage was likely a remobilizing fluid that significantly interacted with the country rocks and possibly did not mobilize additional metals from the basement rocks.

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