Geology and structure of Mobale gold deposit at Kamituga: A contribution to the study of the Kivu Supergroup

A geological investigation of the Kibaran series of Kamituga (Kivu Supergroup) was carried out in the Mobale gold deposit at Kamituga with a cartographic, petrographic and structural approach. The petrographic analyses showed the existence of mesozonal metasediments (quarzite and metapelite) affected by a bimodal magmatism, first of dioritic and later of a granitic nature. Two deformation phases affect the metasediments: (D1) a first E-W folding with subhorizontal axis and axial-plane cleavage (S1) of presumed Kibaran age, and (D2) a second N-S folding with an axis moderately inclined towards the South. The latter could represent a Pan-African influence outside the Itombwe Synclinorium and would therefore be the first evidence, for a Pan-African folding affecting the Kibaran series

Pegmatite zonation and the use of muscovite as a geochemical indicator for tin-tantalum-tungsten mineralization: Case studies from the Kalehe and Idjwi areas, Democratic Republic of Congo.

The central part of the Karagwe-Ankole Belt (KAB) in the Great Lakes area of Central Africa is rich in various rare and strategic elements. The area north of Kalehe and Idjwi island within the Sud-Kivu Province, in the eastern part of the Democratic Republic of the Congo (DRC), contains Nb, Ta, Sn and W mineralization hosted in pegmatites and quartz veins that are related to leucogranite intrusions. In this study, two granite-pegmatite fields were studied: the Bugarula field on the northern part of Idjwi island, and the Kalehe field, along the west coast of Lake Kivu, with a view to determining the co-genetic link between different pegmatites and leucogranites in these regions. Based on the mineralogical assemblages of the pegmatites, five zones were identified, occurring with some spatial overlap: 1: albite pegmatites, 2: two-mica pegmatites, 3: muscovite pegmatites, 4: beryl-bearing Nb-Ta-Sn mineralized pegmatites, 5: tourmaline-bearing Nb-Ta-Sn mineralized pegmatites. The use of petrogenetic parameters for the alkaline elements K, Rb, and Cs, which are involved in the Rayleigh fractionation model, has shown that different leucogranites are found directly associated with the pegmatites, making it difficult to identify the exact parental granite. However, the leucogranites in these areas have an average composition of 3.6 wt.% K2O, 8 ppm Cs, and 300 ppm Rb. The mineralized pegmatites (4 and 5) show high concentrations of Nb, Ta and Sn. Muscovite compositions were analysed from each zone, and used to calculate fractionation trends using a Rayleigh-type modelling starting from the average leucogranite composition. In the Idjwi granite-pegmatite field, the degree of fractionation with respect to the initial composition of the leucogranite varies from 20% to 99%, respectively for the least fractionated and most evolved Nb-Ta-Sn mineralized pegmatites. In the Kalehe area, the fractionation values also range from 22% up to 99% for the least fractionated and most evolved (mineralized) pegmatites, respectively. The geochemical characteristics of the muscovite in these pegmatites are examined to guide Nb-Ta, Sn, and W exploration in these zones. This high in degrees of fractionation associated with the enrichment in economically interesting elements is comparable as reported for other pegmatite fields in the Karagwe-Ankole Belt

Tentative de décomplexification du complexe d Uvira : Cas des secteurs d Idjwi et Kalehe, Sud-Kivu/ RDC. Cartographie, pétrographie et minéralisation.

L équivalent de complexe d Uvira, qui serait d âge paléoprotérozoïque, affleure à Idjwi et sur une partie de Kalehe. Son prolongement Sud-Ouest (au Rwanda) constituent le complexe de Butare avec son extension SE (au Burundi) dans Mwokora, Kabarore, Kimanga et Cohoha. Il a été signalé, dans la région des grands lacs africains, la présence de trois chaines précambriennes dont celle du paléoproterozoique (Ubendienne), du Mésoprotéroizoique de Karagwe-Ankole Belt (KAB) au Kivu, Kibaran Belt (KIB) au Katanga et du néoproterozoique (Panafricaine) (Tack et al. 2010; Dewaele et al., 2015). Les formations de KAB à l Est de la RDC ont été regroupées, quant à elles, dans le Supergroupe du Kivu (Fernandez-Alonso et al., 2015). Dans ces formations, il est signalé des intrusions locales de trois générations de granitoïdes du Mésoprotérozoïque. (1) Le premier groupe est caractérisé par les granitoïdes de type S d âge 1 375 Ma (2) de type A d âge 1 205 Ma et (3) Le second groupe est celui des leucogranites, d âge moyen de 976 Ma (Tack et al., 2010; Fernandez-Alonso et al., 2012; Dewaele et al., 2015). La géologie des secteurs d Idjwi et de Kalehe est constituée des roches magmatiques (leucogranites, granites à deux micas, rhyolite, Basalte, aplite, gabbros, dolérite), métamorphiques (amphibolites, marbre, gneiss, micaschistes, phyllites, quartzite, schsites graphiteux) et sédimentaires (calcaire, travertin, grès). Dans ces régions, les minéralisations de colombo- tantalite et la cassitérite se trouvent dans les filons de pegmatite tandis que celles de cassitérite et wolframite sont liées aux filons de quartz et de greisen. Le granite à Etain consiste en un leucogranite à muscovite (rarement à deux micas) et tourmaline. Sa paragenèse tectonothermale a été concomitante au climax métallogénique du Kibarien d étain. Les pegmatites et veines minéralisées en Au, Sn, W, Nb, Ta, Be, phosphate, tourmaline et U l accompagnent (Fernandez-Alonso et al., 1986; Tack et al., 1994). Ces minéralisations sont présentes à Idjwi et Kalehe et seraient aussi liées à ce type de granite. Dans ces deux zones, les différentes failles seraient donc des zones de faiblesses pour de possibles circulations de fluides magmatiques hydrothermaux. Les structures minéralisées sont orientées parallèlement aux failles et parfois à la foliation. Ces failles présentent une orientation préférentielle de NE-SW, La lithologie joue un rôle indispensable dans la mise en place des minéralisations. Présence de schistes graphiteux et des quartzites pourrait être le marqueur des pièges de la minéralisation de W et Sn dans ces régions. Références bibliographiques Dewaele, S., Hulsbosch, N., Cryns, Y., Boyce, A., Burgess, R., Muchez, Ph., 2015. Geological setting and timing of the world-class Sn, Nb-Ta and Li mineralization of Manono-Kitotolo (Katanga, Democratic Republic of Congo). Ore Geology Reviews 72, 373-390. Fernandez-Alonso, M., Kampata, D., Mupande, J.-F., Dewaele, S., Laghmouch, M., Baudet, D., Lahogue, P., Badosa, T., Kalenga, H., Onya, F., Mawaya, P., Mwanza, B., Mashagiro, H., Kanda Nkula, V., Luamba, M., Mpoyi, J., Decree, S., Lambert, A., 2015.Carte géologique (provisoire) de la République Démocratique du Congo au 1/2.500.000, notice explicative, Ministère des mines, République Démocratique du Congo. Fernandez-Alonso, M., Cutten, H., De Waele, B., Tack, L., Tahon, A., Baudet, D., Barritt, S. D., 2012.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. Precambrien Research 216-219, 63-86. Fernandez-Alonso, M., Lavreau, J. and Klerkx, J., 1986. Geochemistry and geochronology of the Kibaran granites in Burundi, Central Africa: implications for the Kibaran Orogeny, Chemical Geology 57, 217-234. Laghmouch, M., Kalikone, C., Ilombe, G., Ganza, G., Delvaux, D., Safari, E., Bachinyaga, J., Dewaele, S., Wazi, N., Nzolang, C., Fernandez-Alonso, M., Tack, L., Nimpagaritse, G. & Kervyn, F., 2018. Carte géologique du Kivu au 1/500 000. Musée Royal de l Afrique Centrale, Tervuren. Tack, L., Liégeois, J.P., Deblond, A. et Duchesne, J.C., 1994. Kibaran A-type granitoids and mafic rocks generated by two mantle sources in a late orogenic setting (Burundi). Precambrian Research 68, 323-356. Tack, L., Wingate, M. T. D., De Waele, B., Meert, J., Belousova, E., Griffin, A., Tahon, A., Fernandez-Alonso, M., 2010. The 1 375 Ma Kibaran event in Central Africa: prominent emplacement of bimodal magmatism under extensional regime. Precambrian Research 180, 63- 84

Pegmatite zonation and the use of muscovite as a geochemical indicator for tin-tantalum-tungsten mineralization : case studies from the Kalehe and Idjwi areas, Democratic Republic of Congo

The central part of the Karagwe-Ankole Belt (KAB) in the Great Lakes area of Central Africa is rich in various rare and strategic elements. The area north of Kalehe and Idjwi island within the Sud-Kivu Province of eastern Democratic Republic of the Congo (DRC) contains Nb, Ta, Sn and W mineralization hosted in pegmatites and quartz veins that are related to leucogranite intrusions. In this work, two granite-pegmatite fields were studied: the Bugarula field in the northern part of Idjwi island and the Kalehe field, along the west coast of Lake Kivu, with a view to determine the co-genetic link between different pegmatites and leucogranites in these regions. Based on mineralogical assemblages, five pegmatite zones were identified, occurring with some spatial overlap, but with an overall increasing distance from the inferred parental granite: 1) albite pegmatites, 2) two-mica pegmatites, 3) muscovite pegmatites, 4) beryl-bearing Nb-Ta-Sn mineralized pegmatites and 5) tourmalinebearing Nb-Ta-Sn mineralized pegmatites. Muscovite compositions were analyzed from each zone and used to calculate pegmatite fractionation trends by using Rayleigh fractionation modelling. The model uses published partition coefficients (Kd) and measured contents of K, Rb and Cs in muscovite. Muscovites from the consecutive pegmatite zones in both the Bugarula and Kalehe pegmatite fields record a continuous fractionation trend, indicating a co-genetic magmatic evolution. A parental leucogranitic composition of 3.6 wt% K2O, 8 ppm Cs and 300 ppm Rb was iteratively determined to yield a best fit fractionation trend with the data. This composition is within the range of exposed leucogranites sampled in this study, which average at 4.2 wt% K2O, 7 ppm Cs and 400 ppm Rb (n = 12). It should be noted that individually, none of the sampled leucogranites gave satisfactory fractionation trajectories that would suggest a direct co-genetic link between them and the pegmatites. In the Idjwi granite-pegmatite field, the degree of fractionation with respect to the inferred parental leucogranite ranges from 20% to 99% for the least fractionated and most evolved Nb-Ta-Sn mineralized pegmatites, respectively. In the Kalehe area, fractionation values also range from 22% up to 99% for the least fractionated and most evolved (mineralized) pegmatites, respectively. Muscovites from mineralized pegmatites of zones 4 and 5 show high concentrations of Nb, Ta and Sn. As such, we show that muscovite compositions in these pegmatites provide a powerful tool to track Nb-Ta, Sn (and W) enrichment and potential mineralization. These high degrees of fractionation associated with the enrichment in economically interesting elements are similar to reported values for pegmatite fields elsewhere in the Karagwe-Ankole Belt

Zircon dating and mineralogy of the Mokong Pan-African magmatic epidote-bearing granite (North Cameroon)

International audienceWe present the mineralogy and age of the magmatic epidote-bearing granite composing most of the Mokong pluton, in the Central Africa orogenic belt (North Cameroon). This pluton intrudes Neoproterozoic (~830 to 700 Ma) low- to high-grade schists and gneisses (Poli-Maroua group), and is crosscut or interleaved with bodies of biotite granite of various sizes. The pluton is weakly deformed in its interior, but solid-state deformation increases toward its margins marked by narrow mylonitic bands trending NNE-SSW. The magmatic epidote granitic rocks are classified as quartz monzodiorite, granodiorite, monzogranite, and syenogranite. They are medium- to coarse-grained and composed of K-feldspar + plagioclase + biotite + amphibole + epidote + magnetite + titanite + zircon + apatite. In these granites, the pistacite component [atomic Fe+3/(Fe3+ + Al)] in epidote ranges from 16 to 29 %. High oxygen fugacity (log ƒO2 - 14 to -11) and the preservation of epidote suggest that the magma was oxidized. Al-in hornblende barometry and hornblende-plagioclase thermometry indicate hornblende crystallization between 0.53 and 0.78 GPa at a temperature ranging from 633 to 779 °C. Zircon saturation thermometry gives temperature estimates ranging from 504 to 916 °C, the latter being obtained on samples containing inherited zircons. U/Pb geochronology by LA-ICP-MS on zircon grains characterized by magmatic zoning yields a concordia age of 668 ± 11 Ma (2 σ). The Mokong granite is the only known occurrence magmatic epidote in Cameroon, and is an important milestone for the comparison of the Central Africa orogenic belt with the Brasiliano Fold Belt, where such granites are much more abundant

Tentative de décomplexification du complexe de Butare: ses extrémités NW et SE respectivement à Kalehe et Idjwi à l'Est de la RDC et à Zina-Randa et Cohoha au Nord du Burundi.

La chaîne Karagwe-Ankole (KAB) est une ceinture orogénique d âge Mésoprotérozoïque s étendant depuis l Est de la RDC jusqu au Sud-Ouest de l Ouganda et au Nord-Ouest de la Tanzanie en passant par le Burundi et le Rwanda. Elle est caractérisée par des roches méta sédimentaires à méta volcanosédimentaires réparties dans deux domaines structurellement distincts, à savoir un domaine occidental (WD) reposant sur des gneiss et des migmatites paléo-protérozoïques et composé de roches sédimentaires à volcanosédimentaires méso-protérozoïques déformées métamorphosées dans les schistes verts au faciès des amphibolites. Le WD, ainsi qu un domaine oriental (ED) reposant sur un sous-bassement archéen (Tack et al., 2010). Dans ces formations, il est signalé des intrusions de trois générations de granitoïdes, dont un groupe de granitoïdes de type S d âge 1375 Ma ou de type A d âge 1205 Ma et un groupe de leucogranites d âge moyen de 976 Ma (Tack et al. 2010; Fernandez-Alonso et al., 2012 ; Dewaele et al., 2015). Il a été également reconnu dans cette chaîne des formations à positions stratigraphiques incertaines, comme celles constituant ce qui a été cartographié comme « complexe de Butare », du nom de son locus-typicus au Sud du Rwanda, mais qui s étend en direction NW jusqu au-delà de la frontière à Kalehe et Ijwi en RDC avec son appellation locale de « complexe d Uvira, ainsi qu en direction Sud au-delà de la frontière burundaise depuis le Nord-Ouest avec le complexe de Zina-Randa jusqu au Nord-Est avec le complexe de Cohoha, en passant par les granites de Mwokora, Kabarore, Mparamirundi et Murehe ( Baudet et al. 2019, Laghmouch et al. 2019, 2018). La géologie des secteurs d Idjwi et de Kalehe est constituée des roches magmatiques (leucogranites, granites à deux micas, rhyolite, basalte, aplite, gabbros, dolérite), métamorphiques (amphibolites, marbre, gneiss, micaschistes, phyllites, quartzite, schistes graphiteux) et sédimentaires (calcaire, travertin, grès). Des minéralisations de Sn, Nb-Ta, Be se trouvent dans les filons de pegmatite tandis que celles de Sn et W sont liées aux filons de quartz et de greisen, suivant des orientations parallèles à des failles généralement NE-SW et parfois à la foliation. Le complexe de Zina-Randa se situe au Nord-Ouest du Burundi et s étend depuis la rivière Kagunuzi au Nord jusqu au granite de Bubanza au Sud. Les données préliminaires de terrain montrent que ce complexe est caractérisé par de nombreuses intrusions pegmatitiques au sein des métasédiments essentiellement psammitiques et phylliteux. On remarque également plusieurs pointements de granites (leucogranites et granites à 2 micas foliés). Il renferme des zones de minéralisations, comme celle du champ pegmatitique à Sn, Nb-Ta et Li à Ndora. Au sein du complexe de Cohoha au Nord-Est du Burundi, on observe un centre plutôt plus leucogranitique évoluant vers une zone de bordure où on trouve un mélange de lambeaux lenticulaire de métasédiments avec des granites et des pegmatites dont il ne reste parfois plus que des arènes de quartz flottant dans des restes de feldspaths kaolinisés. Alors que jusqu à ce jour il a été seulement signalé dans la région dit « Ikibuye Cya Shyari » au Sud de Butare où il a été observé et daté, on observe dans ce mélande de bordure, à Cyumva non loin de la ville de Kirundo, des affleurements d orthogneiss fort identiques au prototype du socle paléoprotérozoïque du WD. Un échantillon en cours d'analyse géochronologique révélera bientôt son âge. Des minéralisations en Sn, Nb-Ta, W et Au se trouvent en bordure et loin de ce complexe avec une zonéogéographie allant de zones d abord à Sn, puis Sn et Nb-Ta, W, et Au, même si le lien génétique reste à étudier. Réferences Dewaele,S.,Hulsbosch,N.,Cryns,Y.,Boyce,A.,Burgess,R.&Muchez,Ph.,2015.Geological setting and timing of the world-class Sn, Nb-Ta and Li mineralization of Manono-Kitotolo (Katanga, Democratic Republic of Congo). Ore Geology Reviews 72,373-390. Fernandez-Alonso, M., Cutten, H., De Waele, B., Tack, L., Tahon, A., Baudet, D., & Barrit, S. D., 2012. 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. Precambrien Res. 216-219, 63-86. Tack, L., Wingate, M. T. D., De Waele, B., Meert, J., Belousova, E., Griffin, A., Tahon, A. et Fernandez-Alonso, M., 2010. The 1 375 Ma Kibaran event in Central Africa: prominent emplacement of bimodal magmatism under extensional regime. Precambrian Research180, 63- 84. Baudet, D. Fernandez-Alonzo, M. Ntege, A. Ngaruye, J.-C. Kanyana, A. Tuyishimiye, P. Habiyakare, & T. Gabinema, C., 2019. Geological Map of Rwanda 1/100.000 scale series, Karongi S3/29 NW. Laghmouch, M. Nimpagaritse, G. Mudende, L. Minani, M. Ndereyimana, J. Icitegetse, I. Nahimana, A. Ndarihonyoye, P. Niyongabo, J-B. Fernandez-Alonzo, M. Baudet, D. Tack, L. Kervyn, F., 2019. Carte Géologique du Burundi au 250.000ème

Petrogenesis and geodynamic setting of the Bingo alkaline-carbonatite complex, DRC: Constraints from petrography, geochemistry, C-O isotopes and U-Pb geochronology

This study presents petrographic, whole-rock geochemical, stable isotope (C-O) and U-Pb geochronological data for a suite of carbonatite, nepheline syenite and ijolite from the Bingo complex. The Bingo alkaline-carbonatite complex is located within the western branch of the East African Rift System (EARS) in the Democratic Republic of Congo (DRC). The carbonatites are petrographically classified into calcite and magnetite-calcite carbonatite but have similar geochemical compositions. The ∑REE vary from 960 to 2400 ppm and the LaN/YbN ratios fall within the spectrum of 20 to 145. Calcite stable isotope compositions, δ13C(V-PDB) (− 3.51 to − 3.87 ) and δ18O(V-SMOW) (+12.12 to + 13.70 ), plot within the primary igneous carbonatite field and therefore reflect carbonated mantle melts. Based on similarity of geochemical trends between the carbonatite and the alkaline silicate suite, such as negative anomalies in Th, Ta, Pb, Zr, Ti and positive anomalies in Ba, Sr, La and Nb, we suggest that carbonatite and alkaline rocks could have been generated from a carbonated mantle source through extensive fractional crystallization and liquid immiscibility processes. U-Pb titanite dating of the ijolite suggests an emplacement age of 861.5 ± 7.0 Ma during the Neoproterozoic era, consistent with carbonatite magmatism elsewhere along the western branch of the EARS (i.e., Lueshe and Upper Ruvubu Alkaline Complex). This timing is associated with the initial break-up stages of the Rodinia supercontinent, a significant geological event recorded in central Africa during the Neoproterozoic. Additionally, in-situ U-Pb pyrochlore dating of the carbonatite indicates an age of 48.13 ± 0.80 Ma, coupled with a low (radiogenic) 207Pb/206Pb ratio of 0.1554. We discuss three potential interpretations of this anomalous U-Pb age. Firstly, it could represent a primary magmatic crystallization age, implying carbonatite magmatism during the Eocene. A second interpretation is that the age indicates resetting of the U-Pb system due to hydrothermal alteration of the pyrochlore. Thirdly, the pyrochlore could have recrystallized during intense tropical weathering (lateritization). The first and second interpretation are deemed unlikely given the apparent petrogenetic and geochemical relationship between the silicate and carbonatite suites, as well as the fact that this age would predate any recorded rift activity within the western branch of the EARS. Furthermore, the low 207Pb/206Pb ratio points to a highly radiogenic reservoir that is incompatible with a magmatic-hydrothermal source. We thus consider the third option of supergene recrystallization of pyrochlore most likely, possibly from Nb, U and Pb rich fluids released from the overlying weathering profile. As such, supergene pyrochlore might offer a novel approach to determine the absolute age of tropical weathering affecting carbonatites

Causes and triggers of deep-seated hillslope instability in the tropics – Insights from a 60-year record of Ikoma landslide (DR Congo)

peer reviewedStudying the causes and triggers of landslides is essential to understand the key process of hillslope evolution and the hazards they generate. Such understanding is crucial in tropical areas where landslide impacts are high and on the rise, and the dearth of accurate processes characterisation is large. Here we investigate the timing and the mechanisms of relatively slow-moving deep-seated landslides in weathered tropical environments through the analysis of a landslide located in the Kivu Rift (DR Congo). This landslide, developed in weathered basalt, shows obvious deformation features at its surface indicating large deformations during recent years, making it a unique natural laboratory in an underexplored area. High-resolution topographic data, historical aerial photographs, satellite imagery and careful field investigations are used to detail the landslide mechanisms and investigate failure development over a 60-year record. By confronting rainfall time series and earthquake sequences with the different deformation episodes, we show that the relation between instability triggers and slope failure is not straightforward; e.g., the largest instability occurred at the end of a dry season during a period of relatively low seismicity. Instead of direct influence of external triggers, we show that some phases of instability may be caused by the intrinsic evolution of the hillslope associated with weathered-related weakening of the slope strength through time. Our results question the relative weight of the commonly recognized causes and triggers of slope instability in this area. Analysis of landslide causes and triggers provided here should help improve the understanding of how surface processes influence the pace of hillslope evolution. It also contributes to a more accurate evaluation of the landslide hazard in the area and across other regions where similar environmental conditions are met. © 201