Two pulses of Mesoproterozoic intra-plate magmatism in the heart of the Palaeoproterozoic Ubendian Domain of northern Malawi

Two Mesoproterozoic anorogenic igneous events are recorded in the Palaeoproterozoic Ubendian Domain in northern Malawi. The oldest is represented by the Mwakikome orthogneiss, a small peraluminous, sub-alkaline and ferroan syenogranite that has a U–Pb zircon emplacement age of 1411 ± 3 Ma, an initial εNd of 4.5 and a Nd TDM2 model age of 2.27 Ga. These data are interpreted to suggest that the Mwakikome intrusion was derived mainly from the melting of the adjacent Palaeoproterozoic country rocks during Kibaran-aged extension. The second anorogenic igneous event is defined by the more widespread Mwenga Suite, a series of small (<100 km2) NW-SE oriented and elongated plutons that occur in a narrow belt in the north of the Ubendian Domain. Seven samples from different intrusions gave U–Pb zircon emplacement ages between ~1150 and 1110 Ma. The Mwenga Suite is comprised of rocks that are also ferroan, potassic and sub-alkaline in composition and shows a range of major element compositions from monzogranite to alkali-feldspar granite. Nd isotopic data from one sample yielded an initial εNd of 2.2 and a Nd TDM2 model age of 1.88 Ga, suggesting that the suite was also largely derived from melting of the Ubendian host rocks, but with some additions from an enriched mantle source

A New Type of Large Ultramafic Intrusion-Hosted Fe-Ti-V Deposit in the West-African Archean Craton: The N’Guérédonké Complex, Guinea

International audienceA new type of large ultramafic intrusion-hosted Fe-Ti-V deposit has been discovered by BRGM in the West-African Archean craton. The N’Guérédonké intrusive complex (7 x 0.9 km) is composed of a dunite core surrounded by rims of wehrlites and pyroxenites. This syntectonic complex is emplaced within a crustalscale shear zone. The disseminated mineralisation made of ilmenite and magnetite is mainly hosted by pyroxenite units along the northern and southern contacts. Surface and drilling exploration work (47 drill holes totalling 10124 m) demonstrate polyphased injection of successive ultramafic melts from core to rims and also crosscutting of dunite core by a network of successive pyroxenite generations. A 3D modeling constrained by drilling and joined inversion of magnetic and gravity data ascribe the thickness of the intrusion up to 1000 m. A first very conservative titanium resource assessment on a small part of the intrusion (1000 x 100 x 250 m) lead to definition of 141.38 Mt ore @ 10.52 % TiO2. The lack of typical features of Ural-Alaskan type intrusions and strong differences with classical Gabbro-Anorthositehosted Fe-Ti-V deposits demonstrate a new type of large low grade high tonnage Fe-Ti-V deposit occurring in Archean crato

A New Type of Large Ultramafic Intrusion-Hosted Fe-Ti-V Deposit in the West-African Archean Craton: The N'Gueredonke Complex, Guinea

International audienceA new type of large ultramafic intrusion-hosted Fe-Ti-V deposit has been discovered by BRGM in the West-African Archean craton. The N'Gueredonke intrusive complex (7 x 0.9 km) is composed of a dunite core surrounded by rims of wehrlites and pyroxenites. This syntectonic complex is emplaced within a crustal scale shear zone. The disseminated mineralisation made of ilmenite and magnetite is mainly hosted by pyroxenite units along the northern and southern contacts. Surface and drilling exploration work (47 drill holes totalling 10124 m) demonstrate polyphased injection of successive ultramafic melts from core to rims and also crosscutting of dunite core by a network of successive pyroxenite generations. A 3D modeling constrained by drilling and joined inversion of magnetic and gravity data ascribe the thickness of the intrusion up to 1000 m. A first very conservative titanium resource assessment on a small part of the intrusion (1000 x 100 x 250 m) lead to definition of 141.38 Mt ore @ 10.52 % TiO2. The lack of typical features of Ural-Alaskan type intrusions and strong differences with classical Gabbro-Anorthosite hosted Fe-Ti-V deposits demonstrate a new type of large low grade high tonnage Fe-Ti-V deposit occurring in Archean craton

Review of the Senegalo-Malian shear zone system – Timing, kinematics and implications for possible Au mineralisation styles

International audienceThe Kédougou Kénieba Inlier (KKI) of eastern Senegal forms a typical Paleoproterozoic greenstone belt characterised by low-metamorphic sequences of volcanic rocks and volcano-sediments that have been intruded at various stages by gabbroic suites and calc-alkaline granitoids. The main structures of the KKI comprise two anastomosing structures about N–S trending shear belts that are distinguished into the western Main Transcurrent Zone (MTZ) and the eastern Senegalo-Malian shear zone system (SMSZ). These shear belts are taken to define the limits between the western Mako, the central Diale-Kéniebandi and the eastern Daléma-Kofi domains even though transitions exist between their characteristic deposition sequences.Remote sensing analysis of airborne geophysics and satellite data (Landsat, ENVI, SRTM) in combination with geological field mapping, structural analysis and geochronology suggests that the SMSZ represents a Birimian structure that records a main stage of deformation that could characterise the major transcurrent Eburnean tectonics (D2) dated from 2105 Ma (Feybesse et al., 2006a–e). This sinistral transpressive deformation marked by a major constraint oriented NNW–SSE, is accompanied by a NNE–SSW extension leading to the opening of dilational areas such as small pull-apart basins marked by local calk-alkaline volcanic sequences and several coeval intrusions of the Boboti Suite dated 2080 ± 0.9 Ma (Hirdes and Davis, 2002). A post-Birimian to pre-Neoproterozoic deformation post dates the transcurrent phase and is marked by regional N–S extension.Review of the existing Au-mineralisation models in combination with the spatial analysis of soil geochemical data suggest seven possible mineralisation styles that are related to the transpressional tectonics and coeval magmatism

Caractérisation tectono-métamorphique de la zone de cisaillement ductile de Mugesse, Ubendian Belt (Malawi)

International audienc

A New Type of Large Ultramafic Intrusion-Hosted Fe-Ti-V Deposit in the West-African Archean Craton: The N'Gueredonke Complex, Guinea

International audienceA new type of large ultramafic intrusion-hosted Fe-Ti-V deposit has been discovered by BRGM in the West-African Archean craton. The N'Gueredonke intrusive complex (7 x 0.9 km) is composed of a dunite core surrounded by rims of wehrlites and pyroxenites. This syntectonic complex is emplaced within a crustal scale shear zone. The disseminated mineralisation made of ilmenite and magnetite is mainly hosted by pyroxenite units along the northern and southern contacts. Surface and drilling exploration work (47 drill holes totalling 10124 m) demonstrate polyphased injection of successive ultramafic melts from core to rims and also crosscutting of dunite core by a network of successive pyroxenite generations. A 3D modeling constrained by drilling and joined inversion of magnetic and gravity data ascribe the thickness of the intrusion up to 1000 m. A first very conservative titanium resource assessment on a small part of the intrusion (1000 x 100 x 250 m) lead to definition of 141.38 Mt ore @ 10.52 % TiO2. The lack of typical features of Ural-Alaskan type intrusions and strong differences with classical Gabbro-Anorthosite hosted Fe-Ti-V deposits demonstrate a new type of large low grade high tonnage Fe-Ti-V deposit occurring in Archean craton

Caractérisation tectono-métamorphique de la zone de cisaillement ductile de Mugesse, Ubendian Belt (Malawi)

International audienc

Tectono-metamorphic evolution and strain partitioning along the Mugesse mega shear zone, Ubendian Belt (Northern Malawi)

International audienceIn this contribution we focus on a portion of the Mugesse mega shear zone (MGSZ) in the Ubendian Belt, for which details on kinematics and strain partitioning remains poorly documented. We present a field-based structural work, a petrological and thermobarometrical study of mylonites and LA-ICP-MS in-situ U-Pb dating on monazite and zircon. The detailed field mapping highlights that the whole study area shows the same sinistral strain pattern with deformation being partitioned within S-C and C-C’ structures. Syn-kinematic metamorphic mineral assemblages indicate that the MGSZ mylonites recorded a clockwise P-T evolution with peak temperature conditions at upper amphibolite metamorphic facies of 11–15 kbar and 760–790 °C. Retrogression and cooling reached P-T conditions of 1–7 kbar and 330–480 °C. LA-ICP-MS in-situ U-Pb monazite dating suggests that the main activity of the MGSZ was Pan-African, at ca. 547 ± 5 and 545 ± 4 Ma. Zircon dates at 1985 ± 44 Ma and 1127 ± 29–988 ± 10 Ma are interpreted as inherited ages of potentially detrital origin related to the Ubendian and Irumide orogeny, respectively. A 1146 ± 14 Ma date obtained on zircons from the Lwakwa mylonitic orthogneiss is interpreted as the crystallisation age of the granitic protolith that intruded the Paleoproterozoic basement. Strain partitioning and strain softening during the growth of the MGSZ was strongly controlled by feedback relationships between deformation and retrograde metamorphism enhanced by fluid ingress and recrystallisation

The Mesoproterozoic Nampula Subdomain in southern Malawi: Completing the story from Mozambique

International audienceNew field mapping, lithostratigraphic, whole-rock geochemical, U–Pb zircon and radiogenic isotope data on the Nampula Subdomain of southernmost Malawi show good correlation with those published from adjacent NE and NW Mozambique. The oldest rocks, dated between ∼1180 and 1070 Ma form a complexly interlayered sequence of supracrustal layered paragneisses and migmatites (Nampula Group), intimately interlayered with intermediate-basic (Mocuba Suite) and felsic (Mamala Suite) gneisses, likely meta-volcanic and meta-plutonic in origin. The Mocuba Suite has typical calc-alkaline, I-Type granitoid, TTG compositions, with generally positive ϵNd values, NdTdm ages around 1.2 Ga and low initial 87Sr/86Sr ratios. These characteristics indicate generation in a juvenile volcanic arc setting at this time. Minor exceptions include a few samples which show limited input of Palaeoproterozoic crust. The Nampula arc is viewed as one of a number that developed in a closing oceanic basin which lay between the Archaean-Palaeoproterozoic Congo-Bangweulu-Ubendian-Tanzania cratonic blocks in the north and the Zimbabwe Craton to the SW, and which can be traced “southwards” in Gondwana to continue in the NE part of the ∼1 Ga Maud belt in East Antarctica. The Nampula arc-complex was intruded at ∼1040 by syn-to late tectonic sheets of megacrystic granite/augen gneisses of the Culicui Suite and, at ∼1000 Ma, by a number of bodies of late-to post-tectonic, massif-type anorthosite (Tengani Suite), which are unique to the Malawian segment of the Nampula Subdomain. The anorthosites are considered to have intruded at a post-tectonic stage under a late Mesoproterozoic extensional regime. The rocks underwent deformation and high grade metamorphism in both the Mesoproterozoic (Irumide) and Neoproterozoic-Cambrian (Pan-African) orogenies. P-T data suggest the latter attained peak granulite facies conditions of ∼800 °C at a maximum of 13–18 kbar, comparable with Neoproterozoic granulites in adjacent areas. The data presented here do not rule out either Irumide (∼1 Ga) juxtaposition of the Congo and Kalahari cratons (in Rodinia), or Pan-African (∼0.5 Ga) collision (Gondwana). Instead of this “either-or” situation, a recent hypothesis suggests accretion/collision occurred during both orogenies, separated by a period of drift apart, by the “strange attractor” phenomenon. Our data do not rule exclude that contention; rather, they tend to support it