Microchemical characterization of placer gold grains from the Meyos-Essabikoula area, Ntem complex, southern Cameroon

Gold occurs as a native metal, usually containing silver, and in some cases mercury, copper, and palladium. It may also occur as inclusions within sulfur-rich minerals, such as pyrite and arsenopyrite. The style and variety of gold mineralization is influenced by the geological setting, chemistry of the ore fluids, and the nature of their interactions with rocks. Gold grains liberated from bedrock into surficial sediments during weathering and erosion are chemically stable and may be characterized according to their mineralogy: i.e the alloy composition and suite of mineral inclusions revealed within polished sections, characteristics faithful to gold from the hypogene source. This approach has been applied to placer gold grains from the Meyos-Essabikoula area, Cameroon, where the source of gold is not yet confirmed due to poor outcrop exposure. A total of 221 alluvial gold grains from 10 sites, tributaries of Sing and Bivele River over the Ntem Complex have been studied using Electron Probe Micro-Analysis (EMPA) to determine the concentration of minor alloying metals, (notably Au, Ag, Cu, and Hg) and Scanning Electron Microscopy (SEM) in order to evaluate the assemblage of mineral inclusions within the gold. Most of the grains are sub-rounded with pitted surfaces and inclusions of pyrrhotite, acanthite, and chalcopyrite were observed. The grains are AuAg alloys ranging from 54.4 to 99.8 wt% Au, 0.1–48.4 wt% Ag, 0.1–0.8 wt% Hg and 0–0.3 wt% Cu. The presence of Fe oxide (magnetite) inclusions containing Cr and V (to around 5 wt %) has not been reported elsewhere and suggests a strong interaction between hot reducing ore fluids and local mafic lithologies

Neoarchæan crustal evolution in the Congo Craton: evidence from K rich granitoids of the Ntem Complex, southern Cameroon

L'UMR 6530 a été intégrée dans l'ISTO - UMR6113 - CNRS Université d'OrléansThe Neoarchæan (2.6 Gal K rich granitoids of the Ebolowa area (Ntem Complex, Congo Craton) in southern Cameroon form small massifs trending north-northwest-south-southeast to north-south and contain xenoliths of the surrounding tonalite-trondhjemite-granodiorite (TTG) series and of the greenstone belt country rocks. The granitoids range in modal composition from granodiorite to granite and leucogranite. The rocks display high SiO2 (69–76.5%), Na2O+K2O (5.5–9.1 %) and K2O/Na2O (0.5–1.9). They are metaluminous to slightly peraluminous (0.

Crustal origin of Early Proterozoic syenites in the Congo Craton (Ntem Complex), South Cameroon

International audienceTwo petrographically different syenite suites can be distinguished in the mostly Archean Ntem Complex of Cameroon: a suite of two-pyroxene syenites and a suite of clinopyroxene syenites. The syenites occur as discrete intrusions surrounded by Archean gneisses. Based on structural relationships the syenites are of late to post-orogenic origin. The Pb–Pb zircon evaporation ages indicate that the syenites intruded at ca. 2.3 Ga and thus postdate significantly the Archean episode of crust formation and differentiation that generated the surrounding gneiss terrane. The isotope signatures and the trace elements provide unambiguous evidence that the two syenite suites are of crustal origin. The syenite suites do not show simple chemical trends, have highly variable trace element contents and display only a small variation in their major element contents, particularly in their SiO2 contents. The isotope characteristics, including 143Nd/144Nd, 87Sr/86Sr and common Pb, point to a heterogeneous source or multiple sources for the syenites. The Sr isotope compositions calculated for the time of intrusion vary over a wide range from an unrealistically low 87Sr/86Sr ratio of 0.690 to high values of 0.713. In contrast the initial 143Nd/144Nd ratios are more homogenous with var epsilonNd(2320) from −5.8 to −7.5. The Pb isotopes from leached K-feldspars indicate that the sources of the syenites had a history of U depletion and Th/U enrichment. The syenites show the same isotope characteristics as the surrounding gneiss terrane, thus the deep continental crust of the Ntem Complex itself is the most likely source for the syenite melts. This conclusion is also supported by the heterogeneity of the trace elements in the syenites that exclude a homogenous source. In addition, inherited zircons from the syenites yield Pb—Pb evaporation ages that are consistent with ages obtained for zircons extracted from the surrounding gneisses. In contrast to many other syenites worldwide, the syenites in the Ntem Complex do not contain juvenile mantle material, but are the sole products of crustal reworking. Thus, the syenites did not contribute to the crustal growth in the Ntem Complex. However, this conclusion may not preclude the possibility that the syenites were generated in the deep crust during a time of addition of mantle material to the continental crust. The cause for the generation of these large syenite bodies in the Ntem Complex is most likely an episode of deep-seated thermal activity that was so far not known from this part of the Congo Craton

Late Archaean high-K granite geochronology of the northern metacratonic margin of the Archaean Congo craton, southern Cameroon: Evidence for Pb-loss due to non-metamorphic causes.

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Neoarchaean crustal evolution in the Congo Craton: evidence from K rich granitoids of the Ntem Complex, southern Cameroon

L'UMR 6530 a été intégrée dans l'ISTO - UMR6113 - CNRS Université d'OrléansThe Neoarchæan (2.6 Gal K rich granitoids of the Ebolowa area (Ntem Complex, Congo Craton) in southern Cameroon form small massifs trending north-northwest-south-southeast to north-south and contain xenoliths of the surrounding tonalite-trondhjemite-granodiorite (TTG) series and of the greenstone belt country rocks. The granitoids range in modal composition from granodiorite to granite and leucogranite. The rocks display high SiO2 (69–76.5%), Na2O+K2O (5.5–9.1 %) and K2O/Na2O (0.5–1.9). They are metaluminous to slightly peraluminous (0.

Archaean high-K granitoids produced by remelting of earlier Tonalite-Trondhjemite-Granodiorite (TTG) in the Sangmelima region of the Ntem Complex of the Congo craton, southern Cameroon

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Structure and petrology of Pan-African nepheline syenites from the South West Cameroon; Implications for their emplacement mode, petrogenesis and geodynamic significance

International audienceThree late-Neoproterozoic nepheline syenite intrusions crop out close to the late-Pan-African SW Cameroon shear zone, namely the Mont des Eléphants, Eboundja and Rocher du Loup intrusions. They are characterized by magmatic to solid-state deformation structures and microstructures. Their magmas were mainly derived from partial melting of the subcontinental lithospheric mantle. Magmatic differentiation may have occurred through fractionation of clinopyroxene, amphibole, plagioclase and accessory minerals (apatite, sphene, magnetite and zircon). Bulk magnetic susceptibilities are variable in intensity depending of the magnetite content. Their magnetic anisotropies are unusally high, especially in the Rocher du Loup intrusion. The trajectories of magnetic foliations and lineations display an arcuate shape from an E-W direction in the easternmost Mont des Eléphants to a N-S direction in the Rocher du Loup intrusion. These features are consistent with a synkinematic emplacement in relation with the sinistral motion along the SW Cameroon shear zone, whose age is therefore dated by the age of the syenites, i.e. 590 Ma. Magma genesis and ascent was likely favored by a large gradient in lithospheric thickness along the western margin of the Congo craton

TTG magmatism in the Congo craton; a view from major and trace element geochemistry, Rb-Sr and Sm-Nd systematics: case of the Sangmelima region, Ntem complex, southern Cameroon

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Archaean crustal accretion at the northern border of the Congo Craton (South Cameroon). The charnockite-TTG link

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