Study On Metallogeny Of Gold And Nickel Deposit In Mayo Kebbi,Southwestern Chad

The Lere and Pala mineralized district in Mayo kebbi is located in southwestern Chad, within the Precambrian greenstone stone belt of the central African Fold Belt (CAFB). Orogenic gold quartz vein systems constitute a class of epigenetic precious metal deposit; they formed syntectonically and near peak metamorphism of subduction-accretion complex’s of Archean to Cenozoic age, spanning over3billion years of Earth’s history. This class of gold deposits is characteristically associated with deformed and metamorphosed mid-crustal blocks, particularly in spatial association with major crustal structures. Gold-sulfide mineralization occurs often as auriferous quartz veins which filled fractures in fault, or as stockworks cutting the host rocks locally associated with Pb and Cu sulfides, and in the veins and veinlets of quartz, located in brittle to ductile shear zones.These shear zones are correlated with three deformation events:D1are characterised by a pronounced S1foliation related to the axial plane of tight to isoclinal F1folds. The D2deformation is responsible of transposition of the bedding surface So into Si and S2schistosities. D3deformation occurred as the latest brittle deformation. The main weathering processes are sericitization, chloritization, carbonization and sulfidation.The sulfide mineral assemblage is dominated by arsenopyrite, pyrite, pyrrhotite, chalcopyrite, galena and covellite. Gold occurs mostly as free grains (usually<0.5mm in size) in grey quartz associated with pyrite and arsenopyrite. The orebodies are often parallel to the foliation of host metabasite, and more generally strike N15°E and dips60°. The nickel occurrence occurs in igneous rocks, rich in ferromagnesian mineral and oxides. These rocks are hornblende, pyroxenite and amphibol-gneiss. The mineralization occurs in disseminated wall rock along trending, steeply dipping magmatic shear zone.The gold content in veins exceeds33g|t in place, while the wallrock contain up to29.6g|t Au. Primary native gold occurs in quartz and is present in Pyrite, and arsenopyrite,The nickel content in wall-rock exceeds1563ppm in Pala, but in Lere the content in wall-rock range between149.9ppm to173.0ppm. Ni mineralization is associated with Cobalt, Cuivre, Chrome and Zinc. The orebodies are characterized by the development of calc-silicate minerals such as actinolite.The orogenic gold deposit is hosted by different rock types, within the Mayo Kebbi in Neoproterozoic low-grade meta-volcanosedimentay units. Those units are metamorphosed to greenschist and amphibolite facies and intruded by calc-alkalic and alkalic granites. U-Pb dating in zircon from the granite indicates that granitoids emplacement occurred between692.4±6.6-630.4±6.3Ma. The obtaining age from these rocks show that the Mayo kebbi granite is associated with the Pan-African orogen.The pyroxenites host rocks of nickel deposit at South Lere very low PGE contents due to the formation of magmatic sulfides. The800.7±6.7-691.4±7.3Ma ages suggest the deposition of the Mayo Kebbi nickel occurs during late Cryogenian period. These ages suggest that Mayo Kebbi basin was filled by magma, the source of mainly reworked Neoproterozoic protoliths. This age indicates also that the igneous complexes were derived from a juvenile Neoproterozoic source.The composition of magamatic sulfide deposits is highly dependent on the MgO content of the source magma involved, high MgO content being associated with sulfides. For Ni occurrence the Mg/Fe ratio, the values varied from (1.61to2.60). Our result illustrates that mafic rock ratio (m/f) value is basically in accord with that of worldwide PGE-bearing Cu-Ni sulfide deposits and vanadium-titanomagnetite, and that, they belong to ferruginous or ferruginous-rich mafic-ultramafic rocks of gabbro, pyroxenite and peridotite. Nickel occurrence including the Cobalt, copper, Chrome and Zinc suggests a syngenetic origin. The igneous mafic-ultramafic complexes are Precambrian and Neoproterozoic in age.The results from fluid inclusion distribution, microthermometry, and Raman spectroscopy indicate that fluids associated with Au mineralized quartz veins and veinlet have low salinity ranging from0.18to19,5wt%NaCl equiv with density up to (1g/cm3). These veins formed at temperatures from128.8and314.2℃and Pressures of38.83MPa-93MPa; with location depths of about5-9km. Ore-forming hydrothermal fluids responsible for Mayo kebbi gold mineralization in Au-bearing quartz belong to CH4-CO2-H2O-NaCl system. According to the relatively low salinity, the ore forming fluids are mixed in origin, meteoritic water and magmatic fluid. Based on the geological control and intrusion, geotectonic setting of gold mineralization is an orogenic.