Pétrogenèse et étude des minéralisations métamorphosées de grand Zambi et ses environs (région du sud Cameroun)

Abstract

Grand Zambi region lies between latitute 2°55'46 and 3°8'3'' north and longitude 10°16'34'' and 10°25'34'' east. This region (Grand Zambi, Bipindi, Bibindi, Madoungou and Ndoa) is located in the Nyong unit (western part of the Ntem Complex) in the Southern part of Cameroon. Grand Zambi consists essentially of metamorphic lithological units comprising iron formations (BIFs) and host rocks including metasediments (chlorite and epidote shist and chloritoid and pyrite shist) and metabasites or metavolcanites (garnet and pyroxene gneiss, pyriclasite, epidosite and epidotized amphibolite). The petrogenetic characters of these formation are still unknown. The main objective of this work is the petrogenetic characterization of metamorphic formation and associated mineralizations. This specifically involves determining the nature of rocks, their protolith, the paleoenvironment, tectono metamorphic evolution and geological setting. These rocks are characterized by heteroganular granoblastic microstructures. The lithological formations of the region show a polyphase, plurifacial and polycyclic tectonic and metamorphic evolution with three deformation phases. Phase D1 is ductile and compressive in nature. Phase D2 is shearing. The D1 and D2 deformations are non-coaxial with a simple shear regime. Phase D3 is essentially brittle. On the whole, the area under study is affected by a dome and trough structure and thus by two major tectonic events, namely transpression (shear and compression) and transtension (shear and extension). Geochemical analyses reveal that Grand Zambi BIFs are low in Fe (39.32wt%) and can be exploited for steel production. The Al2O3 and TiO2 contents, as well as the Al/Ti ratios (9.38) of the BIFs indicate low crustal contamination. The BIFs studied are metamorphosed ferriferous sediments. These BIFs were formed by the upwelling of hydrothermal fluids. These metamorphogenic fluids leached silica and concentrate iron in the rock. The host shist are ancient iron shales and are believed to come from a platform corresponding to a stable cratonic environment. The metabasites are ancient basalts with toleitic and calc-alkaline affinities, indicating a extensive and compressive environment, respectively.