Mise en évidence des relations entre déformations tectoniques et minéralisations aurifères à l'aide de la méthode audiomagnétotellurique (AMT) dans les régions de Garga Sarali - Ndokayo, Est Cameroun

Abstract

New geological, structural and mining data was put in evidence in the strongly weathered lateritic terrane covering the localities of Garga Sarali and Ndokayo through a tactical multidisciplinary prospecting methodology using GIS technics to define predictive metallogenic targets. In particular, a study of the satellite images to arise the whole of the structures and their orientation, a cartography of the formations and mining sites with an evaluation of the gold grade and finally a geophysical prospection by the audiomagnetotellurics sounding (AMT). The equipment used is a STRATAGEM EH4 receiver from Geometrics, whose frequencies ranging 20 Hz to 50000 Hz. We took precautions to minimize uncertainties as well in the acquisition from the 16 stations AMT along 03 profiles, cross cutting Panafrican formations of the East Cameroon. We used appropriate programs for the interpretation of the collected data and the analysis of the dimensionality tests shows that the structures are 2D to 3D. The lithology of the study area consists of sedimentary formations, granitic intrusions that crossed the metamorphic formations. These intrusions are either in the form of small-insulated solid masses, or in the form of granitic dyke and of pegmatites, which would represent the ultimate magmatic phase; metamorphic granito-gneiss rock made up, migmatite, schists, and enclaves (basic schists and materials). The mapping of the zone enables to identify approximately 126 gold bearing sites with an average content of 1.54 g/t. The interpretation and analysis of LANDSAT OLI 8 image from the February 1, 2019 identified 1149 linear structures. The distribution by directional family led to the individualization of eight (08) main structural family’s trend. The geometry of the structures highlights a senestral and dextral sliding corresponding to the Riedel fractures system. The analysis of AMT data highlights the deeply litho-stratigraphic heterogeneity of geoelectrical formations with irregular shape and the presence up to 1000 meters depth of both conductive and resistive formations. This heterogeneity shows the prolongation of the overlapping of the Congo Craton on Panafrican beyond the parallel 04°N latitude and electric discontinuities assimilated as faults. In particular, on the Nd4 station of the Ndokayo profile and on the stations GS4 and GS6 of the profile Garga Sarali. The correlation therefore made it possible to establish the presence of a network of new deeper faults (( 400 meters) on the Garga Sarali and up to 800 meters on the Ndokayo profile with a NE-SW direction interpreted as the prolongations of the Sanaga Fault. The correlation of all the relevant results of this study enables to identify four (04) targets marked by rather considerable gold bearing contents. The targeting zones that correspond to the intersections of metallogenic structure represent by rivers (Lom, Guingé, Sara) which are the subsurface signature of deeper fractures and metallotect structures considered as metal traps. Finally, we can assume that gold bearing mineralization observed and subject of the intense artisanal exploitation activity in the Garga-Sarali – Bétaré Oya’s area, is controlled by the intersection of geological structures (metallogenic and metallotect), which are the indicators of an intense deformation liable to the tectonic events having affected the formations of the Adamawa – Yade domain (ADY).