Petro-Mineralogical and Geochemical Characterization of the Banded Irons Formations BIFs of the Nimba Range and its Western Extension (Nimba Region)

摘要

The Nimba Range and its western extension are located in the Nimba region on the borders of the Republic of Guinea, Liberia and Cote d'Ivoire. It is a mountainous region made up of magmatic, ortho-magmatic and metasedimentary rocks. Magmatic and ortho-magmatic rocks are gneisses, granites, amphibolites and quartzites, which constitute the lower part of Archean age. The upper part consists of Proterozoic rocks of metasedimentary origin. It contains important deposits of itabirites which occupy the top of the mountains and hills of the region. The petrographic study of the banded iron formations reveals the existence of silicate banded iron formations (SIF) and oxidized banded iron formations (Off). The results of the scanning electron microscope (SEM) and metallogenic analyzes show the presence of iron minerals (magnetites, hematites, pyrites, goethites, martites and siderites). These analyzes also reveal the presence of the metamorphic index minerals associated with the banded iron formations, hence the existence of several types of ferriferous formations (silicate (SIF) and oxidized (OlF) banded iron formations). Overall, there is an increase in the degree of regional metamorphism from east to west of the Nimba region. The geochemical analysis of the banded iron formations reveals that with the exception of Na2O, all the major elements have a negative linear correlation although dispersed with Fe2O3. This correlation is explained by a decrease in quartz, garnet, micas (muscovite and biotite), amphibole, pyroxene, plagioclase, titanium and phosphorus contents. Conversely, there is an increase in iron ore content: magnetites, pyrites, hematites, goethite. But the alkali content remains constant in these banded iron formations. Then, the lower the Fe(2)O(3)content, the higher the FeO content, while those of SiO2 and Al2O3 are constant in all of these formations in the Nimba region except in the chlorite banded iron formation where both are anticorelated. Finally, the ratio SiO2/Fe2O3 vs MgO + CaO + MnO /Fe(2)O(3)of the banded iron formations of the Nimba region compared to the same formations of the whole world allows to give them Proterozoic age. Some itabirites have high levels of magnetite, hematite, and goethite (same feature as itabirites of Lac superieur and Pic de fon) and only chlorite itabirite has a low to medium Mg-Si-BIF content.