Pétrogenèse des formations ferrifères de Kouambo (sud Cameroun)

Abstract

The Kouambo area is located in the southern region of Cameroon and belongs to the Ntem complex, more precisely to the Nyong group (western part of the Ntem Complex). This sector lies between latitude 2°59'24''N and 3°04'48'N' and longitude 10°20'24''E and 10°25'48''E. The work carried out in this sector focuses on the petrogenesis of the iron formations and their host rocks and aims to determine the origin, tectonic setting and age of the iron mineralization. In order to achieve this objective, a total of 40 rocks samples (15 from the iron formations and 25 from the host rocks) were collected in the field. In the laboratory, 20 standard and 20 polished rock thin sections were made. The Electro Probe Micro Analyses (EPMA) were done on magnetite, pyroxene, amphibole, garnet, plagioclase, biotite and opaque minerals. 30 whole rock geochemical analyses were performed by X-ray Fluorescence (XRF) for major elements, and by Induction Plasma Mass Spectrometry (ICP-MS) for trace elements. 2 rock samples (garnet and pyroxene gneiss and chlorite - sulfide BIF) were dated by SIMS U-Pb on zircon method. The results obtained revealed that, petrographically, this area consists of a metamorphic set made of oxide-type (pyroxene BIF and tremolite BIF) and mixed oxide- silicate type (chlorite-sulfide BIF) banded iron formations, as well as host rocks including garnet-pyroxene gneiss, garnet-plagioclase gneiss, phyllite, and epidote-chlorite schists. All these rocks have heterogranular granoblastic microstructures and mineralogical associations ranging from greenschist to granulite facies metamorphism. The thermodynamic estimation has revealed that of the P-T conditions of metamorphic peak range between 777 and 982°C and 4 to 5 Kb, implying a High Temperature-Low Pression metamorphic gradient. The EPMA data have revealed that magnetite is the main iron mineral with very high FeO contents (87.82-92.54%) and very low Al2O3 (0.05 - 2.40%) and TiO (0.01 - 0.34%) contents. Tremolite is the amphibole present in the tremolite BIFs and shows high FeO (12.60 -16.98%), MgO (10.91-16.07%) and CaO (8.67-12.41%) contents. Pyroxene of the pyroxene BIFs has mostly the composition of hypersthene (En57-58 Fs40-41Wo ) with minor proportion 1-2 of augite (En37Fs18Wo44-46). Geochemically, the BIFs of the Kouambo area have high SiO (39.49-56.21%) and 2 Fe2O3 (36.37-55.97%) contents, characteristic of pure chemical sediments. However, Al2O3 (1.17-1.27%) and TiO (0.05-0.12%) contents indicate a slight detrital input into the 2 depositional environment. The PAAS-normalized REE–Y patterns of the Kouambo BIFs are characterized by null to positive Eu anomalies ((Eu/Eu*) PAAS = 0.92-2.22). The average Y/Ho ratios of tremolite BIFs and pyroxene BIFs are 33.09 and 29.26 respectively while the chlorite-sulfide BIFs have an average Y/Ho ratio of 24.92. These values suggest that the Kouambo BIFs would have precipitated from solutions composed of about 30% seawater and 70% hydrothermal components. The presence of negative Ce anomalies in tremolite BIFs is attributed to an oxic depositional environment while, the absence of Ce anomalies in the mixed oxide-silicate facies BIFs and pyroxene BIFs suggests reducing or suboxic conditions. The metavolcanites (garnet-plagioclase gneisses) associated with the studied BIFs have a calco-alkaline affinity and are thought to be derived from a continental margin setting. Geochronologically, the chlorite-sulfide BIFs have a maximum depositional age (MDA) of 2466 ± 62 Ma and mark the onset of the rise of atmospheric oxygen, also known as xvi the "Great Oxidation Event" in the Congo Craton. These BIFs underwent metamorphism/metasomatism at 2078 ± 10 Ma and 688 ± 46 Ma corresponding to the Eburnian/Transamazonian and Pan-African orogenies. The zircons extracted from the garnet pyroxene gneisses gave a dominant Mesoarchean age of 2857 ± 47 Ma which directly constraints the depositional age of the associated pyroxene BIFs.