Geochemistry of pyriteous mudrocks of the cenozoic N'Kapa formation in Douala Sub-basin, western Cameroon (Central Africa): Source rock weathering, provenance, paleo-redox conditions and tectonic settings

摘要

Pyriteous mudrocks of the pyritic deposit situated at 28 km of Douala (pK28) were studied in a tropical forest setting of Douala Sub-basin (Cameroon, Central Africa), using physicochemical analyses, X-ray diffraction and ICP-AES and ICP-MS in order to decipher source rock weathering, provenance, paleo-redox conditions and tectonic setting. Three main types of sediments were differentiated in the area that include mudstones (claystones and siltstones), sandstones (fine sandstones and coarse sandstones) and lenses of conglomerates. All the pyriteous mudstones fades of the study area contain between 88.15 and 98.45 wt% (clay and silt fractions). Siltstones and claystones containing pyrite are more acid and more acid and relatively homogenous, and have organic carbon content higher than the sandstone facies and red laminated mudstone. All these mudrocks are essentially Fe-shales. Mineralogically, they are essentially composed of quartz, kaolinite, smectite, goethite, hematite, K-feldspar and pyrite. The CIA is more than 98% and Th/U ratios are higher than 4. This shows that these mudrocks have been derived from significantly weathered source rock. The Al2O3/TiO2 ratios between 12.19 and 21.69, Cr/Th and Th/Cr ratios respectively 4.59-48.46 and 0.02-0.22, La/Th versus Th/Yb, a negative Eu anomaly and higher LREE/HREE ratios showed that they were mostly composed of felsic rocks with lesser contribution of mafic rocks like source rocks of Bomkoul, Missole, Kumba and Delta of Niger mudstones. The U/Th ratios below 1.25, V/Cr ratios less than 2 and Ce anomaly ranged from 1 to 1.12 are indicators that investigated mudrocks were deposited in an oxic fully marine environment in the area of active continental margin field like those of Bomkoul, Missole, Kumba mudrocks and Delta of Niger mudstones.