Geochemical features of the Matomb alluvial rutile from the Neoproterozoic Pan-African belt, Southern Cameroon

The Matomb region constitutes an important deposit of detrital rutile. The rutile grains are essentially coarse (> 3 mm), tabular and elongated, due to the short sorting of highly weathered detritus. This study reports the major, trace, and rare-earth element distribution in the bulk and rutile concentrated fractions. The bulk sediments contain minor TiO2 concentrations (1-2 wt%), high SiO2 contents (similar to 77-95 wt%) and variable contents in Al2O3, Fe2O3, Zr, Y, Ba, Nb, Cr, V, and Zn. The total REE content is low to moderate (86-372 ppm) marked by high LREE-enrichment (LREE/HREE similar to 5-25.72) and negative Eu anomalies (Eu/Eu* similar to 0.51-0.69). The chemical index of alteration (CIA) shows that the source rocks are highly weathered, characteristic of humid tropical zone with the development of ferrallitic soils. In the concentrated fractions, TiO2 abundances exceed 94 wt%. Trace elements with high contents include V, Nb, Cr, Sn, and W. These data associated with several binary diagrams show that rutile is the main carrier of Ti, V, Nb, Cr, Sn, and Win the alluvia. The REE content is very low (1-9 ppm) in spite of the LREE-abundance (LREE/HREE similar to 4-40). The rutile concentrated fractions exhibit anomalies in Ce (Ce/Ce* similar to 0.58 to 0.83; similar to 1.41-2.50) and Eu (Eu/Eu* similar to 0.42; 1.20-1.64). The high (La/Sm)(N), (La/Yb)(N) and (Gd/Yb)(N) ratios indicate high REE fractionation. (C) 2013 Published by Elsevier GmbH.