Niobium-tantalum oxide minerals in alluvial placer deposits from the Ngoura area, East-Cameroon

This work presents a preliminary mineralogical characterization of Nb-Ta oxide minerals from alluvial placers located in the Ngoura area, eastern Cameroon. The heavy minerals are characteristic of short hydromechanical transport and highlight the stability of Nb-Ta oxides during the weathering and transport. Nb-Ta mineralization in placer deposits from the Ngoura area may derive from local sedimentation. The assemblage of Nb-Ta oxides in the studied placers consists of tapiolite-(Fe) and tantalite-(Mn). Tapiolite-(Fe) exhibits high concentrations in Ta2O5(78.07-80.46 wt%) and FeO (12.18-13.66 wt%) and plots within a narrow range of Ta* (0.900-0.933) and Mn* (0.037-0.119), which correspond to the ranges typically observed in tapiolite-(Fe) worldwide. Tantalite-(Mn) shows Ta(2)O(5)contents ranging from 62.17 to 69.45 wt%, Nb(2)O(5)from 12.09 to 17.37 wt%, MnO from 7.63 to 12.49 wt% and FeO from 1.48 to 6.62 wt%. It is also characterized by a wider range of Mn* (0.538-0.891) and relatively homogeneous Ta/(Ta + Nb) (0.683-0.779) ratios. Texturally, the studied Nb-Ta minerals exhibit oscillatory zoning characterized by bright Ta-rich zones alternating with dark Nb-rich zones. This oscillatory zoning is progressive in some minerals and the alternating bands may appear regular to wavy with gradual transitions. Oscillatory zoning in Nb-Ta oxides from the Ngoura placers is considered to be a primary magmatic feature and is tentatively explained as a result of magmas mixing, rapid cooling or degassing/decompression of the igneous system. The geomorphology and the tropical humid climate of the eastern region offer suitable conditions for the deposition of alluvial placers. Therefore, niobium-tantalum minerals from the Ngoura placers must have been sourced from the weathering and erosion of alkaline granites and pegmatites widespread in the study area.