Mineralogy and geochemistry of recent alluvial sediments from the Ngaye River watershed, northern Cameroon: Implications for the surface processes and Au-PGE distribution

Abstract

In this work, six vertical sequences were selected to understand the petrological features of recent alluvial sediments as well as Au-PGE distribution in heavy mineral concentrates from two terraces of Ngaye River watershed in Northern Cameroon, using mineralogical (XRD) and geochemical (XRF and ICP-MS) instruments. The Ngaye River watershed is located in the tropical region with contrasted seasons. The lithotypes are mainly made up of gneisses, granites, and amphibolites with different textures. Granites contain high average contents (n = 7) in SiO2 (69.54 wt.%), Al2O3 (15.21 wt.%), Na2O (4.39 wt.%) and K2O (2.00 wt.%) contrary to gneisses with moderate average contents (n = 7) in SiO2 (64.32 wt.%), Al2O3 (15.75 wt.%), Fe2O3 (5.03 wt.%), CaO (4.45 wt.%), and MgO (2.79 wt.%). Several major element contents are higher in gneisses than amphibolites except for Fe2O3 (11.38 wt.%), CaO (8.63 wt.%), and MgO (6.41 wt.%). Amphibolites (n = 7) are more enriched in Cr, V, Ni, Zn, and Co whereas granites exceed in Ba, Sr, and Rb. Ngaye rocks have moderate REE content (75–122 ppm) with variable LREE/HREE values (5.88–18.83). The alluvial sediments are mainly clayey sandy and are made up of abundant quartz associated to smectite, kaolinite, feldspars, illite, and accessory rutile, amphibole, olivine, pyroxene, ilmenite, and goethite. The occurrence of olivine in one the sequences approves the occurrence of basalts in the watershed. The SiO2 contents range from 55 to 83.41 wt.%, the contents of this oxide show an increasing tendency while those of Al2O3, Fe2O3, MgO, TiO2, and LOI decrease. Several oxides are negatively correlated with SiO2; this confirms the high proportion of silicate minerals and different weathering of source rocks (ICV ∼0.77–0.96; CIA ∼59–80%; PIA ∼64–83%). These sediments are also highlighted by high contents in Ba, Zn, and Sr, and moderate contents in transition trace metals (Cr, V, Ni, and Zr) which confirm the felsic nature of the source rocks. Meanwhile, selected trace elements correlate negatively with SiO2 and explain the low contents in trace elements. The total REE content is twofold high compared to the lithotypes but the LREE/HREE values show similar trend like those of the rocks. The spider diagrams display (i) homogeneity of patterns; (ii) negative anomalies in U, Nb, and P; (iii) and slight positive Zr anomalies. Chondrite-normalized REE patterns exhibit negative Eu anomalies (Eu/Eu∗ = 0.56–0.89) contrary to the PAAS normalization (Eu/Eu∗ = 1.25–1.37). The total Au-PGE contents are very low (1.49–3.94 ppb) in the heavy mineral concentrates contrary to slightly higher contents in Ngaye amphibolites (8.43 ppb). Despite the low contents, the heavy mineral concentrates are richer in PPGE, particularly in Pd and Pt. This geochemical survey has revealed that further alluvial Au-PGE exploration may not be necessary.