Field studies (location Au pits, sampling of different layers of sedimentary sequences filling the pits, panning of sediments), litho-stratigraphic investigations, grain size analysis and outlining of depositional conditions revealed a characteristic fining upward sequence in all the sections of 08 Au-pits within the Betare Oya basin. A typical profile is 2.5 to 4.5 m thick and is composed of a 30 to 50 cm organic topsoil layer, underlain by a 1.5 m thick reddish brownish gold free saprolite, which is underlain by 1.5 m sandy section. From litho-stratigraphic investigations and grain size analysis results a quartz dominated gravelly 1.4 m thick layer consisting in boulders (275 mm), cobbles (2-<20 mm) and subrounded pebbles (28 -38 mm) mixed with sand grains (0.9 -1.5 mm) and silt (0.01 -0.02 mm). Pyrite, chalcopyrite and arsenopyrite are minor minerals and could be referred as to Au pathfinders (As, Fe, Cu). This layer overlays paleo-Proterozoic basement rocks (Nyong series) and is overlain by a poorly sorted sandy layer containing rounded whitish quartz grains and plagioclase fractions (Ca, Na), minor silts (0.02 -0.2 mm), gravel and feldspar (K) leached from granitic plutons along shear zones in the area. The saprolite horizon overlaying the sandy layer derives from weathering and leaching of basement rocks under warm-humid climatic conditions. From gold grain count, the gravelly layer is the main productive horizon. Au is lithologically controlled and could be an important exploration factor in the study area. The calculated aspect ratios for pebbles from the gravelly and sandy layers disclosed two polynomial trends: 1) y = −0.0148x 2 + 1.2187x + 2.0344, R² = 0.6929 (sandy layer); 2) y = 0.0617x 2 + 1.0849x + 0.8097, R² = 0.8694 (gravelly layer); and y > R 2 is satisfied in both cases. This implies that Au recovery from these layers could be effective through gravity separation using sieves mounted on classifiers of 7 cm (sandy layer) and 10 cm (gravelly layer) diameter.

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The Bambui area is located southwestern part of the Pan-African fold belt in Cameroon, and it is found at the foot of Bamenda Mountains, which is part of the Cameroon Volcanic Line (CVL). The study area is made up of various rocks types such as alkali basalt, trachyte, rhyolite and ignimbrite with a granitic basement of the Pan Africa Fold belt. The main objective of this work was to assess the contamination and risk pose by heavy metals in stream sediments of Bambui area, southern Cameroon. Heavy metal contents in the representative stream sediment samples collected were determined using inductively coupled plasma-mass spectrometer (ICP-MS). The elevated content of Co, Cr, Cu, Mn, Ni, Pb, Th, V, Zn, La, Fe and Ti could be attributed to the geology of the area and anthropogenic metal input sources. The evaluation of contamination factor, degree of contamination, modified degree of contamination, enrichment factor, ecological risk factor and potential ecological risk index revealed that the sediments have low to high ecological risk index. Pollution load index (0.78-1.60), geo-accumulation load index (1.73-5641.91) and anthropogenic metal input (0-5.25) indicate heavy metal contamination of the study area. Geogenic origins, agricultural practices, municipal waste disposal into streams, biosolids and animal manure were identified as the major sources of heavy metals in stream sediments of the study area.

The petrology of Achaean biotite- and amphibole-rich metagranites, the geochemistry of Fe-occurrences and heavy mineral concentrations in stream sediments from Olounou (Ntem complex) help to constrain their inter-relationships. These metagranites display granoblastic textures outlined by microcline, biotite, amphibole and oxides. Ilmenite (60 to 80%), zircon (15 to 20%) and rutile (1 to 2%) are the mineral fractions in pan concentrates. Two types of Fe-mineralization as silicified veins crosscut the above rocks: hematite± magnetite quartzites with primary hematite enclosing magnetite relicts contain high average Fe2O3 (69.05 wt %) and TiO2 (0.73wt %); banded magnetite-rich quartzites with magnetite partially replaced by hematite are characterized byFe2O3 (52.15 wt %), TiO2 (0.27 wt %), relatively high W (286 ppm) and Ni (108 ppm). Both types have Zn, Cu, Ga contents below 10 ppm, positive Eu anomalies, HREE enrichment over LREEs and low ∑REE (9.76 to 12.07). Iron and SiO2 were derived from weathering of Archean biotite and amphibole metagranites, deposited in existing Post-Archean intra-cratonic basins under greenschist facies and impacted by sub-marine hydrothermal solutions. These results are genetically comparable to other Precambrian BIFs and underline the iron potential of Olounou. They equally contribute to iron ore exploration in Cameroon and worldwide.Supplementary material:https://doi.org/10.6084/m9.figshare.c.5426547

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