Characterization of Alluvial Gold Bearing Sediments of Betare Oya District-East Cameroon, Implication for Gold Exploration and Recovery

Abstract

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. Its upper layer (0.5 m) yields 02 Au grains per Kg of sampled materials while the 1 m-tick lower layer contains up to 34 Au-grains per sample and 13 g rains in 1 Kg of sampled materials. The sandy layer follows with averagely 02 Au grains per Kg as indication of partial leaching of primary mineralized rocks. Au concentration at depth (sandy and gravelly layers) is due to physical and chemical remobilization of Au in the secondary environment of deposition. 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 = &minus; 0.0148x 2 + 1.2187x + 2.0344, R 2 = 0.6929 (sandy layer); 2) y = 0.0617x 2 + 1.0849x + 0.8097, R 2 = 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.