Microchemical characterization of placer gold grains from the Meyos-Essabikoula area, Ntem complex, southern Cameroon

Abstract

Gold occurs as a native metal, usually containing silver, and in some cases mercury, copper, and palladium. It may also occur as inclusions within sulfur-rich minerals, such as pyrite and arsenopyrite. The style and variety of gold mineralization is influenced by the geological setting, chemistry of the ore fluids, and the nature of their interactions with rocks. Gold grains liberated from bedrock into surficial sediments during weathering and erosion are chemically stable and may be characterized according to their mineralogy: i.e the alloy composition and suite of mineral inclusions revealed within polished sections, characteristics faithful to gold from the hypogene source. This approach has been applied to placer gold grains from the Meyos-Essabikoula area, Cameroon, where the source of gold is not yet confirmed due to poor outcrop exposure. A total of 221 alluvial gold grains from 10 sites, tributaries of Sing and Bivele River over the Ntem Complex have been studied using Electron Probe Micro-Analysis (EMPA) to determine the concentration of minor alloying metals, (notably Au, Ag, Cu, and Hg) and Scanning Electron Microscopy (SEM) in order to evaluate the assemblage of mineral inclusions within the gold. Most of the grains are sub-rounded with pitted surfaces and inclusions of pyrrhotite, acanthite, and chalcopyrite were observed. The grains are Au-Ag alloys ranging from 54.4-99.8 wt% Au, 0.1-48.4 wt% Ag, 0.1-0.8 wt% Hg and 0-0.3 wt% Cu. The presence of Fe oxide (magnetite) inclusions containing Cr and V (to around 5 wt %) has not been reported elsewhere and suggests a strong interaction between hot reducing ore fluids and local mafic lithologies.

Soil Geochemistry Combined with Particulate Gold Microchemistry Provides Evidence of Eluvial Gold Genesis and Anthropogenic Hg Use in Eastern Cameroon Goldfields

“…This is in agreement with the bulk geochemical data discussed above, which pointed to the redistribution of gold in the solid state in the soil. Iron oxide inclusions containing Cr and V reported for the first time in the Essabikoula area within the Ntem complex have been a ributed to the interaction between hot reducing hydrothermal fluid and local mafic lithologies [55].…”

“…The soil show similarities in mineral composition (as determined by the X-ray diffraction technique) with lateritic soil developed on granite in the eastern region of Cameroon [53,54]. It is worth noting that the formation of the regolith seems to be accompanied by the segregation of elements such as Au-As-Ce-Cu-Pb-Rb-Sr-Y-Zn indicating contribution from felsic bedrock, while the Cr-V content can be a ributed to subordinate mafic lithologies (Figure 8; [9,55]). The in situ character of the saprock/saprolite and mo led zone studied is further supported by enrichments in high field strength elements (HFSE) such as Ga, Th, Hf and Zr representing the least mobile element derived from the chemically homogenous basement rock (Table 1).…”

A New Approach to Characterizing Deposit Type Using Mineral Inclusion Assemblages in Gold Particles

“…The northern Cordilleran sample sets are relatively large and were collected with a view to generating suitably large inclusion suites whereas those from previous campaigns elsewhere were sufficient to characterize alloy variation, but in many cases the inclusion count was small. In addition, only recent studies of detrital gold in West Africa (Dongmo et al, 2019;Fuanya et al, 2019) and Russia (Lalomov et al, 2016;Zaykov et al, 2017;Svetlitskaya et al, 2018;Nevolko et al, 2019) have systematically studied large gold grain populations and their inclusion assemblages. Gold from Precambrian granite-greenstone terranes in northern and central Africa show very low inclusion abundances and generation of radar diagrams has not been possible (Ngomo et al, 2019;Fuanya et al, 2019;Chapman et al 2021) .…”

Tourmaline occurrence and gold mineralization at a granitoid-metasediment contact in the Upper Lom Basin, east Cameroon

“…The identification of primary gold deposits in the eastern, southern, and northern goldfields in Cameroon, on the basis of gold microchemistry, has yielded pertinent information concerning primary gold mineralization. Orogenic primary gold mineralization is suggested in the southern goldfield of Cameroon [21,[37][38][39], based on placer gold studies. Hydrothermal quartz vein style of mineralization is common in the eastern and northern gold terrane as the possible primary source of gold [21,22,34,40].…”