Alluvial gold in the Bétaré Oya drainage system, east Cameroon

Abstract

The morphology and elemental composition of alluvial gold grains from the Bétaré Oya gold district were investigated as part of a district exploration strategy. The morphology and general chemistry of the grains, determined using scanning electron microscopy (SEM)—energy dispersive spectroscopy and electron probe microanalyzer (EMPA), respectively, revealed three categories of grains: (1) gold grains with irregular to regular, bent-up and folded outlines with irregular and pitted surfaces and a flatness index ranging from 2.1 to 4.6; (2) gold grains with regular and polished outlines, smooth surfaces with few or no cavities and a characteristic flatness index range between 3.0 and 8.6; (3) and elongate grains. Rounding of the grains, physical abrasion and bent/folded features suggest that the alluvial gold grains have been transported in a high-energy environment, but not necessarily over long distances from their source rock(s). The gold grains are alloyed with Ag and Cu with concentrations of Ag ranging from 0 to 14.19 wt% whereas Cu concentrations are between 0.03 and 0.15 wt%. SEM images of sites where active weathering of gold is indicated by the presence of colloidal gold, i.e., crevices possessing sedimentary materials on the surface of the grains, revealed the presence of bacteria. All of the gold grains analyzed possess high purity (~ 100% Au) at the water–sediment–gold interface demonstrating that Ag and Cu are highly dispersed in placer systems relative to gold, and that gold can be dissolved and subsequently re-precipitated possibly with the aid of bacteria in alluvial systems.

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Data availability

All data used in this study will be readily available to the public.

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Acknowledgements

This paper acknowledges the contributions from Prof. Lehmann for generating the chemistry of the gold grains at the Technical University of Clausthal, Germany. This manuscript benefited significantly from comprehensive comments by an anonymous reviewer.

Funding

CES and KIA acknowledge support from the Alexander von Humboldt Foundation for the analytical work at TU Clausthal with the support of Prof. Dr. Bernd Lehmann.

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Contributions

KIA carried out the field study, sample collection and processing and, initiated and wrote the first draft of the manuscript. CES designed the project in cooperation with KIA and guided the analytical procedures for all the samples and reviewed, read and approved the manuscript. JS, FR and GS analyzed the gold grains using the SEM in Australia, provided data interpretation insights, read, corrected and approved the manuscript. VA and EMS assisted in the acquisition and interpretation of the EMPA data and read, contributed to and approved the manuscript.

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Correspondence to Kevin Ijunghi Ateh.

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Ateh, K.I., Suh, C.E., Shuster, J. et al. Alluvial gold in the Bétaré Oya drainage system, east Cameroon. J. Sediment. Environ. (2021). https://doi.org/10.1007/s43217-021-00051-w

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Keywords

  • Gold
  • Morphology
  • Microchemistry
  • Bacterioform gold