Supergene gold transformations of the placer gold deposits from the Gamba district in northern Cameroon: Implications for secondary and nano-particulate gold formations

Abstract

The northern part of Cameroon is a Sudanese or Sudano-Sahelian type characterized mainly by a dry climate, a significant landscape position, and distance from the primary gold sources, giving the occasion to explore the study of complex supergene Au transformations. This study on the biogeochemical transformation of Au in Cameroon will expand understanding of this process and provide advantageous results for Au recovery from materials that are ultimately regarded as waste. In this study, alluvial Au particles were collected from four individual sites, Djaba, Bigbal, Bougma, and Mayo Salah, and analyzed using a scanning electron microscope (SEM), electron probe microanalysis (EPMA) and LA-ICP-MS Time of Flight (ToF) to observe nanometer-sized Au grain morphologies, the Au mobility and spatial association between Au and related major and minor elements by element distributions in Au particles, respectively. The results from the examination of Au samples across all sampling sites show that Au particles are formed by the binary Au/Ag alloys and that the formation of Au nano- and micro-particles is associated with Au/Ag dissolution and Au re-precipitation. The abundant nano- and micro-particles of Au morphotypes within the cavities of primary Au grains across all the sampling locations were associated with the polymorphic layers formed by clays, iron-oxides, and silicate minerals. The nano- and micro-particles of Au overgrow as colloids and crystalline phases of triangular, spherical, hexagonal, and octahedral platelet aggregates of bacteriomorphic Au and were indicative of biogeochemical transformation. The high Au concentrations in the rim suggest the dissolution of Ag and Cu involving Au mobility identified in alluvial Au grains across the individual sites. Hence, the minor elements, such as Ag and Cu, occur in solid solution in the Au cores, while trace elements, such as Si, Al, Fe, Cr, Ti, Ga, Ni, Pb, Mn, V, and As, form impurities or micro inclusions in the rim of native gold which was interpreted as evidence of Au mobility. Furthermore, the Au grains are commonly associated with pyrite, chalcopyrite, bornite, and galena. This can only be attributed to the hypogene orogenic precipitation environment, implying that Au grains were transported from primary sources. Overall, the Au particle transformation and mobility of placer Au highlighted in this study was a good indicator for understanding biogeochemical mechanisms affecting gold particles in the northern Cameroon supergene environments.