1 Introduction

Placer gold deposits around the world form through the processes of erosion, transportation and deposition of material from gold-rich rocks and/or structures in supergene fluvial environments. Alluvial gold grain morphology, texture and chemistry have been used successfully in the determination of primary gold mineralization type of mineralization and the potential host rock (e.g., Chapman et al. 2009; Moles et al. 2013; Omang et al. 2015; Townley et al. 2003). According to Boyle (1979) Au grains are a natural alloy of Au, Ag, and Cu in proportions that vary with the conditions of ore formation. Various gold types bear characteristic alloy composition and therefore peculiar microchemical signatures. These signatures have been summarized for orogenic gold signatures (Chapman et al. 2000, 2010), epithermal vein signatures (Chapman et al. 2005; Chapman and Mortensen 2006), skarn- and intrusion-related gold signatures (Potter and Styles 2003) and oxidizing chloride hydrothermal systems (Chapman et al. 2009). Recent experimental works have shown that biogeochemical weathering of gold-bearing ore could be a key phenomenon in the formation of secondary gold deposits. This may be in part due to the action of iron and sulfur oxidizing bacteria and subsequently by sulfate-reducing bacteria that are known to be responsible for the solubilization, precipitation and re-concentration of gold, respectively, under supergene conditions (Lengke et al. 2006a, b, c; Lengke and Southam 2006; Reith et al. 2007; Shuster et al. 2013, 2016; Shuster and Southam 2015).

The link between bacteria and secondary gold precipitation was established in the works of Reith et al. (2006) using scanning electron microscopy (SEM) and confocal scanning laser microscopy (CSLM) combined with nucleic acid staining in which bacterial pseudomorphs and active bacterial biofilms, respectively, were revealed on the surface of these grains. According to Reith et al. (2006) the ability of sulfur oxidizing and reducing bacteria to transform gold complexes into octahedral gold suggests that bacteria contribute to the formation of gold platelets in the supergene environment. Also, Shuster et al. (2016) highlighted the importance of biogeochemical weathering on primary gold-bearing metal sulfide ore which leads to supergene gold enrichment and provide insights on the evolution of the ore deposit over time. According to Shuster and Southam (2015) secondary Au, initially occurring as dispersed nanometer-scale colloids, bacteriomorphic structures, octahedral platelets and micrometer-scale foils continued to develop and aggregate into millimeter-sized grains within sedimentary placer deposits leading to gold grain growth. There is need to back these laboratory results with evidence of secondary gold formation in natural alluvial systems and this is the main thrust of this study.

1.1 Geological setting

The Bétaré Oya alluvial system is located within the Lom Basin which is part of the Central Cameroon Shear Zone (CCSZ), a SW extension of the Central African Shear Zone (CASZ) and constitutes a part of the North Equatorial Central African Fold Belt (NECAFB, Fig. 1a). The fold belt is situated at the northern edge of the Congo Craton (Njongfang et al. 2008) and is essentially composed of Palaeoproterozoic basement, traced through Cameroon across the Atlantic into northeastern Brazil and the Trans-Sahara Belt. The CCSZ and its relay feature known as the Sanaga fault (SF) have a NE-SW orientation and is characterized by the abundance of granitic intrusions associated with major shear zones (Ngatcha et al. 2019; Takodjou Wambo et al. 2020). The Lom Basin is composed mainly of meta-sedimentary rocks, grouped into two main structural and metamorphic units; monocyclic and polycyclic units. The monocyclic unit includes rocks such as volcano-clastic series, orthogneiss, quartzite and polygenic conglomerate (Soba 1989) metamorphosed under greenschist facies and associated with grabens. The polycyclic unit consists of staurolite mica schists, Lom Bridge gneisses, and staurolite-chloritoid mylonites, closely related to the horst structures (Ngako et al. 2003). The mylonites are the key rocks that reveal the presence of the SF (Soba 1989). Quartz veins and granitoids generally intrude both units and show evidence of sinistral deformation.

Fig. 1
figure 1

a Regional geologic map situating Cameroon within the central African fold belt (CAFB), b location of study area within the lower Lom basin sandwich by the Sanaga fault (SF), c aeromagnetic and radiometric survey map of the study area (CAMINCO 2009), d stream sediment sample location map. SF Sanaga fault, Pe Pernambuco Shear Zone, Pa Patos Shear Zone, AS Adamawa Shear Zone, TBF Tcholéré Banyo fault, CCSZ Central Cameroon Shear Zone

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The Lom basin is characterized by intensive artisanal and semi-mechanized alluvial gold exploitation. Primary gold in the Bétaré Oya district is hosted in quartz veins that truncate meta-sedimentary rocks in the vicinity of small local granitoid intrusions (Vishiti et al. 2017). The quartz veins are thought to be a product of hydrothermal fluids which originated from nearby intrusive bodies. The study area was selected on the basis of the presence of gold in the alluvial system in a granitoid-dominated terrane (Fig. 1c). The area extends from Northing 411,000 to 417,000 and from Easting 626,000 to 633,000, zone 33 N (Fig. 1d). The granitoid intrusion in the study area is sub-alkaline, high K calc-alkaline in nature, granodioritic to tonalitic affinity emplaced in a volcanic arc setting (Ateh et al. 2017). This intrusion is Pan African in age similar to other granitoids along the CCSZ with a mean age of 635 ± 5 Ma (Ateh et al. 2017). The wall rock alteration zone and the vicinity of this Pan African granitoid carries primary gold lodes. It has not also been established whether all the gold within the alluvial deposit is placer or authigenic.

2 Method of study

Stream sediment samples were collected at selected sites shown on Fig. 1d and panned for the heavy mineral concentrate and their visible gold count recorded on site. These samples were later air dried and the gold grains in them picked using binocular microscopes at the Geology Laboratory, University of Buea.

The gold grains were divided into two subsets. The first set was sent to the University of Queensland, Australia where each sample was investigated for its morphology using a scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS). The second set was used to study the chemistry of the grains at Technical University of Clausthal, Germany using electron probe microanalyzer (EMPA). For the EMPA study the detection limits for gold was 5 ppb and 0.1–50 ppm for the Zn, Cu and Ag. Laboratory precision based on replicate analyses was better than 10% at 95% confidence level. The same grains were polished using standard procedure and analyzed using a Cameca SX100 electron microprobe for Ag, As, Au, Ba, Cu, Fe, Pb, S and Zn at the Technical University of Clausthal, Germany. The instrument was operated at 20 kV, 40 nA. Pure metals (Ag, Au, Cu, Pd, Se, Pt), pure galena (Pb, S), sphalerite (Zn), HgTe (Hg), GaAs (As), InSb (Sb), cassiterite (Sn), and hematite (Fe), were used as standards. Care was taken to choose analytical points free of cracks. A standard was run after every ten analyses to ensure quality control on the data. The fineness for Au was calculated using the formula (Au*1000)/(Ag + Au) (Hallbauer and Utter 1977).

3 Results

3.1 Gold grain morphology and microtexture

Gold grains from the alluvial system within the Bétaré Oya gold district displayed a variety of shapes and sizes (Fig. 2). Based on morphology, three populations of gold grains were distinguished. The first group of grains was characterized by irregular shapes and regular edges. Their sizes generally ranged between 350 and 600 µm and can go up to 1 mm (Fig. 2a–j). The second gold grain population was characterized by sub-rounded, rounded, ovoid or spherical grains. These grains were generally pitted although some grains demonstrated smooth surfaces. Grain sizes varied from 400 to ~ 700 µm (Fig. 2k–r). Gold grains from this population were hammered with smooth surfaces. Their outlines were regular showing no clear evidence of primary imprints (Fig. 2n, KS-20). The third gold grain population was characterized as elongated, smooth and/or pitted grains with sizes ranging between 400 and 800 µm.

Fig. 2
figure 2

SEM images of representative gold grains from the Bétaré Oya alluvial system, eastern Cameroon investigated in this study

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The SEM study allowed for better appreciation of the surface features of the gold grains (Fig. 3). The microtextures observed on some of the gold grains include cavities, shallow hammer-marks, scratches or furrows, irregular surface pits and abraded embayments up to 20 µm and canals. Most of the cavities were filled with a light gray clayey material and sometimes organic matter. In some cases, dissolution features, often as honey-comb textures were observed (Fig. 3d). Crevices within the gold grains reveal the presence of bacteria and the by-products of their metabolic activities (Fig. 4), i.e., secondary, colloidal and octahedral gold. Bacterial attachment to gold grains was facilitated by exopolymeric substances (Fig. 4b), which also trapped and/or helped form the re-precipitated colloidal gold particles. Surface depressions within the gold grains are extensively covered by bacteria and by bacterioform gold (Fig. 4e, f).

Fig. 3
figure 3

Microtextures of gold grains from the Bétaré Oya alluvial system a cavities, b grooves and scratches as a result of abrasion and smearing, c embayment and corroded surfaces, d dissolution textures and/or cavities, e canal and f cavity filled with clay/organic matter

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Fig. 4
figure 4

Microphotographs of gold grain crevices from the Bétaré Oya drainage system. a Bacterial colonization, b bacterial contact via exopolymer, c gold colloids, d octahedral gold platelet, e secondary gold dispersed in clay-sized materials, f network of bacterioform gold on the surface of a gold grain

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3.2 Gold grain chemistry

Representative EDS spectra of gold grain KS-03 and KS-07 are shown in Fig. 5 while the EMPA data for the grains are presented in Table 1. In addition to Au, other alloy elements include Ag, Cu and Zn. Ag content in the gold grains varied from 0 to 14.19 wt% whereas Cu contents ranged between 0 and 0.15 wt% and Zn in most gold grains was below the detection limit. The concentration of gold in the rims varied from 82.36 to 100.38 wt% and that of the core varied between 83.6 and 100.38 wt%. Gold grain fineness values were calculated and ranged between 859 and 1000. The fineness in the rims varies from 860 to 1000 whereas that of the core ranges from 859 to 1000. On The Au–Cu–Ag plot (Fig. 6) these alluvial gold grains revealed a mesothermal/orogenic origin similar to those reported by Vishiti et al. (2015).

Fig. 5
figure 5

EDS patterns for gold grain KS-03 and KS-07 investigated in this study

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Table 1 Ag–Au–Cu–Zn compositional variation of gold grains from the Bétaré Oya gold district
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Fig. 6
figure 6

Ternary Au–Cu–Ag plots of the compositions of Au grains from the Bétaré Oya gold district compared with those from the Kazikkaya—Turkey lode gold, Hunker dome lode, Grasberg, Indonesia, Santo Tomas II, Philippines, and Cerro Casale gold porphyry northern Chile. The Bétaré Oya gold grain samples show mesothermal, orogenic/or intrusion-related signature

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4 Interpretation and Discussion

4.1 Alluvial gold grain morphology and proximity to source rock

The results of this study show that some of the gold grains have irregular morphology and are interpreted to represent newly liberated grains from the source rock. During progressive transformation and separation in stream, the morphology evolves to semispherical, wafer‐shaped, and finally flake‐shaped. The surface texture of the grains also evolves from smooth and clean to pitted, hackly and, finally to lobate‐textured (Groen et al. 1990). These features depend on the stream’s energy, sediment type, and composition. Flaky and small size particles are easier to transport; consequently distal placers consist of predominantly flake‐shape gold of small size classes. In this study, three main gold populations were distinguished based on shape and outline. The irregular gold grains still preserve outlines, which suggest a relatively short transport distance as compared to the sub-rounded/elongated grains which indicate a moderate distance of transportation. The rounded to oval shaped grains represent long distance of transport. According to Yeend (1975), the features created by hammering and abrasion are recorded by surface texture, related to shape of gold particles showing evolution in response to the changing stream energy and size distribution of host sediments. The microtextures observed within the gold grain population of the Bétaré Oya drainage system such as embayment, scratches and furrows, grooves, etch pits, cavities, corroded surface, honey comb-like textures and canals reveal that the mechanical and corrosion processes are the main processes responsible for their formation. In terms of morphological characterization, Townley et al. (2003) used the shape of placer Au grains to provide a vector to the location of bedrock sources. Based on the physical characteristics of gold particles as outlined by Townley et al. (2003) the gold grains from the Bétaré Oya gold district (BOGD) are potentially derived from two sources.

  • Those with irregular to regular, bent-up and folded outlines; irregular and pitted surfaces (cavity full) and an inferred flatness index range between 2.1 and 4.6 which represent gold grains from a source area between 50 and 300 m radius.

  • Those with regular and polished outlines, regular topography and little or no cavities on the surfaces and a characteristic flatness index range between 3.0 and 8.6 defining gold grains from a distance to source area within a 300–1000 m radius.

Based on Townley et al. (2003), the proximity to a lode gold source in the Bétaré Oya gold district ranges from 50 to 1000 m up-gradient from these sample sites. This distance corresponds to a proximal primary gold mineralized zone characterized by the presence of wall rock alteration such as seritization, silicification and albitization (Vishiti et al. 2017) in the vicinity of the local granodioritic intrusion (Ateh et al. 2017) in the Bétaré Oya gold district.

4.2 Lode gold proximity in relation to microchemical signature of alluvial gold

Morphological characterization and microchemical signature of alluvial gold grains have been used to establish a correlation between the gold grains from bedrock and alluvial populations (Youngson et al. 2002). According to Chapman et al. (2000) the degree of agreement between the microchemical signatures of bedrock and associated alluvial gold is influenced by the geomorphological development of the locality. In the cases where gold-bearing structures are being actively eroded by rivers, the degree of correlation between the characteristics of the bedrock and alluvial gold populations is very high. The abundance of alluvial gold within the drainage system is influenced by the orientation of the mineralization in relation to the river valley, particularly where the intersection is either parallel or perpendicular to the mineralized structure (Chapman et al. 2000). With the mineralization structure lying along the river valley contributing more gold into the stream sediments than when it cuts across. According to Chapman et al. (2000) ‘rough’ grains, with attached quartz particles, are likely to have experienced a smaller degree of fluvial transport than the corresponding ‘smooth’ grains, and are probably derived from more local sources. This hypothesis is supported by the variation in silver content in alluvial gold grains (Chapman et al. 2010 Chapman et al., 2011; Morrison et al.,1991; Nakagawa et al., 2005; Nesterenko et al., 2014). Alluvial gold grains that have traveled short distances from their lode sources are generally rough with little or no variation in its Ag content. Alluvial gold grains from the Bétaré Oya gold district reveal rough edges associated with an almost flatten curve in the Ag % vs Ag cumulative plot (Fig. 7). This plot reveals an almost uniform Ag concentration in the gold grains. The Ag content alongside the gold grain morphology support a close proximity to the lode source in the Bétaré Oya gold district.

Fig. 7
figure 7

Ag content variation of alluvial gold grain populations from the drainage system investigated in this study area, highlighting the enrichment of pure gold at the gold grains surfaces

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4.3 Evidence of secondary gold deposit resulting from weathering of primary ore and microbial activity in the BOGD

Primary gold grains from mineralized quartz veins in the Bétaré Oya gold district were investigated for their chemistry and it revealed a microchemical signature of Au + Ag ± Cu ± Hg ± Se (Vishiti et al. 2018). Alluvial gold from creeks draining the Bétaré Oya gold district were analyzed and revealed a similar microchemical signature Au + Ag ± Cu. The similarity in the gold chemistry suggest that alluvial gold is a direct consequence of weathering of mineralized veins by streams that drain the gold district. Gold grains from the creeks of Bétaré Oya reveal Au-rich rims and Ag rich cores. This is explained by the continuous leaching of Ag and Au, combined with the re-precipitation of Au during gold grain transport thereby improving upon gold fineness (purity).

SEM images of gold crevices for gold grains from the Bétaré Oya gold district reveal the existence of bacteria on these gold grains. According to Reith et al. (2007) gold-associated bacteria are capable of actively catalyzing the reductive precipitation of toxic gold (I/III) complexes, which improves their survival in such environments. Supergene gold enrichment processes and the mobility of gold have been attributed to thiosulphate and chloride ions—two ligands able to form soluble gold complexes depending on the geochemistry of the environment (Boyle 1979). However, according to Lengke and Southam (2006) these soluble gold complexes are destabilized in the presence of chemolithotrophic bacteria such as iron and sulfur—oxidizing bacteria and sulfate-reducing bacteria. These bacteria can act as reducing agent, leading to the formation of elemental gold colloids. SEM micrographs of the Bétaré Oya alluvial gold reveals textural evidence of bioaccumulation of secondary gold precipitates, potentially as a result of microbial activity. These images show the presence of bacteria held together by extracellular polymeric substances which trap precipitated octahedral and colloidal gold. Biogeochemical weathering on primary bearing metal sulfide ore is known to lead to supergene gold enrichment and hence provide insight on the evolution of ore deposits over time (Shuster et al. 2016). The presence of bacteria in the vicinity of lode gold mineralization in the BOGD is suggestive of a potential host of economical supergene gold enrichment. This gold is said to be authigenic as it is formed from in situ biogeochemical processes and different from the processes that led to the formation of the lode gold. Shuster and Southam (2015) investigated and established gold grain growth through the agglomeration of nanophase gold particles to micrometer size and subsequently to mm-scale nuggets. This agglomeration of nanophase gold particles is evident in SEM images of the crevices of gold grains from the BOGD. EDS peaks from re-precipitated gold via biogeochemical weathering possess high Au fineness.

5 Conclusion

Based on shape and microtextures of gold grains recovered from alluvial deposits within the Bétaré Oya gold district, the gold is sourced from a proximal lode gold deposit, i.e., within 1 km. Gold grains from the Bétaré Oya drainage system are generally alloyed with Ag and Cu. However, some small grains have high purity, ~ 100% Au suggesting they are entirely secondary. Many grains reveal rough edges with little or no variation in Ag content. The Ag content alongside the gold grain morphology support a close proximity to the lode source in the Bétaré Oya gold district. SEM images reveal the presence of bacteria that likely contributed to the solubilization, re-precipitation and concentration of high purity gold (authigenic gold) in the supergene environment of Bétaré Oya.