Microchemical signatures of placer gold grains from the Gamba district, northern Cameroon: Implications for possible bedrock sources

Abstract

The Central African Fold Belt (CAFB) in Cameroon hosts numerous alluvial gold deposits derived from known deposit types in the eastern and southern part of the country. In contrast, in northern Cameroon, primary source of gold remains enigmatic. Based on the morphological and geochemical signature of alluvial gold grains from northern Cameroon, this study discusses the distance-to-source and the style of gold mineralization of the Gamba district. Three hundred and fifty gold grains collected from the four locations were primarily analyzed by scanning electron microscopy (SEM) to investigate their morphological characteristics, and subsequently the composition was examined through the SEM and electron microprobe analysis (EMPA). The gold grains from the four locations have irregular to rounded morphology, with a microtextures indicating a proximal source at less than 5 km. Most of the gold grains display evidence for mechanical hammering and abrasion. EMPA results show mainly Au-Ag alloys, dominated by high Ag values in the cores from different location. Thus, Djaba is characterized by a range of 1.1–22.1 wt% Ag with 77.2–99.3 wt% Au; Bigbal includes a range of 10.3–32.9 wt% Ag with 66.1–88.7 wt% Au; Bougma is distinguished by a range of 7.3–18.3 wt% Ag with 80.8–91.9 wt% Au; and Mayo Salah is characterized by a range of 2.8–22.8 wt% Ag with 77.3–96.8 wt% Au. These range in compositions suggest multiple deposits of the same type in different locations. Gold grains from the four locations show the Te (0.1–0.3 wt%) content, Bi (0.1–0.7 wt%) content, and Cu (0–0.05 wt%) content, suggesting a source of orogenic gold mineralization. Most of the gold grains are Ag-depleted from both the cores and rims and show lack of Au-rich rims indicate no evidence of supergene precipitation, which suggests that the grains were transferred directly from hypogene source to surficial and fluvial system. EMPA data on the gold grains, plotted in the ternary diagram of Au-Ag-Cu, suggest that gold from the Gamba district is hydrothermal-orogenic in origin. Mineral inclusions include silicates (zircon, titanite, quartz, muscovites, biotite, actinolite, titanite, albite and almandine), oxides (magnetite, ilmenite and hematite), and phosphates (apatites), which suggest the magmatic and metamorphic conditions of formation for the hypogene source. This indicates the granitoids and metasediment of schists which bear multiple quartz veins as the potential bedrock sources. The microchemical signature of placer gold used in this study is a useful tool to explore the hypogene environment of an unknown primary source that can greatly help in mineral exploration in northern Cameroon.