Morphological, mineralogical signatures, and biogeochemical cycle of gold from the Nyema and Zingui placer gold deposits in southern Cameroon: Implications for primary bedrock source and secondary gold formation

Abstract

• Primary source of placer deposits from Nyema and Zingui is proximal. • Primary gold has been subjected to secondary transformation. • Gold particles display the marks of physical mechanism. • Primary gold derived from orogenic deposit type with an elevated Cu (0.01–0.5 wt%) content. • Primary gold is associated with amphibolites and numerous quartz veins. Placer gold (Au) from hypogene deposits is generally subjected to physical, chemical, and biological mechanisms in the surface environments involving the transformation of Au grains with the formation of authigenic Au in the placer deposits. Placer Au collected from Nyema and Zingui deposits in southern Cameroon was examined using a combination of advanced micro-analytical techniques, including scanning electron microscope (SEM), electron probe microanalysis (EPMA), and Laser-ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) Time of Flight (ToF) to determine the possible bedrock source(s) and to assess the (bio)geochemical transformation of Au. The results show that the gold grains from the two locations have irregular to rounded morphology, with the microtextures indicating a proximal source that could be related to amphibolites and numerous quartz veins. The mineral inclusion of gold grains includes Fe-Mg amphibole, quartz, biotite, and K-feldspar, supporting a hydrothermal mineralizing system under medium–high grade metamorphic conditions. The EPMA results show mainly Au-Ag alloy, with some amounts of Cu, Al, Te, and Bi. Thus, Nyema is characterized by a range of 2.85–35.30 wt% Ag with 63.33–97.15 wt% Au; Zingui includes a range of 1.08–29.78 wt% Ag with 69.22–98.06 wt% Au. The chemical composition of the Au particles from both areas plotted in the ternary diagram Au-Ag-Cu shows that all the Au grains were derived from an orogenic deposit type with an elevated Cu (0.01–0.5 wt%) content. The Au-rich rims by >20 µm surrounding Ag-rich cores are observed on some Au grains from both studied areas. The high-purity Au rims (∼99.9 wt% Au) on Nyema and Zingui placer gold deposits suggest the Ag depletion and/or Au addition for some gold grains. The chemical dissolution and re-precipitation of Au are supported by porous textures that occur nearly at the outer edge surfaces of acutely distinct Ag-depleted rims. In conclusion, the primary Au grains from the Nyema and Zingui placer deposits in southern Cameroon have been exposed to short distance to the source, chemical de-alloying, and biogeochemical mechanisms with dissolution and re-precipitation of Au involving the formation of secondary Au and the spreading of microparticulate Au, suggesting that the processes of biogeochemical precipitation of hydrothermal Au deposits occur in tropical environments.