Mineralogical and Geochemical Characterization of Gold-Bearing Minerals from Placers in the Les Saras Area (Mayombe, Congo Republic)

Abstract

With the aim to contribute to the search for the source of placer gold, the Les Saras sector, located in the south-western part of the Republic of the Congo in the Mayombe chain, has been the subject of a study aimed to characterize the mineralogical composition of artisanally mined gold placers. Sediment samples collected from the local drainage system were analyzed using scanning electron microscopy (SEM) and electron microprobe techniques. Results show that the placers are mainly composed of oxides (ilmenite, cassiterite, columbo-tantalite and rutile), followed by silicates (garnets), rare earths (monazite) and native elements (gold). The ilmenites exhibit high TiO 2 (54 - 57 wt%) and FeO (37 - 43 wt%) content, while rutiles are chemically pure. Garnets are classified as almandine- and pyrope-type, and monazites show enrichment in lanthanum (21 - 28 wt%) and cerium (8 - 11 wt%). The angular morphology of gold grains suggests limited transport. Geochemical analyses reveal the presence of Ag, Bi, and Cu as associated elements, along with a peripheral enrichment in gold attributed to selective silver leaching. Two distinct types of gold grains were identified based on fineness: 1) high-fineness grains (up to 988) with low Ag content (<5%), and 2) low-fineness grains (as low as ~670) enriched in Ag (20% - 33%). These compositional differences, along with the geochemical signatures of the associated minerals, suggest two potential sources for the gold: high-temperature hydrothermal systems for the first type and epithermal deposits for the second. However, the limited number of analyzed grains for some species points to the need for further analyses, including fluid inclusion studies, to refine the interpretation of the deposit’s origin.