The rivers of Rio Muni are worked by local artisans using simple panning and wooden sluice technologies. Previous commercial investigations have concentrated on the three main areas of artisanal workings - Coro, Aconibe, and Mongomo and there are several other known occurrences that have been worked. Historical records are incomplete, but at least 2300 kg (74,000 0z) of gold was produced from the Coro area alone in the mid-70's.
The alluvial gold is relatively coarse grained, occurring as either dendritic or rounded nuggets, mostly in the 350-180um size range, although larger nuggets up to 4mm x 8 mm are relatively common. The common occurrence of the gold with vein-quartz, clays, and lateritic minerals attests to the proximity and variable types of bedrock gold mineralisation which have yet to be delineated.
Typical Coarse gold from Coro
Heavy-mineral sampling results have identified zinc-rich chromites in the Nsork area, similar to those found in the diamondiferous lamproite dyke swarm at Mitzic (Gabon), 50km to the southeast. Remote sensing and limited outcrop mapping show the dyke swarm trending into the southeastern corner of Rio Muni. Furthermore, exploration in Gabon for both gold and diamonds has identified trails of indicator minerals and diamond right up to the southern borders of Rio Muni. Detailed exploration work is required in the Nsork area to establish the primary sources of the diamonds.