- © 2017 Society of Economic Geologists.
West Africa, with presently an approximate 10,000-metric ton (t) gold endowment, is one of the world’s great gold provinces and the largest Paleoproterozoic gold-producing region. The gold resources are concentrated within the 2250 to 2000 Ma greenstone belts of the Man-Leo shield, forming the southern part of the West Africa craton. Most of the major orebodies are best classified as orogenic gold deposit types, although there are paleoplacer and porphyry-skarn deposits within some of the greenstone belts, and perhaps local intrusion-related gold systems. The gold-hosting, mainly greenschist metamorphic facies greenstone belts are dominated by tholeiitic volcanic rocks, with clastic and chemical sediments filling adjacent subbasins. The Paleoproterozoic sequences formed in what was likely a rift or series of rifts in a Precambrian cratonic block; it is not clear whether significant Late Archean lithospheric roots occur below these Paleoproterozoic arcs that formed in the resulting ocean subbasins. Although diachronous across West Africa, the Eburnean orogeny is typically indicated to have been initiated at ca. 2130 Ma, with closure of the subbasins, amalgamation of the Paleoproterozoic arcs, and their accretion back to the continental margin of Archean rocks. Compressional tectonics took place for about 25 to 30 m.y., with widespread crustal thickening along orogen-parallel, commonly NE-trending, first-order thrust fault systems. This was followed by more than 100 m.y. of transcurrent tectonism and associated exhumation; gold ores mainly formed late during the Eburnean deformation.
The most productive orogenic gold deposits are located in greenstone belts in Ghana, Mali, Senegal, Burkina Faso, Cote d’Ivoire (or Ivory Coast), and Guinea. Yielding about 200 t Au per year, West African production has exceeded that of the Yilgarn craton of Western Australia since 2007 and, if grouped together, current annual production from these relatively small countries would only be surpassed by China, Australia, and Russia, demonstrating the global significance of the gold mineralization within this region. With an endowment of >2,500 t Au, the Obuasi deposit represents the largest single Precambrian gold deposit discovered in the world to date, exclusive of the Witwatersrand paleoplacers. More than 25 of the deposits in the Man-Leo shield contain resources in excess of 100 t Au. The gold-bearing brittle-ductile quartz veins, stockworks, breccias, and disseminated orebodies are located adjacent to major faults, typically in areas of second-order shears, large dilational jogs, regional fold systems, and rheological contrast. Mineralogy, alteration, structural geology, stable isotope geochemistry, and P-T conditions of gold deposition are typical of those observed in most orogenic gold provinces. Well-constrained absolute ages for much of the orogenic gold formation remain lacking, but there seems to be a temporal association with the 2100 Ma onset of transpression/strike-slip and exhumation. However, ore deposition was likely spread over at least 130 m.y. throughout the shield, with some ores as old as ca. 2160 Ma (e.g., Wassa) and some no older than ca. 2030 Ma (e.g., Damang).
It is unclear as to exactly why such a large gold endowment is present in West Africa and, in particular, the Birimian sequences of the Man-Leo shield, although it is likely the consequence of a combination of key regional factors. Relative to other Paleoproterozoic orogens, the abundance of carbonaceous oceanic sediments that overly the Birimian basalts may represent an exceptionally fertile source of both fluid and metal. The fact that many of the orogenic gold deposits formed from ore fluids with at least 70 to 80 mol % CO2, quite different from deposits elsewhere in the world where H2O >CO2, hints at an atypically large volume of carbon being released during metamorphic devolatilization. A series of closing and subsequently inverting basins, which were important sinks for the carbonaceous material, also resulted in a favorable structural architecture with development of many orogen-parallel, deep-crustal shear zones. Two hundred million years of orogenesis represented a lengthy period of deformation, including strike-slip reactivation events along the older thrusts that may have allowed for diachronous gold-forming events throughout the Eburnean.