Description: The Gaoua mining district, underlain by Paleoproterozoic volcanic and intrusive rocks of the Boromo-Goren greenstone belt in southern Burkina Faso, is a rare case where economic copper and gold resources occur together in the Birimian terrane of West Africa. The main country rock lithologies consist of flows and volcaniclastic sediments of basaltic to andesitic compositions. These are intruded by subvolcanic diorite and quartz diorite stocks and dikes, granophyre dikes and, less commonly, gabbroic dikes. The stocks are porphyritic in texture, locally brecciated, and characterized by development of a carbonate, anhydrite, and quartz stockwork system. Structural observations, detailed petrography, and sulfide geochemistry reveal that copper and gold in the stockworks and surrounding altered rocks cannot be associated with the same mineralizing event. Copper mineralization, mainly in the form of chalcopyrite, is typically observed in hydrothermal breccia and is associated with early arsenic-poor and gold-free pyrite. The geochemistry of the plutonic rocks and associated basalts and andesites implies formation in an arc setting. Textural evidence indicates that gold mineralization is closely tied to a second generation of sulfides, with the later pyrite hosting visible gold and minor chalcopyrite that clearly postdates the copper ore. Analyses of these pyrites by LA-ICP-MS reveal compositions rich in Ag, As, Au, Bi, Cu, and Pb. Pyrite with this signature was only observed in local shear and fracture zones that crosscut the host rocks and copper-rich breccia. Therefore, our data indicate the strongly deformed and metamorphosed porphyry copper mineralization in the Gaoua district is overprinted by orogenic gold mineralization. Formation of the early porphyry copper deposit took place at 2165 ± 24 Ma based upon Re-Os dating of the early pyrite; the age of the younger gold event, which is similar in style to that described throughout the West African craton, is not known but may be related to one of the lode gold episodes that are widespread through the West African craton at ~2150 and ~2100 to 2040 Ma. The Gaoua copper deposit, one of very few Precambrian porphyry copper deposits known in West Africa, formed in a volcanic-arc setting that predated the Eburnean orogeny in the Boromo-Goren greenstone belt.

Description: The Archean-Proterozoic craton of West Africa hosts numerous gold deposits, which are spatially and temporally related to the Eburnean orogeny that took place between 2250 and 1980 Ma, and included multiple deformation events. The majority of these gold deposits are located along shear zones. The structural history is relatively well established for most gold deposits, but absolute timing of the mineralization is commonly lacking. Five deposits hosted in the Baoulé-Mossi domain, located in the southwestern part of the West African craton, were studied to better constrain timing of gold events: Nassara and Kiaka in Burkina Faso and Wassa, Damang, and Obuasi in southern Ghana. Gold mineralization was structurally characterized at each deposit, and dated by the Re-Os method on pyrite, arsenopyrite, and pyrrhotite grains that were coeval with the gold deposition. Combined structural and Re-Os geochronological constraints allow two groups of gold deposits to be distinguished. Early orogenic gold formed during the Eoeburnean orogeny, i.e., between 2190 and 2125 Ma, as represented by Kiaka1 at 2157 ± 24 Ma, and Wassa1 at 2164 ± 14 Ma. These early gold occurrences are expressed as early disseminated gold enrichment (1–3 g/t Au) at Kiaka, and gold hosted in folded and boudinaged quartz veins that are transposed along the primary S 1 foliation at Wassa. These Re-Os ages on the early sulfides at Wassa and Kiaka are significant because they definitively provide the first direct age constraints on gold-only Eoeburnean mineralization in the Birimian of West Africa. Late orogenic gold, hosted by brittle structures and with higher gold grades (up to 60g/t at Kiaka), formed during late Eburnean deformation (D 3 –D 5 events) between 2120 and 2000 Ma, as represented by the Re-Os dating of these ores at Wassa at 2055 ± 18 Ma and Obuasi at 2045 ± 40 Ma. Similar-style ores dated at Nassara at 2131 ± 99 Ma and Damang at 2080 ± 135 Ma, but both with large uncertainties, and an undated younger event at Kiaka, are also suggested to be part of this later mineralizing episode. The high uncertainties on the Nassara and Damang ages may be directly linked to the low rhenium and osmium contents of the studied samples. These results highlight the polyphase character of the widespread gold mineralization in the West African craton. Although the late stages of the Eburnean orogeny constitute a prolific period for the formation of high-grade gold mineralization, identification of less well-studied early-stage gold deposits, which can also contain large quantities of gold, is critical for mineral exploration in the West African craton.

Description: 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 % CO 2 , quite different from deposits elsewhere in the world where H 2 O 〉CO 2 , 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.