Abstract Obuasi, with a total mineral resource plus past production of 70 Moz, is the largest gold deposit in West Africa, and one of the largest in the world. It is hosted by ~2135 Ma siliciclastic rocks of the Eburnean Kumasi Basin, which were obliquely shortened along an inverted boundary with the older Eoeburnean Ashanti belt to the east. Greenschist facies metamorphism was coeval with mineralization and related alteration at ~2095 Ma. The steeply dipping, ENE-plunging lodes extend over an 8-km strike length and to depths of >2.5 km. They include paragenetically complex gold-rich quartz veins surrounded by refractory auriferous arsenopyrite and closely associated carbonate-muscovite alteration halos in deformed carbonaceous phyllites and subordinate metaigneous host rocks. Gold and arsenic were initially precipitated during deformation-assisted interaction with reduced host rocks at ~350°C and 100 to 200 MPa. The mineralizing fluids were derived primarily from deeper, As-rich metasedimentary sources by basinal fluid expulsion and metamorphic devolatilization triggered by inversion and shortening, followed by transpression. Continued fluid injection during and after the metamorphic peak produced changes in gold fineness, sulfide assemblages, repeated dissolution (stylolites) and reprecipitation of mineralized veins, and a change from early deformed shear-related, sulfide-rich lodes to later quartz-rich lodes that plunge down or across the axes of younger transpressional folds. Channelized fluid flow due to reactivation of basin-edge transfer structures, and/or irregularly distributed gold source rocks, may explain the variation in gold endowment along the former basin boundary.

Abstract Paleoproterozoic (Rhyacian) gold deposits of the Loulo district in western Mali contain >17 million ounces (Moz) Au and form part of the second most highly endowed region within West Africa. The deposits are located within siliciclastic, marble, and evaporitic rocks of the ca. 2110 Ma greenschist facies Kofi series, which were folded and inverted between ca. 2100 and 2070 Ma, prior to gold mineralization. Deposits at Yalea and Gounkoto are located along discontinuous, low-displacement, albite- and carbonate-altered shear zones, whereas Gara is confined to a tourmaline-altered quartz sandstone unit. Lodes typically plunge gently to moderately, reflecting the attitude of folds in the adjacent rocks and bends in the host shear zones, both of which influenced their location. Gold mineralization in the Loulo district was broadly synchronous with emplacement of the Falémé batholith and associated Fe skarn mineralization, which intrude and overprint the western margin of the Kofi series, respectively. However, hydrothermal fluids generated during metamorphic devolatilization of the Kofi series rocks appear responsible for gold mineralization, albeit within a district-wide thermal gradient associated with emplacement of the Falémé batholith. The regional-scale Senegal-Mali shear zone, commonly cited as an important control on the location of gold deposits in western Mali, is absent in the Loulo district.

Abstract The Kibali district in the Democratic Republic of Congo hosts the large Karagba-Chaffeur-Durba (KCD) deposit and smaller satellite deposits that together contained 20 million ounces (Moz) of gold when mining recommenced in 2013. An additional 3 Moz of gold was probably mined from the district before 2013. Gold deposits in the Kibali district are located along the KZ trend, a series of folds, contractional shear zones, and altered lithostratigraphic units that coincide with the margin of an earlier 2630 to 2625 Ma intraorogenic basin within the Neoarchean Moto belt. Fluids first responsible for barren carbonate-quartz-sericite alteration, and later for siderite and/or ankerite (±quartz, magnetite, pyrite, and/or chlorite) alteration with associated auriferous pyrite ± rare arsenopyrite veinlets, infiltrated and replaced the siliciclastic, banded iron formation (BIF), and chert host rocks via fold axes, shear zones, and reactive BIF horizons. The complex shape and gentle northeast plunge of the lodes across the Kibali district reflect the shape and plunge of coincident folds that formed during early barren alteration. Many other folded BIF horizons across the wider Moto belt remain barren or only weakly mineralized, suggesting deep extensional structures that may have developed in the vicinity of the KZ trend during basin opening and prior to gold mineralization, were important fluid pathways during later contractional deformation and mineralization.

Abstract Paleoproterozoic terranes of the Man-Leo Shield in the southern part of the West African craton host one of the world’s largest gold provinces with an overall endowment >10,000 metric tons (t). Although gold deposition commenced by ca. 2170 Ma, most deposits formed later, either during the inversion and metamorphism of intraorogenic sedimentary basins between ca. 2110 and 2095 Ma, or during later transcurrent deformation and associated widespread high K plutonism following docking of Archean and Paleoproterozoic domains within the craton at ca. 2095 Ma. Deposits formed between ca. 2110 and 2095 Ma include those with free gold in quartz veins and refractory gold in arsenopyrite and/or pyrite, and are associated with halos of carbonate, sericite, chlorite, and albite alteration. Most are located in bends and intersections between shear zones, minor faults, folds, and entrained blocks of relatively reactive igneous rock. Conglomerate-hosted gold deposits of the Tarkwa district formed early in the 15-m.y.-long period. Gold deposits that formed subsequently between ca. 2095 and 2060 Ma have a wider variety of styles, geologic settings, and metal assemblages. District-scale albite, carbonate, and tourmaline alteration, hydrothermal breccias, and a close relationship to high K granitoids characterize some of these deposits, whereas others are more typical orogenic gold deposits that are similar to those formed earlier during the craton evolution.