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 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.

Abstract The West African craton is fringed along its western side by a 3000 km long orogenic belt subdivided into three separate orogens: the Bassaride (Pan-African I orogeny), Rokelide (Pan-African II orogeny) and Mauritanide (Hercynian orogeny) thrust belts. The Bassarides are cut to the north by the Mauritanides and to the south by the Rokelides but parts of this Bassaride belt are incorporated in the other two younger belts. This review presents the main geological, geophysical and geochronological results from the western side of the West African craton, collected over the past 90 years, concentrating on those from the last 15 years. Former geological investigations underlined the thin-skinned structure model within these thrust belts, whereas the geophysical results gave prominence to the major importance of block faulting resulting from the Pan-African I orogeny and its strong influence on the subsequent orogenic belt features. The geochronological data allow us to distinguish major tectonothermal events related to the Pan-African I (660–650 Ma), Pan-African II (550–530 Ma) and Hercynian (330–300 Ma and 280–270 Ma) orogenies. However, they also reveal five other tectonothermal events (at 1200–1000, 750–700, 600–580, 510–480 and 450–380 Ma), which are still very poorly understood. The 1200–1000 Ma tectonothermal event recently revealed in the northern Mauritanides may correspond to a remanent orogenic belt segment that witnessed the Grenvillian orogeny.

Abstract The Massawa gold project is situated on the Senegalese side of the highly prospective/productive Palaeo-Proterozoic (Birimian) Kédougou–Kéniéba inlier, which hosts several world-class orogenic gold deposits/districts in western Mali (e.g. Loulo and Sadiola). The Massawa ore body has a strike length of at least 4 km and a current resource of 3.61 Moz at a grade of 2.8 g t −1 . The ore body is structurally controlled and located within a package of low-grade regionally metamorphosed volcaniclastic sediments (agglomerates, tuffs and ash-tuffs), quartz–feldspar and lithic wackes, carbonaceous shales, hydrothermal breccias, and gabbro and porphyry sills. These rocks have undergone pervasive silica alteration followed by a sericite–ankerite–chlorite alteration event related to mineralization. Two major styles of mineralization are recognized at Massawa from field and laboratory studies. The first stage of sulphide–Au mineralization is associated with disseminated arsenopyrite–pyrite, which follows shear zones in the sedimentary and volcano-sedimentary host rocks. The second stage consists of quartz–stibnite±tetrahedrite veining distinguished by coarse visible gold and represents a late stage overprint on the primary mineralization. The two stages of gold mineralization are separated by a phase of quartz–molybdenite veining. A distinctive base metal trace assemblage is linked to stibnite formation including multiple Sb phases such as chalcostibite, zinkenite, roshchinite, aurostibite, jamesonite and robinsonite. Secondary ion mass spectroscopy-based gold deportment data indicate that up to 90% of stage 1 gold is held as a solid solution within either arsenopyrite or arsenian pyrite. Stable isotope data yield δ 34 S sulphide values of between 0 and 4.1‰ and δ 18 O H2O values of 5.5–10.9‰ for all stages of mineralization, suggesting a magmatic fluid influence. This is consistent with field data that suggest that mineralization is synchronous with emplacement of a sequence of concordant felsic sheets. That mineralization occurred at shallow (<6 km) depths is suggested both by the presence of stibnite and by fluid inclusion studies. Low-temperature (homogenization temperatures between 150 °C and 230 °C) H 2 O–NaCl fluids (<6 wt% NaCl equiv.) and coeval CO 2 –CH 4 inclusions, observed in both phases of mineralization, indicate trapping conditions of 220–315 °C at 1–1.65 kbar. A combination of phase petrology, fluid inclusion and stable isotope data suggests deposition of gold from low-salinity, magmatic fluids, most probably released from felsic rocks similar to those emplaced into the Massawa sequence during mineralization.