Sadiola Hill: A World-Class Carbonate-Hosted Gold Deposit in Mali, West Africa

Abstract

International audienceThe ~8-Moz Sadiola Hill gold deposit is located in the Kédougou-Kénieba inlier, a window of deformed ca. 2200 to 2050 Ma rocks that crop out in eastern Senegal and western Mali. The geology of the inlier differs from other Paleoproterozoic granite-greenstone belts and sedimentary basins by the abundance of carbonate rocks. The Sadiola Hill gold deposit occurs within 3 km of the Senegal-Mali shear zone, and country rocks in the region have undergone polycyclic deformation. The lithostratigraphy of the open pit consists of impure limestones overlain by younger sequences of detrital sedimentary rocks, which include arenite, wacke, and siltstone. The sedimentary rocks have been subjected to regional greenschist-facies metamorphism and have been intruded by multiple generations of synkinematic, calc-alkaline stocks and dikes. The bulk of the ore is hosted within a N-S- to NNW-trending, 10- to 50-m-wide, brittle-ductile dilational shear zone defined as the Sadiola shear zone. Gold lodes also occur along an array of steep NNE-trending shear zones. Geometric and kinematic analyses indicate that the ore-hosting structures were undergoing sinistral displacement at the time of mineralization, locally defined as the D3s NNW-SSE-shortening event. Hydrothermal alteration is polyphase and includes an early high-temperature calc-silicate phase (i.e., porphyroblastic growth of actinolite-tremolite) followed by a potassic phase (i.e., biotite-calcite-quartz ± K-feldspar-tourmaline-actinolite) that was synchronous with ore mineral deposition. Paragenetic studies reveal a multistage ore development that includes an As-rich sulfide stage, followed by an Au-Sb stage. The ore is associated with a metal enrichment suite of Au-As-Sb ± Cu-Fe-W-Mo-Ag-Bi-Zn-Pb-Te. The Sadiola Hill deposit shares a similar relative timing and structural setting to that of other world-class orogenic gold systems in the West African craton (e.g., Ashanti, Loulo), with gold deposition occurring during a period of transcurrent tectonics, after the cessation of region-wide compressional deformation. The high-temperature paragenesis at Sadiola Hill is atypical of gold mineralization in the craton and indicates that the late Eburnean tectono-magmatic activity between ca. 2080 and 2060 Ma played an important role in the dynamics of hydrothermal fluid circulation along the Senegal-Mali shear zone

Evidence for Two Stages of Mineralization in West Africa’s Largest Gold Deposit: Obuasi, Ghana

“…The Birimian terranes are unusually well endowed and contain a number of world-class deposits (>6 Moz as defined by Schodde andHronsky, 2006), including Sadiola, Morilla, Syama, Siguiri, Damang, Prestea, andTarkwa (e.g., McFarlane et al, 2011;Perrouty et al, 2012 ;Masurel et al, 2017;Lebrun et al, 2017). It has been demonstrated in this study that the Obuasi gold deposit formed mainly during two mineralizing events.…”

Chemical and boron isotope composition of tourmaline from the Kiaka orogenic gold deposit (Burkina Faso, West African Craton) as a proxy for ore-forming processes

“…Masurel et al submitted). Large transpressional shear zones cross-cut the Baoulé-Mossi domain contemporaneously with the emplacement of high-K calc-alkaline to shoshonitic granitic plutons (Feybesse and Milési 1994;Egal et al 2002;Parra-Avila et al 2019) in syn-to post-collisional settings (Eglinger et al 2017;Masurel et al 2017;Thébaud et al 2020).…”

“…2.14 and 2.13 Ga (Lawrence et al 2017;Le Mignot et al 2017). They include the ~5.8 Moz Kiaka gold deposit, hosted by volcano-sedimentary rocks which were deformed and metamorphosed under amphibolite facies conditions (Fontaine et al 2017 (Eglinger et al 2017;Masurel et al 2017).…”

Stable C, O, and S Isotope Record of Magmatic-Hydrothermal Interactions Between the Falémé Fe Skarn and the Loulo Au Systems in Western Mali

“…Later small-scale (< 1 km) folding and development of an associated steeply dipping axial planar cleavage, and Au mineralization occurred during D2 sinistral transcurrent deformation between ca. 2085 and 2060 Ma (Hirdes and Davis, 2002;Schwartz and Melcher, 2004;Lawrence et al, 2013a;Masurel et al, 2017a;Masurel et al, 2017b). The Falémé Batholith, and associated outlying plutons and dikes, intruded the Kofi Series between 2084 ± 8 Ma and 2075 ± 6 Ma, after inversion of the Kofi Series during D1 contraction.…”

“…The ages of the youngest detrital zircon populations indicate that the sedimentary basins developed from 2135 to 2095 Ma (Davis et al, 1994;Oberthür et al, 1998;Hirdes and Davis, 2002;Vidal et al, 2009;Lebrun et al, 2016). Emplacement of younger granitoid plutons (Parra-Avila et al, 2018;Masurel et al, 2017a), further contractional deformation and metamorphism, late strike-slip deformation, and widespread Au mineralization occurred during the subsequent Eburnean orogeny (Oberthür et al, 1998;McFarlane et al, 2011;Parra-Avila et al, 2015;Fontaine et al, 2017;Fougerouse et al, 2017;Masurel et al, 2017b). Greenschist facies mineral assemblages dominate in most Paleoproterozoic rocks across West Africa, but amphibolite and granulite facies assemblages are present locally within both the Eoeburnean belts and Eburnean sedimentary basins (John et al, 1999;White et al, 2014;MacFarlane et al, 2019).…”