The Alamoutala Carbonate-Hosted Gold Deposit, Kédougou-Kénieba Inlier, West Africa

Abstract

The Alamoutala gold deposit is located in the Kedougou-Kenieba inlier, a window of Paleoproterozoic rocks that crop out in eastern Senegal and western Mali. The deposit is part of the ~3-Moz Au Yatela district and produced 308,400 oz Au between 2002 and 2012. Country rocks in the Alamoutala open pit consist of carbonate rocks, arenites, wackes, and siltstones. The sedimentary rocks have been subjected to polycyclic deformation and regional greenschist-facies metamorphism. Synkinematic, calc-alkaline, intermediate quartz-feldspar porphyry stocks intruded the metasedimentary rocks at 2083 ± 7 Ma (U-Pb zircon). Amphibolite-facies contact metamorphism of carbonate rocks resulted in the formation of a high-temperature-low-pressure magnetite-bearing skarn. Field relationships and microtextural data, however, indicate that economic gold mineralization is shear hosted and occurred during retrograde contact metamorphism. Geometric and kinematic analyses indicate that the ore-hosting structures were undergoing sinistral-reverse displacement at the time of mineralization. Ore minerals deposited synchronous with a potassic hydrothermal alteration assemblage defined as biotite + calcite + pyrite ± K-feldspar, actinolite-tremolite, quartz, muscovite, and tourmaline. Petrographic studies reveal that ore-related sulfides largely consist of pyrite with lesser pyrrhotite and chalcopyrite. The ore also contains accessory Ag-Sb-As-Hg-Bi-Te-Mo-W-Zn-Pb-Ni-Co-U-bearing mineral species. The Alamoutala deposit represents two distinct hydrothermal systems in which a locally developed magnetite-bearing skarn was overprinted by a region-wide orogenic gold event in the late Eburnean.