Chapter 7: Paleoproterozoic Gold Deposits of the Loulo District, Western Mali

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.