The Loulo Gold Field, which includes the Gara, Yalea and Gounkoto deposits, is located in westen Mali, <10 km east of the border with Senegal (marked by the Falé river). The district is 350 km west of Bamako and 220 km south of Kayes.
The gold field is located within the West Mali gold belt, which stretches for 180 km north-south along the Senegal border. Operational mines include Gara, Yalea, Gounkoto, Sadiola, Yatela, Tabakoto and Segala. These deposits are all linked to the Senegal-Mali shear zone, a major crustal scale lineament. The West Mali gold belt contains total resources (as known in 2012) of more than 1100 of gold.
The district lies on the eastern side of the Palaeoproterozoic Kédougou-Kéniéba Inlier, which covers eastern Senegal and western Mali, representing the westernmost exposure of the 2050 to 2200 Ma Birimian Supergroup of the West African Craton. This inlier is bounded on its western margin by the Hercynian Mauritanide orogenic belt (Villeneuve, 2008) and is surrounded on all sides by unconformably overlying flat-lying sandstones of the Neoproterozoic Taoudeni intracratonic basin (Villeneuve and Cornée, 1994).
The Birimian domain of the Kédougou-Kéniéba inlier has been subdivided into a western granite-greenstone belt (the Mako Series) and an eastern sedimentary basin containing the Dialé-Daléma Series, separated by a regional-scale Main Transcurrent zone shear zone (Bassot, 1987). The Mako belt is composed of tholeiitic basalt and andesite lavas, with intercalated volcanic agglomerates (between 2160 and 2197 Ma; Boher, 1991; Dia et al., 1997) and banded tuffs (Gueye et al., 2008), Calc-alkaline volcaniclastic units and derived sedimentary rocks occur along its eastern margin. The Mako Series rocks are intruded by a complex of pre- and syn-tectonic mafic and felsic plutons (Dia et al., 1997; Dioh et al., 2006; Gueye et al., 2008), emplaced between 2210 and 2070 Ma (Bassot and Caen-Vachette; 1984; Dia, 1988; Dia et al., 1997; Hirdes and Davis, 2002; Gueye et al., 2007). The Dialé-Daléma basin contains folded sandstones and siltstones, interbedded with calc-alkaline ash- and lapilli-tuffs (Bassot, 1987; Hirdes and Davis, 2002), and is generally considered to be younger than the Mako Series (Bassot, 1987; Dia, 1988; Abouchami et al., 1990). Siliciclastic rocks within the Dialé-Daléma sequence have detrital zircon ages ranging between 2096±8 and 2165±1 Ma (Milési et al., 1989; Hirdes and Davis, 2002). The Dialé-Daléma Series is centrally intruded by the 2079±Ma (Hirdes and Davis, 2002) Saraya quartz monzonite-granite (Pons et al., 1992).
The Falémé Series, a calc-alkaline volcanic-plutonic complex in the eastern part of the Kédougou-Kéniéba inlier, is considered to be separate from the Dialé-Daléma Series (Hirdes and Davis, 2002; Lawrence, 2010). It comprises of a sequence of carbonate-rich sedimentary rocks, minor basalts and andesites, rare rhyolites, and syntectonic granitoids (Ndiaye et al., 1997; Hirdes and Davis, 2002). In the southern parts of the Falémé Series, a rhyolite flow has been dated at 2099±4 Ma, with the surrounding Boboti and South Falémé plutons dated at 2080±1 and 2082±1 Ma, respectively (Hirdes and Davis, 2002). The eastern margin of the Falémé belt is defined by the Senegal-Mali shear zone, which separates it from the Kofi sedimentary basin in Mali, which contains sandstones, argillites and platform carbonates intruded by syntectonic, S-type, peraluminous biotite-bearing granites (similar in composition to the Saraya batholith).
Detrital sedimentary rocks at Loulo are between 2093±7 and 2125±27 Ma (Boher et al., 1992), cut by the 2045±27 Ma Gamaye leucogranite, in the south of the sedimentary basin (Bassot and Caen-Vachette, 1984). Birimian rocks of the Kédougou-Kéniéba inlier are polycyclic deformed, with a metamorphic history related to the Eburnean orogenic event (Ledru et al., 1991; Dabo and Aïfa, 2010; 2011). The initial deformation (D1) is compressive, linked to the initial accretion of the Birimian terranes (Milési et al., 1989, 1992; Ledru et al., 1991). D2-D3 deformation is associated with transcurrent movement and the formation of the regional-scale, north-south, shear zones (Main Transcurrent zone and the Senegal-Mali shear zone) and subsidiary structures (Bassot and Dommanget et al., 1986; Ledru et al., 1991). Transcurrent deformation is synchronous with gold mineralisation and the emplacement of several calc-alkaline granitoids (e.g., the Saraya, Yatia, Gamaye peraluminous granites, the Falémé granitoids, and the youngest generation of subcircular Mako plutons; Ledru et al., 1989; Pons et al., 1992; Gueye et al., 2008). Regional greenschist facies metamorphism is associated with both compressive and transcurrent phases of deformation.
(Source: Porter Geocunsultancy, http://www.portergeo.com.au/, 2013)
