Loulo Gold Field - Gara, Yalea, Gounkoto
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The Loulo Gold Field, which includes the Gara, Yalea and Gounkoto deposits, is located in westen Mali, <2 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. Yalea is 6.5 km south of Gara, and Gounkoto is 16 km south of Yalea (#Location: 13° 5' 10"N, 11° 24' 59"W [Gara], 13° 1' 42"N, 11° 23' 34"W [Yalea], 12° 52' 18"N, 11° 23' 19"W [Gounkoto]).
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 and Yatela These deposits are all linked to the Senegal-Mali shear zone, a major crustal scale lineament. The Tabakoto and Segala deposit are part of a parallel belt ~20 km to the east. 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 the regional-scale, curvilinear, Main Transcurrent shear zone, which trends from NE-SW in the SW, to north-south in the north (Bassot, 1987).
The Mako Series 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 Belt, a ~20 km wide, north-south trending 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 Belt, 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, separating it from the Kofi sedimentary basin in Mali, which contains the Kofi Series, a sequence of sandstones, argillites and platform carbonates, intruded by syntectonic, S-type, peraluminous biotite-bearing granites (similar in composition to the Saraya batholith). Detrital sedimentary rocks of the Kofi Series 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 and 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 (Lawrence et al., 2013).
The geology and structure of the Loulo Gold Field is dominated by the terrane-bounding Senegal-Mali shear zone which influences the course of the Falé river. Rocks of the Falémé Belt and a small portion of the Dialé-Daléma Series lie to the west of the shear zone.
The sedimentary rocks of the Dialé-Daléma basin outcrop in the northwestern part of the gold field, where they are intruded by the Moussala syenogranite, which is strongly kaolinised, and represents the northern extension of the Saraya batholith (Pons et al., 1992).
The Falémé Belt is predominantly composed of I-type, calc-alkaline and metaluminous granitoids which share similar compositions to the "belt-type" plutons of the Ghanaian Birimian (e.g., Leube et al., 1990). In the northern part of the area, there are several individual granitoids of various compositions and sizes, composed of fine to medium grained hornblende-bearing granodiorites, biotite + hornblende-bearing microdiorites and rare aphanitic clinopyroxene rich tonalites. Along the Senegal-Mali shear zone, these rocks have been strongly deformed and sodic altered to albitites. The plutons to the south have similar primary compositions (Ndiaye et al., 1997; Hirdes and Davis, 2002). Overall, the Falémé granitoids have volcanic arc geochemical signatures (Schwartz and Melcher, 2004; Lawrence, 2010), similar to the rocks of the Mako Series (Pawlig et al., 2006; Dioh et al., 2006).
The Falémé Belt volcano-sedimentary rocks comprise siliciclastic rocks and marbles, with undifferentiated intercalated volcaniclastic sedimentary rocks in the north, with local minor pillowed and massive basaltic-andesite lavas. Ferrugineous carbonate breccia units outcrop along the eastern contact of the belt, whilst contact metasomatic skarns of the Falémé iron ore district (Schwartz and Melcher, 2004) are found on the margins of some of the plutons of the belt. These skarn altered iron deposits (28 individual bodies) are hosted by dolomites, and collectively total 750 Mt of ore reserves, comprising primary magnetite and supergene hematite (ArcelorMittal 2007 press release). The primary mineralisation comprises magnetite with associated pyrite + chalcopyrite + pyrrhotite, and an alteration assemblage of albite + clinopyroxene + biotite ± garnet ± serpentine ± amphibole ± calcite (Wade, 1985; Milési et al., 1989; Bassot, 1997; Schwartz and Melcher, 2004). See the separate Faleme record for more details.
The Kofi Series outcrop to the east of the Senegal-Mali shear zone, generally striking NNE with dips of 60° E to vertical. They are interpreted to represent a fore-arc environment, with a sequence of shelf carbonates and calcareous clastic rocks to the west, and deeper water argillites and turbiditic sedimentary rocks towards the east. Arenaceous facies include tourmalinised quartzwackes, feldspathic sandstones and calcareous greywackes, with argillite intercalations. The sequence, which has been metamorphosed to greenschist facies and subjected to series of D1 thrusts causing stacking and repetition of the strata during early Birimian orogenesis, is cut by late crosscutting tholeiitic dolerite dykes (Lawrence et al., 2013).
In the eastern parts of the Kédougou- Kéniéba inlier, D1 compression produced NNE to NE trending F1 recumbent and overturned folds, NW-verging thrusts and axial plane schistosity (Milési et al., 1989; 1992; Ledru et al., 1991; Dabo and Aïfa, 2010). D2 deformation is superimposed upon, or refolds, F1 structures, to produce upright F2 folds, as well as north-south brittle-ductile faulting, including the Senegal-Mali shear zone, which was initiated by sinistral transpressional. The major Senegal-Mali shear zone provided the main regional conduit for hydrothermal fluids, with gold ores in second order, NNE-striking sinistral shear subsidiary structures (P-shear geometry), within the Kofi Series along its eastern margin. To the east of the Senegal-Mali shear zone, in Mali, a third stage of deformation, D3, is associated with transtensional movement along the D2 structures and the development of sinistral, north and NW trending, third and fourth order Riedel shears. Gold mineralisation in the Loulo Gold Field is largely linked to this late D3 transtensional event (Lawrence et al., 2013).
To the west of the Senegal-Mali shear zone, in Senegal, NW-SE and SW-NE trending post D2 dextral shear bands, and associated Z-shaped folds, are mapped within the Kofi sedimentary rocks (Dabo and Aïfa (2011). The relationship of this event to the D3 transtension in Mali is unclear (Lawrence et al., 2013).
Under construction - to be completed soon
The most recent source geological information used to prepare this summary was dated: 2013.
This description is a summary from published sources, the chief of which are listed below.
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References to this deposit in the PGC Literature Collection:
Lawrence D M, Treloar P J, Rankin A H, Boyce A and Harbidge P, 2013 - A Fluid Inclusion and Stable Isotope Study at the Loulo Mining District, Mali, West Africa: Implications for Multifluid Sources in the Generation of Orogenic Gold Deposits : in Econ. Geol. v.108 pp. 229-257|
Lawrence, D.M., Treloar, P.J., Rankin, A.H., Harbidge, P. and Holliday, J., 2013 - The Geology and Mineralogy of the Loulo Mining District, Mali, West Africa: Evidence for Two Distinct Styles of Orogenic Gold Mineralization: in Econ. Geol. v.108, pp. 199-227.|
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