Summary:
The Loulo and Gounkoto deposits can be classified as a typical shear hosted Birimian style mesothermal gold deposits.
The Loulo Mine comprises two main deposits, Gara and Yalea, and multiple satellites deposits.
Gara
Gara is located six kilometres NNW of the Yalea deposit and extends for 2.5 km within a tourmaline sandstone/greywacke unit, which outcrops over 800 m as QT forming low (10 m scale) topographic highs. Mineralisation averages 9 m width and 1.1 km depth. The mineralisation is hosted by a stockwork of quartz- carbonate-pyrite-tourmaline veinlets and associated disseminated pyrite, both of which are confined to a single bed of QT generally less than 30 m thick. The deposit lithologies from west to east are: limestone and argillaceous quartzite (SQR) in the hanging wall (HW), mineralised QT ranging from 5 m to 20 m thick (on average 15 m), and a coarse to medium grained greywacke unit in the footwall (FW). The sedimentary package is also crosscut by three un-mineralised late east-west trending dolerite sub-horizontal dykes that dip shallowly from north to south.
The geometry of the deposit is dominated by the strike slip shearing and the development of associated conjugate sets of antithetic structures. This shearing has resulted in folding, fracturing, brecciation, and development of a quartz vein stockwork within the QT which behaved in a more brittle manner during deformation. The deposit spans the hinge of a broad open fold with a gently plunging north-south trending axis. The upper limb of this fold dips moderately to steeply west, whereas the lower limb dips moderately to steeply east. A second generation of younger, smaller scale, SW plunging, and distinctly non-cylindrical folds are developed in the upper part of the deposit. Vein orientations within the limbs and hinges of the folds are similar, and poles to the veins do not appear to scatter around a great circle centred on the fold axis at Gara (30° towards 213°). This implies that veining occurred late, after the folding of the sandstone host. However the overall gentle SSW plunge of the higher-grade mineralisation shoots in the deposit is close to coaxial with the fold, implying the fold geometries had some influence on the distribution of the mineralised veins. Gara is characterised by intense tourmaline alteration. This tourmalinisation appears gradational from the hanging wall contact to the footwall, varying from strong to weak. It is also associated with carbonate and silica veining. Chloritisation is mainly observed in the hanging wall. There is also a correlation between the intensity of tourmalinisation, sulphide intensity, silica-carbonate veins, brecciation, and gold mineralisation.
Gold mineralisation is stratabound and hosted predominantly within the QT sandstone unit, which is enveloped within FW greywackes and HW schistose sandstone. Higher gold grades typically occur in areas where the veining is most intense and the range of vein orientation, more complex and are mostly in association with carbonate-pyrite. The sulphide assemblage predominantly consists of disseminated auriferous pyrite with minor chalcopyrite, scheelite, and nickeliferous sulphides.
Yalea
Yalea is located 5.7 km SE of Gara, in a 2.5 km long, north-south striking, steeply dipping shear zone with pervasive albite altered sandstone and breccias that are partly overprinted by zones of carbonate alteration. Mineralisation averages 11 m width over 1.3 km depth. Host lithologies at Yalea (from west to east) consist of grey quartzite (black sandstone) in the hanging wall, with tectonic breccias in the north. Immediately above the main body of mineralisation is a thin (0 m to 5 m) sequence of banded schistose greyish limestone, with alternating white and grey calcitic layers and dark grey to black phyllite units. The main mineralised body is a hydrothermally brecciated argillaceous pink quartzite that becomes more argillaceous (and less altered) towards the footwall. A higher grade ‘Purple Patch’ zone is observed in a dilatational strain transfer zone formed as the dip of the mineralised package steepens, forming hydraulic breccias. The footwall package is a thick sequence of argillaceous quartzite and black sandstone. This sedimentary package is intruded locally by thin (0.1 m to 2 m) acidic intrusives of mostly granitic composition. The country rocks are also crosscut by a late E-W trending dolerite dyke that is generally subhorizontal, with a gentle southward plunge.
Alteration in the Yalea deposit is characterised by intense carbonate alteration, albitisation, silicification, sericitisation, and chloritisation. Alteration is invariably associated with sulphide but also forms a halo within the FW and the HW units. The alteration can be identified by the localisation of coarse-grained aggregates of carbonate, quartz, and albite, whereas the mineralisation is associated with chloritisation, sericitisation, and ferrous carbonate. Weathering at Yalea extends up to 300 m below the surface due to the strong carbonate alteration associated with the mineralised structure. Four main weathering types have been characterised in the rock: saprolite, transition, fresh, and high-grade fresh.
At Yalea, the main mineralised body is hosted by a north-south striking brittle-ductile shear, the ‘Yalea Shear’, that transects the Yalea Structure, which is a NNE striking shear zone that links Yalea with the Loulo 3 deposit. The majority of high-grade (exceeding 8 g/t Au) mineralisation is located south of the intersection between the above-mentioned structures. The main highergrade shoots, ‘Purple Patch’ and ‘Transfer Zone’, are observed in a dilatational strain transfer zone between a major change in the dip of the Yalea Shear Zone (Upper and Lower inflexion). The plunge of this mineralised shoot is sub-horizontal to gently southward and locally coincides with the thickest sheared limestone unit along its eastern side. The steeply south plunging subsidiary mineralisation shoot between the ‘Purple Patch’ to the north and the ‘transfer zone’ to the south is parallel to the intersection of the Yalea Shear Zone and minor SSW striking, steeply ESE dipping altered shears on its western side.
Economic levels of gold mineralisation are almost exclusively associated with paragenetic late sulphide veins, breccias, and zones of massive sulphides. The auriferous sulphides occur as disseminations, stringers, and networks of fracture fill pyrite and arsenopyrite along the margins of the mineralisation shoots, while the high-grade mineralisation (exceeding 8 g/t Au) consists of zones of massive pyrite and arsenopyrite associated with areas of high strain. Late gold at Yalea is associated with tennantite and chalcopyrite. Yalea mineralisation remains open at depth and to the south with potential for significant high-grade extensions.
Gounkoto
The Gounkoto deposit is located 17 km and 24 km south of the Yalea and Gara deposits, respectively. Various stacked mineralised zones within the Gounkoto deposit (four principal lodes referred to as Main Zones) are localised along or adjacent to a NNW striking angular unconformity called the ‘Domain Boundary’ structure consisting of early pervasive pink albite-ankerite alteration and brecciated rocks. On average, mineralisation is 25 m wide, strikes over 2.2 km, and continues to over 700 m depth. The hanging wall of the deposit is composed of a thick sequence of fine-grained argillaceous sandstones and a thin unit of limestone, whereas the footwall consists of argillites, polymictic breccias, and coarse- grained sandstone with microdiorite intrusive rocks and narrower mafic equivalents.