Summary:
The Loulo and Gounkoto deposits can be classified as typical shear hosted Birimian style mesothermal gold deposits.
Loulo-Gounkoto is located within the Kedougou-Kenieba erosional inlier. The inlier is unconformably overlain by Upper Proterozoic sandstones towards the east and further south. Loulo-Gounkoto is predominantly underlain by the Kofi formation consisting of greywacke, sandstone, argillaceous sandstone, calcareous sandstone and tourmalinized sandstone, sheared greenstone units.
Loulo Mine
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.
Gara (previously known as Loulo 0) is hosted within an intensely tourmaline greywacke unit which outcrops on the surface due to its high resistance to weathering. Gold mineralization is strata bound and hosted by a stockwork of quartz-carbonite-pyrite veinlets that is enveloped by footwall greywacke and hanging wall (“HW”) Sheared Quartzite Rose. The higher gold grades generally occur where the veins are most intense and the range of vein orientations more complex.
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.
The Yalea main mineralized body is hosted by the Yalea Shear, where it is intercepted by the Yalea Structure. The Yalea Shear is a 1.4-kilometer long brittle-ductile, north-south striking, mineralized fault that transects the Yalea Structure, which is a complex, north to north-northeast striking shear zone. The Yalea mineralization is predominantly hosted in hydrothermally brecciated argillaceous pink quartzites.
Baboto is a shear hosted deposit situated along a north-south striking shear structure located approximately 14 kilometers north-northeast from the Yalea deposit. Baboto is dominated by a thick sequence of metasediments and structural breccias. Gold mineralization is mainly associated with the finely disseminated pyrite occurring in the brittle-ductile shear breccias.
Loulo 3 is located 4 kilometers north-northeast of the Yalea mine. Loulo 3 consists of four mineralized zones: a north-northwest trending main zone (“MZ1”), which is situated on the Loulo 3 structure and is transected by the north-northeast striking main zone (“MZ2”), which is situated on the Yalea structure, and two sub- parallel northwest striking footwall zones. The stratigraphy at Loulo 3 (inclusive of Loulo 2) comprises three major lithological sub units, which from east to west include: the HW package (subunits HW1 to HW5), the host package (subunits HP1 to HP4), and the footwall (“FW”) package (subunits FW1 to FW2). The mineralization in Loulo 3 is hosted in the HP4 subunit of the Main Sandstone package with a dominant vein-hosted mineralization style within MZ1 or quartz-tourmaline veins in MZ2. These vein arrays locally transition into bedding-parallel hydrothermal breccias with local semi- massive to massive pyrite, which can also include arsenopyite and hematite, and form the high-grade shoots within the Loulo 3 deposit. The position of the high grade shoots is controlled by pre-existing competence contrasts within the host rock package.
Gounkoto Mine
The Gounkoto gold mine consists of two mineral deposits: Gounkoto and Faraba.
Gounkoto
Gounkoto is a large north-northwest trending shear zone, with a complex assemblage of ductile shear breccias, shears and faults characterized by a stepped geometry, with wider zones of mineralization generally seen on the northwest trending structures and narrower zones on the north-south trending structures.
Gold mineralisation at Gounkoto is hosted in silica-albite-sericite altered wall rocks with high grade mineralisation associated with overprinting chlorite-hematite within hydrothermal breccias and high strain chlorite-sericite shears and subordinate limestone. Several phases of alteration are evident at Gounkoto. An earlier generation of barren albite alteration, peripheral to the deposit, is deformed by a system wide shear zone, while the albite associated with mineralisation is superimposed on this shear zone. Several phases of oxidation occurring at various stages relative to gold mineralisation deposition resulted in iron mineralisation, producing characteristic zones of red oxidised hematite alteration.
Faraba
The Faraba deposit strikes north-northwest and is comprised of several zones of gold mineralization hosted within and along the contacts of north-south striking, coarse-grained, gritty sandstone units (lithic wackes) and polymictic breccias, flanked by packages of sheared argillaceous sediments. Lithological layering (transposed bedding) dips steeply westward; however, the mineralized zones (with associated silica, silica-carbonate, and late overprinting hematite alteration) dip steeply to the east. The mineralization terminates up against the west-dipping Faraba Structure at depth. Mineralization is predominantly pyrite, with subordinate arsenopyrite, local magnetite, rare chalcopyrite and pyrrhotite. The mineralization is rheological competency contrast controlled and is typically vein-hosted (i.e. massive, stringers and blebs), or occurs as dissemination in strongly altered hosts (i.e. blebs and fine grains), with semi-massive to massive sulphides typically within the lower parts of the system adjacent to the Faraba Structure.