Summary:
The Loulo and Gounkoto deposits can be classified as a typical shear hosted Birimian style mesothermal gold deposits.
The Gounkoto Mine comprises three primary sources of ore, namely the Gounkoto open pit, underground, and the Faraba open pit.
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.
Two main high-grade mineralised shoots have been identified in the Gounkoto system. The first high-grade (exceeding 3.15 g/t Au) mineralised shoot is located in the south and corresponds to a left step jog along the Domain Boundary. It is associated with intense silica-albite-hematite and chlorite alteration known as the ‘Wrench Zone’ in the MZ1 lode. The second high-grade mineralised shoot is located to the north and corresponds to the junction between MZ2-MZ3 with stacking of the two main lodes, possibly due to transpression and faulting. The zone is referred to as the ‘Jog Zone’. Smaller mineralised lodes are also located along the ‘Iron Structure’, a hematite altered fault overprinting an older zone of altered and locally mineralised rocks, as well as the hanging wall zone, structurally above MZ3 along Domain Boundary toward NNW. Foliation in the hanging wall is parallel to bedding and strikes approximately 020° and dips 50° to 60° to the east. However, the foliation and bedding dip to the west in the footwall. This unconformity adds complexity and favours an economic mineralisation shape with main lodes dipping to the east, concordant in dip with HW stratigraphy and multiple small scale adjacent lodes dipping to the west parallel to FW stratigraphy called ‘finger mineralisation’. This type of finger mineralisation is more developed in MZ1 and MZ2.
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.
Gold is strongly associated with sulphide mineralisation, dominantly pyrite. Magnetite, chalcopyrite, arsenopyrite, and pyrrhotite are also present locally and have a strong association with gold. Gold is also commonly found in gangue in zones of strong silica-carbonate alteration, suggesting that remobilisation played a role in gold (re)distribution at Gounkoto. Initial stages of mineralisation involved the deposition of euhedral and equant grains of magnetite in stringer networks, veinlets, and aggregates, and accompanied by chlorite mineralisation. The magnetite mineralisation was subsequently subjected to a strong oxidation and hydration event resulting in almost total replacement of magnetite by hematite and annealing of the stringer network to form aggregate, almost massive, bodies of hematite, leaving magnetite as small relict grains within hematite.
The onset of the second stage of mineralisation at Gounkoto was synchronous with the oxidation of magnetite and growth of hematite, occurring initially as pyrite (with minor gold and chalcopyrite) which are locally included within the aggregate bodies of hematite. The pyrite mineralisation event was pervasive in nature and was accompanied by strong silica-carbonate alteration, creating the dominant silica-carbonate-pyrite mineral assemblage at Gounkoto. This early pyrite is typically surrounded and enveloped by later stage pyrite with a more cellular and filamentous habit with which gold is commonly associated.
Faraba
The Faraba deposit is located 19 km south of Yalea and 2.5 km SE of Gounkoto and consists of a series of discrete shears and hydrothermal breccias developed generally at rheological contrasts between competent coarse-grained sandstone units and relatively ductile polymictic breccias, associated with silica-albite alteration, overprinted by hematite and chlorite. Mineralisation consists of quartz- carbonate veins with pyrite and arsenopyrite in both fracture fill and dissemination styles. Mineralisation averages 10 m width over 2.4 km strike and 320 m depth. At Faraba North, the mineralisation is sub-parallel to stratigraphy with dips varying from 50° to 60° to the east. At Faraba Main, the east dipping mineralised shears and veins crosscut the primary bedding/foliation which is now more west dipping. As such, a change in bedding and foliation dip direction from an easterly dip at Faraba North to a westerly dip at Faraba Main is observed. The mineralisation system of the Faraba deposit has been truncated by a depositscale gold bearing structure known as the ‘Faraba Structure’ and corresponds to the geological and geochemical contrast between the Eastern Kofi and Western Kofi domains. The Faraba Structure is a brittle-ductile fault zone striking over 10 km along a NNW trend, separating fine-grained, laminated sediments to the east and a package of coarse-grained sandstone conglomerate/breccia and intensely deformed shales that host the mineralisation system in the Western Kofi stratigraphic domain. The mineralised lodes of Faraba Main and Faraba North have been constrained to the west by the bounding Faraba Structure, which is located to the east.
The Faraba Structure has been recognised in geophysical data, and several exploration targets are located on or adjacent to this structure (Faraba Main, Faraba North, and Toronto and Namila). The Faraba Structure has been interpreted in the Faraba deposit area over 2.3 km strike length and consists of a west dipping (67° to 73°) ductile-brittle fault with associated sub-parallel splays in both HW and FW domains.
Mineralisation in the Faraba System is generally related to veins associated with a variety of alterations (silica-albite, hematite, and chlorite), localised exclusively to the west domain stratigraphy of the Faraba Structure. Three mineralised vein styles have been identified:
- Style 1: Centimetric shears/foliation veins of silica-carbonate with semi-massive to stringer pyrite, locally associated with hematite and chlorite. Style 1 is associated with the highest grades in the system.
- Style 2: Fracture fill network veins of quartz-carbonate-chlorite associated with patchy and stringer pyrite + arsenopyrite. Style 2 is commonly associated with hydrothermal breccia, related to high- and low-grade mineralisation in the system.
- Style 3: Spaced foliation veins of quartz-carbonate-chlorite, associated with stringer pyrite, and subordinate hematite. These veins are distributed system wide and delineate the mineralisation envelope. Style 3 is related to medium- (between 1.0 g/t and 3.0 g/t Au) and low-grade mineralisation (below 1.0 g/t Au) in the system.