Summary:
The Property covers a sedimentary package belonging to the Kofi Formation, which comprises argillaceousbanded carbonates and an overlying suite of arkose or greywacke where the variable content of feldspar grains is the most abundant detrital mineral rather than quartz or lithic fragments. Irrespective, both lithologies may be mineralised.
The Sadiola deposits are hosted in the Kofi Formation on the northern portion of the Kédougou-Kéniéba Inlier, on the eastern side of the SMSZ. The Kofi Formation comprises impure limestones, sandstones, black shales, pelites and greywackes as well as minor felsic to intermediate hypabyssal intrusions intruded as numerous felsic to intermediate dykes and stocks. Southwest of the main Sadiola deposit, the Kofi Formation is capped by intermediate to mafic diorites. Seven kilometres east of the Main pit, the carbonate sequence is overlain by sandstones of the Neoproterozoic Taoudeni basin that form a prominent escarpment along the strike of the inlier.
Mineralisation
Currently known gold mineralisation within the Sadiola Exploitation Permit is hosted within two major trends, the Sadiola trend in the west of the permit area and the Farabakouta East (known locally as “FE”) trend in the east.
On the Sadiola trend, gold mineralisation is associated with the north-striking Sadiola Fracture Zone (SFZ) that exploits the faulted contact between the greywacke to the west and the carbonate on the east along a 4 km strike, with mineralisation mostly hosted by the carbonate. This zone is also associated with third order north-northeast to northeast trending mineralised fault splays. The SFZ dips steeply towards the west, with localised flexures to the east, whereas the north-northeast to northeast trending splays dip moderately towards the southeast. The intersection of the splays on the plane of the north-trending shear results in a shallow south-plunge to the elevated mineralisation at the junctions.
The greywacke-carbonate contact was also exploited by intrusive lenses of diorite, which formed a significant “cap” to the mineralisation. Subsequent movement along the SFZ has sheared and deformed the diorite, which also become mineralised. Towards the northern end of the Sadiola Main pit, gold mineralisation occurs as discrete lenses and/or shoots within the carbonates along north-northeast trending structures.
Within the north-northeast striking FE trend, gold mineralisation occurs along the contact between carbonates of the western flank footwall and the overlying graphitic metapelites. The contact is brecciated and folded, which may indicate a possible unconformable relationship between the strata. Gold mineralisation is hosted within the weathered carbonates, in a horizon that stretches over at least a 10 km strike. At the southeast end of the horizon, the mineralisation dips approximately 45° northwest because of fault complications, but along the western and northern portions, the unit dips shallowly eastward.
Gold mineralisation is mesothermal or shear-hosted and is mostly associated with lens-shaped breccia zones with both arsenic and antimony-dominated sulphide assemblages of arsenopyrite, pyrrhotite, pyrite, stibnite and gudmundite (sulphide content of between 0.5-3%).
Hydrothermal alteration assemblages include calc-silicate, potassic, chlorite-calcite, carbonate and silicification.
Supergene processes acting on the mineralisation resulted in enrichment of gold in the oxidized lateritic and saprolitic material. The saprolite zone extends to 200 m depth, especially along the permeable structures and sheared fabric of the SFZ, which facilitated groundwater ingress and the oxidation of the primary sulphides.