Summary:
The Lefa gold mine lies within the Siguiri Basin, part of the Paleoproterozoic Birimian terrain of the West African Craton. Host lithologies are typically Paleoproterozoic (2.2-2.1 Ga) volcanosedimentary greenstone belts, metamorphosed to greenschist facies (or higher) and intruded by felsic plutons. Deformation is associated with a system of NE-SW trending crustal-scale ductile shear zones, related to the Eburnean orogeny, a protracted series of orogenic events spanning 2.2-2.0 Ga. These deformation zones are considered fundamental to the development of gold mineralisation.
Localised finely-laminated white and pale yellow limestones occur towards the north of the licence. Three post-Birimian sedimentary packages occur within the Lefa area, namely:
- Neoproterozoic fluvial and shallow marine sandstones;
- Eocene paleo-valley fill loosely consolidated sandstones; and
- post Post-Eocene transported colluvium.
Intrusive lithologies within the basin consist of basic to intermediate sheet intrusives and the Maléa intrusion. Two sets of dykes are identified, corresponding to a swarm which exhibits trends of circa 105-285° and 055-235°. In addition to the dykes, the basin is also intruded by numerous dolerite sills ranging from a few metres to tens of metres in thickness.
Mineralisation at Lefa is hosted within the “Lefa Corridor”, which lies within the Siguiri Basin. This is a zone which is some 10 km wide, underlain by an upper clay rich formation and a lower coarser arkosic layer, with gold occurrences more common in the latter. Apart from younger dolerites and sandstones, there is virtually no fresh outcrop. Often, the stratigraphy is affected by folding which is observed within the pits. Host lithologies for the mineralisation are typically a mixture of sandstones to finer grained mudstones and claystones. Bedding is moderately to steeply dipping and deformation is dominated by discrete faults. The entire stratigraphy has been intruded by massive dolerite dykes and sills, which typically form prominent hills and bluffs. These intrusions are typically thin (<1 m to >10 m) and tend to be fresh dolerites.
Mineralisation typically occurs in more permeable, altered, coarser grained sediments, within and adjacent to structures and fracture zones. Mineralisation is localised by a combination of lithological and structural controls, and as such, the dip and strike of mineralised zones, and to a lesser extent the style of mineralisation, varies considerably between individual deposits. Gold is often associated with stockwork and sheeted quartz-carbonate sulphide veining, stockworks of albite-carbonate-sulphide veinlets, or as sulphide rich haematitic breccias.
Pyrite is the dominant sulphide species. Gold is largely developed within fractures in pyrite grains, associated with quartz carbonate veining (± sulphides), and albite-carbonate-sulphide veinlets. The degree of sulphidation and silicification can vary. Traces of other sulphides, principally chalcopyrite, galena, pyrrhotite, arsenopyrite, bornite, tennantite, linneite and mackinauwite are present as vein, fracture fill and localised disseminations. Gold grains are rarely larger than 50 µm and is non-refractory. The typical alteration assemblage is albite, quartz, carbonate, pyrite, chlorite, and graphite. Albitisation is known to develop on the contact of faults in the coarser grained sediments but not in finer sediments, due to the reduced permeability.
Deformation and metamorphism appear to have been substantially more subdued within the Lefa Corridor when compared with other West African Birimian terrains. Within the Corridor, the basement stratigraphy is typically sub-horizontal, and fault offsets are rare. Primary mineral assemblages reflect low-grade regional metamorphism and are characterised by broad monoclinal folding.
Extensive weathering and lateritisation has occurred across the project area. Both transported and residual laterites, which can be up to 15 m thick can host gold mineralisation. The base of oxidation extends to over 100 m below surface and may be locally deeper in zones of fracturing and brecciation. The width and grade of primary mineralised zones appear to be little different from their equivalents within the saprolite profile.