Summary:
The Project is in an area of greenstones near the southwest margin of the Siguiri Basin, situated in upper Guinea and southwest Mali. The Siguiri Basin contains metasediments and related volcanic and plutonic rocks of the Early Proterozoic Birimian Supergroup, which hosts most of West Africa's gold deposits. Prolonged weathering has led to extensive lateritic duricrusts and deep saprolite profiles. Vertical remobilisation of gold during lateritic weathering is common, and primary gold deposits are often overlain by lateritic or supergene gold deposits.
Regionally, mineralisation has been focussed on the intersection of NNW striking and NW striking structures on the margin of a regional granitic batholith. Numerous anastomosing NNE striking structures have been interpreted from the aeromagnetic data. Smaller granitic intrusions in the greenstones are structurally controlled and provide evidence for significant heat and fluid flow late in the orogenic history, likely to be part of the gold mineralisation process.
These granitic intrusions partially host the two Bankan deposits: NEB and BC.
NEB
NEB has been developed at the hangingwall contact of a small tonalitic intrusion, structurally controlled by a NNW striking shear (Main Shear Zone or STMZ), which is part of a network of anastomosing NNW to NNE striking structures. The NEB deposit includes a small satellite deposit, Gbengbeden, located approximately 250 m north of the main NEB deposit.
The STMZ dips approximately 40° to the west and has been intersected by drilling over a strike length of at least 800 m and 1,150 m down dip. It is open at depth and along strike to the south. The STMZ typically consists of a zone of shearing, strong mylonite fabric and sericite alteration, often with significant quartz veining, at or just above the hanging wall contact of the main tonalite intrusion. The STMZ is typically a single mylonite zone with associated alteration ranging from 4 m to 7 m thick. Still, it may be up to 36 m thick locally or comprise up to four separate mylonite zones.
Sulphide mineralisation largely comprises pyrite with minor chalcopyrite. In the altered felsic igneous rocks, the sulphide mineralisation is generally associated with the later stage veining, with minor amounts disseminated through the rock texture. In NEB, higher grade mineralisation is characterised by higher pyrite and covellite, and arsenopyrite and sphalerite contents. Low grade mineralisation lacks covellite, galena, sphalerite, and bismuth species. Other sulphides that have been noted include tennantite-tetrahedrite, hessite, gersdorfitte, bornite and cobaltite.
BC
The second deposit, BC, is hosted in the carapace of a small tonalitic intrusion, which has intruded a structurally complex greenstone sequence of clastic and carbonate metasediments, volcanics and marbles. The structural controls for BC are much less well known. From the drillhole logging, two shears have been interpreted, a major one dipping moderately to the southwest and a second order structure dipping moderately to the northeast; these appear to constrain both the small tonalite intrusion and the mineralisation that is localised in the carapace of the intrusion. Foliations generally dip parallel to the major shear, whereas the veins have several preferred orientations and a greater scatter than the veins at NEB. Bedding planes and contacts broadly dip parallel to the foliations and shears.
Dimensions
The NEB resource covers a strike length of approximately 1,500 m, and has been estimated to approximately 1,100 m below the natural surface. The plan width varies from 50 m to more than 220 m wide. The laterite mineralisation is near the natural surface, with saprolite mineralisation directly below the base of the laterite.
BC covers approximately 650 m long in strike and to approximately 350 m below the natural surface, with a width of the Low Grade domain of up to 240 m.