Summary:
The Project is in an area of greenstones near the southwest margin of the Siguiri Basin, which is 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. The gold deposits within the region are principally orogenic lode 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.
The Project area is deeply weathered, with a thick saprolite and a pisolitic and nodular lateritic cover which hosts remobilised gold, generally above the primary deposits or dispersed a few tens of metres laterally. Outcrops are sparse, and the underlying bedrock geology is known largely from regional scale geophysics and drilling completed by PDI.
Regionally, mineralisation has been focussed on the intersection of north-northwest striking and northwest striking structures on the margin of a regional granitic batholith. Numerous anastomosing north-northeast 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 Project main deposits. NEB has been developed at the hanging wall contact of a small tonalitic intrusion, structurally controlled by a north-northwest striking shear (main shear zone or STMZ), which is part of a network of anastomosing north-northwest to north-northeast striking structures. The NEB deposit includes a small satellite deposit, GBE, located approximately 250 m north of the main NEB deposit.
In the footwall, a very well developed second order shear, 3 m to 5 m thick, (STSZ01) has very similar structure and alteration characteristics to the STMZ and forms a step over, or jog, from the STMZ to a more weakly developed structure and hence it is a locus for dilation and fluid flow associated with mineralisation. The STSZ01 nearly outcrops, whereas the STMZ terminates below the surface above its intersection with STSZ01. This fault duplex is interpreted to represent a soft-linked overlapping shear system, where a component of strain is accommodated by rotation or folding between the main bounding shear segments, as well as at the termination of the segments.
Below the STSZ01 shear, four other parallel structures have been interpreted with similar relationships to the STMZ, however, these are less well constrained by drilling and, hence, have a greater degree of uncertainty in their location and extent.
Higher grades are found in and on the immediate footwall of the STMZ, with lower grade mineralisation in both the tonalitic footwall and the greenstone hanging wall. Mineralisation comprises wide zones of structurally controlled chlorite, silica and sericite alteration with associated pyrite and quartz veining.
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 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 understood. 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.
The weathered profile in the Project area comprises:
• Cemented ferricrete layer, composed of in-situ or transported ferruginous concretions in a ferruginous matrix;
• Mottled clay layer, composed of variably ferruginous residual clays formed by intense weathering and consequent profile collapse;
• Saprolite zone, composed of highly weathered bedrock, where there has not been sufficient leaching to initiate the collapse of the profile, and original rock textures are recognisable even though most original rock forming minerals have been weathered to clays. There may be a transition or saprock zone at the base of the saprolite zone into the fresh zone, where weathering is either patchy or restricted to favourable structures. Levels greater than 40% fresh rock defines this saprock zone;
• Underlying essentially un-weathered fresh zone.
The complete laterite profile is preserved at NEB under a ridge capped with resistant ferricrete. At BC, recent erosion has incised the active river valley and the mottled zone and saprolite are largely exposed at the surface in the artisanal workings with a thin veneer of transported soil and alluvium elsewhere. A few small patches of remnant ferricrete have also been identified.
Dimensions
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.
Fouwagbe mineralisation covers a strike length of approximately 400m, and has been estimated to approximately 300m below the natural surface. The plan width varies from 5m to more than 40m wide. The laterite mineralisation is near the natural surface, with saprolite mineralisation directly below the base of the laterite.
The Mineral Resource is constrained by an optimised open pit; this does not capture the entire mineralisation but is approximately 220m deep.
Sounsoun mineralisation covers a strike length of approximately 550m, and has been estimated to approximately 250m below the natural surface. The plan width varies from 5m to more than 25m wide.
The Mineral Resource is constrained by an optimised open pit; this does not capture the entire mineralisation but is approximately 150m deep.