Summary:
The Koné deposit is considered to be a lode gold-style system, hosted by brittle ductile shearing within a quartz diorite / mafic volcaniclastic package. The Gbongogo gold deposit is a mesothermal, lithologically constrained deposit hosted by a plunging quartz diorite that has undergone brittle deformation. As such, both are typical of the region.
Much of the Project area is covered by duricrust interpreted to represent remnant peneplain surfaces with only very rare outcrop and deep weathering. The local geology is not yet fully understood.
Regional mapping indicates the Project overlies Birimian sediments, volcaniclastics and volcanics flanked to the west by basement tonalite and diorites. The rocks have been metamorphosed to greenschist facies. Regional aeromagnetic data shows strong northeast–southwest trends interpreted to reflect the distribution of underlying rock units.
Koné Deposit Geological Setting and Mineralisation
Koné is a mesothermal, structurally controlled gold deposit hosted within a north-south trending, westerly dipping (50°), composite package of sheeted 20 m to 30 m thick diorite intrusions which have been emplaced by multiple intrusive pulses. These diorite intrusions are of the same composition and genetically associated but display a variety of textures. The package as a whole is up to 350 m in true thickness and can currently be traced along strike for 2.4 km.
The diorite bodies at Koné have intruded into the contact zone between two different sequences of mafic volcaniclastic rocks which form the hanging wall and footwall of the deposit. The diorite intrusions have been age dated to 2168 ± 5 Ma, indicating they were emplaced during the later stages of the Eburnian orogeny (2200 to 2100 Ma).
The hanging wall volcaniclastics are characterised by a polymictic volcaniclastic sequence of proximal volcanic facies. The footwall volcaniclastics contain smaller clasts, are foliated and display strong deformation at the footwall contact of the diorite domain.
Koné is interpreted to have formed as part of a stacked thrust-shear under a compressional tectonic regime. The major thrust is located at the footwall of the diorite body where a 5 m to 20 m wide zone of shallowly plunging/horizontal tight folds can be observed within volcanic rocks immediately below the footwall contact of the diorite. This folding progressively decreases as you move outward from the contact into the footwall volcanic sequence. The upper contact of the diorite displays little to no deformation and has no mineralisation related to it.
Within the diorite, higher gold grades (>1 g/t) are associated with swarms of foliation parallel, 1 to 2 mm quartz + pyrite ± chalcopyrite veinlets which form distinct corridors of mineralisation. Recent observations from drill core indicate these higher grades are related to discrete zones of more intense shearing and localised slivers of highly deformed volcanic material within the diorite domain. These features are beginning to be resolved as secondary thrust shears peeling off the main footwall thrust structure. In between the high grade zones, lower grades (0.2 g/t to 1.0 g/t) are associated with disseminated pyrite mineralisation and, in general, the diorite package is mineralised over most of its width, averaging >200 m over the main part of the deposit, with a maximum of up to 330 m.
Regional metamorphism in the Boundiali belt is greenschist facies, however the mineral assemblages observed at Koné consist of chlorite, biotite, amphibole, magnetite and pyrite with peripheral epidotequartz-amphibole vein assemblages. These higher temperature, higher pressure mineral assemblages are interpreted to be related to the major thrust structure controlling the Koné deposit.
The deposit is intruded by multiple dykes with the majority post-dating mineralisation by as much as 50 Ma and seen cutting foliation. One set of dykes, known as the early green dyke, displays deformation and folding.
The Koné Gold deposit is hosted by the composite diorite body and is characterised by very large mineralised true widths, averaging >200 m over the southern half of the deposit, with a maximum of up to 330 m (MDD015B, 330.7 m grading 0.58 g/t), i.e. almost the entire width of the composite diorite body.
Higher grades (1 to 1.5 g/t) are associated with high density ‘swarms’ of 2 to 5 mm thick, foliation parallel translucent white to smoky quartz veinlets containing fine grained sulphide. Lower grades are related to disseminated fine grained pyrite. Importantly, no significant silicification of the host rocks is associated with the mineralisation resulting in positive comminution characteristics.
Mineralisation at Koné is interpreted to be controlled by a major thrust shear at the footwall contact of the diorite contact. The volcaniclastic rocks in the footwall of the thrust have deformed plastically whereas the more rheologically competent diorite has developed brittle / ductile shears. This is expressed in drill core as brecciated zones of diorite, associated slivers of internal volcanic rocks, and localised shears with very localised folds of foliation within the diorite and footwall volcanic.
Gbongogo Deposit Geological Setting and Mineralisation
The Gbongogo gold deposit is a mesothermal, lithologically constrained deposit, hosted within a single, north plunging (50°), quartz diorite intrusion which is an approximately elliptical cylinder. The intrusion has a surface expression of 120 m x 75 m and dimensions of 180 m x 100 m in section, perpendicular to plunge. The intrusion has been intercepted in drilling to a vertical depth of 320 m and is open at depth.
The quartz diorite is interpreted to have been emplaced in a single event, based on its homogeneous mineral composition, low variability in pXRF analysis, and lack of internal contacts. The quartz diorite is intruded along the contact of two stratigraphic packages, the fine grained, volcanic to volcaniclastic hanging wall package, and the medium to coarse grained siliciclastic and volcanic sediments of the footwall group. The contact between these packages dips approximately 47° to 275°. The quartz diorite intrusion has been dated to 2094 Ma (± 4 Ma).
The quartz diorite is interpreted as being intruded, at least in part, along the contact of the eastern package of medium to coarse grained siliciclastic between the eastern domain coarse grained sedimentary, and western domain fine grained volcanic to volcaniclastic rocks. The contact of the quartz diorite and the wall rocks and are sharp, often accurately measurable.
Gold mineralisation in the Gbongogo quartz diorite is associated with a stockwork of quartz / tourmaline / quartz-tourmaline veins, and their associated orthoclase-albite alteration haloes. The deposit is believed to be controlled by the rheological contrast of the quartz diorite host and surrounding volcaniclastics, where accommodation space for mineralisation formed by brittle deformation of the quartz diorite.
The majority of mineralisation of the Gbongogo deposit is constrained to within the Gbongogo quartz diorite intrusion. Minor gold mineralisation occurs within the volcano-sedimentary country rocks and along the contact with the intrusion.
Mineralisation correlates with the presence of a quartz and/or tourmaline stockwork and alteration selvage. An absence of vein stockwork and alteration is an indicator of lower or absent grade within the IQD. Mineralisation rarely extends into the volcano-sedimentary host rock, and when present is usually in the form of narrow tourmaline veinlets with pyrite and small alteration selvages.