Summary:
Deposit Types
The Bomboré Gold Deposits are orogenic gold deposits (Goldfard and Groves, 2015; Groves and Santosh, 2016. Like gold deposits globally and in the late Proterozoic Birimian terrains of West Africa, the Bomboré Gold Deposits exhibit a structural control and associated hydrothermal alteration mineral assemblages. The Deposits represent a large tonnage, low-grade gold mineralization system characteristics similar to other Birimian gold deposits, such as Kiaka in Burkina Faso, Damang, YamfoSelwi and Obuasi in Ghana, and Sadiola in Mali.
The Bomboré Gold Deposits are essentially stratabound disseminated sulphide bodies hosted preferentially in the meta-gabbro, meta-argillite, and granodiorite lithologies. These rock units are interpreted to have acted as preferential gold traps during syntectonic deformation and metasomatism, due to their strong geochemical and rheological contrasts with the surrounding rocks (Hagemann and Cassidy, 2000).
The wet paleoclimate that preceded the current semi-arid climate in Burkina Faso resulted in extensive surface oxidation of bedrock and a deep weathering profile. Oxidized, weathered bedrock can occur up to a vertical depth of 100 m below surface. The gold mineralization occurs in both the surface oxide zone and the underlying sulphide zone. In the oxide zone, gold typically occurs in a free milling form. However, gold is grind-sensitive in the sulphide zone.
Mineralization
The Bomboré gold deposits occur within the regional BSZ, a major north to northeast trending structure considered as a subsidiary to the Tiébélé-Dori-Markoye Fault. Eleven separate auriferous deposits have been delineated by drilling within the 14-km segment of the BSZ located within the Property. The gold deposits were discovered by tracing gold-in-soil anomalies to bedrock by drilling.
The gold mineralization in the Property area is associated with arrays of structurally controlled quartz veins and veinlets and attendant silica, sulphide, and carbonate alteration developed within the BSZ. Most quartz veins are oriented sub-parallel to the foliation and exhibit strong strain, however, the presence of relatively unstrained quartz veins and breccia in drill core attest the protracted history of vein formation and deformation. Late west trending veins crosscutting the main foliation fabric are also observed. Locally, there is evidence suggesting that gold mineralization was remobilized into northeast and southeast dilation zones associated with late faults.
The quartz associated with the gold mineralization is milky white to smoky, locally vitreous and may contain tourmaline. The widths of the veins range from two centimetres to one metre, with an average of ten centimetres. The near surface gold mineralization with grades of up to 0.2 g Au/t is pervasive regardless of quartz veining.
In the fresh zone, the gold mineralization is associated with fine disseminated sulphides, predominantly pyrite. Most of the quartz vein material is barren, as demonstrated by the scrubbing metallurgical test work completed on the Project.
Generally, the gold occurs as fine grain electrum (< 10 µm) but can be visible in outcrop. Artisanal mining over the 1990-2023 period attests to the existence of coarser gold locally. Gold occurs in places as free gold in late smoky quartz tension veinlets and stringers, and is mainly associated with pyrite, pyrrhotite, chalcopyrite, and arsenopyrite. Most sulphides occur as disseminations and fine stringers sub-parallel to the foliation fabric, which suggests development in an active shear zone or re-mobilization. Magnetite and graphite are present locally. Although the sulphide content can be as much as 5%, it is on average only 1% to 2% in fresh (i.e., non-weathered) mineralized rocks.
Gold mineralization is hosted by various rock types, but most commonly in the biotite schist (metagabbro), meta-sandstones, and also the granodiorite dikes that intrude the gabbros, although in the Maga North, P16 and P17N areas the meta-argillites are the main host rocks. The syn-tectonic granodioritic intrusives are also mineralized, although to a smaller extent than the biotite schist and the meta-argillites. The meta-conglomerate and meta-peridotite are unfavourable hosts. The meta-gabbro might represent the best chemical trap, given its high iron content (if gold was transported hydrothermally as a thio-complex), as suggested by the pervasive fine pyritic assemblage that is associated with the gold mineralization in the sulphide zone. Although much of the gold Mineral Resources defined within the Project area are hosted in the meta-gabbro unit, the deformed granodiorite, and its contact zone with the metagabbro host is where the higher-grade mineralization is concentrated.
Petrographic work on fresh rock samples in 2008 (Schandl 2008a, b and c) revealed that the gold mineralization is predominantly associated with silica and iron carbonate, although sericite is a ubiquitous and commonly an abundant alteration mineral in many of the gold-enriched rocks. Gold occurs as electrum, native gold, and gold telluride (calaverite). Small gold grains are included in pyrite, in fractures of pyrite grains, and as free gold in the fine-grained quartz-goethite matrix in the weathered zone. The major sulphides are pyrite and pyrrhotite with subordinate amounts of chalcopyrite, covellite, galena, and arsenopyrite. Pyrrhotite and chalcopyrite are found mainly in the biotite schist and arsenopyrite in the metasedimentary rocks. The gangue of the saprolitic mineralization consists of an assemblage of quartz, sericite, kaolinite, hematite, and goethite.
At a cut-off grade of 0.15 g Au/t, the gold mineralization exhibits reasonable continuity over a strike length of approximately ten kilometres. At this cut-off grade, the gold mineralization forms more restricted corridors (500 m to 1,000 m in length and 10 m to 100 m in width), defining anastomosing patterns parallel and slightly oblique to the general trend of the BSZ.
These higher-grade corridors formed the basis for defining geostatistical domains within each lithodomain considered for Mineral Resource estimation. One of the benefits of the 2010 to 2013 infill drilling programs was the delineation of higher-grade sub-domains based on a cut-off grade of 0.5 g Au/t with the broader low-grade domains based on a lower cut-off grade of 0.20 g Au/t. The higher-grade subdomains have a strike length of up to 500 m and a width typically between 5 m and 30 m.