Summary:
The Bonikro gold mineralization shares several characteristics with intrusion-related gold deposits, such as the:
• Relatively reduced, calc-alkaline to alkaline composition of the Bonikro granodiorite (Ouatarra, 2015)
• Presence of aplite and pegmatite dykes
• Sulphide-poor mineralogy of the sheeted veins
• Narrow 0.1–1 cm-wide alteration selvages around the sheeted veins implying a low fluid/rock ratio during vein formation
• Gold ± bismuth ± tellurium ± tungsten ± lead ± molybdenum metal association (Masurel et al., 2019).
The Oumé-Féttékro greenstone belt is located in south-central Côte d’Ivoire. The greenstone belt extends ~300 km along strike, parallel to the north-northeast trending regional structural grain. The southern part of the greenstone belt hosts the Bonikro, Hiré and Dougbafla gold deposits.
The south-central part of the Oumé-Féttékro greenstone belt, in the Bonikro district, consists of Toumodi Group supracrustal rocks and the Kan River Plutonic Complex, which consists primarily of the Loukouyakro suite of TTG orthogneisses containing mafic enclaves and local migmatitic orthogneisses.
The Kan River Plutonic Complex is interpreted as the mid to lower crustal remnant of a volcanic arc, metamorphosed and unroofed during the assembly of Baoulé- Mossi Domain (Mortimer, 1990). Similar TTGlike plutonic rocks in the Dabakala area, in the northern part of the Oumé-Féttékro Greenstone Belt, were emplaced at ca. 2150 Ma.
The Toumodi Group is inferred to have been deposited in an evolving volcanic arc, with the basal Zaakro Formation representing its immature stage and the Bofia Formation its most evolved stage (Mortimer, 1990). The following four lithostratigraphic formations are less than 1,500 m thick:
• Basal Zaakro Formation, which comprises pillow basalts and mafic, lithic tuffs with subordinate rhyolite flows
• Bomba River Formation, which consists of epiclastic sandstones, flow-banded rhyolites and ignimbrites • Akaoka Formation, which includes a basal epiclastic conglomerate overlain by mafic lithic tuffs and basalts with subordinate epiclastic sandstone horizons
• Bofia Formation, which comprises felsic volcaniclastic rocks, intercalated lapilli tuffs, laminated rhyolitic tuffs and minor discontinuous black shale, manganiferous and cherty horizons.
Numerous elliptical to circular calc-alkaline plutons of various sizes and compositions, such as the Toumodi granite, intruded the Toumodi Group and Kan River Plutonic Complex after ca. 2100 Ma. The metamorphic mineral assemblages in the Toumodi Group generally include chlorite-epidote-sericite ± biotite-actinolitecalcite, indicative of regional greenschist-facies metamorphic conditions (Ouatarra, 2015).
The north-northeast trending structural grain of the Bonikro district is a result of D1 and D2 deformation. Early D1 west-northwest to east-southeast directed compression resulted in the formation of tight to isoclinal, north to north-northeast trending, gently plunging folds (F1), and a penetrative axial planar cleavage (S1). Incremental bulk deformation during D2 (west-northwest to east-southeast directed shortening) resulted in the formation of the regional-scale Boni-Andokro shear zone and eastwards directed thrusting of the Toumodi Group over the Kan River Plutonic Complex. Higher-order, steeply dipping shear zones and back-thrusts also formed during D2 shortening.
Most gold mineralization in the Bonikro district is interpreted to have occurred during D3 sinistral and/or sinistral-reverse displacement along high order north-northeast trending brittle-ductile shear zones (Houssou, 2013; Ouatarra, 2015). Gold mineralization is hosted in granitoids at Bonikro, Hiré and Dougbafla. In contrast, gold-(molybdenum) mineralization at Bonikro has been interpreted to have a magmatic affinity (Masurel et al., 2019).
Mineralization
Bonikro mineralization comprises auriferous sheeted quartz veins within the Bonikro porphyritic granodiorite. The gold mineralization shares several characteristics with intrusion-related gold deposits, but this has likely been overprinted by orogenic-style gold mineralization unrelated to the granitoid host and much later in relative timing. Bonikro is 1,300 m along the shear and 1,500 m northeast-southwest in the felsic body, the termination of which has not yet been identified.
Gold mineralization at Hiré is hosted by fault-fill auriferous quartz veins, formed within solid plutonic host rocks during D3 transcurrent faulting. The fault-fill character of the mineralized veins at Chapelle and AkissiSo imply that the brittle rheology of the host granitoids was the critical feature that focused fluid flow.
The variably quartz-veined mineralized structures at Akissi-So are linear, dip to the northwest, strike for 1,200 m and extend down dip for greater than 100 m with widths up to several metres. Within Akissi-So, there are two lodes with mineralization stepping from the southeast lode to the northwest lode that extends beneath Hiré town. In Chapelle, there are several sub-parallel structures in close proximity, one group of which strikes east-west (1,600 m long), whereas the other curves to the west-southwest (1,800 m long). Mineralization pinches and swells along the lodes which can have various dip directions in the third dimension.
Gold mineralization at Dougbafla appears to be a blend of the two mine areas. There is certainly a pluton association, although mineralization appears to lie on its margins rather than disseminated throughout. Sheeted(?) granitoid bodies intrude the mafic-felsic host suite, and mineralization lies on lithology boundaries, possibly due to rheological contrasts. The difference in timing for the different styles of mineralization may reflect the changing tectonic regime from compression to transcurrent.