The Gbongogo gold prospects, northern Cote d’Ivoire: constraints on ore genesis and structural evolution

Abstract

The Palaeoproterozoic Baoule-Mossi domain of the West African Craton hosts numerous economic and world-class gold ´ deposits within Birimian volcano-sedimentary belts. The Mankono Prospect Area is located at the southern extent of the junction between the regional Boundiali-Bagoe and Senoufo greenstone belts in northern Cote d’Ivoire. The Gbongogo Main ˆ Tonalite, host to an approximate 1 Moz at 2 g/t gold endowment, occurs within the Mankono Prospect Area. This work explores the geological framework and structural setting of the Mankono Prospect Area, investigating an atypical style of intrusion-hosted gold-telluride mineralisation preferentially hosted by the Gbongogo Main Tonalite intrusive body. The Mankono Prospect Area is characterised by two lithologically and geochronologically contrasting meta-volcanosedimentary domains, the Western Domain (ca. 2180-2135 Ma) and Eastern Domain (ca. 2100-2090 Ma), having been juxtaposed along the deep-seated Gbongogo Thrust. Four deformation events are locally recognised in the Mankono Prospect Area, termed D1 - D4, active during tectonic events of the Eburnean Orogeny (2130-1980 Ma). The bulk of the deformation observed in the Mankono Prospect Area, and cogenetic greenschist metamorphism, occurred during D2 wherein NE-SW trending structures formed broadly under NW-SE directed shortening at ca. 2100 Ma, folding early D1 structures. The D3 event has been subdivided into two discrete periods of deformation, D3A and D3B, during largely a change in compressive shortening from NW-SE (D2) to E-W (D3). D3A has been associated with a period of brittle-ductile deformation in which the juxtaposition of the Eastern and Western domains took place. In contrast, D3B deformation is brittle and associated with the development of mineralised, sub-vertical, E-W trending veins forming preferentially within rheologically competent units. A final deformation event, D4, is recognised by chlorite shear zones cross-cutting the gold-telluride mineralised veins within the Gbongogo Main Tonalite. The timing of the gold mineralisation event remains poorly constrained. However, cross-cutting relationships indicate mineralisation succeeds emplacement of the Gbongogo Main Tonalite at 2094 ± 4 Ma. Mineralisation is spatially and temporally associated with quartz-tourmaline and tourmaline veins preferentially cross-cutting the Gbongogo Main Tonalite as semi-sheeted, subvertical, E-W trending veins. Gold is concentrated within the Gbongogo Main Tonalite largely due to competency contrasts, having acted as a favourable structural site along which hydrothermal fluids could easily migrate. Native gold occurs intimately associated with a co-precipitated suite of Bi-rich tellurides, forming late in the mineralisation paragenesis as fracture-filled veinlets, void infillings of pre-existing pyrite, and concentrating preferentially along pyrite crystal boundaries.Mineralisation styles and assemblages, similar to that forming within the Gbongogo Main Tonalite, are additionally present within quartz-tourmaline veins cross-cutting regionally competent units including the mafic sills and meta-rhyodacites. Fluid inclusion studies conducted on the mineralised quartz-tourmaline, gold-bearing veins show co-existing carbonic-aqueous populations of low-salinity (<7 wt % NaCl equiv), H2O-CO2-NaCl fluid compositions, and with P-T conditions of fluid inclusion entrapment estimated at approximately 180-350◦C and 1-3 kbars. Fluid inclusion characteristics associated with mineralised veins in the Gbongogo Main Tonalite have demonstrated similarities to that of orogenic, intrusion-hosted deposits in the Baoule-Mossi domain, strongly contrasting that of the fluid-systems associated with more commonly noted shear- or ´ fault-hosted gold mineralisation in the West African Craton. Analyses conducted on the regional quartz-tourmaline vein system indicated comparable fluid inclusion and tourmaline trace element characteristics to that of the mineralised veins cross-cutting the Gbongogo Main Tonalite. This may suggest the presence of a more regionally-extensive, gold-telluride mineralisation system.