Intrusion-related affinity and orogenic gold overprint at the Paleoproterozoic Bonikro Au–(Mo) deposit (Côte d’Ivoire, West African Craton)

Abstract

The Bonikro gold deposit and satellite Hiré open pits are located at the southern tip of the Oumé-Féttékro granite–greenstone belt in Côte d’Ivoire. Country rocks in the region have undergone polyphase deformation and prolonged arc magmatism during the Paleoproterozoic Eburnean orogeny. Intrusive host rocks at Hiré and Bonikro have been dated at 2180\u2009±\u20096 and 2086\u2009±\u20094\xa0Ma (U–Pb on zircon), respectively. These plutonic bodies acted as favourable sites for fluid flow due to their brittle rheological characteristics. Gold mineralisation at Bonikro is hosted by (i) a sheeted quartz vein array characterised by an Au–W–Bi–Te–Ag metal association, in the cupola of a porphyritic granodiorite; and (ii) an overprinting swarm of fault-fill smoky quartz–(molybdenite) veins. Gold mineralisation at Hiré is shear-hosted and shares a similar relative timing with the latter set of auriferous veins. This second phase of gold mineralisation occurred during a period of transcurrent tectonics late in the Eburnean tectono-magmatic history, soon after the cessation of compressional deformation. It is responsible for the bulk of gold resources in the region (e.g. Bonikro, Hiré, Agbahou) and has been dated at 2074\u2009±\u200916\xa0Ma (Re–Os on molybdenite). The integration of structural–paragenetic relationships with high precision dating of magmatic and hydrothermal events highlights the superposition of late Eburnean orogenic gold mineralisation on an earlier intrusion-related gold system linked to the local emplacement of a porphyritic granodiorite at Bonikro and illustrates the genetic diversity of Paleoproterozoic granitoid-hosted gold mineralisation in the West African Craton. It also further supports that gold mineralisations in the West African Craton occurred diachronously throughout the Eburnean orogeny through a variety of deposit types including Au-skarn, intrusion-related Au, orogenic Au, and porphyry Cu–Au.