Polyphase deformation and structural controls on Rhyacian gold mineralization at the Oko West deposit, Guyana

The Oko West deposit, in north-central Guyana, is hosted in supracrustal rocks of the Barama-Mazaruni Group greenstone belt. This new orogenic gold deposit has been discovered in a steeply east-dipping volcano-sedimentary sequence bounded by the hanging wall Oko Pluton to the east and a footwall sill to the west. The Oko Pluton and related intrusive bodies, which intruded along the western edge of the Bartica Gneiss Complex, created strain partitioning and local changes in the deformation regime. The mineralized volcano-sedimentary sequence features a shear zone that incrementally developed at the contact with the footwall sill. This manuscript focuses on the detailed structural features observed around the main body of mineralization, in the northern Kairuni Zone. The tectonic history of the study area involves a shortening phase (D 1 ) associated with near-isoclinal coaxial folding (D 1a ) and partitioned, high-angle reverse shearing and faulting (D 1b ), followed by a further but differently oriented compressional phase (D 2 ) causing yet a second, more open fold set, as well as zones of significant brecciation. The local volcano-sedimentary sequence hosts sulfidized gold mineralization. Gold mainly occurs as inclusions, fracture- or grain boundaries-infilling within pyrite. Gold-hosting pyrite is related to several stages of deformation. It is associated with: (i) D 1 EV-SV; (ii) pre-existing (reworking) S 1/0 -parallel structures overprinted by D 2 fluid circulation and sulfidation; (iii) D 2 fault-fill veins, cataclastites and breccias. The presence of early intrusive bodies played a key role for strain localization, together with ductile to brittle transition. Metamorphic fluid circulation within high strain domains during D 1 modified the rock strength, creating favorable conditions for intense fracturing and brecciation during D 2 . This brittle state of deformation enhanced gold-bearing fluid flow and entrapment along D 2 and reactivated D 1 structures. The contact with the footwall intrusive body was the site of the highest strain, which developed simple shear conditions that were later brecciated and further permeated to trap gold-bearing metamorphic fluids. This research provides evidence of a multi-stage gold deposit, at a relatively confined scale where strain was focused in the immediate contact of a pluton. This structural framework, which created progressive changes in fluid pathways and structural traps, favored the formation of a significant deposit. • Oko West (Guyana) is a multi-stage gold deposit. • Polyphase deformation is associated with gold enrichment and endowment. • Strain partitioning and brittle deformation enhanced gold-bearing fluid circulation. • Local changes participated in producing a major gold deposit in a confined setting.