Summary:
The Oko West gold mineralization can be classified as a structurally controlled, orogenic gold mineralization. Nearby in French Guiana, orogenic-type gold deposits are mainly related to D2 tectono-metamorphic deformation (between 2.1 and 2.0 Ga). The mineralization occurs along shear zones in greenstone belts and is associated with granitic magmatism.
The Oko West Project straddles the north-south striking contact between rocks of the Barama-Mazaruni Supergroup greenstone belt to the west and a granitoid pluton to the east (the Oko pluton). The Barama-Mazaruni Supergroup sequence comprises mafic volcanic flows, volcaniclastics, and siliciclastic and carbonaceous sediments. The following sections describe each of these geological units and their role in gold mineralization.
Regolith
Long chemical weathering typical of humid equatorial paleoclimate produced a thick lateritic profile down to a depth below 100 m from the surface. This profile is typically composed of a veneer of pisolitic colluvium and latosols overlaying a massive clay zone, which pass into a mottled zone and then saprolite / saprock before reaching unweathered rocks at depth.
At the topographic highs in the centre of the project area, the profile is different; characterized by a superficial duricrust, either in situ or broken, passing into a massive clay zone and then the mottled zone. This duricrust, remains of a peneplain, can be several metres thick and form a protective cap, slowing the erosion of the underlying rock.
Gold mineralization at Oko West rocks outcrop on the eastern flank of a north-south trending ridge, covered by a veneer of pisolitic colluvium up to two (2) metres thick and blocks of duricrust that survived erosion. The clay layer and mottled zone below vary in thickness up to three (3) metres and do not allow the recognition of lithologies, although they are considered to be in situ. The saprolite allows the recognition of lithologies and geological structures. The transition zone or "saprock" to unweathered rocks exhibits partially preserved mineralogy and structural features typical of the protolith. It is noticeable that weathering is deeper overlying gold mineralization, facilitated by the abundance of easily dissolved carbonate minerals and faults and shear zones that allowed weathering to reach greater depths. Overall, the lateritic or weathered profile ranges from 16 m to 118 m over the area (Exploration Blocks 1, 4 and 5), and averages 65 m in thickness. The thickness of the saprolite zone averages 43 m, and the transition zone averages 22 m.
Granitoids
The pluton flanking gold mineralization to the east and acting as its hanging wall does not appear to belong to the Bartica Gneiss complex. It is interpreted as a later intrusion of unknown dimensions, being less foliated and lacking evidence of partial melting, hereby called the "Oko pluton" Tedeschi (2022) dated a sample from the "Bryan pit" area at 2,107 ± 6 Ma. The rock is primarily coarse-grained and slightly foliated, described petrographically by Thompson (2022) as a metamorphosed quartz-monzodiorite and granodiorite. At the deposit scale, its contact with the sedimentary sequence is sharp and locally sheared. There is no evidence of pre- or syn-orogenic contact metamorphism. The locally fractured and sheared zones near its contact can be mineralized, but do not represent significant thicknesses.
Smaller intrusive bodies (decimetric) with the same granitoid composition and texture can often be observed within the volcano-sedimentary units. Mostly parallel to the bedding, they highlight the preferential path created by slippage during folding. Larger intrusive bodies (decimetric to metric) can also be locally observed at the contact between units with different rheological characteristics, such as massive volcanic rocks and sediments. These larger intrusions are often associated with strong alteration (K-feldspath or epidote assemblages) and metasomatism affecting surrounding volcano-sedimentary rocks.
Volcano-Sedimentary Sequence
The sequence hosting the bulk of the Oko West gold mineralization is composed of clastic rocks: siliclastic, volcanoclastic, and carbonaceous, and is better understood in the Kairuni zone, the 2.5 km northernmost extent of known gold mineralization. The sequence is 100 to 200 m wide and has an overall tabular geometry, dipping steeply to the east and "sandwiched" between the Oko pluton to the east and the footwall granitoid to the west. The units are intercalated and strongly deformed. The mineralized sequence has been intercepted by drilling to a depth of 600 m from the surface (drillhole D22-112) but is known to continue much further down-dip. This sequence straddles the contact with the Oko pluton for the entire length of the project area within the Kairuni and Takutu zones.
Volcanoclastics
Volcaniclastics are herein defined as clastic rock containing predominantly volcanic particles of any shape or size and does not imply any specific clast-forming, transport or depositional process (McPhie et al.,1993). They show local evidence of bedding and crossbedding as well as more massive facies with possible perlitic textures suggesting both sedimentary and effusive origins respectively. They present numerous imbricated quartz-carbonate veinlets transposed to the foliation.
Siliciclastic Sediments
The siliciclastic sediments correspond to interlayered sandstones and siltstones. They present a pale grey / beige colour and are usually highly carbonatized and moderately sericitized, with quartz and probably fully carbonitized and/or sericitized feldspar.
Carbonaceous Sediments
Carbonaceous sediments correspond to siliciclastic turbidite-like facies, alternating fine carbonaceous bearing sandstone / siltstone, and fine carbonaceous-rich shale layers (graphitic schists).
Gold Mineralization
Gold mineralization mainly occurs within volcanoclastic, siliciclastic, and carbonaceous sediments, which have an overall tabular geometry dipping to the east. Strong evidence of silica and carbonate alteration can be observed within the mineralized zone, with more intense sericitization, as well as with the presence of multiple sulphides (pyrite, chalcopyrite, sphalerite) disseminated within the altered rock, along bedding / laminations or small fractures / veinlets, or as envelopes around the quartz and quartz-carbonate veins. Despite the spatial association, most of the alteration is pre-mineral and served to harden the rock allowing for brittle deformation and dilation during the D2 mineralization stage.
Other hydrothermal alteration does not seem directly related to the mineralization, or their association with it is unclear. Chloritization moderately to highly affects all lithologies in mineralized and non-mineralized areas. Epidotization was only observed in non-mineralized areas, mainly in the upper and intermediate granitoids, in the upper volcano-sedimentary sequence between them, and locally in the lower main volcano-sedimentary package. Magnetization has been observed in mineralized and non-mineralized areas, with strong variability, even within the same unit (probably due to alteration to other oxides).
The hydrothermal fluids responsible for sulphide and gold emplacement produced a network of Sulphide bearing quartz and Sulphide-only stringers spatially associated with EV quartz-carbonate vein systems and their hydrothermal alteration that partially or totally overprint the parent rock.