Summary:
The Bissa and Bouly gold deposits all occur within an approximate 90 km NW-SE zone, comprising Birimian volcano-sedimentary rocks of the Boromo greenstone belt. The belt consists of mafic volcanic rock, minor felsic volcanic rock and extensive sedimentary units, intruded by Eburnean granitoids, all of which have undergone incipient to low greenschist facies metamorphism (Huot and Sattran, 1987).
The Bouly deposit, approximately 9 km east of the Bissa-Zandkom corridor, is hosted by shear zones within metamorphosed metavolcanics, diorites and porphyritic diorites. The volcanosedimentary sequence is well foliated, with a NE-SW striking fabric. The meta-volcanics are intruded, and mostly replaced, by later diorite, granodiorite and porphyritic diorite bodies, which are not well foliated except in areas where extensive shearing has occurred. As per the BokenZandkom corridor, two phases of deformation are recognised. The earliest deformation phase is associated with NW striking, moderately SW dipping thrust structures. The second deformation phase is associated with NE striking, steeply SE dipping strike slip structures.
Mineralisation at the Bissa and Bouly deposits is primarily orogenic and structurally controlled, with a secondary lithological control. The most significant mineralised zones are associated with anastomosing networks of quartz and quartz-carbonate veins (or stacked, parallel “arrays” of veins) developed within major brittle-ductile shear zones.
Gold mineralisation is most commonly developed in either quartz-sulphide veining ± carbonates and tourmaline, or disseminated and fracture filling sulphides and magnetite. Across most deposits, the primary sulphide species is pyrite, with minor chalcopyrite and arsenopyrite and very rare native gold.
As is typical in the region, the Bissa and Bouly deposits are variably oxidised. The oxidation sequence typically comprises a thin (0 to 20 m) lateritic layer at surface, which overlies a welldeveloped oxidised saprolitic zone. This normally progresses into a transitional zone, comprising a mixture of saprolite and fresh bedrock, with un-altered fresh rock typically around 50 m to 125 m below surface. Secondary enrichment of gold within the laterite layer is common.