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 Bissa Hill, SW, IOSE, Z51, Z52 and Boken Zandkom deposits all fall within an approximate 9km NE-SW oriented corridor (“the Bissa-Zandkom corridor”) in the northeastern portion of the Boromo greenstone belt. This corridor comprises a series of interbedded argillaceousarenaceous sediments, alongside sequences of mafic volcanic rock. The thick metasedimentary sequence represents the principal host to mineralisation. Structurally, the lithological assemblage at Bissa-Zandkom appears to be tightly folded, with a broad NE to ENE trend, and is offset by several large-scale faults that trend NW-SE. At least two phases of deformation can be identified at the mine-scale, including an earlier D1 and later D2, resulting in a re-folded fold sequence.
Bouly, which lies to the southeast of Bissa, appears to represent a large, low grade gold-copper porphyry.
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. Two main types of mineralised veins are recognised within the deposits of the Bissa-Zandkom corridor; namely a) the main gold-bearing quartz veins in the central part of steep shear zones, which are often axial planar shears; and b) complex vein systems of gently inclined and steep extensional fractures, developed both within the shear zones barren wall rocks, and along folded bedding planes and lithological contacts. Similar mineralisation styles are observed across the Bissa-Bouly deposits.
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.
Boken Zandkom, IOSE, SW, Bissa Hill and Yimiougou all strike NE-SW, dipping moderatelysteeply to the NW. Bouly, Z51 and Z52 are also NE-SW striking, but dip steeply to the SE. All of the deposits of the Bissa-Zandkom corridor have a moderate north-easterly mineralisation plunge. Trends associated with the satellite deposits of Zinigma, Samtenga, Ronguen and Gougre are varied.
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.