Summary:
The deposits are characterized by multiple quartz and quartz–carbonate vein sets and stringers. Vein arrays occur in the east limb, fold hinge (or fold axis), and west limb lithostructural domains. Arsenopyrite and pyrite tend to be late, and are concentrated near the margins of the veins or in cross-cutting stringers.
Faults reactivated during the D1 and D2 regional deformation events provide the structural control on the mineralization.
Gold mineralization is associated with thrust faults or shear zones with brecciated, banded, sheared quartz veins and boudins within highly silicified zones.
Mineralized bodies form as subvertical, or slightly inclined to the east, and consist of lenses, quartz stockwork and/or quartz–carbonate veins. The preferred emplacement is on the fold hinge or the limbs (EMZ, Tassiri, Gourara) or along shear corridors (Gossey, Korizena, Sokadie).
Essakane (EMZ, Essakane Nord, Gourouol)
The EMZ deposit is about 3,000 m long. Mineralization has an average thickness of approximately 200 m. Mineralization has been intercepted at 600 m vertically below surface; however, the deposit remains open at depth and along strike.
The Essakane Nord and Gourouol deposits are situate immediately north of the EMZ deposits. The Essakane Nord deposit is mined out. The mineralized zone was approximately 400 m in length, averaged about 40 m in thickness, and was intercepted to 200 m depths below surface. The Gourouol deposit is being infill drilled. It is approximately 300 m in length, averages about 30 m in thickness, and has been intercepted to 125 m depths below surface.
The EMZ deposit is characterized by multiple quartz and quartz-carbonate vein sets and stringers.
Vein arrays occur in the east limb, fold hinge (or fold axis), and west limb lithostructural domain.
Except for vein sets located in the turbidite-hosted Birimian sills, all recorded vein sets are mineralized. The east–west and north–south vein sets both appear to exhibit higher average gold grades than the other vein sets; however, they are also more variable, with a higher proportion of the lower values also appearing in the same vein sets. Tests conducted in December 2010, over three areas inside the pit with oriented grade control drilling, have demonstrated that the grade difference between holes oriented 242° (initial drill pattern) and 120° (later pattern that intersects more of both vein sets) can be as high as 9%.
Continuity of mineralization in the steep west limb is poor. The mineralization is usually low grade due to the frequency of white, late-stage extensional quartz veins with visible gold; however, there are a few east–west extensional veins cross-cutting the west limb that have been worked by artisanal miners. Gold dissemination into the wall rock is rare, and gold is largely confined to the early stage, bedding parallel, and conjugate veins sets.
Gold occurs as free particles within the veins and is also intergrown with arsenopyrite ± tourmaline on vein margins or in the host rocks. Gold is typically coarse. Visible gold particles have been recorded during core logging within and on the margins of quartz veins, intergrown with coarse arsenopyrite, and as isolated grains in the host rock.
Gold particles occur without sulphides in the weathered saprolite.
The gold is free milling in all associations. Disseminated arsenopyrite in the host rock rapidly decreases away from the veins and is strongly associated with the gold mineralization. The same relationship is seen away from lithological contacts, which generally show higher densities of bedding-parallel veining. Oriented core drilling shows that significant concentrations of gold with arsenopyrite can be found in the arenite-argillite lithological contacts in association with quartz veining or in veinlets of massive arsenopyrite. Deeper below the main arenite unit, significant concentrations of gold are found in association with coarse arsenopyrite in the argillitic unit.
In the Essakane North ant Gourouol deposits, gold mineralization is associated with quartz carbonate stockwork veins hosted by a folded turbidite succession of arenite and argillite.
Lao
The deposit is about 900 m long. Mineralization has an average thickness of 60 m. The deposit has been drill tested to 300 m. It remains open at depth and along strike.
Gold mineralization is associated with zones of complex networks of fracture systems filled by quartz and quartz-carbonate. Pyrite and arsenopyrite are observed associated with gold.
Gossey
The Gossey deposit is located about 15 km northwest of the Essakane Mine.
The deposit is about 2,700 m long. Mineralization has an average thickness of 40 m, and has been drill tested to about 150 m depth. The deposit remains open at depth and along strike.
Gold mineralization is associated with brecciated, banded, sheared quartz veins and boudins within highly-silicified zones.
Mineralized bodies occur as subvertical, or slightly inclined to the east, lenses of quartz vein stockworks, and quartz-carbonates associated with pyrite, arsenopyrite, and more rarely, pyrrhotite. The mineralized structures are typically oriented at N10° with a subordinate direction of N35°.
Mineralogical studies identified interstitial gold (with traces of silver) in the quartz grain edges, associated with sulphides such as pyrite, arsenopyrite, and pyrrhotite. Many grains of gold occluded in these minerals were also observed. Gold grain sizes were variable, ranging from as small as small as <5 µm to 10–30 µm.