Roxgold Sanu S.A. operates the mine, which is a 90% direct subsidiary of Fortuna Silver, with the remaining 10% held by the State of Burkina Faso
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Summary:
Primary gold deposits in Burkina Faso occur within the Paleoproterozoic Birimian terrain. Mineralization was synchronous with regional metamorphism and deformation. Gold deposits found within the Birimian greenstone belts of the West African shield are typically late orogenic hydrothermal deposits that exhibit a strong relationship with regional arrays of major shear zones.
The gold mineralization is typically associated with an organized network of quartz veins containing subordinate amounts of carbonate, tourmaline, sulphides, and native gold. In these deposits, the gold is typically free milling. Alternatively, gold mineralization can also be associated with disseminated sulphides in strongly deformed alteration zones.
Gold mineralization is related to regional arrays of alteration and deformation zones, commonly located at major lithological discontinuities. The local controls on the distribution of the gold mineralization are structural and lithological.
The gold mineralization found at the 55 Zone and Bagassi South deposits is associated with shear hosted low sulphide quartz veins typically associated with the contact between local volcanic and granitic intrusive units and is free milling in both the oxidized and fresh rock zones. The weathering profile over the deposit is shallow and ranges from approximately 10 m to 40 m.
Mineralization
Gold is the main mineralization of economic interest found on the Yaramoko Gold Mine with the main areas of gold mineralization the 55 Zone, Bagassi South, and the 109 Zone, along with severalsmallerscale areas. The 55 Zone and Bagassi South host the current mining operations, with advanced exploration underway on the 109 Zone.
Both the 55 Zone and Bagassi South deposits occur along dextral shear zones with gold primarily associated with quartz veining. The bulk of the gold mineralization occurs in dilational segments of the shear zone where quartz veins are thicker and exhibit greater continuity. Gold typically occurs as coarse free grains in quartz and is associated with pyrite. The gold bearing veins range in size from a few centimeters to over 5 m in width and contain only minor concentrations of disseminated pyrite (frequently less than one percent). Adjacent sheared vein wall rock locally contains a small percentage of pyrite. 109 Zone mineralization is considered generally analogous to both the 55 Zone and Bagassi South.
At the Bagassi South, the gold mineralization is associated with laminated quartz-carbonate veins developed in two shear zones: QV1 and QV'. The average thickness of the gold mineralization at QV1 varies from less than one meter to over 18 m and extends from the surface to over 300 m depth; gold mineralization remains open along strike and at depth. Gold mineralization at Bagassi South is associated with quartz and pyrite alteration in similar structural settings to the 55 Zone.
Four mineralogically distinct hydrothermal veins were defined from samplesfrom the 55 Zone: quartzrich veins, iron-dolomite rich veins with quartz and muscovite, iron-dolomite and quartz veins with albite, and albite rich veins with quartz and iron-dolomite (GeoMinEx, 2013). Native gold is present in each vein type, with accompanying sulphides of pyrite and trace tellurides. The most abundant sulphide mineral, pyrite, occurs in veins and altered wall rock. Textural and chemical complexity of pyrite document a protracted period of crystallization from a compositionally evolving hydrothermal fluid. Native gold occurs in numerous textural associations and at a wide range of grain size ranging from less than 1 and up to 300 micrometers.
The second type of gold mineralization encountered is also associated with pyrite, occurring in zones of conspicuous shearing primarily in the volcanic rocks, with minimal to no significant quartz veining. These two styles of mineralization represent two end-members of brittle-ductile deformation within the 55 Zone where coarse gold in veining, usually seen in a granitic host, defines a more brittle environment while pyrite and shearing in the volcanic rocks is typical of a ductile domain.
55 Zone and Bagassi South QV1 and QV’:
• The mineralization is hosted within a steeply dipping shear-hosted quartz vein, at depth it comprises a broader shear zone hosting quartz veining of variable continuity along strike. Mineralization dip for the 55 Zone is in the range of 70 to 85 degrees with Bagassi South (QV1 and QV’) in the range of 60 to 75 degrees.
• The quartz veining of both deposits tends to pinch and swell to the extremities in the east and west direction with evidence of splay and parallel veins. Vein thickness vary between 1.0 m to 6.0 m at 55 Zone and 0.5 m to 7.0 m at Bagassi South.
• The steeply dipping nature and rock strengths of the mineralization has permitted the adaptation of a longhole open stoping mining method with cemented backfill, which has been in place since production commenced for the 55 Zone mine and Bagassi South mines QV1 lode and suitable for shrinkage stoping mining method for the Bagassi South QV’ lode.
• The narrow nature of the vein dictates small hole size for production blasting (64 mm diameter) to maintain burden and spacing requirements and a close sublevel spacing (20 meters and 17 m respectively for 55 Zone and Bagassi South QV1) to limit the impact of hole deviation on external dilution and equipment capabilities, and the narrow vein of Bagassi South QV’ justifies the use of handheld mining method with production blastholes in the range of 32mm to 42mm.