The Séguéla open pit gold mine is operated by Roxgold Sango S.A. (Roxgold Sango), a 90 % indirectly-owned Fortuna subsidiary, with the remaining 10 % interest held by the government of Côte d’Ivoire.
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Summary:
The deposits at the Séguéla Project are considered orogenic lode style systems. The Antenna, Agouti and Boulder deposits are hosted by brittle-ductile quartz–albite vein stockworks, often associated with flow banded rhyolite units or porphyritic intrusions. The Ancien, Sunbird and Koula deposits are hosted by brittle-ductile quartz and quartz–carbonate vein networks associated with strongly to intensely sheared tholeiitic basalt.
Antenna Deposit
The Antenna deposit occurs within a greenstone package that comprises (west to east) an ultramafic hangingwall, which isin presumed fault contact with an interlayered package of felsic volcaniclastic rocks and flow banded rhyolitic units, which are then in contact with a mafic (basaltic) footwall unit.
The Antenna gold deposit is a brittle-ductile quartz-albite vein stockwork predominantly contained within the flow banded rhyolite units. The stockwork lode varies in width roughly in proportion with the widths of the rhyolitic units which host it (approximately 3–40 m) and extends over a strike length of approximately 1,350 m. Stockwork veins which host mineralisation show two principal orientations; steep east-dipping and steep west-dipping. Veins in the steep west-dipping orientation range from being ptygmatically folded to undeformed, while veins in the east-dipping direction may be variably boudinaged to undeformed. This evidence suggests syn-deformational emplacement of the vein sets during west over east movement along the main fault structures within the region. Mineralisation occurs as free gold, associated with pyrite and pyrrhotite. Alteration assemblages associated with this mineralisation assemblage vary from proximal intense silica – albite ± biotite ± chlorite alteration, through medial silica-albite-sericite ± chlorite assemblages, to more distal sericite-carbonate (ankerite/calcite) and carbonate-magnetite assemblages. Pyrite isthe dominantsulphide associated with higher-grade mineralisation within proximal alteration zones, while sulphide mineralogy is pyrrhotite dominated in medial and distal assemblages and is associated with lower-grade gold mineralisation.
Agouti and Boulder Deposits
The Boulder and Agouti deposits are located within a distinct northerly-trending litho-structural corridor that extends from Boulder in the south to Gabbro in the north. The deposits consist of a stockwork array of veins and are variable in width up to 40 meters with a cumulative strike length at Boulder of approximately 1.5 kilometers and 1.3 kilometers at Agouti.
Gold mineralization is associated with strongly foliated or mylonitized, quartz/quartz-carbonate veined basalt and the margins of the felsic intrusives. Generally lower grade mineralization occurs internal to the felsic intrusives where they are brecciated or extensively veined. The highest gold grades generally correlate with the intersection of NNE and NW-trending structures. Mineralization occurs as free gold within a network of milky white quartz veins, and associated with foliation or quartz/quartz-carbonate vein controlled pyrite and minor pyrrhotite.
Ancien Deposit
The high-grade Ancien deposit is located within a thick package of magnetically quiet pillow basalts, tholeiitic basalts and minor mafic sediments that form the westernmost part of the East Domain. The deposit is associated with an interpreted D2 sinistral shear zone, informally labelled the Ancien Shear and comprises (from west to east) a chloritic pillow basalt footwall overlain by a foliated/sheared tholeiitic basalt unit, which is in turn overlain by a second chloritic pillow basalt hangingwall unit.
Significant mineralization is restricted to the more reactive and competent tholeiitic basalt unit and is best developed in zones of strong brittle-ductile brecciation and shearing, with selective sericite+/-silica alteration and intense quartz and quartz-carbonate veining. Mineralization occurs as free gold, predominantly as small grains within microfractured milky- white quartz veins and associated with pyrite and lesser pyrrhotite.
Generally, lower-grade mineralization is also developed at the margins of felsic porphyries that intrude the tholeiitic basalt, and in zones of increased brecciation and veining within these porphyries. Significant mineralization has been intersected over a strike length of greater than 350m and to a vertical depth of greater than 300m and has a moderately to steeply south-plunging core of high-grade mineralization. This high-grade core of the deposit is associated with the most intense deformation and veining and is interpreted to be associated with the hinge zone of the postulated anticline. The deposit remains open down-dip and down-plunge.
Koula Deposit
The high-grade Koula deposit is situated within the same package of mafic rocks as the Ancien deposit located 7km to the south,which is informally labelled the Ancien-Koula corridor.
The Koula deposit is hosted within a strongly foliated/sheared tholeiitic basalt unit with a chloritic pillow basalt hangingwall and footwall.
Significant mineralization at Koula is restricted to the tholeiitic basalt unit and is best developed in discrete zones of strong shearing, biotite-sericite-(silica) alteration and intense recrystallized quartz and quartz-carbonate veining. Mineralization occurs as free gold, predominantly as small grains within recrystallized and microfractured milky-white quartz veins, and associated with disseminated to blebby, foliation controlled pyrrhotite and lesser pyrite. The predominance of biotite and pyrrhotite at Koula is indicative of higher temperature hydrothermal fluids compared to Ancien, where sericite and pyrite are the more dominant species. This change in mineral species suggests a temperature gradient (increasing) from south to north; which isimportant for ongoing exploration of the Ancien-Koula corridor.
Drilling to date has defined the Koula deposit over a 650m strike length and to a depth of greater than 350m vertically. The deposit remains open at depth and down plunge to the south, presenting a priority target for ongoing exploration.
Sunbird Deposit
The Sunbird deposit is hosted within the same package of mafic rocks as both the Ancien and Koula deposits, within the Ancien–Koula corridor. Analogous to the mineralization setting of Koula, Sunbird is hosted within a strongly foliated/sheared tholeiitic basalt unit with a chloritic pillow basalt hanging wall and footwall.
Mineralization appears to be hosted along two preferential planes; that of the general stratigraphy, and that of the pervasive shear foliation within the area, which occurs at a low angle to the stratigraphy. The deposit is hosted predominantly by a near vertical north-striking shear zone within the generally sub- vertical, north–northeast-striking stratigraphy. Additional drilling is required to confirm this interpretation.
Significant mineralization at Sunbird is predominantly restricted to the tholeiitic basalt unit, though minor lenses of mineralization have been interpreted within the rhyolite, diorite and also the pillow basalts. Mineralization is best developed in discrete zones ofstrong shearing, biotite-sericite-(silica) alteration and intense recrystallized quartz and quartz–carbonate veining. Mineralization occurs as free gold, predominantly as small grains within recrystallized and microfractured milky-white quartz veins, with individual veins up to 5 m wide. The highest grade portions of the Sunbird deposit exhibit a moderate southerly plunge, which is concordant with the intersection lineation of the prevailing shear orientation and the stratigraphy within the area.
Drilling to date has defined the Sunbird deposit over a 1,200m strike length and to a depth of greater than 450m vertically. The deposit remains open at depth and down plunge to the south, and drilling is continuing to test the deposit’s extents.