Summary:
Tasiast gold deposits are hosted in Archaean volcanic-sedimentary sequences that have been deformed and metamorphosed to lower amphibolite peak metamorphic grade. Mineralization is both structurally and lithologically controlled, epigenetic in style and was coincident with early stages of post-peak metamorphic retrograde Greenschist P-T conditions.
The district scale geology is characterized by basement rocks, largely composed of orthogneiss, overlain by deformed north-striking metavolcanic and metasedimentary successions intruded by stocks and plutons of mafic to intermediate composition (granite-greenstone belts). All the rock units are cut by unfoliated and post-mineral mafic (gabbroic) dykes. Two significant Archaean greenstone belts are exposed within the Tasiast District:
- Aouéouat (+75 km long x 8 km wide); and
- Imkebden-Kneiffissat (+60 km long x 9 km wide).
All the significant mineralized bodies defined to date dip moderately to steeply (45° to 70°) to the east and have a south–southeasterly plunge. Gold deposits on the Tasiast trend are associated with second order shear zones and splays cutting the hanging wall block of an inferred thrust. The volcano-sedimentary stratigraphy has been tightly to isoclinally folded and is cut longitudinally by sub-parallel shears that are sub-parallel to the predominant foliation.
The main Tasiast gold trend includes the West Branch and Piment-Prolongation deposits. Gold mineralization is spatially associated with the west vergent Tasiast shear system that places mafic to felsic volcanic and intrusive rocks of the Aouéouat assemblage, including the host rocks of the West Branch deposit, on top of the younger metasedimentary rocks of the Tasiast assemblage. The Tasiast trend passes northsouth through the Guelb El Ghaîcha mining permit and extends to the north and south onto adjacent licences.
The Tasiast Mine consists of two deposit sets hosted within distinctly different rock types, both situated within the hanging wall of the west-vergent Tasiast thrust.
The Piment deposits (Piment and Prolongation) are hosted within metasedimentary rocks including metaturbidites and banded iron formation where the main mineral association consists of magnetite - quartz pyrrhotite ± actinolite ± garnet ± biotite. Gold is associated with silica flooding and sulphide replacement of magnetite in the turbidites and in the banded iron formation units.
The West Branch deposit geological succession comprises mafic to felsic volcanic sequences, iron- rich formations and clastic units that have undergone mid greenschist to lower amphibolite facies metamorphism and multiple deformation events. The main units recognized at West Branch are:
• GDI – A diorite to quartz diorite intrusion(s) of the Aouéouat Assemblage. Historically, the GDI was referred to as the Greenschist Zone (GST).
• FVC – Felsite of the Aouéouat Assemblage.
• SVC and BIM – Siliciclastic metasedimentary rocks of the Tasiast Assemblage.
• MDO – Mafic dykes, post schistosity and post mineralization.
Most of the gold mineralization at West Branch is hosted by quartz–carbonate veins within the sheared and hydrothermally altered meta-diorites that constitute the Greenschist Zone. All the significant mineralized bodies defined to date dip moderately to steeply (45° to 70°) to the east and have a south–southeasterly plunge.
Structural Geology
The Tasiast deposits are hosted within a package of strongly foliated and folded rocks in the hanging wall block of an assumed thrust fault or thrust fault system referred to as the Tasiast thrust system. Modelling and interpretation of high-resolution gravity data shows deep geometry suggestive of a thrust system underlying the Aouéouat belt. The Tasiast thrust system displays zones of strong deformation typically 0.5 m to 10 m wide and characterized by laminated foliation with locally preserved mylonitic textures. Hydrothermal alteration assemblages, sulphides, and quartz veins are commonly spatially associated with the zones of intense deformation.
All the Tasiast deposits host an intense, generally north-south striking, variably dipping, penetrative foliation, S1, which is axial planar to tight isoclinal folds in the host sequence (Davis 2018). The foliation fabric within the main mine sequence, at West Branch, dips moderately to the East at 40° to 50°, steepens to the north to 55° to 65° at Piment, and becomes sub-vertical, at the north end of the mine sequence, near Prolongation.
Mineralization and Alteration
All the rocks in the property area have undergone lower amphibolite facies metamorphism. Given the metamorphic grade, it is challenging to identify the rocks’ alteration as it has been largely, if not totally, overprinted by the metamorphism.
The bulk of the mineralization at the West Branch deposit is hosted within the GDI. The GDI typically shows a zonation from a barren garnet-amphibole assemblage at its margins to an auriferous quartz-biotite-ankerite-pyrite-pyrrhotite assemblage and back into the barren garnet-amphibole assemblage. This zonation likely reflects a metamorphosed alteration assemblage with the garnet-amphibole assemblage representing a chlorite alteration precursor and the biotite-quartz-sulfides representing a quartz-sericite precursor.
Ore control and pit mapping identified a high-grade quartz-carbonate-chloritetourmaline-gold vein which is coincident with an interpreted Tasiast splay (locally termed the central fault). The vein has been mapped over several benches striking 330° to 340° and dipping at approximately 55° to 60°to the east.
Quartz-carbonate vein sets occur sub-parallel and oblique to foliation and range in style from boudinaged, buckled, folded to planar. The veins formed in extensional and/or Riedel shear orientations and were progressively folded, rotated, locally boudinaged, and partially or wholly transposed parallel to the foliation. Density of veining is typically higher in the GDI (averaging between 2% to 5%) than in the meta-basalt.
Gold occurs as both microscopic grains and coarse visible gold. When observed in hand specimen, grains are commonly spatially associated with hairline fractures in quartz veins and margins of sulphide minerals. The majority of the quartz veins containing coarse visible gold cut the foliation at a slightly oblique angle and mainly dip gently to the east. Most gold grains occur along the margin of the gangue and ore minerals, with 98% of the calculated volume/mass of the grains occurring in liberated and partially liberated forms.