Summary:
The Tasiast district is situated in the south-western corner of the Reguibat Shield, which is a northeast-trending crustal block of the West African Craton. The Reguibat Shield contains the oldest rocks in Mauritania and consists of two major subdivisions separated by a crustal-scale shear zone representing a major accretionary boundary. The southwestern part (which hosts the Tasiast deposits) consists of Mesoarchean to Paleoproterozoic rocks that include high-grade granite-gneiss and greenstone belt assemblages. The north-eastern part of the shield consists of younger Paleoproterozoic to Neoproterozoic successions, which hosts many orogenic gold occurrences in the West African Craton. This region is characterized by a series of volcano-sedimentary belts and associated batholithic-scale granitic intrusive suites of different ages cut by major shear zones.
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 of the rock units are cut by unfoliated and post-mineral mafic (gabbroic) dikes.
The Tasiast Mine consists of two deposits hosted within distinctly different rock types, both situated within the hanging wall of the Tasiast thrust. The Piment deposits are hosted within metasedimentary rocks including metaturbidites and banded iron formation. The West Branch geology 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 Tasiast gold deposits fall into the broad category of orogenic gold deposits. The regional geological setting and deposit features at Tasiast are similar to other well known Archaean lode gold deposits hosted along greenstone belts in granitoid greenstone terranes.
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 of the Tasiast deposits host an intense, generally N-S 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 40-50°, steepens to the North to 55-65° at Piment and becomes sub-vertical, at the north end of the mine sequence, near Prolongation.
Pit mapping at Tasiast includes the collection of structural fabric measurements for structural geology and geotechnical application. Numerous consultants have assisted Kinross in developing a structural model for the Tasiast deposits along with developing pit mapping procedures.
Quartz-carbonate veins sets occur sub-parallel and oblique to foliation and range in style from boudinaged, buckled, folded to planar. The veins clearly formed in extensional and/or Riedel shear orientations and were progressively folded, rotated, locally boudinaged and partially or wholly transposed parallel to the foliation. In the core of the West Branch Greenschist Zone vein, densities are typically higher in the meta-intrusive dioritic unit (averaging between 2% to 5%) than in the meta-basalt (<2%). This higher density suggests the coarser-grained feldspar-rich dioritic facies focused stresses and readily developed brittle-ductile shears, as expected for quartzofeldspathic rocks under retrograde Greenschist metamorphic conditions. Along the margins of the West Branch deposit, both the dioritic and meta-mafic volcanic units have a low vein density (<1%). Quartz-carbonate veins also developed locally within FVC that envelops the Greenschist Zone and within the footwall meta-sedimentary units.
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. The majority of 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. By volume/mass calculations, the majority (greater than 70%) of the volume is associated with the coarser (plus 75 µm) size fraction. Encapsulated gold grains are rarely observed. When present they are predominantly a very fine grain size. Semi-quantitative scanning electron microscopy analysis of gold grains indicated low silver (less than 15%) and trace iron (less than 3%) content.