The Tasiast 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 Tasiast gold trend includes the active Piment and West Branch pits. It 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 north-south through the El Ghaîcha mining permit and extends over tens of kilometres to the north and south.

The Pantaloon gold trend is located about 3 km to the east of the Tasiast trend. It is spatially associated with the north to north-northwest striking Pantaloon shear system. Mineralization is hosted by quartz-carbonate veins within sheared metasedimentary rocks of the Tasiast assemblage in the hanging wall of the thrust, and sheared ultramafic schist in the footwall of the thrust. The Pantaloon trend is the locus of several exploration prospects on the El Ghaîcha mining permit and also on the Tmeimichat exploration permit to the north.

Mineralization at the Piment deposits (South-South, South-North, Central, North, Mars, Prolongation is hosted by iron-rich metasedimentary formations that belong to the Tasiast assemblage and that have been metamorphosed to upper Greenschist and lower amphibolite peak grades. Gold is mainly associated with the sulphide replacement of Fe-oxide minerals, such as magnetite. At the West Branch deposit, the mineralization is hosted by quartz-carbonate veins within strongly sheared, fine- to medium-grained intrusive rocks of diorite and quartz diorite compositions that belong to the Aouéouat assemblage.

The West Branch and Piment deposits are hosted within a package of strongly folded and sheared rocks in the hanging wall block of the Tasiast thrust system. The Tasiast thrust is the master (lowermost) structure, and higher level shear zones are interpreted to be splays.

Fold repetition is evident within the Tasiast thrust system and to the east of it. The dominant foliation in the mine sequence is moderately to steeply east-dipping (40° to 65°). Overall, the dip angle of the thrusts and of the foliation steepens from the southern part of West Branch toward the north where it is subvertical near Prolongation.

The Tasiast thrust and its splays are typically 0.5 m to 10 m thick and are characterized by a laminated foliation with locally preserved mylonitic textures. The fault splays are commonly situated at the contacts of lithologic units. Hydrothermal alteration assemblages, sulphides and quartz veins are commonly spatially associated with the principal thrusts and splays.

Gold mineralization and alteration assemblages at the Tasiast deposits have been defined over a strike length of approximately 30 km and to vertical depths of at least 740 m. Alteration profiles through the West Branch deposit at 71,411N, 71,811N and 71,061N are provided in the lower panels of Figure 7-7 to Figure 7-9. Alteration mineral associations within the Greenschist Zone at West Branch include a central core of biotite-carbonate-quartz-pyrrhotite-pyrite ± magnetite ± garnet ± tourmaline ± leucoxene (coded as BST alteration zone) that is mainly developed within the mafic to intermediate intrusive rocks (Pollard, 2011). Intense silicification is also locally well developed in the same rocks where it is associated with sheeted quartz veins. A strong correlation exists between the +2 g/t Au grade shell and quartz vein densities of 1% to 5%, sulphide volume of 0.5% to 2.0% (pyrrhotite dominated) and a medium abundance of carbonate in both the veins and groundmass.

The BST alteration grades outward to a biotite-amphibole-carbonate-quartz-pyritepyrrhotite ± magnetite ± garnet alteration assemblage that itself grades into an amphibole-biotite-garnet ± carbonate ± quartz ± pyrite ± pyrrhotite assemblage. These two alteration assemblages are respectively coded as GST2 and GST1 and are hosted mainly in the mafic meta-volcanic unit, and to a lesser degree in the mafic to intermediate intrusive rocks.

Volumetrically minor mineral alteration associations include sericite-zoisite ± quartz ± carbonate detected on the margins of the GST2 and GST1 and a late overprinting albitesericite-carbonate assemblage observed locally throughout the West Branch deposit. The FVC units have been variably altered by albite, biotite, muscovite, pyrite and quartz, and contain variable quartz vein densities.

At West Branch, several types of veins are recognized within the Greenschist package.

The deposits can be approximated as slightly flattened prolate ellipsoidal shoots that have moderate dips to the east and a shallow to moderate (approximately 30°) southerly plunge. Notable variations between the West Branch and Piment deposits are the rocks hosting mineralization and the continuity of individual mineralized shoots. The majority of mineralization at West Branch is hosted in the altered and veined Greenschist Zone that is bound by footwall and hanging wall FVC.

A strong correlation is established between the BST altered Greenschist Zone rocks and a continuous +2 g/t Au shell at West Branch. The +1 g/t Au grade shell correlates with the portions of the mafic meta-volcanic rocks, whereas the 0.6 g/t Au shell extends locally into the FVC. The low-grade halo (ranging from 0.25 g/t Au to 0.60 g/t Au) extends further into the FVC and is also developed over approximately 100 m horizontal widths in the SVC footwall.

At West Branch, individual shoots that define +2 g/t Au grade shells are continuous over a strike length of at least 1,000 m and have a down dip extent between 100 m and 400 m. In comparison, Piment mineralization is largely hosted along faults and within the adjacent altered rocks. Individual mineralized shoots at Piment have been continuously defined over a 300 m strike and range in thickness from 5 m to 40 m.

Metallic minerals within the Greenschist Zone are dominated by pyrrhotite, pyrite and native gold that occur as vein infill and alteration spots commonly in and around the foliation planes in proximity to veins. Pyrrhotite and pyrite occur together in many places, but in variable ratios. Zones of pyrite-only and pyrrhotite-only are rare.

The oxide zone is characterized by leaching of quartz-carbonate veins and sulphides, and precipitation of calcrete and iron hydroxides in voids and fractures. No well-defined transition zone was identified as the strongly weathered upper portion of the deposits grades into fresh rock at depth. The depth of oxidation is in the order of 30 m to 60 m, at an average of 40 m. No supergene enrichment of gold is apparent in the upper profile.

Ore and waste rock is currently mined by conventional open pit methods from two pits (West Branch and Prolongation East). Prior mining has taken place in West Branch, Piment and several other completed pits at Tasiast. Since Kinross acquired the property in late 2010, a total of approximately 458 million tonnes of material have been mined from various pits, including 51 million tonnes in 2011, 78 million tonnes in 2012, 82 million tonnes in 2013, 55 million tonnes in 2014, 63 million tonnes in 2015, 54 million tonnes in 2016 and 75 million tonnes in 2017. Drilling and blasting is performed with regards to wall control and fragmentation using the same methods in both ore and waste material. The excavation fleet on site is made up of five Caterpillar 6060 hydraulic shovels, two RH340B Bucyrus hydraulic excavators and six Komatsu PC1250 hydraulic excavators. The truck fleet is made up of 42 Caterpillar 793D 220 t haul trucks (with 6 more to be commissioned in Q1 of 2018) and 7 Komatsu 785 90 t haul trucks. The larger shovel and haul truck pairing (CAT6060/RH340 and CAT793) is used at West Branch and Prolongation, while the smaller shovels and trucks combination (PC1250 and KOM785) is used primarily for wall and drill pattern cleanup activities. The introduction of larger mining equipment has shifted the mining strategy from a highly selective mining practice to a combination of both bulk and selective mining.

The current conventional open pit truck and shovel mining method will continue to be used. Varied blasting techniques, such as presplit and buffer holes, are employed to protect the pit slopes. Blasted material will be routed based on the application of cut-off grades. Cut-offs are initially based on the net block value at the pit optimization stage and later on the gold grades during scheduling. Applying cut-off controls ensures that the highest-value materials are routed to the CIL process, while lower-grade materials are routed to the stockpiles or, if appropriate, to the dump leach. Materials below the cut-off threshold are sent to the waste destinations.

Waste rock is used for haul road and tailings dam construction as needed. The road network currently in place is well developed, but will require continued maintenance. Additional roads will also be required throughout the life of mine. When not needed for infrastructure purposes, waste material is disposed of in constructed waste rock dumps situated at least 100 metres radially away from the final crest of an open pit and outside the zone where there is potential for dilation, cracking and subsidence related to the pit walls. Mining is expect to finish at Tasiast in 2029.

The 8 kt/d (approximately 3 Mt/a) capacity processing plant uses the CIL process producing approximately 70-80% of the gold shipped from Tasiast. The remaining gold is produced from the dump leach operations via a dedicated ADR plant. All produced gold is in the form of doré and is transported regularly to a refinery for final refining and sale.

Crushing.
Mined ore above plant feed cut-off grade is transported from the open pits to the plant by truck and deposited onto the run-of-mine (ROM) pad. The material is blended according to grade and competency. To aid blending, material is transferred to the crushing plant feed bin by front end loader.

Crushing of the mineralized material takes place in three stages; a primary jaw crusher that reduces rock to less than 150 mm; a secondary cone crusher and two tertiary cone crushers. Screens located before the secondary crusher remove material that is at the final product size, nominally passing 16 mm. Material that is greater than 16 mm is fed to the secondary crusher.

Secondary crushed material is conveyed to a screening section before two tertiary stage crushers. Oversized rock is subjected to further crushing and returned to the screens, the tertiary screens and crushers forming a closed circuit. Material passing through the tertiary screens joins the secondary screen undersize as final product, of 80% passing 16 mm screen, which is then transferred to two fine ore bins.

Grinding.
Crushed material is transferred from the fine ore bin at a controlled rate to each of two ball mills by means of a conveyor belt, and water is added to maintain a fixed slurry density in the mills. The ground ore slurry is pumped to hydrocyclones for classification. The target grind size is 80% passing 90 µm. Cyclone overflow containing solids of the required size flows to the leaching circuit. Coarser solids are returned to the mill for further grinding, via a gravity circuit to capture liberated, coarse gold particles. The gravity concentrates flow periodically to a Leach Reactor where the gold is dissolved from the concentrates and the resulting solution pumped for gold recovery in the gold room.

CIL.
The leaching circuit at Tasiast comprises a single leach tank and 6 CIL (carbon-in-leach) tanks where the dissolved gold is recovered on activated carbon. Material that exits the grinding circuit has an approximate slurry density of 42% solids, by weight. The slurry gravitates to the first of the seven agitated tanks via a trash screen to remove oversize tramp particles. Lime is added to increase the slurry pH to 10.2 then dilute sodium cyanide solution is added to maintain a fixed cyanide concentration. Pure oxygen is injected into the first 4 tanks to enhance gold leaching kinetics. The oxygen demand in the remaining tanks is satisfied by compressed air injection.

Activated carbon granules contact slurry in the CIL tanks to adsorb the dissolved gold from solution. Carbon that has achieved the target gold content, typically around 2,500 g/t, is termed "loaded" and is transferred daily to the elution circuit for recovery of the gold on a batch basis. After each such transfer, carbon in the remaining tanks is advanced counter-current to the slurry flow on a sequential basis, and fresh or “barren” carbon is added to the last CIL tank to maintain the carbon inventory.

After maximising gold recovery from the solution and ore particles in the CIL process, the resulting slurry flows via carbon retention screens to the thickener, where the solids settle to achieve a density of approximately 55% solids and residual solution is returned to the process.

Carbon Elution and Electrolysis.
Loaded carbon recovered from the CIL slurry by screening is first washed to remove entrained ore particles and then washed with hydrochloric acid solution, in a dedicated wash vessel, to remove lime scale from the carbon surfaces. The acid-washed carbon after being neutralized is transferred to the elution pressure vessel. To recover gold from the loaded carbon, batches of approximately 6 tonnes of carbon are subjected to a high pressure and temperature leaching process, called elution. Tasiast is using the Anglo American Research Laboratories (AARL) strip process with no cyanide addition. A hot caustic soak under pressure followed by a rinse with demineralized water removes the gold from the carbon and into solution. After gold removal, the "barren" carbon is transferred to a regeneration kiln for thermal reactivation of the carbon. Reactivated carbon is returned to the last CIL tank. Gold is recovered from the caustic solution by electrowinning onto stainless steel wire wool cathodes in electrowinning cells, located within the Plant gold room. The gold is removed as a sludge by pressure washing the cathodes at intervals. The sludge is dried and mixed with fluxing materials and charged to a diesel-fired crucible furnace. After melting, the slag is poured, followed by pouring the gold into bullion moulds. Bullion or “doré” contains 94% or higher gold content together with a minor content of silver. The doré bars are transported by a security firm to a commercial refinery for further purification and sale.

Dump Leach Pads (Heaps).
The dump leach operation has recently been re-designed to process up to 11 Mt/a of low-grade oxide mineralization using two dump leach operations: one pad (Piment) with five cells separated by a raised berm for solution drainage control, and the other pad (West Branch) with 8 cells. The design of each pad allows for three 10 m lifts for a final stack height of 30 m. All solution collection ponds are plastic-lined with installed leak detection systems and bird netting protection. The gold-containing (pregnant) solution produced by the dump leach operations is pumped to a dedicated ADR plant to recover the gold and reactivate the carbon.

Mine haulage trucks deliver low-grade ROM material directly to the pads, where the material is dumped and levelled as required by bulldozer. Once sufficient material has been placed to an initial height of 10 m, a layer of lime is spread on top, with typical usage of 4.0 kg/t, and the surface is ripped. The heap is then available for irrigation.

At each of the two pads, barren solution is pumped from the barren pond to irrigate the heap. The first drainage collection of “intermediate” solution is collected in a pond, then pumped to another section of the pad to contact freshly placed ore. The resulting drainage is captured in the pregnant solution pond adjacent to each pad and pumped at a controlled fixed rate to the ADR plant for gold recovery. Make-up water is added to the systems from the raw water pipeline system connecting the bore field with the CIL mill.

The lime added during stacking maintains a minimum pH of 10. Irrigation is targeted at 10 L/h/m2 and is applied using a system of plastic pipe headers and dripper pipe. Antiscalant solution is added at pumping points to inhibit scale formation. Pregnant solution contains approximately 0.5 g/t Au.

The pregnant solution produced by the dump leach operations is received in a single train of six carbon containing columns at the ADR plant.

After acid washing, the carbon is advanced to the elution vessel, where the same AARL process as in the Plant elution system is used to elute gold from the carbon. The gold rich “pregnant” solution is stored in a surge tank prior to electrowinning. After elution of the gold, the barren carbon is re-activated thermally in a diesel fired rotary kiln, stored in a surge bin and eventually returned to the column train via the last tank in series.

Gold is recovered from the pregnant solution by electro-winning onto stainless steel wire wool cathodes in a single electro-winning cell. The gold is washed from the cathodes after each elution cycle, dried in an oven, mixed with fluxes and melted in a crucible furnace.