The Essakane Gold Mine occurs in the Paleoproterozoic Oudalan Gorouol greenstone belt in northeast Burkina Faso. The stratigraphy can be subdivided into a succession of lowergreenschist facies meta sediments (argillites, arenites, and volcaniclastics), conglomerate, and subordinate felsic volcanics, and an overlying Tarkwaian-like succession comprising siliciclastic meta-sediments and conglomerate. Each succession contains intercalated mafic intrusive units that collectively comprise up to 40% of the total stratigraphic section.

The Essakane Gold Mine consists of one mining permit (the Essakane Mining Permit), which contains the Essakane main zone deposit (EMZ deposit) and the Falagountou deposit, and seven exploration permits (the Essakane Exploration Permits), all located on contiguous ground.

The addition of the heap leach process in 2018 has increased the EMZ Mineral Reserve inventory. The Falagountou deposits have no Mineral Reserves attributed to the heap leach process due to the distance they are located from the EMZ pit and the short mine life, which ends in 2020 for the West pit and 2021 for the East pit.

The EMZ deposit is a greenstone hosted orogenic gold deposit. Specifically, it is a quartzcarbonate stockwork vein deposit hosted by a folded turbidite succession of arenite and argillite (Figure 7-6). The laminated sedimentary units are part of turbidite sequences. The regular laminated unit is composed of very regular alternating sandstone, siltstone, and grey-black argillite. The lateral extension of this unit is limited. The irregular laminated unit is thicker than the regular bed and is mainly composed of an argillite unit (more than 65% of the whole rock). This irregular laminated unit is also made of an alternating sequence of sandstone, siltstone, and poorly sorted argillite.

Gold occurs as free particles within the veins and is also intergrown with arsenopyrite +/- tourmaline on vein margins or in the host rocks. Disseminated arsenopyrite in the host rock rapidly decreases away from the veins and is strongly associated with the gold mineralization. The same relationship is seen away from lithological contacts, which generally show higher densities of bedding-parallel veining. Oriented diamond core drilling shows that significant concentrations of gold with arsenopyrite can be found in the arenite-argillite lithological contacts in association with quartz veining or in veinlets of massive arsenopyrite. Deeper below the main arenite unit, significant concentrations of gold are found in association with coarse arsenopyrite in the argillitic unit. The gold particles occur without sulphides in the weathered saprolite. The gold is free-milling in all associations.

The EMZ deposit is an anticlinal fold with flexural slips between layers and is westward thrusting along weakness planes parallel to bedding, with minor displacement.

The quartz veins fill brittle extension and shear deformation structures caused by the folding with at least three distinct sets of veins (Figure 7-8) and two phases of quartz veining and gold mineralization.

The vein arrays in the EMZ deposit are complex and consist of the following:
• Early bedding parallel laminated quartz veins caused by flexural slip and showing ptygmatic folding;
• Late, steep extensional quartz veins as vein filling in extension and shear joints formed by the folding;
• Axial-planar pressure solution cleavage (with pressure solution seams normal and parallel to bedding). All veins may be displaced by two sets of late opposing thrusts as shown in Figure 7-9.

The vein arrays occur in the east limb, fold hinge (or fold axis), and west limb litho-structural domains. The geology and economic potential of the EMZ deposit is dominated by the persistent east limb main arenite. The top contact of the east limb domain is a sharp, sheared contact with no significant gold mineralization above it. The shearing appears to be parallel to bedding, however, some loss of vertical succession has occurred. The main arenite below this contact is the lower coarse grained part of a Bouma cycle. The locus of bedding parallel deformation and alteration is within the east limb main arenite. Graphitic argillite occurs immediately above the contact. The deformation shifts into the hanging wall argillite unit to the north of the EMZ deposit.

Mineralization has been confirmed to over 550 m vertically below surface, however, the full depth extent in the fold hinge and east limb is still unknown. The geometry of the fold hinge zone is an anticlinal flexure that is easily recognized in the pit and oriented drill cores. The fold closure is sharp and sometimes truncated by thrusts and the transition from east limb to west limb takes place over a few metres. The position of the fold axis is often marked by a breccia in the arenite unit. The fold hinge zone in the argillite unit is marked by tight kink structures and sheath folds with rapid transitions from east dipping footwall rocks to nearvertical west limb beds below the fold axial plane.

Arsenopyrite and pyrite occur within and adjacent to quartz veins as well as disseminated throughout areas of wallrock alteration. Traces of chalcopyrite, pyrrhotite, galena, and hematite occur with arsenopyrite. Minor amounts of tourmaline with rutile are found in the main arenite and in interbedded arenite stringers in the footwall argillite. Remobilized graphite can be found associated with tourmaline.

The fine-grained argillites can be strongly enriched in tourmaline and have also been subjected to quartz, carbonate, sericite, and quartz alteration. Fine needles of rutile are generally associated with the tourmaline. Sulphide mineralization preferentially occurs in the coarser arenaceous layers.

The EMZ deposit is characterized by multiple quartz and quartz carbonate vein sets and stringers. Arsenopyrite and pyrite tend to be late and concentrated near the margins of the veins or in late cross cutting stringers. The paragenetic sequence of veining is thought to be as follows:
• Early quartz-carbonate-albite-(sericite) veins.
• Quartz veins with tourmaline and pyrite containing gold.
• Diffuse quartz-albite-carbonate veins with arsenopyrite.
• Later tourmaline-rutile-arsenopyrite stringers with gold.
• Late skeletal pyrite and carbonate-quartz-pyrite stringers.

Mining is carried out using a conventional drill, blast, load, and haul surface mining method with an owner fleet.

Current mining production is approximately 50 Mtpa, however, the mine expansion will result in an increase to 70 Mtpa, which will require additional mining and auxiliary equipment.

The Essakane Gold Mine consists of several operating sites. The Essakane main pit is mined in several mining phases and accounts for over 80% of the production. The Falagountou and Essakane North satellite pits provide additional ore and operational flexibility.

The Essakane main pit comprises a total of seven mining phases. Mining by phases provides a sufficient quantity of ore by postponing and scheduling the mining of waste evenly over time. The average width of push-backs is 120 m and is limited to 30 m in certain areas. Mining by phases also provides effective operational flexibility through the simultaneous opening of several fronts, and the optimization of truck cycle times through good ramp system management.

Disposal on dumps is carried out in 7 m layers. This technique optimizes dump density and reduces time spent using bulldozers.

Ramp systems are planned at a 6% grade and have a width of 35 m. This configuration increases the speed of waste cycles.

Most of the major mining and auxiliary equipment are already in operation and only a minor amount of additional equipment is required for the heap leach project.

Based on the trade-off studies and the first round of column testing, the Essakane heap leach (HL) project has been designed as an open-pit mine with HL operation utilizing a multiple-lift, single-use pad. Engineering and design of a 10.0 Mtpa processing plant was undertaken for complete crushing, leaching, and carbon adsorption systems. Material will be crushed using a three stage high pressure grinding roll (HPGR) crushing plant. Crushed material is heap leached conventionally as a multiple lift heap in 10 m lifts. Gold and silver are leached using a dilute cyanide solution and recovered from the solution using a carbon adsorption process. The loaded carbon will be transported to the existing strip plant for dore production.

With the start of the heap leach (HL) operation, it is planned that a daily batch of 4.5 tonnes of gold loaded carbon will be transported by truck from the HL facilities to the existing Essakane concentrator.

Modifications to the concentrator will be required in the gold desorption and recovery circuits in order to process the additional carbon from the HL carbon-in-column (CIC) circuit. New carbon handling equipment, for both the loaded and the fresh/regenerated carbon, will also be required at the concentrator.

Crushing for the Project is accomplished by a three-stage crushing system with an open primary crushing circuit, a closed secondary crushing circuit, and an open tertiary crushing circuit operating seven days/week, 24 hours/day at a rate 27,397 tpd. The crushed ore bin is fed onto a 1,524 mm primary crusher discharge conveyor by an apron feeder. The 50-65 primary gyratory crusher is designed to crush ROM ore to 100% passing 314 mm.

Primary crushed ore is combined with the secondary cone product and is fed to the secondary screen feed bin. Material is reclaimed from the secondary feed bin with a 1,829 mm apron feeder onto the secondary screen. The secondary screen is a 4.2 m by 8.5 m double deck banana screen. The secondary screen oversize feeds directly to the MP 1250 secondary cone crusher. The secondary cone crusher discharge recycles back to the secondary screen. The secondary circuit is designed to crush ore to 80% passing 38 mm (100% passing 50 mm).

Secondary screen undersize material is placed on the secondary stockpile by the 1,219 mm secondary discharge conveyor, reclaimed using 1,829 mm apron feeders and is transferred to the tertiary crushing circuit.

The tertiary crushing circuit consists of small bin to choke feed a 170/140 HPGR crusher operated in open circuit with a final crushed product of 100% passing 19 mm. The product from the HPGR is conveyed to a fine ore bin.

The crushing system is also set up to provide the drainage gravel for the HL pad over-liner. The screen middlings can be diverted straight onto a mobile conveyor for stockpiling while the oversized material is recirculated to the cone crusher and the fines are passed through a temporary stockpile.

Crushed ore from the fine ore bin is reclaimed using belt feeders and is transferred to the 1,219 mm heap feed conveyor by the fine ore discharge conveyor. Quicklime is added to the ore on the heap feed conveyor at a rate of 1.3 kg/t ore from a 100 t silo equipped with bin activators, a screw feeder, a variable speed feed conveyor, and a dust collector. The quicklime addition rate is controlled by the output of a weightometer mounted on the conveyor belts.

The heap feed conveyor discharges onto a 1,219 mm overland conveyor which conveys the crushed ore to the HL stacking system.

The HL stacking system will be constructed in 80 m wide, 10 m high lifts using a 1,219 mm mobile conveyor stacking system. The Phase 1 leach pad conveying and stacking system will consist of an overland conveyor, four mobile ramp conveyors, 22 mobile grasshopper conveyors, an index feed conveyor, a horizontal index conveyor, and a radial stacker. The overland conveyor transfers material to the mobile conveyors which feed the conveyor stacking system. As the radial stacker progresses, the system is periodically stopped to add or remove grasshopper conveyors, as needed. Phases 2 and 3 will increase the leach area without any additional equipment required.

Once a lift of cells has finished leaching, and is sufficiently drained and dry, a new lift can be stacked over the top of the old lift. The old lift will be cross-ripped with a dozer prior to stacking the new lift to break up any compacted ore sections and to redistribute material that may have been broken down by the irrigation solution or rainfall. Stacked lifts will progress in a stair-step manner.

Following stacking, the material is irrigated with a dilute sodium cyanide barren leach solution and the resulting gold and silver bearing solutions are collected into the pregnant solution pond. The Essakane project has been designed as a single pass system with no recycle of intermediate solutions to the HL.

The total leach cycle of 90 days has been designed for the HL system, which is based upon metallurgical test work to date. Leach solutions will be applied to the ore at a nominal application rate of 10 L/h/m2 with an approximate cyanide concentration of 150 ppm to the heap.

Gold and silver bearing solutions draining from the leach pad are collected by a network of perforated drainage pipes and are directed to the pregnant solution pond. Pregnant solution is pumped from the pregnant solution pond by submersible pumps to the head tank of the carbon adsorption columns. The pregnant solution will flow by gravity through the carbon in the columns before being returned to the barren solution tank.

Precious metals in the HL pregnant solution will be adsorbed on to activated carbon in the carbon adsorption circuit. Loaded carbon from the carbon adsorption circuit is then transferred by truck to the existing adsorption-desorption-recovery (ADR) plant at the Essakane CIL facility to recover the gold and silver.

A 4.5 tonne batch of gold loaded carbon from the HL facilities will be trucked to the concentrator every day. At the concentrator, water will be added to the truck bin to transfer the carbon slurry to the dewatering screen. The dewatered carbon will be stored in a tank until a batch of at least 17 tonnes is obtained (after approximately four days).

The existing stripping circuit is designed to process 17 tonnes of carbon batches. Striping frequencies are currently at one per day with an elution time of 11 hours. To accommodate the additional carbon from the HL, the strips frequency will be increased from one per day to 1.3 per day, thus reducing the available downtime between strips.

To allow the 1.3 elutions per day to be achieved, the existing loaded carbon screen will be replaced by a larger unit. By increasing its capacity, the transfer time between the CIL head tanks and the acid wash column will be reduced.

In between regular CIL carbon strips, an HL carbon elution cycle will be performed. The HL gold loaded carbon will be re-slurried and pumped to the existing acid wash column. It will undergo the identical acid wash and elution treatment as the CIL carbon and the cycle times will be the same.

Considering the frequency of the elutions of 1.3 per day, two additional electrowinning cells of the same model as the existing ones will be added to increase the flexibility of the operation. This will bring the number of electrowinning cells to seven units in parallel.