The Ahafo mine is composed of three orogenic gold deposits that have oxide and primary mineralization. The gold is hosted in brittle shear zones cutting granitic intrusives that have kilometer-scale vertical and lateral extent. Gold occurs primarily in pyrite and secondarily as native gold in quartz veins.

Regionally, there are 12 known deposits in the Ahafo district, localized along multiple northeaststriking structural zones. Discrete mineralization styles are recognized within the Ahafo district, which are termed Kenyasi-style, Subika-style, and Subenso-style. The Kenyasistyle and Subika-style mineralization are identified within the Ahafo South Operations area and Subenso-style mineralization is identified within the Ahafo North area.

The Kenyase style is hosted in structures on or parallel to the regional Belt boundary separating basinal metasediments from Dixcove type granodiorite. High-grade zones occur in hydrothermal breccia and quartz veins accompanied by intense silica-albite-carbonate-sericite-pyrite alteration. Teekyere type deposits occur solely within folded metasediments intensely altered by carbonate, quartz, feldspar, pyrite, chlorite and sericite. They lack veins, instead occurring in pervasive alteration. Yamfo style deposits are similar to Teekyere type but occur in discrete veins. Newmont geologists Enders (2004) and Williams (2005) lump the Teekyere and Yamfo deposits together calling them Subenso type. Grades within the deposits are consistent, with the gold occurring with very fine disseminated pyrite. Arsenopyrite is absent; the ore is non-refractory above and below oxidation, which generally occurs to a depth of 50-75 metres (Griffis, 2004).

All of the “shear zone” deposit types appear to be part of the same mineralized system. As with many deposits located in tropical climates, a saprolite zone, typically between 5–50 m thick, is developed at surface. The saprolite zone gives way at depth to a sulfide zone, within which gold occurs in structurally-controlled zones of hydrothermal alteration.

Ahafo South Operations area.
The Apensu deposit is located on the main Kenyasi Thrust Fault zone at the southern edge of the Ahafo trend. It is considered to be a Kenyasi-style deposit. When open pit mining commenced, the deposit had dimensions of 3.8 km x 600 m and had been tested to 500 m vertical depth.

Mineralization was developed in mylonitic to cataclasite units along the sheared contact between footwall Birimian volcano–sedimentary units and hanging wall granodiorite. Footwall units included phyllonite (PHY), meta-volcano–sedimentary units (MV) and mixed mylonitic volcano– sedimentary units (GVM). Late-stage fine-grained aplite dikes that are sub-parallel to the Kenyasi Thrust Fault were logged but could represent fine-grained mylonite zones.

Mineralization is characterized by an association of silica–albite–carbonate–white mica–pyrite alteration, quartz veining and brittle chlorite-filled fractures. Better gold mineralization is developed in quartz–calcite veins associated with pyrite grains that can vary from fine disseminations to 1.5 mm in size. Gold occurs as single grains 1–20 µm in diameter but also commonly occurs in clusters of grains from 5–10 µm. There does not appear to be an association of gold with either arsenopyrite or rutile, and the gold is generally silver-poor, with <5 ppm Ag.

Visible gold occurs in the veined cataclasite. Locally, 0.2–2.0 cm wide quartz veins can return assays with more than 32 g/t Au from coarse gold. In the oxide zone, gold is associated with coarse goethite pseudomorphs after euhedral pyrite. Gold grains in the oxidized zone range from 5–10 µm. Manganese oxides are also observed in oxide mineralization.

Apensu Deeps
Apensu Deeps has dimensions of 3.9 km x 600 m and is tested to about 800 m vertical depth. The Apensu Deeps area is subdivided into four zones, Apensu South, Apensu Gap, Apensu Main, and Apensu North. Mineralization remains open at depth in all zones, and to the north in Apensu North.

Shear zone fabrics and fault geometries were inherited from early compressional deformation and include a strong cataclastic deformation of the hanging wall granitoids interpreted to be analogous to a crush breccia.

The Apensu Gap area is different to the Apensu South and Apensu Main zones, as the area lacks the mafic unit that is associated with Apensu South, and the cataclasis is very weak. In this area, it appears that low-angle faults control and limit the extent of better grade gold mineralization.

Subika
The Subika deposit is located about 2 km southeast of the Apensu Main deposit. It is developed in the hanging wall of the Kenyasi Thrust Fault but lies on a separate and parallel fracture zone (MFZ) to the fracture that hosts the Kenyasi-style deposits.

The Subika deposit has horizontal dimensions of approximately 2.2 km x 400 m, and is tested to about 1 km in vertical depth. Subika mineralization remains open at depth and along strike.

There is little development of either duricrust or saprolite, due to erosion associated with the old Tanoso River. Oxidation is limited to a thin (5–15 m) zone of complete oxidation of bedrock, followed by an irregular zone of partial oxidation extending as much as 20 m into primary bedrock.

Better grades of gold mineralization occur in dilatant zones (MFZ), ranging in width from 1–60 m. Hanging wall lower-grade mineralization tends to extend only about 30 m from the dilatant zones. Higher grade shoots within the dilatant zones plunge south at 20º to 70º. The high-grade zones appear to be controlled by dilatant left jogs in the MFZ created by offsets across the mylonite zones.

Four granitoid subset lithologies are recognized: diorite, gabbro, microdiorite, and diorite–gabbro mixed. Aplite and pegmatite dikes cross-cut the granitoid material.

Mineralization is hosted in the MFZ, which typically contains >2–5 g/t Au over widths of 5–50 m. Quartz and carbonate veinlets are common with thickness between 1–50 mm. They form stockworks in some instances and most of the veins are impregnated with pyrite, and in some cases with sparse visible gold at the contact with the host rock.

Awonsu
The Awonsu deposit is located approximately 1 km to the northeast of the Apensu deposit in a right-hand jog of the Kenyasi Thrust Fault and is a continuation of the Apensu mineralizing system.

The Awonsu deposit had horizontal dimensions of approximately 1,800 m x 150 m, and was tested to 450 m vertical depth. The mineralization remains open at depth and towards the north along strike.
The primary lithological units in the Awonsu deposit were altered to a depth of 40 m to saprolite, intensely oxidized, leached, and mottled and contain saprolite clay and quartz fragments.

Awonsu mineralization was typically more disseminated than that at Apensu.

Amoma
The deposit has horizontal dimensions of 1,500 m x 170 m, and is tested to approximately 300 m vertical depth. Mineralization remains open at depth.

Footwall rocks comprise a mixture of mafic volcanic units, and pelitic to turbiditic sedimentary units of the Birimian succession. Granitoids of dioritic to tonalitic composition comprise the hanging wall. Overlying the deposit is a layer of duricrust, which can be 8 m thick, comprising iron pisolites and transported alluvial cobbles. Saprolite is from 20–50 m thick.

Gold mineralization is developed primarily in the cataclasite unit. Mineralized zones that host gold grades >0.5 g/t Au range in width from 10–110 m. Higher-grade material (>1.5 g/t Au) is developed in the cataclasite, but lower-grade (0.5–1.5 g/t Au) mineralization locally occurs as a 20–50 m wide halo in the hanging wall granitoids. A narrower, lower-grade halo also occurs in the footwall mixed mylonite units, ranging from 0–30 m in width.

Ahafo has two active open pits, Subika and Awonsu. Subika added an underground operation, which reached commercial production in November 2018, and Awonsu completed a layback in November 2019.

Open pit mining is conducted at Ahafo South using conventional techniques and an Owneroperated conventional truck and shovel fleet.

Underground mining is currently conducted using conventional stoping methods, and conventional mechanized equipment. Underground mining is conducted by contractor African Underground Mining Services (AUMS).

The Subika underground mining operations are split into two areas:
• The Upper mining zone, above the 840 relative level (RL); also known as the upper Yoda area;
• The Central mining zone (corridor) below the 840 RL; also referred to as the Central area.

The mine plan assumes the use of a number of different mining methods, including:
• Sub-level open stope (SLOS): to be used above 840 RL;
• Long-hole open stope retreat (LHOSR): to be used above the 750 RL in the Central zone;
• Long-hole retreat with rockfill (LHSRF): to be used in the transition zone between the 750 and 665 RLs in the Central zone;
• Single lens retreat (SLR): to be used below the 665 RL in the Central zone, and in the North and South mining zones; stopes will be paste-filled.

Mining operations in the Sub level open stope (SLOS) zone will use existing infrastructure and spirals created on a 40 m level spacing to access the stopes. These stopes are mined from the lowest stope level upward in stope groups to create large open stopes. The ore on these levels is loaded directly from the mining extraction level to trucks and hauled up the existing main decline to the surface, and placed on stockpiles. Surface haulage equipment transports stockpiled material to the process plant.

To access the lower ore, below the 840 RL, a set of twin declines will be developed off the existing main haulage decline. The twin declines will be developed as a figure eight or elongated spiral configuration with one full rotation at 50 m intervals. The declines will be connected via a link drive that will act as a ventilation, escapeway and haulage connection between the two declines.

Both declines will act as a primary ventilation circuit with fresh air. Additional ventilation will be sourced through fresh air ventilation raises connected at the link drives and the foot wall drives below the 750 RL. The fresh air raise will deliver refrigerated air from the surface refrigeration system. The return air will be taken from the access drives above the 750 RL and from the ends of the foot wall drives below the 750 RL, using return air raises connected to the main fans at the surface and Portal 2.

Level accesses will be created off the decline at 25 m intervals to cross the ore zone from levels 800 to 700 RL. Below the 700 RL, the level interval will be increased to 35m.

Above the 750 RL, infrastructure such as substations, pump cuddies and sumps will be developed on these access drives. On these levels the ore drives will be developed from the access drives along the ore lenses identified as containing the mineralization for eventual stoping. The ore drives will be driven to the extents of the defined mining corridor and stoping will retreat from the end of the orebody towards the accesses. These stopes will be mined top-down. Stopes will be mined from the end of the ore drives back to the level access. Pillars will be left between stopes along the mine level for regional stability. Stopes will be mucked using remote mucking equipment back to an ore pass created between levels. Trucks will be loaded from the level below the mining extraction level via the material placed in the ore pass. The trucks will travel up the spiral declines to the main haulage decline and exit and enter via the North portal.

Below the 725 RL, the access drive from the decline will connect to a footwall drive that will be offset from the ore zone by 30 m. Stope access drives will be driven off the footwall drives to develop the stopes in the mineralized zone. The footwall drive will be used for infrastructure to connect ventilation returns, ore passes, substations, sumps and other infrastructure to support the mining on the levels. The mining direction for stopes in the areas below the 750 RL is centerout. Thus, stopes will be mined from a specified centralized location out to the extents of the orebody. These stopes will be mined bottom-up.

For stopes between the 750 and 665 RL, pillars will be left between stopes along the mine level for regional stability. Stopes will be mucked using conventional and remote mucking equipment back to the foot wall drive. Trucks will be loaded from the level below the mining extraction level via the material placed in the ore pass. When stope mining is completed, the stopes will be backfilled with rockfill using truck tips and remote loading. Stopes must be backfilled before adjacent stopes can be mined in the sequence.

For stopes below the 665 RL, no pillars will be left between stopes along the mine level for regional stability. Stability will be provided by backfilling the stopes with paste fill after the stopes have been mined. Stopes must be mined in sequence from a center-out, bottom-up approach with the mine sequencing a critical factor in controlling the high stress potential of these stopes. Stopes will be mucked using conventional and remote mucking equipment back to the footwall drive.

Trucks will be loaded from the level below the mining extraction level via the material placed in the ore pass. When stope mining is completed, the stopes will be backfilled with paste fill from a surface plant facility. Paste fill will be directed to the stopes through fill pipes from the surface to underground. Once a stope is filled and the backfill cured for the time required, the adjacent stope can be mined.

The daily production rate is approximately 95,000. tonnes.

The primary source of ore for 2019 will be from stockpiles and Subika, which will produce significant tonnes of high-grade ore. Three 9400 diggers will be located in Subika from May 2019 to the last quarter of 2019, when mining is planned to start in Awonsu Phase 3. Subika mining will constitute stripping of large quantities of waste from pit Phase 4. Mining will be completed in Subika by 2024, while Awonsu Phase 4 will be the last pit to be completed in 2029.

The Ahafo Mill Expansion was completed in October 2019 that expanded the existing plant by approximately 3.5 million tonnes per year through the installation of a new crusher, a single stage SAG mill and two leach tanks.

Crushing Circuit
ROM ore is dumped from haul trucks or a front-end loader into a feed hopper which feeds a 54- inch x 75-inch gyratory crusher. Primary crushed material is discharged into a surge hopper directly underneath the crusher. From the surge hopper, the primary crushed material is withdrawn to a reclaim stockpile via an apron feeder, conveyors and a transfer station. Where oxide ore is available, the oxide ore is loaded onto the tail end of the SAG mill feed conveyer. Primary ore from the reclaim stockpile is discharged to the SAG mill feed conveyor through apron feeders. A regulated amount of lime from a lime silo is also added to the SAG mill feed conveyor.

Grinding Circuit
The grinding circuit consists of a single 10.36 m diameter (Ø) x 5.00 m effective grinding length (EGL) SAG mill followed by a ball mill in closed circuit with hydrocyclones. The SAG mill and two MP800™ pebble crushers are in a closed circuit. The SAG mill discharge is classified via a pebble dewatering screen. The oversize from the screen is crushed via the pebble crushing circuit and returned to the SAG mill.

The undersize from the pebble dewatering screen reports to the cyclone feed hopper and is pumped to a cluster of classification cyclones. The cyclone underflow reports to the ball mill for regrinding. The cyclone overflow reports to the pre-leach thickener via the trash screens.

The processing plant was commissioned in 2006 to process 7.5 million tonnes of primary and oxide ore per year. With the depletion of oxide ore, the current plant throughput has decreased to 6.5 million tonnes per year. The processing plant consists of a crushing plant, a grinding circuit, carbon in leach tanks, elution circuit, counter current decantation circuit and a tailings disposal facility.

The Ahafo Mill Expansion which was completed in October 2019, expanded the plant capacity to process approximately 11 million tonnes per year through the installation of a new crusher, a single stage SAG mill and two leach tanks.

Wet Circuit
The undersize of the trash screens reports to the 42 m Ø pre-leach thickener with the thickener overflow recycled to the milling circuit as process water. The pre-leach thickener underflow is pumped to CIL leach and adsorption tanks in series. Carbon is added to the CIL tanks and flow countercurrent to the process slurry. The CIL tailings is discharge onto the carbon safety screens before being pumped to the countercurrent decantation (CCD) circuit.

The CCD circuit consist of two 42 m Ø thickeners. The overflow from the thickeners is recycled back to the process as process water while the underflow is pumped to the tailings disposal tank. Tailings are discharged via a spigot system into the TSF.

Stripping and Dore Production
The loaded carbon is recovered via a carbon recovery screen and treated in the elution and electrowinning circuit. Loaded carbon is acid washed with dilute hydrochloric acid in an 18 t acid wash column prior to transfer into an elution column where it is presoaked in a cyanide/caustic solution for 30 minutes to elute gold. The pregnant eluate is then rinsed from the carbon by as many as 10 bed volumes of water heated to 130º C. The resultant pregnant solution is pumped to electrowinning cells in which the gold is deposited on cathodes. The gold sludge on the cathodes is washed, dried and smelted in a furnace to produce doré. Doré is shipped to Switzerland to be refined to bullion at Valcambi.

A CCD circuit was commissioned in 2008 to recover cyanide from CIL tailings prior to discharge to the TSF. Recovered cyanide is effectively re-used in the CIL circuit and weakly acid-dissociable cyanide (CNWAD) levels in the plant tailings are effectively controlled to ensure the discharge limit of 50 ppm CNWAD is not exceeded.

A gravity circuit that was initially included in the plant was decommissioned in 2010.