The deposits that comprise the Ahafo Operations are considered to be examples of orogenic gold deposits.

Two distinct deposit styles are recognized within the Ahafo Operations. Kenyasi-style deposits, comprising Apensu, Awonsu, and Amoma, are associated with the Kenyasi Thrust Fault, along a sheared thrust contact between Dixcove Suite granitoids and footwall volcano-sedimentary units. Subika-style mineralization comprises gold that is entirely hosted in Dixcove granitoids, in the hanging wall of the Kenyasi Thrust Fault. To date, the only recognized deposit of the style is Subika.

Gold typically occurs as native gold, associated with pyrite. Alteration associated with the deposits includes silicification, albitization, pyritization, and carbonation.

Mineralization in Kenyasi-style deposits is associated with mixed (meta)-pelitic sedimentary rocks and (meta)-mafic volcanic units along the footwall of the Kenyasi Thrust Fault. Dixcove Suite granitoids form the hanging wall to the thrust, and appear to be overthrust onto the volcano–sedimentary sequence. Multiple thrust fault duplexes developed along the thrust contact between the granitoids in the hanging wall and volcano/sedimentary rocks in the footwall and are favorable sites for gold deposition.

In Subika-style deposits, mineralization is hosted in Dixcove Suite granitoids. The granitoids are cut by multiple mylonite zones that occur as imbricate thrusts and vary in thickness from < 1 m to as much as 10 m. Zones of brittle fracturing and dilatant breccias are commonly developed over the mylonite zones and are favorable loci for gold deposition.

Apensu
The Apensu deposit, a Kenyasi-style deposit, is located on the main Kenyasi Thrust Fault zone at the southern edge of the Ahafo trend.

The Apensu deposit had horizontal dimensions of approximately 4,200 x 600 m, and has been drill tested to 800 m vertical depth. The mineralization remains open at depth and towards the north along strike.

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, meta-volcano–sedimentary units, and mixed mylonitic volcano–sedimentary units.

The shear zone varied in width from about 10–75 m in true width, with gold mineralization grading >0.5 g/t Au and varying from 30– 150 m in width. Higher gold grades (>5 g/t Au) were hosted in, or immediately adjacent to, strongly-altered quartz–calcite veined cataclasite. The veins ranged from veinlets of 0.1–3.0 cm in width to silica-rich veins that ranged from 2–10 cm in width.

Awonsu
The Awonsu deposit, a Kenyasi-type deposit, developed on the sheared contact between mafic volcanic rocks, metasedimentary rocks and Dixcove granites. It is a continuation of the Amoma deposit, with the two mineralized zones separated by a zone of lowergrade, sub-economic mineralization.

The Awonsu deposit had horizontal dimensions of approximately 4,100 m x 150 m, and has been drill tested to 600 m vertical depth. The mineralization remains open at depth and towards the north along strike.

Footwall to the mineralization is a mixture of mafic volcanic and pelitic to turbiditic sedimentary units. The hanging wall is composed of granodiorite. Mixed mylonitic and cataclasite units and dilatant breccias, developed during plastic and ductile deformation occur in the sheared contact between the footwall and hanging wall.

Awonsu mineralization was typically more disseminated than that at Apensu. The shear zone varied in true width from 5–100 m, with gold mineralization >0.5 g/t Au ranging from 5–150 m in width.

Higher gold grades (>1.5 g/t Au) were hosted in, or immediately adjacent to, strongly-altered cataclasite, forming zones from 5–60 m in width. Grades >5 g/t Au were rare, but high-grade zones could be as much as 30 m wide. Gold grades of 0.5–1.5 g/t Au were more commonly developed in the fractured, moderately-altered hanging wall granodiorite.

Lower-grade material typically formed a halo of 2–50 m in thickness. Locally, particularly on the northern side of the deposit, highergrade areas within the hanging wall alteration zone occurred in discontinuous mylonite zones, and in fine stringer quartz veins. A narrower low-grade halo, ranging in width from 5–30 m, occurred in the footwall. As with Apensu, higher-grade shoots were associated with a southward plunge. Typically, the shoots averaged about 2–5 g/t Au versus >5 g/t Au in Apensu.

Awonsu is the only deposit within the Ahafo Operations where multiple generations of cross-cutting milky to opaque quartz veinlets with open-space filling of minor pyrite and gold mineralization were observed. Distinct, sheeted, sub-parallel milky quartz veins, 0.1– 2 cm in width, with minor pyrite and occasional coarse gold, cross-cut fresh to weakly-altered hanging wall granodiorite. The milky veins generally occurred in sets of 2–10 veinlets that were separated by 10 cm to 1 m.

Subika
The Subika deposit, to date the only example of Subika-style mineralization, developed in the hanging wall of the Kenyasi Thrust Fault but lies on a separate and parallel fault zone to the thrust fault complex that hosts the Kenyasi-style deposits.

The portion of the Subika deposit being exploited in the open pit has horizontal dimensions of approximately 2.2 km x 400 m, and is tested to about 800 m in vertical depth. The portion of the deposit being exploited from underground is the continuity of mineralization below the open pit. This portion of the deposit has horizontal dimensions of approximately 3.7 km x 400 m, and is tested to about 1,600 m in vertical depth. Subika mineralization remains open at depth and along strike.

Alteration is controlled by the 5–40 m wide “Magic Fracture Zone” (MFZ), a continuous zone of quartz–albite–sericite–carbonate– pyrite alteration.

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.

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 display sparse visible gold at the contact with the host rock.

Ahafo has two active open pits, Subika and Awonsu. Two open pits, Apensu and Amoma, were mined out in 2016 and 2017 respectively. The Apensu open pit is being used for water storage. 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 using conventional techniques and an Owner-operated conventional truck and shovel fleet. Underground mining is currently conducted using conventional stoping methods, and conventional mechanized equipment. Underground mining is conducted by a contractor.

Open pit
Subika mining is in the final stage with limited potential to grow at depth because of the underground crown pillar. No changes were made to the Phase 4 pit design for Subika. The minimum mining width between the Subika Phase 4 and the mined-out third phase is 50 m. All operating pits are mined on 8 m benches. Mining dilution and recovery are included in the block model, based on historic reconciliation.

Open pit design uses defined geotechnical domains together with rock mass quality ratings for the principal lithologies and appropriate pit design criteria that reflect expected conditions and risk. Inter-ramp angles vary by deposit and pit wall lithology, and range from 30–55º.

The active pits are currently mining below the water table. Pit dewatering uses a combination of perimeter and in-pit dewatering wells, in-pit sumps, and horizontal drains. A network of monitoring piezometers is installed around all of the operating pits.

The LOM plan currently envisages mining at an average rate of approximately 26 Mt/a for nine years and peaking at 32.2 Mt/a in 2022 with a maximum rate of advance by pit stage of eight benches per annum and an average of six benches (48 m) per year. The mine life will extend to 2030 with material mined from the open pit. Milling will cease in 2032 after treatment of stockpiled ore.

Ore Loss and Dilution
All operating pits at Ahafo South are mined on 8 m benches. The Subika model is a 24 x 12 x 8 m model to account for the 8 m mining. Block models for Awonsu, however, are produced using a 12 x 12 x 8 m block dimension to reflect the increased selectivity in ore zones

Pit design assumptions include haul road widths for two-way travel of 30 m, maximum ramp grades of 10% and minimum pit-bottom widths of 30 m in deep pits as a safety measure. For the last couple of benches to the pit bottom where good grades are located, the haul road widths are reduced to a 21 m one-way traffic to allow for maximum ore mining recovery.

Production drilling and blasting for the open pits is conducted on 8 m benches with a subdrill of 1.2 m, using a 165 mm diameter bit. The pattern for production drilling is 4 x 4.5 m in both ore and waste, with powder factors varying by material type and geological conditions. Bulk emulsion is loaded into both production and buffer holes; the stemming length varies according to rock type and other geologic conditions but it is generally at 3.3 m. Pre-splitting is conducted on all pit wall areas with power-split explosives supplied by the explosives provider, Orica. The powder factor for open pits is 0.92kg/m .

Underground
Mining levels are based on the mining method to be used, which varies by depth from surface. A set of twin spiral declines was developed off the existing main haulage decline. Level accesses were created off the decline at 20–25 m intervals, depending on mine elevation to intersect the ore zone. The ore drives have been driven to the extents of the defined mining corridor and stoping being retreated from the end of the orebody towards the accesses. These stopes are being mined top-down.

Mining was initially envisaged as long-hole open stoping mining method; however, an improved understanding of the geotechnical setting led to the selection of SLS in preference. A transition zone between mining methods at 450 m below surface was required to migrate the different stoping types. The mine will completely transition to the SLS mining method when the long-hole open stopes are complete, but currently the two mining methods are being used together.

Stopes in the central mining zone, 800–700 RL, are being mined using the sub-level open stoping mining method through a set of twin spiral declines that were developed off the existing main haulage decline. Level accesses were created off the decline at 25 m intervals to intersect the ore zone.

The ore drives were driven to the extents of the defined mining corridor and stoping is being retreated from the end of the orebody towards the accesses.

These stopes are being mined top-down. The stopes were mined in panel with the maximum span up to 100 m vertical distance. Stopes were mined using a combination of longitudinal and transverse retreat methods in 25 m sublevels. A section of the mining area on the 800–750 RL will be mined in 50 m panels to increase productivity. Stopes were mucked using a combination of free and remote bogging. The ore on these levels was loaded directly from the mining extraction level to trucks or/and stockpiles, hauled up through a designated(one-way) main decline to surface, and placed on the run-in-mine (RIM) pad. Surface haulage trucks transported material from the RIM pad to the process plant ROM pad.

The open stopes were backfilled with unconsolidated rock ones the stope panel was completed.

The declines were connected via a link drive that acted as a ventilation, escapeway and haulage connection between the two declines. To increase productivity, there was a one-way traffic in and out of the mine with the aid of the two declines and decline links.

Below the 725 RL, the access drive from the decline connected to a footwall drive that was offset from the ore zone by 30 m. Stope access drives were driven off the footwall drives to develop the stopes in the mineralized zone. The footwall drives were used for infrastructure to connect ventilation returns, fresh air, sumps and other infrastructure to support mining on the levels.

The second mining method being used was the SLS method. This started from the 680 RL. The mine will totally transition to the SLS mining method in a few years when the open stopes are complete, but currently, the two mining methods are being used together.

A 50 m sill pillar was established from the 750–700 RL to separate the two mining methods. The sill pillar houses infrastructure such as the 15 fill passes for backfilling and the geotechnical instrumentation monitors.

The first two levels, 680 RL and 660 RL, have 20 m sublevels and from 635 RL, the sublevels are every 25 m.

Apart from the 680 RL that is using the longitudinal mining approach, the rest of the levels are/will be mined using the transverse method. Mining commences from the center of the orebody out towards the draw point extremities, thus splitting the mining fronts into two halves. Production rings are being fired adopting the chevron mining pattern. This will ensure the mine achieves multiple mining fronts to maximize production.

A structured draw percentage strategy by level was used for the extraction of the blasted material which started from 45% draw in high-grade rings and 30% draw in low-grade rings on the 680 RL.

Current plans are to mine two sublevels concurrently due to geotechnical seismicity guidelines. Unconsolidated backfill material are introduced through the fill passes to ensure wall stability and maintain the integrity of the sill pillar. The backfill material is currently being sourced from the underground waste development headings and later, the yet to be developed waste pass from surface. The waste pass from surface will make use of the open pit WRSF material.

Blasting and Explosives. For optimal drilling efficiency a burden of 2.8 m with ring toe spacing of 3.2 m is used for stopes.

- subscription is required.

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 process consists of: primary crushing, semi-autogenous grind (SAG) milling, carbon-in-leach (CIL), Anglo American Research Laboratory method (AARL) elution circuit to strip gold from loaded carbon, smelting to produce doré, and counter-current decantation to recover cyanide from CIL tailings prior to discharge to the tailings storage facility (TSF).


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.