The ore bodies are located within the Tarkwaian System, which is an important stratigraphic component of the Ashanti Belt in south-western Ghana. The Ashanti Belt is a north-easterly striking, broadly synclinal structure made up of lower proterozoic sediments and volcanics underlain by the metavolcanics and metasediments of the Birimian System. The Tarkwaian unconformably overlies the Birimian, and is characterised by lowerintensity metamorphism and the predominance of coarse grained, immature sedimentary units

Mineralisation hosted by conglomerates
- Laterally continuous with mid- to longrange predictability
- Clear patterns of mineralisation governed by sedimentary characteristics
- Exploration programmes ongoing to test homogeneity of geology and grade domains
- Targets are both palaeoplacer and hydrothermal.

The mine utilises selective surface mining methods to optimise the extraction of the mineralised conglomerate units. Short-term mine planning and forecasts are based on grade controlled models. During mining, fresh rock and transitional zones are drilled and blasted in six-metre lifts, with excavation in three metre flitches.

The mine is owner-operated and has its own load and haul fleet of 13 excavators, which range from 120 to 400 tonnes. The haul fleet includes 67 dump trucks with a payload of 146 tonnes, eight dump trucks with a payload of 240 tonnes and nine dump trucks with a payload of 90 tonnes. The load and haul fleet are supported by an ancillary fleet consisting of dozers, graders, water carts and compactors. A total of 22 owneroperated drill rigs are used for blast hole drilling. Maintenance of the fleet is carried out by Tarkwa mine, while maintenance of the excavators is carried out by contractors.

Tarkwa is in the process of assessing and potentially changing its business model from owner to contractor mining. The current outlook for longer hauling distances and increased drilling costs due to rock hardness will drive further cost pressures at site. A transition from owner mining to contract mining provides an opportunity to abate cost increases and maximise cash-flow, and in conjunction with an accelerated exploration campaign assist in underpinning the mine’s sustainable long-term future.

Ore is processed utilising a conventional CIL plant, with a gyratory crusher feeding a SAG mill and ball mill. Gold is recovered from solution by electro-winning and smelted in an induction furnace. Current plant capacity is 13.5Mtpa.

Since 1999, all ore has been sourced from open pit mining operations and is currently processed at the North Heap Leach facility and the CIL plant.

The North Heap Leach employs three stages of crushing prior to agglomeration. RoM ore is directly tipped by dump trucks or reloaded by front-end loaders into a 50” x 65” Sandvik Superior primary gyratory crusher. The crushed product (100% passing 250mm and 80% passing 150mm) gravitates to a 1.83m x 6.82m D4 Jacques A Terex apron feeder, which feeds two secondary screens via a conveyor.

The oversize from the two 2.44m x 7.32m Nordberg double deck scalping screens of aperture sizes 50mm and 32mm for the upper and lower deck respectively feed two Superior gyratory secondary crushers (S6000). The undersize joins the final product to the agglomeration stockpile via a series of overland conveyors.

Secondary crushing product is fed to the tertiary hopper via series of conveyors. Ore withdrawn from this hopper is fed by six 0.75m x 1.80m Nordberg Vibratory feeders to six 2.44m x 7.32m Nordberg double deck scalping screens operating in closed circuit with six tertiary H4000 hydrocone crushers. The oversize from the upper and lower decks (32mm and 22mm respectively) feed the tertiary crushers and the discharge from the tertiary crushers is conveyed back to the tertiary bin. The undersize from the tertiary screens join the final crushing product to the agglomeration stockpile.

The agglomeration process involves the addition of 4 kg/t of cement to the crushed rock to bind the fine material and produce an agglomerate that remains stacked and porous on the heap leach pads. Following agglomeration, the ore is transferred by conveyor and stacked on the leach pads by a stacking conveyor. The heaps are irrigated with a cyanide solution which dissolves the gold as it percolates through the heaps. The pregnant solution is collected on the layer of geotextile that lines the base of the heaps and is pumped through a series of ponds to the adsorption/ desorption/recovery (ADR) plant where the gold is adsorbed onto activated carbon, removed from the carbon by acid wash, and recovered using electrowinning. The North facility has a smelt house with diesel-fired furnaces to smelt all gold produced at the North facility.

The CIL process route has a 54” x 75” Gyratory crusher that feeds two crushed ore stockpiles that have a live capacity of 45,000 tonnes (30 hours). Underneath each stockpile is a reclaim tunnel, with apron feeders that feed onto a conveyor belt, which in turn feeds the milling circuit. The milling circuit consists of a SAG and ball mill with recycle crushing in closed circuit with the SAG mill. The SAG mill has an effective grinding length of 42” with an internal diameter of 27” and 14 MW of installed power (2 x 7,000 kW twindrive motors). The ball mill has an effective grinding length of 36’ with an internal diameter of 26” and 14 MW of installed power (2 x 7,000 kW twin drive motors) and is in close circuit with the cyclone cluster. The milling circuit it operated at a capacity of 1,450 tph. The CIL circuit consists of two trains of eight tanks in series fed from a common leach tank. The loaded carbon passes into a 15-tonne acid wash column. The gold is recovered from the loaded carbon in two 15-tonne elution circuits. Gold is recovered from solution by electrowinning and smelted in the CIL smelt house in an induction furnace.