Gold mineralisation is associated with, and occurs within, graphite-chlorite-sericite fault zones. These shear zones are commonly associated with pervasive silica, carbonate and sulphide hydrothermal alteration and occur in tightly-folded Lower Birimian schists, phyllites meta-greywackes, and tuffs, along the eastern limb of the Kumasi anticlinorium.

Two main ore types are mined, namely quartz vein and sulphide ore. The quartz vein type consists mainly of quartz with free gold in association with lesser amounts of various metal sulphides containing iron, zinc, lead and copper. This ore type is generally nonrefractory. Sulphide ore is characterised by the inclusion of gold in the crystal structure of arsenopyrite minerals. Higher gold grades tend to be associated with finer grain arsenopyrite crystals. Sulphide ore is generally refractory.

The mine is located within the Obuasi concession area in south-western Ghana along the north easterly-striking Ashanti volcanic belt. The deposit is one of the most significant Proterozoic gold belts discovered to date. The Ashanti belt predominantly comprises sedimentary and mafic volcanic rocks, and is the most prominent of the five Birimian Supergroup gold belts found in Ghana. The belt is a 300km wrench fault system that propagated from Dixcove in the south-west to beyond Konongo in the north-east.

The Birimian was deformed, metamorphosed and intruded by syn- and post-tectonic granitoids during the Eburnean tectonothermal event around two billion years ago. Folding trends are dominantly north-northeast to north-east. Elongate syn-Birimian basins developed between the ridges of the Birimian system and these were filled with the Tarkwaian molasse sediments made up primarily of conglomerates, quartzose and arkosic sandstones and minor shale units. Major faulting has taken place along the same trends.

Mine designs are done to delineate development layouts and production stopes by taking into consideration economic cut-off grade and geotechnical design parameters for each mining block, mining level and section. The underground development system extends to a depth of 1,500m from surface. Mining levels lie between 15m and 20m intervals with major levels between 30m-60m intervals. Underground production was by open- stope mining (both longitudinal and transverse), and sub-level caving method; with future designed production by longhole open stope mining methods with paste fill.

The processing facilities currently consist of a Sulphide Treatment Plant (STP) and an Alternate Ore Treatment (AOT) Plant which treats low grade oxide material and tailings. The operation has two Tailings Storage facilities (TSF’s), one of which is no longer receiving tailings material and the other is due for closure by the end of 2015. A new TSF is currently under design and is planned to come into operation to take over from the one currently receiving STP tailings.

The Sulphide Treatment Plant (STP) has a design capacity of 274 tons per hour (tph) of refractory transitional and primary sulphide sulphur ores yielding 30tph of sulphide sulphur concentrate for processing in its BIOX® facility. The treatment process flow includes primary crushing, semi-autogenous/ ball milling, flash flotation and conventional flotation, bio-oxidation and carbon in leach (CIL) gold recovery. Gravity Recoverable Gold (GRG) is recovered at the Mill and BIOX® Circuits via Knelson concentrators.

The milling circuit consists of two semi-autogenous (SAG) mills, SAG 1 and SAG 2, in open circuit and a Ball Mill in closed circuit with a cluster of cyclones. The SAG and Ball mills discharge is pumped to a set of primary cyclones whose under flow reports to the Knelson concentrators for free gold recovery while the overflow is screened, conditioned and subjected to conventional flotation.

Flash flotation cells are fed with the Knelson concentrator tailings, to recover freshly liberated coarse sulphides which are further milled in a regrind mill and classified to obtain a finer material as product. Tailings of the flash cells gravitate into the ball mill whose discharge joins the SAG discharge.

The flotation circuit consists of three 100 m3 capacity OK Tank cells in series, seven 42.5m3 capacity conventional scavenger cells, and five 16m3 capacity OK cleaner cells. The generated flash concentrate combines with the final concentrate from the conventional flotation circuit in the concentrate thickener and the thickened pulp is pumped into a surge tank to serve as feed to the BIOX® plant.

The tailings from the scavengers is thickened and pumped to join neutralized effluent for backfill preparation for the Underground backfilling operations.

Concentrate from the Knelson is treated by intensive cyanidation in an In-line Leach Reactor (ILR) and the generated pregnant solution is electro won in the Gold House. The ILR tailing is pumped into a settling cone at the AOT Carbon–in –Leach (CIL) circuit. The settled solid is drained into the AOT CIL head leach tank.

The STP BIOX® Plant designed capacity is 1050 dry tons of flotation concentration per day. Flotation concentrate in the storage/surge tank is fed to the BIOX® reactors at 20% solids after automatic dilution. The feed is split into three trains of six reactors (895m3 each). Nutrient which is a source of Nitrogen and Phosphorus is added at this stage. Potassium which is also required by the bacteria is naturally available in the concentrate. The bacteria also require oxygen and carbon for cell growth, which is available from the air sparged into the reactors, and from carbonates in the ore.

Each of the three trains consists of three Primary reactors in parallel and three secondary reactors in series. The fourth reactor in each module also serves as a primary when any of the dedicated primaries are off line for maintenance. The reactors are made of stainless steel and equipped with air sparges and cooling coil baffles. Sulphur and iron oxidizing bacteria in the reactors oxidize the sulphides and liberate the occluded gold during the process to render it amenable to cyanidation. Major factors which influence bacterial activity are temperature, pH, dissolved oxygen and pulp densities.

The oxidized pulp from the BIOX® circuit which is acidic and has high dissolved iron and dissolved arsenic content is pumped into a series of four Counter Current Decantation (CCD) high rate 20m diameter thickeners. Sufficient amount of water is added to the last thickener to wash out the dissolved salts from the solids. The decanted liquor or effluent is pumped to the neutralization circuit. In addition to the washing of slurry, the CCD circuit thickens the pulp to about 40% solids prior to cyanidation at the BIOX® CIL circuit. The reduction in acid content reduces the lime consumption at the CIL and the reduction in iron and arsenic reduces cyanide consumption. Incorporated in the CCD circuit is a gravity circuit between the third and fourth stages and it consists of a cluster of 10 inch cyclones and a 30 inch Knelson Concentrator with a G-5 cone. The concentrate from the Knelson concentrator is subjected to intensive cyanidation in a batch In- line Leach Reactor (ILR). Gold solution from the ILR process is pumped to the eluate tank for electrowinning in the Gold house. 5 to 8% of the total plant gold production comes from this circuit.

The BIOX® effluent contains mainly arsenic (V), as well as ferric sulphate and sulphuric acid. This solution is treated in a series of six agitated neutralization tanks equipped with air sparges.

The newly commissioned BIOX® CIL circuit consists of three (3) pre-oxidation tanks with live volume of 372m3 each, and eight leach tanks of live volume of 717 m3 each and eight (8) CIL tanks. The pre-ox step employs a two stage pH conditioning with lime and high DO levels of 25 ppm to precipitate Fe and S ion species. A 10 tpd oxygen plant has been installed to generate enough oxygen for the pre-ox and leach tanks. The design of the new plant incorporates all of the cyanide code compliant features. Installed also is a cyanoprobe to provide online cyanide concentrations. The Flotation CIL is made up of six tanks each with a live volume of 1600m3. All of the flotation CIL tanks are currently employed to leach Diawuoso tailings material from the North of the Mine. The final leach residues from the two streams are pumped to the holding tank at the Backfill plant and subsequently pumped to the Sansu Tailings Storage facility (TSF).

Gold-laden carbon from the CIL circuits are treated in a desorption circuit employing the Anglo America Research Limited (AARL) system. Gold-rich eluate is passed through electro winning cells situated in the Gold house in which the gold is recovered from the eluate by electroplating onto steel wool cathodes. Gold loaded steel wool cathodes are calcined in electric ovens. The calcined cathodes are then smelted in a diesel furnace to produce bullion bars.