The Banket Series in the mine lease areas form prominent, arcuate ridges extending southwards from Tarkwa, westwards through Iduapriem and northwards through Teberebie to Mantraim. The ridge segments are supported by a massive lithological unit known as the Footwall Quartzite, which is a strongly-bedded rock of blue-grey colour exhibiting a sub-parallel haematitic/black sand banding and which locally forms the basal stratigraphic unit to the Banket Reef Zone (BRZ).

The BRZ comprises a sequence of individual beds of quartz pebble conglomerates (Banket beds), breccia conglomerates, metasandstones (also called quartzites and grits). All known gold mineralisation within the Banket Formation is associated with the conglomerates and is found within the matrix that binds the pebbles together. Gold content is presumably a function of the size and amount (packing) of quartz pebbles present within a conglomeratic unit- more pebbles present suggest more gold.

Gold mineralisation is hosted by conglomeratic sedimentary packages referred to as zones or reefs and is considered to be of primary palaeoplacer origin. There are four recognized reefs, namely A, B, C and D and which are equivalent to the Tarkwaian Sub-Basal, Basal (or Main), Middle (or West) and Breccia Reefs respectively. The B and C reefs are oligomictic, and consist of well sorted conglomerates and have been mined underground in some areas for over a century. The A and D reefs have a lower gold tenor and are polymictic containing both well rounded and angular fragments. The gold is fine-grained, free and not associated with sulphides. The main ancillary mineral is magnetite.

At Iduapriem, the entire Tarkwaian Group has been folded into a broad syncline, locally referred to as the Tarkwa Syncline. The reefs are stratigraphically located within the Banket Formation and are the locus of the mine towards the southern closure of the Tarkwa Syncline. With the gold mineralisation being stratabound, the characteristics of the mineralisation are defined by the reefs. The Tarkwa Syncline covers the eastern portion of the lease (Blocks 1-8), with the Ajopa deposit located in the north-western lease on the western limb of the Ajopa anticline.

For blocks 1 to 8, the pits are numbered sequentially from east to west across both limbs of the Tarkwa Syncline. These limbs, which outcrop and express themselves as topographic ridges, trend NE on the easterly limb and NNE on the westerly limb. The closure of the syncline results in an east-west cuspate shape to the reefs at the southern limits of the Banket formation. Reef dips are variable and reflect the structural complexity of the basin. The eastern limb of syncline dips steeply 70-80 degrees towards the west, however this changes to the south where the dips become steeper, eventually becoming overturned and southerly dipping at the southern margin of the basin in the vicinity of blocks 1 and 2. Moving westwards to block 3, the dips again dip into the syncline and flattening to approximately 50 degrees by Block 3 east. At Blocks 7 and 8, which are the dominant constituents of the Ore Reserve, the reefs dip 35 to 30 degrees to the west. Reef packages vary in thickness, with those in Blocks 7 and 8 being between 3 and 10m thick, but in the vicinity of Block 1 reefs thin, onlap and thicken due to thrusting.

At Ajopa, where the reefs are expressed as a topographic ridge striking NE-SW, dips are generally 50 to 60 degrees to the west. All four reefs are present, however the B and C reefs are more dominant and thicknesses vary between 3m and 8m.

Faulting is common and major faults are believed to be contemporaneous with the folding. Within the Tarkwa Syncline, faults form bounding structures to the blocks and separate the individual pits from each other.

Iduapriem is an open pit mine, making use of contract miners. The mine fleet consists of two – Lieb 9250 and one – Lieb 984 excavators and 26 – CAT 777s for Block 7 and 8 mining. An additional fleet of one – CAT 390D and one – Volvo EC 750D and 14 – Volvo ADT for Ajopa mining. A further 12 articulated haul trucks are used to haul ore from the Ajopa pit to the plant.

The Iduapriem Mine started operating in June 1992. The original plant, designed to treat about 2.4Mtpa of ore, has undergone several expansions. The most recent expansion work was completed and commissioned in 2009 which incorporated a new 800tph 2-stage crushing plant, a second ball mill at the CIL plant together with a 24 meter diameter thickener, a second Knelson concentrator, and an upgrade of the oxygen plant capacity from 4.0 to 8.0 tonnes per day. The main objective of the expansion project was to increase plant throughput to 4.3Mtpa.

A gravity circuit comprising of 2 XD48 centrifugal (Knelson) concentrators and an In-line Leach Reactor (ILR – Series 1000) recovers free gold (gravity gold) by processing the dense particles from the cyclone underflow slurry. Each circuit has 1 Knelson and part of the cyclone underflow is bled to feed the Knelson.

The concentrator operates in batch mode. Once the preset time has elapsed, the unit goes through a flush cycle time to discharge accumulated concentrates. The concentrates are flushed to the concentrate tank for further treatment by intensive cyanidation through a batch-type In-line Leach Reactor (ILR-Series 1000), where the gold is dissolved and electrowon onto steel wool cathodes in a dedicated electrowinning cell. Gravity Gold accounts for approximately 35-42% of gold produced from the Iduapriem gold treatment plant.

There are four leach tanks and seven adsorption tanks in the CIP circuit. Cyanide concentration of 250-300ppm and an average dissolved oxygen level of about 24ppm are maintained in leach tanks #1and #2. Additional cyanide and oxygen points are located in leach tanks #3 and #4 to ensure leaching of the gold into solution is maximized before the slurry enters the adsorption circuit. On average the leach contact time is between 18-22 hours and gold recovery about 93.5% - 95.0%. Oxygen supply to the leach tank is from an 8 tonnes per day plant which injects the oxygen to the leach tanks through oxygen spargers.

Carbon is used in the adsorption tanks to recover the gold cyanide complex ions out of solution as the carbon moves in counter current direction to the flow of the slurry. Carbon is continuously moved from tank to tank via recessed impeller pumps accumulating higher gold values in the process. Carbon is then recovered from adsorption tank #1 for elution. Carbon can also be recovered from adsorption tank #2 should adsorption tank #1 be isolated or taken out of circuit for maintenance purpose or any other reason.

The Anglo American Research Laboratory (AARL) method is employed to get the adsorbed gold on the carbon back into solution form using about 3-5% caustic solution. The gold is then electroplated onto steel wool cathodes in an electrowinning cell. The loaded carbon is recovered from adsorption tank #1 into the acid wash column. This is then acid washed using 3% strength hydrochloric acid followed by water rinsing. The rinsed carbon is then transferred to the elution column where caustic solution of 3-5% is circulated through the
elution column via the heat exchangers until an elution temperature between 110°C – 120°C and an operating pressure of 300-400Kpa is attained in the column.

The caustic solution is then directed into a pregnant electrolyte tank and hot water is used to rinse the solubilised gold from the carbon into the pregnant electrolyte tank. The pregnant electrolyte is pumped through two electrowinning cells for gold deposition. Once or twice weekly the loaded steel wool cathodes are removed, calcined and smelted into Doré bullion.

The barren carbon is transferred to the adsorption circuit or to the carbon regeneration kiln where it is thermally reactivated at 650°C in a horizontal diesel-fired kiln. The reactivated carbon is collected in a quench tank and hydraulically transferred into adsorption tank #7 through a carbon sizing screen.

On the mine site, sodium cyanide, can be found either in the solid form as briquettes or in solution. The cyanide arrives on site as solids in briquette form and is stored in the Orica Tarkwa Facility yard. (The Orica Tarkwa Facility (OTF) is located on land owned by Iduapriem mine.) The OTF reagent yard is fenced off and is under lock and key to restrict public access. On request by the process plant for cyanide as scheduled, 20 tonnes of solid cyanide is charged into an isotainer at the OTF and transported to the process plant under security escort. At the plant, the 20 tonnes of solid cyanide is sparged or dissolved into a solution state using the new cyanide sparging facility. The solution cyanide can then be used for gold cyanidation processes. On the treatment plant, cyanide solution is used in the leaching process in the leach tanks and gravity circuits, desorption process and metallurgical test work.

During the 2017 year the processing facilities were converted from a carbon-in-pulp (CIP) to a carbon-in-leach (CIL) plant with a gravity circuit.