Bissa Group Properties are underlain by geology considered to be similar to that of other Birimian-age volcano-sedimentary sequences. Hence, this region is considered prospective for orogenic gold deposits, which typically exhibit a strong relationship with regional arrays of major shear zones.

The gold mineralisation is typically associated with regular networks of quartz veins containing subordinate amounts of carbonate, tourmaline, sulphides and native gold. In these deposits, the gold is typically free milling. Alternatively, the gold mineralisation can be also associated with disseminated sulphides in strongly deformed alteration zones. In the latter case, gold may be free milling but also refractory.

In Burkina Faso, the weathering is deep and typically results in extensive surface oxidation of bedrock to depths reaching more than 100m locally. In such areas, gold deposits typically comprise a surface oxide zone, an intermediate transition zone and a deeper fresh rock zone. Gold is typically free milling in the oxide zone.

The gold mineralisation consists of disseminated sulphides, sulphide stringers, quartz veining and attendant hydrothermal alteration and quartz breccia bodies developed in highly strained sedimentary and volcanic rocks, loosely sub-parallel to the dominant foliation fabric of the Sabce shear zone.

Bouly, which lies to the southeast of Bissa, appears to represent a large, low grade goldcopper porphyry.

The bedrock comprises massive non-foliated porphyritic mafic metavolcanics, brecciated by younger porphyritic intrusive dykes and gold is associated with quartz-pyrite-chalcopyrite veinlets with potassic alteration along the vein walls.

The operation envisages open pit mining at an annual rate of approximately 3 Mtpa, utilising
drill & blast with a truck and shovel approach to ore and waste haulage.

A CIL process route was proposed for the Bissa mine at a nominal rate of 3 Mtpa, although the new design target throughput is nearer to 4Mtpa.

The Bissa process flowsheet consists of a conventional carbon-in-leach (CIL) design involving fine grinding of the ore followed by leaching of the gold in agitated tanks using sodium cyanide.

The gold is initially removed from the solution using activated carbon after which it is stripped and recovered onto steel cathodes using an electrolytic process. These cathodes are then melted, refined and poured into gold doré bars.

Run of mine ore will be delivered to the plant by truck where it will be discharged into a hopper equipped with an 800mm trash screen. Oversize will be broken up using a hydraulic breaker.

From the hopper, material will be fed using an apron feeder onto a 150mm aperture grizzley with all oversize material reporting to a single toggle primary jaw crusher. The jaw crusher will crush this oversize to a P80 of 170mm before being recombined with the undersize.

The material will then either be conveyed to the grinding circuit supply silo which will have an operating retention time of 4 hours or to an emergency stockpile which will be used to feed the grinding circuit in the event of breakdown or maintenance to the crushing circuit.

Crushed ore will initially be ground using a 8.5m diameter by 4.9m long (6,000kW motor) semi autogenous (SAG) mill operating in closed circuit with 10mm aperture vibrating screen.

Screen oversize material will be conveyed back to the SAG mill feed conveyor, with provision for pebble crushing, whilst all undersize material will go for further grinding using a 6.1m diameter by 9.1m long (6,000kW motor) ball mill operating in closed circuit with hydrocyclones.

The hydrocyclones will operate at a target P80 of 75µm with all underflow returning to the feed of the ball mill. Cyclone overflow will gravitate through a trash screen to remove any grit or remaining organic material after which it will be pumped to the leach circuit.

Leaching will be performed using a standard circuit consisting of six agitated tanks which will provide a total circuit residence time of 24 hours.

Cyanide will be added to the first tank at a rate of between 0.2-0.6 kg/t ore and allowed to fall at the pulp passes through the remaining five leach tanks. Each tank will contain an air injection system to, if necessary, allow the levels of dissolved oxygen to be increased in order to promote leaching.

In order to adsorb the gold, barren activated carbon will be added to the sixth tank from where it will be advanced using a system of pumps against the flow of the pulp. Carbon is prevented from leaving each tank using inter tank screens.

The loaded carbon will then be recovered from the pulp using a screen and sent for elution in order to recover the gold.

The leached tailings material will be pumped, at a density of 40% solids, to the tailings management facility.

Gold will be stripped from the activated carbon using an AARL (Anglo American Research Laboratory) elution circuit.

The elution circuit is designed to take the gold content of the activated carbon from a loaded value of circa 2,200g/t Au to a barren concentration of 100g/t Au.

This will be achieved initially by subjecting the loaded carbon to an acid wash using a 3% hydrochloric acid solution after which it will undergo pre-soaking using a 2% sodium cyanide/3% sodium hydroxide solution. Finally, the gold will be stripped from the carbon using fresh water heated to a temperature of 130oC.

Each elution cycle will be able to treat 6 tonnes of activated carbon and will take approximately 12 hours to complete. One complete elution will be performed each day.

Following elution, the pregnant eluate will be sent for electrowinning in order to recover the gold within solution whilst the barren carbon will be sent for carbon regeneration.

The gold contained within the pregnant eluate solution will be recovered onto stainless steel cathodes by means of electrowinning.

The electrowinning circuit will consist of two 3.5m3 electrolytic cells each containing a total of twenty two 1m2 stainless steel cathodes. An electrical current will be applied to the cathodes in order to remove the gold from solution where it will become plated onto the surface of the cathodes whilst the barren solution will be returned to the elution circuit for reuse.

In order to minimise the total elution time, the pregnant eluate will only undergo a single stage of electrowinning of between 4 and 6 hours in length with the circuit designed to produce approximately 20kg of metal each day.

At appropriate intervals, the gold plated cathodes will be removed from the electrowinning circuit and washed using a high pressure water jet in order to remove the deposited gold. This, along with an electrolytic sludge, will then be dried and calcined in order to remove impurities.

Finally, the calcined product will be mixed with fluxes and smelted to produce doré bars. These will then be stored securely until they are transported off-site for further refining.

The Bouly deposit represents a large, low-grade gold mineralisation, favourable for heap leach treatment. Bouly is a single open pit operation. The ore is crushed with primary jaw and secondary cone crushers, followed by agglomeration before stacking on HDPE lined pad for irrigation with sodium cyanide leach solution. Bouly utilises mining equipment and jaw crushers similar to that of Bissa, to standardise stock holding and optimise maintenance costs.

Bouly has its own all-season heap leach facilities with final processing stages, including desorption, electro-winning and smelting taking place at the Bissa’s processing plant.