The Sadiola deposit is considered a mesothermal shear-hosted gold deposit and can be correlated with an Ashanti-type orogenic gold model.

The Sadiola gold deposits are located within the Malian portion of the Keniéba-Kedougou Inlier, a major early Paleoproterozoic Birimian window along the north-east margin of the Kenema-Man shield. The deposits are in the north of the inlier and positioned in the Kofi formation, just east of the Senegalo-Malian Shear Zone terrane boundary. Regional metamorphism is to greenschist facies with amphibolite facies metamorphism observed in the contact aureoles around major intrusions.

The Sadiola gold system displays the Sadiola Hill-style Au-As-Sb mineralisation. Within the Sadiola Main Pit the bulk of the ore is hosted within the brittle-ductile Sadiola Fracture Zone and impure footwall carbonates. Mineralisation also occurs along the array of NNE-trending shears although gold grade decreases with increasing distance from the Sadiola Fracture Zone.

Mineralisation is shear-hosted and associated with a polyphase hydrothermal alteration history comprising an early calc-silicate phase followed by a potassic alteration stage. The metal associations of the ore typically comprise As-Au Sb and minor to trace amounts of Cu-W-Mo-Ag-Bi Zn-Pb-Te-Fe-bearing mineral species.

The geometry of the extensive, soft, oxide deposit and its supergene enrichment of gold relates almost exclusively to the weathering history of the primary mineralisation. Intense tropical weathering has produced deep troughs of white to grey, decarbonated, kaolinrich saprolite, locally abundant nontronite and relative gold enrichment. Penetration of groundwater has caused oxidation of the primary sulphides and the formation of sulphuric acid, further promoting deeper argillisation of the bedrock.

Drilling of the (unweathered) primary mineralisation has allowed detailed investigation of major and minor hydrothermal alteration processes that were active during the formation of the deposit. Primary gold is extremely fine grained, dominantly less than 15µm, with rare grains approaching 50µm. Visible gold is rare.

The gold mineralisation in the Sadiola main pit is related to the interaction of the north striking Sadiola Fracture Zone (SFZ) and a northnortheast-striking fault array. The SFZ follows the competency contrast between the brittle hangingwall greywacke and the ductile footwall marbles and is mineralised over a drilled strike length of approximately 2,500km. The stratigraphy is intruded by discontinuous diorite and quartz-feldspar porphyry dykes. Mineralisation occurs in all four rock types although most of the mineralisation is hosted in the footwall adjacent to the SFZ. The deposit has been intensely weathered to a maximum depth of 200m.

Current operations are focused on the mining of oxide material from the FN pits, north of the Sadiola main pit, which is supplemented with the low/marginal grade ore from the stockpiles. Mining from the Sadiola main pit has stopped as the oxide Ore Reserve is depleted although this pit remains a key project in the extension of the life of mine plan with the Sadiola Sulphide Project (SSP) awaiting board approval.

Gold processing uses the conventional plant consisting of crushing, milling, leach adsorption and elution. The elution uses AARL process. The plant consists of two twin streams capable to process soft oxide and soft sulphide with a limited amount of hard component.

There are two primary mills receiving fresh ore from the ore reception by conveyor belt transportation. The primary mills can run on open or close circuit. The mills are running in open circuit when the underflow of the primary mill cyclones feeds the regrind mill. But when the cyclone underflow returns back into the mill, thus the mill is on close circuit.

A gravity plant installed since December 2008 consists of a screening facility, a falcon concentrator and a Gekko unit used for intensive cyanidation in the leach reactor. A second falcon concentrator installed in 2010 is used in conjunction with the first falcon to extract coarse gold which is fed into the Gekko unit. The pregnant solution from the Gekko unit is pumped to electrowinning in the smelt house and the solids residue back to the regrind mill.

The cyclone overflow passes through linear screens prior to discharging into the preoxidation tanks. There are EDR pumps connected to the preoxidation tanks to supply oxygen through injection points on the pipeline. During sulphide ore treatment it is necessary to add some hydrogen peroxide in order to increase the dissolved oxygen concentration to the right level and maintain it.

The lime addition is performed in tank 1. An EDR pump is fitted to that tank as well. The slurry pH is controlled at 10.0 – 10.2 before sodium cyanide addition. The cyanide is added into the leach tank 2. There are ten leach tanks on each stream to assure the required residence time. Automatic cyanide analyzers are in line to control the cyanide concentration and dosing rate as close as possible to the set point. No more cyanide is added downstream to maintain the concentration.

The slurry overflows by gravity into adsorption vessels containing activated carbon except the first vessels used as grit catchers. The carbon movement upstream is performed with the carbon transfer pumps installed in each tank. There are height adsorption tanks in series with gravity flow. The last tanks are used for detoxification of residue slurry prior to pumping to the tailings facility.

After detoxification the residue is pumped from the residue tank to the tailings facility via a steel pipeline. The slurry is cycloned to remove the coarse fractions for impoundment wall building. The fine fractions from cyclone overflow run down to the pool. The decanted water is pumped to the plant via the return water dam or directly from a barge decants system with four submersible pumps capable of pumping 300 m3/hr each. There are underdrains and elevated filter drains for stability of the wall. Piezometers installed along the starter wall and vibrocores allow determining the level of water at the wall foundation.

The Loaded carbon is pumped from second adsorption tanks to elution on a daily basis. The carbon is acid washed with hydrochloric acid, neutralized with caustic soda and washed with clean water before elution. The elution consists of heating the carbon, soaking it in hot caustic cyanide solution and rinse with hot soft water. The gold bearing solution is pumped to electro winning in the smelt house and the eluted carbon is regenerated and pumped back to adsorption circuit.