The Siguiri orebodies are early Proterozoic orogenic quartz-vein hosted deposits located in the Siguiri Basin of West Africa. Generally poorly exposed, the basin sediments have been subject to greenschist facies metamorphism and consist of a well-bedded turbiditic sedimentary sequence, with some brecciated and possibly volcanic members. Mineralisation also occurs as secondary gold in alluvial and colluvial gravels in laterite cover.

Three main sedimentary packages are recognised in the Siguiri district, the Balato, Fatoya and Kintinian formations. The Balato Formation is dominated by centimeter scale alternations of shale-siltstone and greywacke. The overlying Fatoya Formation consists of meter scale beds of greywacke fining towards the west. The Kintinian Formation is a thick package of shale and sandstone with a basal clast-supported conglomerate.

The orebodies are structurally controlled and the area has undergone at least three distinct phases of deformation, with initial N-S compression developing minor folds, the second and largest deformation event is associated with E-W to ENE WSW directed compression leading to N-S structural architecture, and the third event was a NW-SE compression that led to refolding of existing structures.

A deep oxidation (weathering) profile is developed in the region, varying between 50m to 150m. The mineralised saprolite currently provides the main oxide feedstock for the CIP processing plant, although a new treatment option has been approved to mine the fresh-rock extensions of the ore deposits.

Primary gold mineralisation occurs in all three lithostratigraphic units of the Siguiri region although the majority of known mineralisation is found in the central and more competent Fatoya Formation. In some deposits the mineralisation shows strong lithological control and is preferentially developed in coarser-grained units that have higher fracture/vein densities relative to fine-grained rocks.

Two styles of primary mineralisation have been recognised at Siguiri. The first is characterised by precipitation of gold-bearing pyrite associated with proximal albite and distal carbon alteration, and opening of carbonate-pyrite veins. The second style corresponds to ENE-WSW trending native gold bearing quartz veins with carbonate selvages which crosscut carbonate pyrite veins and show arsenopyrite (±pyrite) halos.

Siguiri is currently a multi-pit oxide gold mining operation, operated with a contract miner. The mining method is selective conventional techniques using excavators and trucks on 3m high flitches. Liebherr 994s and 984s excavators are the main loading equipment matched with CAT 777 dump trucks. A minimum mining unit (MMU) size suitable for selective mining and nominated mining equipment of 5m x 5m x 3m based on historical mined out grade control model is used simulate the expected mining dilution and ore losses and built in the geologic block models.

Processing began in 1997 via heap leaching and was continued for eight years, after which the long term potential of the site as a heap leach became limited a CIP plant was built.

On the CIP plant, ROM ore is reduced by a toothed roll crusher and then transported overland to a mill feed stockpile. The crushed ore is withdrawn at a controlled rate, via apron feeders, and conveyed for treatment through a scrubber and conventional ball milling circuit. Mill product is pumped from a common sump through a cluster of 660mm cyclones with the underflow reporting to a 6MW Ball Mill. A bleed of the cyclone underflow is treated in a gravity circuit incorporating a Knelson concentrator and an Intensive Leach circuit which contributes about 25% of the final gold produced.

Overflow from the cyclone cluster gravitates to a leach train consisting of ten tanks where lime, cyanide and hydrogen peroxide are added to effect gold dissolution. Slurry from the leach circuit is then fed into a carbon adsorption train, via a tramp screening step, in between the two tank farms. Carbon is recovered upstream of the adsorption train and eluted through a 12 tonne AARL elution circuit. Pregnant liquors from both the ILR and elution circuits are electrowon in separate cells at the gold room, with the resultant cathode sludge being dried, calcined and smelted. Eluted carbon is regenerated in a diesel fired kiln and returned to the adsorption circuit. Tailings slurry discharges from the last adsorption tank and gravitates to tailings screens for fugitive carbon recovery. Screens underflow then drop into a tank from where the slurry is pumped into a one-sided embankment tailings dam situated 8km south of the plant.