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 fresh rock and oxide gold mining operation, mined by contract miner, Mota-Engil. The mining method is selective conventional mining using excavators and trucks on 3m high flitches. Three Caterpillar 6020B excavators are the main loading equipment matched with Caterpillar 777G dump trucks. In some deposits, a selective mining unit (SMU) of 10 x 10 x 3m has been defined based on historical grade control, the deposit type, and the mining equipment used to simulate the expected mining dilution and ore losses.

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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. As the percentage of heap-leachable ore declined (and to be able to exploit saprolitic ores that extended below the base of the existing pits), a CIP (Carbon-in-Pulp) plant was built.

The current processing plant treats both oxide and fresh sulphide material via a hybrid CIL circuit plant converted from CIP in 2018. The plant is capable of processing blends of hard and soft ore post commissioning a new ball mill and three-stage crushing plant in 2019. Unit operations include comminution, leaching, carbon adsorption and desorption, smelting and tailings disposal. Further modification of three leach tanks to CIL tanks was done in the fourth quarter of 2020 giving a total of seven tanks in the hybrid circuit.

Since February 2019, the Siguiri combination plant was commissioned with following updates:
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