The Loulo and Gounkoto deposits can be classified as typical shear hosted Birimian style mesothermal gold deposits.

The Loulo project area is characterised by a sequence of sandstone, limestones and greywackes which are truncated by three major north to NE trending shear zones. Two major mineralised bodies have been discovered within the Project area, Gara and Yalea. Several other satellite deposits are also present.

Yalea is located in a north-south striking, steeply dipping package of metasedimentary rocks. Host lithologies at Yalea (from west to east) comprise of quartzite/grey in the hanging wall, with tectonic breccias in the north. Immediately above the main body of mineralisation is thin (0 m to 5 m) sequence of banded schistose greyish limestone, with alternating white and grey calcitic layers and dark grey to black phyllite units. The main mineralised body is a hydrothermally brecciated argillaceous pink quartzite that becomes more argillaceous (and less altered) towards the footwall. A higher grade ‘Purple Patch’ zone is observed in a dilatational strain transfer zone formed as the dip of the mineralised package steepens forming hydraulic breccias. The footwall package is a thick sequence of argillaceous quartzite and black sandstone. This sedimentary package is intruded locally by thin (0.1 m to 2 m) acid intrusives of mostly granitic composition. The country rocks are also cross cut by a late EW dolerite dyke that is generally sub-horizontal, with a gentle southward plunge.

Gara is located 6 km NNW of the Yalea deposit and is hosted within 800 m long tourmaline sandstone/greywacke unit which outcrops on surface as black quartzite forming small (10 m) topographic highs. The host lithologies are (from west to east): chemical (limestone or carbonate altered) sediments and alternating argillitic and quarzitic bands (argillaceous quartzite or SQR) in the hanging wall, mineralised quartz tourmaline (QT) ranging from 5 m to 20 m thick (average 15 m) and a coarse to medium grained greywacke unit in the footwall. The sedimentary package is also crosscut by three unmineralised late EW dolerite sub-horizontal dykes that plunge shallowly from north to south.

Gounkoto is a shear zone-hosted deposit consisting of an east-dipping mineralised package of fine-grained sedimentary sandstone units termed QR (after the French term “quartz rosé” meaning pink quartzite), with rare limestones. It is bound in the hanging wall by black sandstone and the footwall by west dipping sediment. The deposit is characterised by a stepped geometry, with wider zones of mineralisation generally seen on the NW trending structures and narrower zones on the north-south trending structures. This is believed to be related to dilation across these structures in a strong sinistral strain environment.

The hanging wall of the deposit is characterised by a thick sequence of fine grained ‘black sandstone’ with or without narrow units of limestone and QR (Figure 7-8). The mineralisation is generally hosted in a siliceous QR unit and the footwall consists of argillaceous quartzites (SQR) and/or greywacke with narrow mafic intrusives including micro diorites.

The Gounkoto system is characterised by strong foliation and shear fabrics with different orientations. Foliation in the hanging wall black sandstone is parallel to bedding and strikes NNE and dips 50° to 60° SE. This fabric intensifies to a shear in the immediate hanging wall of the deposit. Fabrics in the QR- or limestone-hosted mineralisation are generally vertical to steep west-dipping. Black sandstone, SQR and greywacke in the footwall exhibit moderately to steeply west-dipping foliation.

Gold is strongly associated with sulphide mineralisation, dominantly pyrite. Magnetite, chalcopyrite, arsenopyrite and pyrrhotite are also present locally and have a strong association with gold. Gold is also commonly found in gangue in zones of strong silica-carbonate alteration, suggesting that remobilisation also played a role in gold (re)distribution at Gounkoto.

The initial stage of mineralisation involved the deposition of euhedral and equant grains of magnetite in stringer networks, veinlets, and aggregates, and accompanied by chlorite mineralisation. This magnetite mineralisation was subsequently subjected to a strong oxidization and hydration event resulting in almost total replacement of magnetite to haematite and annealing of the stringer network together to form aggregate almost massive bodies of haematite, leaving magnetite as small relict grains within haematite.

The onset of the second stage of mineralisation at Gounkoto was synchronous with the oxidation of magnetite and growth of haematite, occurring initially as pyrite (with minor gold and chalcopyrite) which are locally included within the aggregate bodies of haematite. The pyrite mineralisation event was pervasive in nature and was accompanied by strong silica-carbonate alteration, creating the dominant silica-carbonate-pyrite mineral assemblage at Gounkoto. This early pyrite is typically surrounded and enveloped by later stage pyrite with a more cellular and filamentous habit with which gold is commonly associated.

The Loulo-Gounkoto mining complex, located in the west of Mali, comprises the Loulo underground mines – Yalea and Gara – and the Gounkoto open pit mine.

Production from Loulo started in 2005 as an open pit operation, followed by the development of the underground mines.

Gounkoto, a greenfields discovery, poured its first gold in 2011, with its ore processed at the Loulo plant under a tolling agreement.

The Loulo Permit are comprised of the Baboto open pit, Loulo 3 open pit, and the Gara West open pit. Baboto oxide and free dig transition ore is scheduled to be mined in 2018, however the rest of the Baboto open pit fresh ore and the other pits will be mined from 2024 onwards, after the larger Gounkoto open pit is depleted. The production from these pits has not been extensively detailed. Randgold intends to use the same contractor that has already mined part of the Baboto open pit, so the fleet requirement is known. The Loulo 3 open pit was mined previously in 2013 and the eastern push back forms the remaining Ore Reserve. Both Loulo 3 and Gara West open pits will use the same equipment as they are timed to be mined consecutively.

Gounkoto is an on-going operation that applies selective mining. Ore is classified as either Full Grade Ore (FGO), delivered to primary crushers, or Marginal Ore (MO), which is stockpiled close to the primary crushers, and blended with the FGO to provide a consistent feed grade. The Gounkoto mining plan has been sequenced in such a way that the south pit will be mined out in advance of the north pit. This will allow the waste mined from the north pit to be backfilled into the south pit resulting in a shorter haul than the conventional route of tramming out of the pit and onto a waste dump. This, however, means excess ore must be mined in 2019 and stockpiled to ensure the plant feed requirements are maintained during the waste pushback mining in years 2020 to 2022.

Open pit mining is conducted by the contractor Gounkoto Mining Services (GMS), a local subsidiary of DTP Terrassement (DTP Mining), using either free-dig or conventional drill, blast, load, and haul methods.

The Yalea and Gara underground mines are currently operating and are accessed via portals located in open pits and a box cut. The declines were originally developed as twin declines. One decline containing a conveyor and the second decline is used for mobile equipment access. The lower part of the mines has been developed as single declines with truck haulage up to crushers which feed ore and waste onto the conveyors.

Three mining methods are used at the Yalea and Gara underground mines. They are long hole transverse open stoping; long hole longitudinal retreat open stoping; and stoping under rock fill (SURF). SURF mining is planned to be introduced in the weathered areas of Yalea South Upper.

The SURF method is planned for the portion of Yalea South Upper that has weathered transition or saprolite present in the mineralised zone, or the immediate hanging wall. The method will be used is expected to provide a more consistent production in the less competent ground.

From an operational perspective SURF is very similar to longitudinal sub level caving (SLC). The exception is that in SURF waste rock is tipped into the stope to fill mined voids and prevent caving of the hanging wall. In contrast SLC works by the hanging wall caving to fill the mined void.

In 2014 paste fill plants were commissioned at both mines. The paste fill plants at each of the mines produce a mixed paste / aggregate fill. With the introduction of paste fill the lower areas of the mines are being mined using an underhand (top down) long hole stoping method which retreats to central accesses in an echelon format. Panels are 50 m long, mined as single level stopes (25 m high) and filled with cemented paste fill. The paste fill is exposed by the mining of both the panel below and the next panel on the same level. Transverse long hole open stoping is used in the wider (>15 m wide) parts of Yalea North. Transverse stopes are 15 m to 30 m along strike and mined from hanging wall to footwall.

Crushing – Hard Ore
Hard rock is delivered to the tipping bin. Tipping can be completed from two directions.

The hard rock crusher plant consists of a primary gyrator crusher (FFE Minerals 1,300 mm by 1,750 mm) driven by a 450 kW electric motor. A tramp iron magnet is installed over the product conveyor to remove steel. A metal detector trips the conveyor if metal is detected.

The primary crusher operated at a rate of 700 tph, feeds the secondary crushers which operate in open circuit and then the tertiary crushers which operate in closed circuit. An additional metal detector is installed on the feed to the tertiary crushers

The secondary circuit consists of two Sandvik CS6800 Hydrocone crushers and the tertiary circuit consists of four H6800 Hydrocone crushing units, all are powered by a 315 kW electric motor. The secondary and tertiary crushers are housed in separate buildings connected via transfer conveyors.

The final product from the tertiary crushers discharges onto a reclaim ore stockpile, via a conveyor system, and has a live capacity of about 40,000 t and a total of approximately 115,000 t. This is the feed to the SAG mill.

Crushing - Soft Ore

A separate facility exists to crush the soft ore when it is available. The soft ore crushing consists of a tooth roll crusher (MMD 625 - 3 tooth) driven by a 250 kW electric motor. This feeds a separate stockpile area and the roll crusher product is fed directly onto the ball mill conveyor feed.

Grinding and Classification

The milling section produces a leach feed with approximately 75% passing 75 microns. The milling circuit comprises of a SAG mill operating as the primary mill (6.1 m dia. and 9.5 m EGL and installed motor power of 7,000 kW to be upgraded to 8,000 kW) operating in open circuit. The oversize or scats are re-handled to the stockpile and re-crushed. The primary mill discharge feeds two parallel single stage overflow type ball mills (5.5 m dia. by 8.0 m EGL) with a power rating of 4,500 kW each, operating in closed circuit with a dedicated cluster of twelve 250 mm hydrocyclones of which eight are typically in use.

An additional High Pressure Grinding Roll (HPGR) has been installed to treat scats at a rate of 100 t/h.

The Loulo processing plant uses a conventional carbon in leach (CIL) gold extraction process with a budget throughput in 2017 of 4.8 Mt, which is in line with the plant design capacity.

Since 2014 multiple optimisation projects have been undertaken, resulting in increased throughput and improved recoveries. It is expected that the improvement initiatives will continue but are considered part of operations and not necessarily confined to capital projects. The expected throughputs and recoveries are based on historical metallurgical testwork and the actual operational performance.

The upgraded process plant remains a conventional crushing, milling, gravity, CIL, and tailings disposal circuit. It will process an average of 580 tph using the following circuits:

• Crushing – three stage crushing for the hard rock sulphide ores and a single stage roll toothed crusher for the soft weathered oxide ores.
• Milling – one primary mill (7 MW); two identical single stage ball mills (4.5 MW), one scat conveyor system is inserted to return all mills scats back to primary mill via a cone crusher.
• Gravity - four centrifugal primary concentrators followed by two intensive leach reactors to treat primary concentrates.
• CIL recovery process.
• Zadra elution process.
• Electrowinning.
• Tailings pumping/deposition split between slime dam and paste plant.

Gravity Concentration

A bleed stream from each cyclone cluster underflow is diverted to its respective gravity concentration circuit.

In each primary concentration circuit, the feed slurry from the cyclone underflow is fed on a horizontal vibrating screen to remove material greater than 2 mm from the concentrator feed that would otherwise plug the concentrator bowl riffles.

Screen undersize from each cluster feeds Knelson XD-48 and QS-40 gravity gold concentrators after which the concentrates pass to a common feed hopper and Gemini dressing table located in the gold room. There is also the presence of an ILR1000 and ACACIA CS2000.

Leaching and Adsorption

The CIL circuit consists of fourteen tanks that operate in series, each having a nominal capacity of 2,500 m3 giving a retention time of approximately 40 hours. Cyclone overflow, at a density of 35% solids, is introduced onto a linear trash screen (0.7 mm by 0.7 mm apertures) and the undersize goes to the leach feed thickener, the thickener underflow at a density of 50% presented as leach feed. Two pre-oxidation tanks receive the CIL circuit, each with six high-shear reactors (Aachen REA-400). Cyanide is added into Tank 2 and doses automatically to control concentration around the desired value with ± 2%. An oxygen dispersion system is installed to maintain optimal dissolved oxygen across CIL to assist leaching consisting of power mixers (EKATO in front tanks #3 to 7). The oxygen plant, operated by Air Liquide, supplies 44 tpd. Hydrogen peroxide is injected at the thickener underflow, as required, to maintain the required dissolved oxygen levels necessary for the process. Each CIL tank is fitted with a mechanically swept cylindrical inter-tank screen (0.8 mm) complete with pumping mechanism.

Fresh carbon is introduced to tank fourteen and is advanced to tank three in counter current flow to the pulp, using recessed impeller vertical pumps.

Elution and Gold Recovery

Loaded carbon is recovered from CIL tank 3 into the acid wash cone. After elutriation, it is acidwashed using a 3% hydrochloric acid solution, followed by a caustic neutralisation/water flushing step. The rinsed carbon is transferred to one of two elution columns where the caustic/cyanide solution is circulated at elevated temperature (135°C) and pressures using the Zadra process. Loulo carbon stripping consists of two parallel circuits from harvest - elution column – heater and exchanger to electro-winning cells at the gold room.

The pressure Zadra method utilises a pressure strip vessel that reverses the chemical equilibrium of the adsorbed gold-cyanide complex on the activated carbon resulting in desorption of the goldcyanide complex from the activated carbon into the strip solution.

The pressure Zadra process is conducted in a batch-by-batch process and requires approximately eight to 16 hours to complete.

The gold is then recovered downstream from the strip column by electro-winning. Six electrowinning cells are provided in the gold house for the electro-winning circuit where the pregnant electrolyte is introduced for gold deposition.

After each elution, the gold loaded stainless steel mesh cathodes are removed from the electrowinning cells and hosed down with high pressure water stream. This removes the plated gold onto a hopper where it is collected, settled, decanted and the sludge smelted after it has been dried in one of two electrically heated calcine ovens to produce doré bullion.

The barren carbon is now transferred to the adsorption circuit or to the carbon regeneration kiln where it is regenerated at C 700°C in a horizontal gas fired kiln.

The regenerated carbon is charged back into the CIL circuit.