The Perkoa deposit lies in a felsic to intermediary series of volcanic and volcaniclastic rocks, within the Paleoproterozoic Birimian Supergroup of West Africa. The prospective Birimian-aged rocks in Burkina Faso are the same sequences that host major gold deposits in Burkina Faso and in the neighbouring countries of Ghana and Mali. The Birimian Supergroup of West Africa is renowned for their gold mineralization; however, known occurrences of base metals are scarce. The Perkoa deposit represents the only significant zinc-silver massive sulphide mineralization discovered in the Birimian to date and it is also the first zincsilver massive sulphide mineralization discovered in this region. Only zinc is being recovered at the Perkoa Mine.

The Perkoa deposit has been classified as a volcanogenic massive sulphide (“VMS”) deposit. VMS deposits are lenses and sheets of massive sulphide that form from seafloor hydrothermal systems where metal rich fluids precipitate on (exhalative) or near the seafloor (sub-seafloor replacement.). The Perkoa mineralization occurs as a series of stacked, northeast-southwest striking tabular VMS lenses hosted, and separated by, tuffaceous material that has been overturned with an average dip of approximately 70°. The deposit is unusual for its high concentrations of zinc and barium mineralization, and relatively low levels of lead and copper. Mineralization occurs within two continuous VMS lenses, the main Perkoa lens and the Hanging Wall lens. The main lens outcrops on surface and extends to a depth of approximately 620 metres below the topographic surface, characterized by a strike length that varies between 300 and 470 metres with a width that varies between 5 metres to 25 metres. The Hanging Wall lens extends from approximately 100 metres below the topographic surface down to a depth of approximately 820 metres with a strike extension that varies between 250 metres to 350 metres. Thickness of the Hanging Wall lens is narrower than the Main lens, varying from 5 metres to 15 metres.

The Perkoa mine is an underground operation, however, a small open pit was mined to reach near surface material during initial start-up to increase plant throughput as the underground mine ramped up production. The pit is now complete. Underground mining with contractors carrying out all aspects of the mining process commenced in February 2013.

The mine is accessed via an 85 m long decline in the box cut prior to entering the portal.

The decline and associated cross cuts are excavated in the competent quartz diorite of the footwall. The 5.3 m wide x 5.5 m high decline is a spiral and the level spacing is set at 30 m. Each level is identical with a footwall drive, sumps, and services cubbies.

A ventilation shaft connects all levels of the mine and acts as an emergency egress via the inking bucket (kibble). A further secondary ventilation ramp egress exists into the completed open pit to allow personnel to escape on foot in an emergency situation. A 35 m crown pillar has been maintained between the open pit and underground production workings.

Longhole stoping is being used as the primary extraction method. There are several variations on this mining method employed such as longitudinal and transverse, with both bottom-up and top-down mining sequences. The exact method chosen is dependent on the orebody geometry. Stopes are backfilled with either cemented rock fill (CRF) or waste.

Longitudinal bottom-up stoping has been used above 190 level. Due to the orebody width and mine sequencing, stopes between 190 level and 280 level are mined using the transverse method with primary and secondary stopes, with some longitudinal retreat mining occurring at the extremities of the orebody. All of the primary stopes above 280 level have been mined out with the remaining ore to be mined as secondary stopes.

When the full stope area has been mined out, voids are backfilled with CRF for primary stopes. Secondary stopes are filled with waste. The fill serves both to support stope walls and to provide a working platform for equipment when mining the next slice. Run of mine (ROM) waste and crushed material (100 mm all-in) is mixed with different binder percentages, depending on the application. The CRF is mixed in dedicated underground sumps.

Production drilling is by one Sandvik DL421 longhole drill with either 89 mm or 102 mm holes being drilled. Drill productivity is approximately 350 m/day. Burden is 2.8 m and spacing is 3.2 m.

Ore and waste is loaded by three scooptrams. The scooptrams are remotely operated using line of site and full tele-remote operation technology. Approximately 70% of each stope is mucked with the tele-remote scooptrams. Three 60 t trucks take the ore to the ROM pad (ore stockpile), and waste to the waste dump. In addition, there are a dozen or so pieces of assorted heavy mobile equipment to support the mining and processing operations.

Explosives, ammonium nitrate, slurry mix, and accessories are transported to the site from Ouagadougou by BME, the explosives supplier which is a member of the Omina Group. All charging underground is provided by the mining contractor using its own equipment.

Permanent excavations, such as declines, remuck bays, level accesses, ventilation accesses, etc., are supported with nine 2.4 m double corrosion protection (DCP) grouted expansion bolts every two metres along the drift, plus wire mesh to within 3.7 m of the floor, and 2.4 m long 47 mm splitsets every two metres along both walls of the drift. Temporary excavations, such as ore drives, are supported with eleven 2.4 m long 47 mm splitsets every two metres along the drift plus wire mesh to within 3.05 m of the floor.

The mine has an established stockpile plan, which includes high grade (18% to 20% Zn), medium grade (14% to 18% Zn) and low grade (~10% Zn). Each day, the geology department issues stockpile blending instructions to the processing plant with the aim of achieving a more consistent head grade in the range of 13% to 14% and 13% to 14% Fe as well as a Zn:Fe ratio of 1.0. As of January 18, 2018, the total ore on the stockpile was 51,500 t grading 16.7% Zn and 16.3% Fe.

The process plant includes crushing, screening, and grinding, followed by zinc flotation and filtering to produce a zinc concentrate.

The ROM ore is fed through a grizzly to the primary jaw crusher at a rate of approximately 150 tonnes per hour (tph). The product is sent to a secondary cone crusher circuit operating in closed circuit with a screen, splitting at 10 mm. The screen oversize (+10.0 mm) is sent to the cone crushers. Product from the cone crushers is returned to the primary screen. The undersize (-10.0 mm) from the primary screen is sent to the mill feed stockpile.

From the mill feed stockpile, the ball mill (10.5 ft x 16 ft) is fed at a rate of 85 tph to 88 tph solids feed pending on variations in mineral hardness. The ball mill operates in closed circuit with cyclones to produce a zinc flotation feed with a minimum of 60% of the cyclone overflow product passing 200 mesh (75 µm)(P60 75µ).

The process plant at Perkoa is a conventional sulphide flotation plant capable of processing 2,000 tpd and is currently operating at approximately 1,900 tpd. The process plant includes crushing, screening, and grinding, followed by zinc flotation and filtering to produce a zinc concentrate. The process plant originally included a lead recovery circuit, however, this circuit is no longer used and has been reconfigured to increase capacity in the zinc circuit due to higher zinc head grades.

The flotation feed is sent to a conditioner where reagents (hydrated lime, copper sulfate (CuSO4), xanthate, and frother) are added before it passes on to rougher tank cells. High grade concentrate from the first three roughers is sent to final concentrate and the low grade concentrate from the last rougher is sent to the cleaner circuit.

The rougher tailings go to the scavenger circuit, where the concentrate from the first and second scavenger cells is sent to final concent ........