The Sentinel deposit is a sediment-hosted stratiform copper deposit. Mineralisation is predominantly primary sulphide copper, with sheet-like horizons of ore dipping north at 20-30o . The mineralisation is parallel to dominant foliation, within the structurally deformed carbonaceous phyllite host.

Copper mineralisation at Sentinel is limited to the strongly deformed phyllite unit, with rare lowgrade mineralisation extending only 1-2 metres into the hanging and foot-wall from the contact. The ore-body strikes approximately east-west for 11 km and mineralised horizons dip 20 to 30 degrees in a northerly direction, generally parallel to the dominant foliation.

The dominant copper-bearing mineral is chalcopyrite and typically occurs within bedding/foliation parallel quartz-kyanite-carbonate mm-scale veinlets. Within folded zones, veinlets tend to be thicker (mm scale), blebby, and more irregular, and often contain a relatively higher proportion of chalcopyrite. Late sulphide-bearing cross-cutting veinlets and disseminated or blebby chalcopyrite are less common.

The oxidised horizon, up to approximately 70 m in depth, contains non-primary sulphide Cuminerals, predominantly chalcocite, and tarnished chalcopyrite. The top 5-15 metres from surface is typically leached of copper, or contains mixed refractory copper and trace oxide minerals.

Nickel-cobalt mineralisation exists predominantly in the form of cobalt-pentlandite, with trace amounts of vaesite. Apart from rare sporadic metre-scales lenses (likely related to structures) the Ni-Co mineralisation occurs as a discrete horizon within the ‘footwall’ phyllite. ‘Footwall’ phyllite refers to the lowermost portion of phyllite that tends to be barren, or very low in copper mineralisation. Ni-Co mineralisation is best developed in the NE extent of the deposit, proximal to the Kalumbila Fault.

The Sentinel Pit is being mined in a series of terraced phases, using large-scale mining equipment, and with mining costs expected to be minimised through the adoption of bulk mining and ore handling methods featuring electric shovels and drill rigs, trolley-assisted (TA) haulage, and in-pit primary crushing and conveying (IPCC). Waste and ore haul cycle times, and hence fuel consumption, are expected to be reduced through the adoption of TA and IPCC.

Open pit mining at Sentinel commenced in two surface box-cut areas of the Phase 1 Pit; ie, in the north west boxcut in April 2013, and then in the south boxcut from early 2014. Since 2013-2014, mining has proceeded in the Phase 1 pit to a current depth of approximately 200 m.

The Phase 2 pit, immediately to the east, was progressively cleared and grade control drilled from 2016. Mining from the southern pit crest limits, 800 m across to the Musangezhi River, now extends along a strike length of 1.2 km and to a depth of approximately 25 m.

Mining capacity will eventually increase to around 68 million bcm of ore and waste mined per annum. The ultimate 5.7 kilometres long, 1.5 kilometres wide and 390 metres deep pit will be mined in stages, with ore crushed in-pit and conveyed overland to the Sentinel process plant.

Three in-pit crushers and associated overland ore conveyors have been installed and are operational within the main pit. The conveyors extending across to the plant crushed ore stockpile via a surface transfer bin. A fourth in-pit crusher is scheduled to be commissioned in late 2021. Pit expansion continued eastward with ongoing mining of the second mining stage during 2020.

Mining follows conventional drill and blast, shovel and truck mining practice. The sequence of mining activities is also conventional and is generally as follows:
- RC grade control drilling delineates the ore zones
- a grade control model is developed from which blast limits and digging blocks are designed
- ore and waste blocks are blasted to design, according to layouts based on varying hole patterns and powder factors to suit prevailing ground conditions
- specific blast designs are engineered to suit excavations in close proximity to in-pit crushers and conveyors
- trim blasts and perimeter blasting techniques are used to ensure pit wall profiles are cut to the correct angle and to minimise wall damage
- electric and diesel/hydraulic shovels and excavators load the blasted rock into a fleet of 330 to 360 tonne and 240 tonne capacity haul trucks
- ore is hauled direct to IPCs or to active and long-term stockpiles, whilst waste is hauled to surface dump tip heads
- trolley assisted haulage is currently in use for waste hauls, and is proposed for future ore hauls from increasingly deeper mining elevations.

Noteworthy changes from the 2015 Technical Report (FQM, May 2015) production plan are:
- the proposed expansion of cupriferous ore processing to 62 Mtpa, commencing in 2022
- a commensurate increase in total mining movement capacity to about 180 Mtpa from 2022, and 190 Mtpa from 2026
- installation of a fourth IPC during 2021, near-surface, in the Phase 2 pit

Primary crushing
The primary crushing circuit consists of three semi-mobile, independent gyratory crushers (IPCs) operating in open circuit. The crushers operate with a nominal open side setting of 165 mm. With all three crushers operating, running times average 16 hours per day, although 24 hour operation is possible. Each crusher is located in-pit, to minimise haulage distances, with crushed ore conveyed to a pit top bin from where the ore is conveyed either directly to the mill feed stockpile, or to secondary crushing. In late 2021, a fourth IPC will be installed, thereby enabling three crushers to continue in operation whilst one is being relocated to a new position deeper in the pit.

A fourth crusher, IPC4A, is now required to ensure crushing continuity when any of the other three are being relocated, and also importantly, to supplement crushing capacity for the proposed 62 Mtpa processing expansion.

The nominal crushing rate of each of IPC1A, IPC2A and IPC3A is 4,000 tonnes per hour. After availability and utilisation factors, plus operational downtime, this rate equates to approximately 18 Mtpa crushed per IPC. The larger capacity IPC4A crusher, when operational, will have a nominal crushing rate of 5,500 tonnes per hour. After similar allowances and factors, this equates to approximately 25 Mtpa crushed.

Crushed ore stockpiling and reclaim
Two parallel shuttle conveyors are used to deliver crushed material to the stockpile and to distribute the crushed ore along the length of the stockpile. The stockpile has a live capacity of at least 12 hours, approximately 60,000 tonnes. The total capacity of the stockpile can be utilised, if required, by bull-dozing the dead load into the stockpile discharge chutes. Ore is recovered from the crushed ore stockpile by four apron feeders (per mill) located in a tunnel underneath the stockpile.

Milling, pebble crushing
The mills were selected on the basis of two milling trains, each comprising the largest SAG and ball mills currently proven at other operations, at the time of design. On that basis, a 28 MW SAG mill (internal diameter of 12.19 m (40 ft)) and a 22 MW ball mill (8.53 m internal diameter (28 ft)) were selected for each milling train. All the mills are equipped with gearless drives.

Each milling train is designed to grind 3,500 tph of material from a feed size of 80% passing 130 mm to a product size of 80% passing 212 µm. A pebble crusher is included in the circuit to crush pebbles ejected from the SAG mills, down to minus 12 mm before recycling them to the stockpile feed conveyor.

Each SAG mill has a dedicated cyclone cluster, with cyclone underflow being directed to the ball mill. Each ball mill has two cyclone clusters, with cyclone overflow mixing with SAG cyclone overflow and gravitating to rougher flotation.

Sentinel copper ores have an average copper grade of approximately 0.50% Cu. At a processing rate of 55 Mtpa and with 90% recovery, the annual production of copper (in concentrate) is approximately 247,500 tpa Cu.

The processing plant design is based on a conventional sulphide ore flotation circuit designed to treat 55 Mtpa of ore, with a separate 4 Mtpa circuit designed to process nickel ore feed from Enterprise project or additional copper ore feed from Sentinel.

62 Mtpa processing is now proposed, commencing from 2022.

The concentrator circuit comprises:
- in-pit crushing of run of mine (ROM) ore
- conveying of primary crushed ore to secondary crushing and to ore stockpiles
- partial secondary crushing of primary crushed ore prior to stockpiles
- SAG and ball milling of crushed ore, with size classification by hydrocyclones. A grind size of 80% passing 212 µm is targeted
- flash Flotation for fast floating coarse chalcopyrit ........