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.

Open pit mining at Sentinel will follow conventional drill and blast, shovel and truck mining practice.

The Sentinel orebody dips towards the north and strikes approximately east-west. The footwall contact is defined by the mineralisation host rock (ie, phyllite), with a general dip trend of 20o to 30o

The Sentinel Pit will be mined in a series of stages, 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) waste 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, respectively.

The primary crushing circuit consists of three semi-mobile, independent gyratory crushers operating in open circuit. The crushers operate with a nominal open side setting of 165 mm. With all three crushers operating, running times will average 16 hours per day, although 24 hour operation will be possible.

Each crusher will be 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.

One set of conveyors from the pit top bin will deliver primary crushed ore to one of two secondary cone crushers, with a combined capacity of 6,000 tph at a closed side setting of about 40 mm. Secondary crushed material will be returned to the stockpile feed conveyors.

Two parallel shuttle conveyors are used to deliver crushed material to the stockpile and 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 utilized, if required, by bull-dozing the dead load into the stockpile discharge chutes.

The mills were selected on the basis of two milling lines, each comprising a SAG and a single ball mill, with each mill being similar to equivalent sized mills proven at other operations. On that basis, a 28MW SAG mill (internal diameter of 12.19m (40ft)) 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 product size of 80% passing 212 µm. A pebble crusher is included in the circuit to crush pebbles ejected from the SAG mills 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. The underflow from each ball mill cyclone cluster can be directed to the ball mill feed chute, or alternatively part of the stream can be directed to a dedicated 57 m³ flash flotation cell (two per ball mill).

Flash flotation has been included in the circuit because of the presence of fast floating coarse chalcopyrite. The flash float concentrate is pumped to the third stage of cleaner flotation.

Four banks of 7 x 300 m3 rougher/scavenger floatation cells are installed; each bank proceeded by a conditioning tank and fed from a surge tank.

Rougher concentrate is cleaned in a multi-stage cleaning circuit comprising 7 x 300 m3 and 10 x 200 m 3 cells to produce final concentrate, with rougher tails directed to tailings thickeners and then to the tailings impoundment.

Collector, frother and gangue depressant are added to the flotation circuit as required via dedicated systems.