The Kemess Underground deposit is a large copper-gold porphyry deposit and is typical of calc-alkaline porphyry copper-gold deposits in the western cordillera. The deposit has a low grade ore zone at a depth of 150 metres below the surface on its western flank and a higher grade zone 300–550 metres below surface on the eastern side, which forms the Kemess Underground Project. The Kemess Underground deposit is hosted by potassic altered Takla Group volcanic rocks and Black Lake plutonic rocks. The deposit is centered on a mineralized porphyritic monzodiorite/diorite pluton and associated WSW trending dykes, which extend to the southwest. Higher-grade copper-gold mineralization is characterized by secondary biotite alteration in volcanic and the eastern plutonic host rocks.

Porphyry style copper-gold mineralization occurs within the Takla volcanic rocks and intermediate intrusive rocks associated with weak to pervasive propyllitic, phyllic, and potassic (biotitic) alteration assemblages. The latter is associated with better copper and gold grades. Alteration of Toodoggone assemblages ranges from fresh to weak propyllitic and is generally barren of significant sulphides and ore grade mineralization.

Gold-copper mineralization forms an inclined tabular zone that is centred on the East Cirque porphyritic monzodiorite, which from structural contours, strikes east-west and dips 20° to the south. The quartz diorite/quartz monzonite intrusive exhibits an irregular upper contact with various peaks and troughs. The general east west strike and shallow south dip geometry is consistent for over 400 metres (mine grid 10660E to 10180E). Between 10260E and 10160E the tabular morphology disappears and the monzonite occurs as wide dykes (10–100 metre) within the Takla volcanics. The change in geometry for the monzonite could be due to the effects of cross-faulting that have down dropped the tabular upper contact present in the East Cirque, or the rheologic conditions during intrusion changed going towards the west whereby steep fracture infilling was preferred over stopping.

Alteration and mineralization is associated with and zoned both vertically and laterally from the quartz diorite/quartz monzonite intrusive and its associated dykes intersected at depth beneath the Central and East Cirques.

The sulphate leach zone, which consists mostly of iron oxides, sericite-chlorite-quartz-pyrite forms an extensive broken zone beneath bright orange-red outcrops at surface (hematite limonite; McKinley 2006). Pyrite is common throughout (5–7%) as both disseminated and within vuggy quartz veining. This alteration zone is mostly barren of any significant copper and will often show a slight increase in gold with depth. The zone is a result of dissolution of the sulphate and carbonate minerals by highly acidic ground waters present currently and probably during the Jurassic.

Present over the entire area in all rock units except the late mafic dykes are barren pinkish zeolitecarbonate veins, which post-date and crosscut the above vein types and rock units. The zeolite-carbonate veinlets are low temperature phenomena.

Overall, sulphide mineralization throughout the deposit consists of 2–3% pyrite, with lesser amounts of chalcopyrite and traces of molybdenum. Pyrite occurs as disseminations, fracture fillings, and veins up to a few centimetres wide generally associated with quartz-gypsum-magnetite veins and zones of quartzmagnetite replacement. The mode of occurrence of chalcopyrite is similar except that veinlets are rare and significant disseminations occur in zones of stronger quartz magnetite stock work and quartz magnetite replacements. Gold and copper grades variably diminish outward into the hanging wall and footwall. Total sulphide content in the core of the deposit averages 3–5%, rising to 10–12% in the phyllic halo.

The opportunity to exploit the Kemess Underground (KUG) gold-copper-silver deposit by underground mining methods such as longhole open stoping, sub-level caving, and block cave mining methods, with the block cave approach recommended due to superior economics. As a result, AuRico (then Northgate Minerals) elected to conduct a Feasibility Study (FS) for the Kemess Underground (KUG) deposit based on block caving.

The KUG panel cave mine was designed using Gemcom’s PCBC™ software. The planned production schedule calls for 107.3 Mt of ore mined at a head grade of 0.27% Cu and 0.54 g/t Au and 1.99 g/t Ag.

Triple declines will be developed from surface comprising access, intake air, and conveyor declines. The access decline will provide access for personnel, equipment, and materials/consumables.

The final design establishes a single extraction level at the 1140 mRL (Figure 5.3) that includes 582 drawpoints (291 drawbells) over a footprint that is approximately 570 m eastwest and 90 m to 300 m north-south. The cave will be initiated in the highest grade ore in the east of the orebody and progress to the southwest over the life of the mine. An 18 m high undercut will be established above the undercut level, with this level at the 1158 mRL and accessed via ramps from the extraction level.

Ore will be recovered on the extraction level using 8.6 m³ (17 t) load-haul-dump (LHD) machines and dumped into one of four 55” x 42” jaw crushers located to the south of the footprint. Transfer conveyors will then transport ore to the main underground conveyor.

The main underground conveyor, 3.2 km long and rising 305 m vertically, will transport ore from the transfer conveyors and transfer ore to a 4.9 km surface conveyor that will drop 98 m vertically overall. A total of 9 Mt per year (Mt/y) of ore will be discharged onto the existing crushed ore stockpile located at the existing Kemess South process plant site.

The Kemess Underground (KUG) project intends to process 9 Mt/y (24,600 t/d equivalent) through one of the two original Kemess South (KS) grinding circuits that processed KS ore. The “A side” grinding circuit has been removed. The existing Kemess “B side” grinding circuit will be used to process the KUG ore following primary crushing underground and stockpiling ahead of the process plant. The original flotation, thickening, and concentrate handling facilities remain, while the tailings will be pumped to the KS open pit (as the KUG Tailings Storage Facility, TSF).

The KS process plant circuit generated a concentrate grade of 20-23% Cu at 70-85% Cu recovery and 55-70% Au recovery. For the last few years of KS operation, grades were 0.1- 0.2% Cu and 0.2-0.4 g/t Au compared to KUG estimated averages of 0.27% Cu and 0.54 g/t Au.

For KUG ore, the process plant will produce a single concentrate at an estimated grade of 22% Cu, and is expected to achieve recoveries of 91% Cu, 72% Au and 65% Ag.

Four single-toggle jaw crushers will be installed underground in the KUG mine. Ore will be conveyed from the crushers to the conveyor decline portal where it will be transferred to an overland conveyor for delivery to the existing process plant ore feed stockpile.

Based on the metallurgical testwork that has been completed for the KUG project, the existing Kemess South process plant flowsheet has the unit operations required to process the ore.

There are some additional equipment requirements, primarily for fine regrinding, but the main preparation efforts will focus on reconditioning the existing equipment.

One SAG mill/ball mill grinding line was removed from the KS grinding circuit. The targeted process plant throughput for the KUG project is estimated to be 24,600 tpd on average. The simplified flowsheet in Figure 6 illustrates the revised flowsheet. A copper flotation concentrate containing gold and silver values will be produced and transported to smelters for processing. Tailings will be discharged to the KS open pit which will be the KUG tailings storage facility (TSF) and which has sufficient volume capacity for the duration of the KUG operations. Process reclaim water will be pumped to the KS process plant facility from the KUG TSF.

Primary jaw crushers will be located underground, with up to four 55” x 42” (1.4 m x 1.07 m), single-toggle jaw crushers operating to deliver 9 Mt/y.

Ore will be transferred from the overland conveyor to the coarse ore stockpile adjacent to the KS process plant using the existing stacker conveyor. The coarse ore stockpile has a live storage capacity of 74,200 t. Ore will be reclaimed from the stockpile by three existing 6 ft x 20 ft (1.83 m x 6.1 m) apron feeders delivering ore to a 60" (1.52 m) wide SAG mill feed conveyor.

The grinding circuit will utilize the existing 10.36 m x 4.65 m (34 ft x 15.25 ft ) SAG mill driven by twin 4,474 kW (6,000 hp) motors. The SAG mill is fixed speed and operated in series with a closed-circuit 6.7 m x 11.1 m (22 ft x 36.5 ft) ball mill, also driven by twin 4,475 kW (6,000 hp) motors. Coarse material or pebbles discharging from the SAG mill (screen oversize) is returned to the mill without being crushed. Slurry discharging from the SAG mill (screen undersize) and ball mill are collected in the cyclone feed pump box and pumped to the ball mill cyclopac for closed circuit ball mill operation. The grinding circuit will have a nominal capacity of 24,600 t/d.

The existing cyclopac in the grinding circuit had been retrofitted with gMAX® 660 mm (26”) cyclones prior to the shutdown of KS operations. The cyclopac consists of 12 cyclones of which nine are typically operational. There will not be any changes to the existing cyclone pumps or cyclopacs for processing of KUG ore.

Cyclone overflow from the aforementioned grinding circuit will be processed in the 16 existing 127 m3 Wemco rougher flotation cells. The circuit layout has two lines of two parallel rows each containing eight flotation cells. During operation of KS, some of the Wemco rougher flotation cells were refurbished with Outokumpu tank cell mechanisms. There are a total of four banks of eight rougher cells (32 cells) and a feed distribution tank which permits selection and operation of each rougher flotation bank independently. No additional rougher flotation modifications are required for processing ore from the KUG deposit.

Rougher flotation concentrate will be pumped to the concentrate regrind circuit to produce a P80 of 20 µm product for final cleaning. The existing 4,475 kW ball mill regrind will not be sufficient to handle the regrinding\ requirements that result from the increased mass pull and the fine target product size. A two-stage regrind circuit will be configured with the existing ball mill regrind operated as the first stage. Cyclones will be used to increase mill feed density and remove the fines component prior to the regrind ball mill. For this feasibility study, the second regrind stage is based on using two 3,000 kW IsaMill™ units to take advantage of the higher efficiencies offered by stirred mills. These fine grinding mills will operate as a single pass unit in open circuit without classification.

Product from the rougher concentrate regrinding circuit will be pumped to the first cleaner mechanical flotation cells. These cells were purchased used, and were installed and retrofitted into the KS process plant. Allowance has been provided to rebuild the first cleaner flotation cells for use on the KUG project. The first cleaner concentrate will be pumped to the two existing 3.4 m diameter x 12.2 m high column cells which are arranged in series. Column cell final concentrate will be pumped to the concentrate thickener. Column cell tailings will be delivered to one line of the existing mechanical column scavenger cells. The line contains 12 Denver cells, each 17 m3 capacity, that will need to be rebuilt. First cleaner tailings will flow to the cleaner scavenger flotation cells from which the concentrate will be pumped back to regrind circuit.

The combined tailings from the cleaner scavenger flotation cells and the column scavenger cells will be pumped to the tailings facility in the KS open pit (KUG TSF) for subaqueous disposal. This will be a new pumping system for the KUG project included in the plant CapEx and Opex estimates as well as tailings discharge in the KUG TSF. Rougher flotation tailings will be pumped to the KUG TSF using the existing tailings pumping system.

Final copper concentrate will be pumped to an existing 18.3 m diameter high capacity WesTech thickener circuit where it will be thickened to 70% solids w/w. The original 10.8 m diameter thickener acts as a clarifier for the concentrate thickener before the overflow water is returned to the process. Thickened concentrate will be stored in the two existing 4.88 m diameter x 4.00 m high agitated stock tanks prior to being pumped to the filter presses.

Two existing Baker Hughes plate and frame filters each containing 135 m2 of filter area will dewater the concentrate to a target moisture content of 8%. The dry concentrate will be conveyed to the existing concentrate storage area before being loaded onto trucks via a front-end loader.