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 ........