Mount Polley is an alkalic porphyry copper-gold deposit. It lies in the tectono-stratigraphic Quesnel terrane or Quesnellia, which is characterized by a Middle Triassic to Early Jurassic assemblage of volcanic, sedimentary and plutonic rocks which formed in an island arc tectonic setting outboard of the ancestral North American continental margin. Mount Polley itself is a complex of alkalic intermediate porphyritic intrusions and related magmatic-hydrothermal breccias.

In the deposits, the degree of brecciation and associated hydrothermal alteration is usually a reliable guide as to grade. The transition from breccia into unbrecciated intrusion forming the ‘wall rock’ roughly corresponds to ore/waste contacts. There is relatively little post-mineralization dike dilution. Chalcopyrite is the dominant copper mineral, typically accompanied by pyrite; bornite is relatively uncommon in the centre of the MPIC. Here, copper sulfides occur as disseminations or veins and fracture coatings in brecciated intrusion, or they are disseminated in the matrix of breccias, in both cases precipitated along with alteration minerals. This is characteristic of the Springer, Cariboo/C2, and WX zones, and in the now mined-out Bell and Southeast zones. The only significant oxide mineralization in the MPIC is in the upper 100-150 m of the Springer deposit, but it is treatable by heap leach for copper extraction, and can be later milled for gold recovery. Mineralization has been traced by deep drilling in the Springer zone to a depth of around 900 m (from pre-mining surface).

In the north of the MPIC are much higher grade orebodies, namely the Northeast (mined in the Wight pit, 2005-2009) and Boundary zones, where copper grades can reach several percent per tonne and are currently supporting underground mining. Chalcopyrite and significant bornite form coarse-grained infill in breccias, and intense vein and microvein stockworks. As in the zones in the centre of the MPIC, gold and silver occur mainly as microscopic inclusions in the copper sulfides and in pyrite. Throughout Mount Polley, copper and gold typically occur in a roughly 1:1 ratio (in terms of copper percent versus gold in grams per tonne).

Mount Polley Mine is an open pit copper-gold mine with a developing underground project. Surface loading equipment included two P&H 2100 electric shovels, a P&H 2300 electric shovel, and an Komatsu PC1800 diesel excavator. The haulage fleet included twelve Caterpillar 785 trucks, and three Caterpillar 777 trucks. The primary crusher pocket has capacity to accept material from Caterpillar 785 trucks. Drilling is performed with an electric Atlas Copco 351 Pit Viper, an electric BE 60R drill and two smaller wall control drills. Underground development is performed in the Boundary zone with two 6-cubic yard scoops, three 30-tonne trucks, a twin-boom jumbo, a bolter, and a scissor deck.

Dredging of tailings in the Springer pit (deposited in the pit in 2015-2016 to allow for restart of milling operations prior to repair of the tailings storage facility) recently commenced. Once mining operations in the Caribo pit are completed in mid-2018, Mount Polley will rely on low grade stockpiles to provide mill feed, until the dredging of the Springer pit is completed.

In the Mount Polley mill, run-of-mine ore from the open pits is hauled to the crusher. The crusher has three stages of crushing involving five crushers, twenty conveyors and four sets of screens. The ore is dumped into the feed pocket of the primary gyratory crusher, crushed in three stages to produce a product at finer than 16mm for the grinding circuit.

The grinding circuit consists of two parallel rod mill/ball mill circuits and a pebble mill circuit. Crusher product is first split between two rod mills where water is added to form slurries. The slurries are pumped to hydrocyclones that classify the particles by size. The larger particles flow to feed the ball mills while the fine particles are discharged to the second stage of grinding: the pebble mill circuit. The ball mills are in “closed circuit”, meaning that the discharge is pumped to the classifying units (hydrocyclones) and the particles will not pass to the next grinding stage until they are fine enough to feed the pebble circuit. The second stage grinding circuit (the pebble mill circuit) also consists of mills, pumps and hydrocyclones. Pebbles obtained from the crusher are used as grinding media for grinding. The coarse particles classified by hydrocyclones reports to three pebble mills for further size reduction. The pebble mills are in “close circuit” and product that is sized at 65% finer than 200mesh is fed to the flotation circuit.

The flotation circuit separates the valuable minerals from the rest of the crushed rocks. With the addition of reagents, the valuable minerals, mostly in the form of sulphides, are separated by floatation and being collected and upgraded to produce a concentrate. Initial separation is done in a rougher/scavenger circuit, where the remaining minerals are discarded as tailings (which flow by gravity to the TIA). Rougher concentrate is reground in a regrind mill and further upgraded in a cleaner circuit to produce the final concentrate product. Cleaner tailings are recycled to the scavenger circuit.

The concentrate from the flotation circuit is dewatered in two stages. The thickener houses settling of particles and decanting of process water so that the settled particles forming a sludge have a reduced water content of roughly 35-40% water while pressure filtration further reduces water content to approximately 7.5%. The water removed is utilized as process water. The filtered concentrate is stored in the load-out building and loaded onto 40-tonne trucks for shipping. Periodically the crusher also used for the production of aggregates used in tailings construction and other tasks.