Mount Milligan is a tabular, near-surface, alkalic copper-gold porphyry deposit that measures some 2,500 metres (m) north-south, 1,500 m east-west and is +400 m thick. It consists of two principal zones, the Main Zone and Southern Star (SS) Zone. The Main Zone includes four contiguous sub-zones: MBX, WBX, DWBX and 66, all of which are spatially associated with the MBX monzonite stock and Rainbow Dyke. The SS Zone is centred on a monzonite stock of the same name and is some 1000 m south of the Main Zone.

Main Zone mineralization and associated alteration are primarily hosted in volcanic rocks, whereas in the SS Zone, mineralization is hosted equally in monzonite stock and volcanic rocks. Mineralization in both zones consists of pyrite, chalcopyrite and magnetite with bornite localized along intrusive-volcanic contacts. The pyritic 66 sub-zone is notable in that it is comparatively gold-rich.

Copper-gold mineralization is primarily associated with potassic alteration which decreases in intensity outwards from the monzonite stocks. Pyrite content increases significantly outward from the stocks where it occurs in association with propylitic alteration, which forms a halo around the potassic-altered rocks.

The mining operation is a conventional shovel and truck open pit mine feeding a 60,000 t/d (permitted throughput) processing plant. The planned mine life is 9 years with a Proven and Probable Reserve of 191.0 million tonnes at a grade of 0.23% copper and 0.39 g/t gold. The pit has been planned as a series of discrete pushbacks and is scheduled to maximize the production of high-value ore, especially during the next three years. This strategy may necessitate mining at a higher cut-off value and the stockpiling of lower value material for processing later in the mine life. The average stripping ratio for the LOM is estimated to be 1.24:1.

The mine currently uses two electric rotary blasthole drills which drill 311 mm diameter holes, two diesel blast hole drills for 203 mm holes, two 41 m³ electric rope shovels and two 19 m3 front end loaders loading haul trucks of 220 and 190 tonnes capacity from 15m high production benches. The operation is supported by standard ancillary equipment including track and rubber tire dozers, graders and a fleet of service vehicles.

The mine operates continuously, seven days per week, using a roster schedule of predominantly 14 days on, 14 days off, with 12-hour shifts.

The overall mining sequence was developed through a series of mining pushbacks (phases) specific to the MBX, 66 and WBX subzones (collectively, the Main Zone), and the Southern Star (SS) phases.

Mineralized material with a value greater than $7.64 (C$9.55) per tonne and with a copper grade greater than or equal to 0.12% is designated as standard ore. Material with a value greater than $7.64 (C$9.55) per tonne with copper grades less than 0.12% are designated as HGLC (high-gold low- copper) ore.

The ultimate pit haul roads have been designed with an overall 34 m width that will include an outside berm (4.84 m wide, 1.82 m high) and ditches (1.00 m) for dual lane passing of 227-t haul trucks (8.28 m) with a one-half truck width (4.12 m) for truck separation. On each side of the truck, an allowance of 4.23 m for rubble buildup and safety berm creep has been included. Roads have been designed with a maximum 10% down grade.

The current LOM Plan consists of nine Phases, building off previous Phases, 1 and 2, which are now complete. The mine is currently operating in Phases 3 and 4 with initial stripping started in Phase 8 in the Southern Star Pit. Phases 3, 4, 5, 6a, 6 and 7 are focused on the Main Zone while Phases 8, 9a and 9 will focus on the Southern Star Pit. The phases are not mined sequentially. The LOM schedule is focused on returning the highest value ore as early as possible while honouring metallurgical constraints.

A secondary crushing circuit, installed in 2016, together with process plant optimization projects, increased the throughput to a nominal rate of 62,500 t/d. Key process equipment consists of:

• Primary crushing plant with a 1.525 m x 2.794 m gyratory crusher
• Secondary crushing plant with two cone crushers prior to the grinding circuit, each powered by one 1,000 kW motor
• SAG/ball mill/pebble crusher grinding circuit:
- one SAG mill with one 23.5 megawatt (MW) gearless motor drive
- two ball mills each driven by two 6.5 MW variable speed synchronous motors (26 MW total installed ball mill power), operated in closed circuit with cyclones
- two cone crushers, each powered by one 750 kW motor
• Regrinding and gravity concentration circuits:
- one 1,125 kW tower mill operated in closed circuit with six 500 mm and two 375 mm cyclones
- two 3,000 kW IsaMills™ operated in closed circuit with twenty-six 250 mm cyclones

Run-of-mine ore is crushed to a top size of 150 mm. Up to 100% of the gyratory crusher product is diverted to the secondary crushing circuit by a moveable gate. Prior to being introduced into the SAG and ball mill circuits, a variable percentage of the ore, depending on hardness, is sent to the secondary crushing circuit targeting a 60 mm, or finer, product. The SAG discharge screens’ oversize is recycled to the pebble crushing circuit targeting a 12 mm, or finer, product. The design for the final feed to flotation from the ball mill circuit is a product size of 80% passing 212 µm. Grinding efficiency improvements in the SAG and ball mill circuits have led to a decreased product size feeding the rougher flotation circuit, averaging 175 µm for 2019 at nominal throughput rates.

The Mount Milligan process plant was originally designed to process ore at a nominal rate of 60,000 t/d, producing a marketable concentrate containing copper, gold, and silver. Key process equipment consists of:

• Flotation circuits:
- rougher flotation: two parallel trains of two 200 m3 tank cells
- rougher-scavenger flotation: two parallel trains of three 200 m3 tank cells
- first cleaner flotation: two 100 m3 tank cells
- second cleaner flotation: four 30 m3 tank cells
- third cleaner flotation: two 30 m3 tank cells
- one 4 m x 12 m flotation column
- cleaner scavenger flotation: five 100 m3 tank cells
• Regrinding and gravity concentration circuits:
- one centrifugal gold concentrator.

The rougher/rougher-scavenger flotation circuit produces a higher-grade rougher concentrate and a lower-grade rougher-scavenger concentrate. These concentrates are separately reground and then upgraded in three cl ........