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 Mount Milligan Mine is a conventional truck-shovel open-pit copper and gold mine.

The mining operations equipment fleet comprises two 311 mm electric blast hole drills, two 41 m3 electric cable shovels, one 19 m3 front end loader and eleven 217 t trucks. These major units are supplemented with a back-up equipment fleet of graders, track and rubber-tired dozers, backhoes, and water trucks. A 15 m bench height has been selected for mining both ore and waste. The overall mining sequence was developed through a series of mining pushbacks specific to the MBX, 66 and WBX Subzones (collectively, the Main Zone), and SS phases.

The Mount Milligan Mine deposit is being mined using conventional open-pit equipment, with the ore being processed through a conventional flotation plant, producing a marketable concentrate containing copper, gold, and silver.

Metallurgical investigations conducted by various research laboratories conclusively showed that froth flotation is the optimum process for the recovery of copper, gold, and silver. The life-of-mine (LOM) average recoveries are estimated at 84.9% for copper and 72.5% for gold. Concentrate grade varied with head grade and ore type with a LOM average of 25.0% Cu. The impurity element contents in the concentrate were below the penalty levels normally imposed by most smelters; therefore, no penalties are expected.

The concentrator was designed to process 60,000 t/d and consisted of a primary crushing plant, coarse ore stockpile, SAG/ball mill/crusher (SABC) grinding circuit, rougher flotation circuit, regrind and cleaner flotation circuit, gravity separation circuit, and concentrate dewatering plant.

The ore is ground to 80% passing 220 µm and processed in the flotation circuit to produce a marketable-grade copper concentrate. A gravity circuit treats a portion of the rougher concentrate regrind cyclone underflow to recover any coarse metallic gold that tends to circulate in the regrind circuit and could be lost in the flotation cleaner circuit because of its very high specific gravity. The gravity concentrate is combined with the final flotation concentrate and pressurefiltered to produce a final product containing approximately 9% moisture.

The company is moving forward with the construction of the permanent secondary crushing circuit, which is expected to be completed in the third quarter of 2016, followed by commissioning in the fourth quarter of 2016. Once completed and commissioned, design average daily mill throughput is expected to increase to 62,500 tonnes per day and above.