Magmatic nickel-copper-platinum group element (PGE) deposits occur as sulphide concentrations associated with a variety of mafic and ultramafic magmatic rocks. The magmas originate in the upper mantle, and an immiscible sulphide phase occasionally separates from the magma as a result of the processes occurring during emplacement into the crust. The sulphide phase generally partitions and concentrates nickel, copper and PGE elements from the surrounding magma. The heavy sulphide droplets once concentrated and separated from the magma tend to sink towards the base of the magma, and form concentrated pockets or layers of sulphides that crystallize upon cooling to form mineral deposits.

The Dumont mineral deposit comprises olivine + sulphide cumulates that comprise differentiated layers of the Dumont sill, an Archean komatiitic intrusion contained within the Archean Abitibi Greenstone Belt of northwestern Quebec. As such, it is usually classified (Naldrett, 1989) with its most analogous counterpart, the Mt. Keith mineral deposit located in the Agnew-Wiluna Greenstone Belt within the Archean Yilgarn craton of West Australia.

Nickel-bearing sulphides and a nickel-iron alloy are enriched (grades > 0.35% nickel) in stratiform bands within the dunite subzone and are also broadly disseminated at lower concentrations throughout the dunite and lower peridotite subzones. The number and thickness of these bands varies from place to place in the deposit. Nickel sulphide and alloy concentrations decrease gradationally away from the centre of these bands toward the interband zones where mineralization continues at lower concentrations. The total nickel contained in these rocks occurs in variable proportions in sulphides, alloy and silicates depending on primary magmatic nickel mineralogy and the degree of serpentinization of the rock.

Disseminated nickel mineralization is characterized by disseminated blebs of pentlandite ((Ni,Fe)9S8), heazlewoodite (Ni3S2), and the ferronickel alloy, awaruite (Ni2.5Fe), occurring in various proportions throughout the sill. These minerals can occur together as coarse agglomerates, predominantly associated with magnetite, up to 10,000 µm (10 mm), or as individual disseminated grains ranging from 2 to 1,000 µm (0.002 to 1 mm). Nickel can also occur in the crystal structure of several silicate minerals including olivine and serpentine.

Mineralized zones containing pentlandite, awaruite, and heazlewoodite, are classified into the following mineralization assemblages; sulphide dominant, alloy dominant and mixed.

The sulphide mineralization assemblage occurs in higher-grade bands (grades > 0.35% nickel) that are subparallel to the dip of, and principally in the centre of, the sill. Sulphide mineralization is dominated by pentlandite (Pn) and/or heazlewoodite (Hz) with lesser awaruite (Aw). Pentlandite and heazlewoodite occur as medium to coarse-grained blebs occupying intercumulus spaces in a primary magmatic texture, sometime exhibiting secondary overgrows within magnetic blebs. These blebs are often intimately associated with magnetite ± brucite ± chromite ± awaruite, in intercumulus spaces. Where awaruite is present with sulphides, it often observed to be a secondary overgrowth on pentlandite within the primarily textures intercumulus magnetite blebs. Up to three sulphide bands are found within the dunite where it is the thickest in the central southeast region of the sill.

The alloy mineralization assemblage is characterized by the presence of awaruite with little to no sulphides. Awaruite occurs as fine grains (generally <1 mm) associated with small intercumulus magnetite or chromite blebs. Awaruite can also be observed as a secondary overgrowth on serpentine within the pseudomorphed grain. Alloy mineralization zones occur where primary sulphides are not present and serpentinization is near complete.

The mixed mineralization assemblage typically represents a transition from sulphide to alloy or sulphide (pentlandite) to sulphide (heazlewoodite) mineralization. The mixed mineralization assemblage contains varying amounts of sulphide (pentlandite and heazlewoodite) along with similar quantities of awaruite. Mineralization can occur as coarse sulphide-magnetite blebs associated with awaruite or as finely disseminated discrete grains.

The Dumont pit will ultimately measure approximately 4.9 km along strike, 1.4 km at the widest point and reach a maximum depth of 520 m below surface. A total of 2,080 Mt of material will be excavated, using large surface mining equipment that will operate at high production rates.

The pit is comprised or three distinct areas:

• The “Quarry”, at the south east limit of the deposit hosts the only outcrop and is thus where mining initiates. The Quarry is initially excavated to a volume of approximately 5 Mm3 and serves as contingent storage for water during the period the Main Pit is operational. Following completion of operations in the Main Pit, the Quarry is expanded to its full limits of approximately 30 Mm3.
• The “South-East Extension” (SEE), measures approximately 92 Mm3. Development of the SEE commences immediately following that of the Quarry, from which it is separated by a saddle of rock. After the SEE reaches its final limits, it is backfilled to just below surface with waste rock from the Main Pit.
• The “Main Pit” represents approximately 85% of the total excavation.

The ore body is covered with overburden of varying depth. Overburden, which comprises 8.3% of the total material that will be excavated, is comprised of differing material types. Overburden will be stripped in advance of the ore mining operation and impounded in different areas depending on the geotechnical parameters of the specific material type. Waste rock, which represents 42.3% of the total material excavated, will mainly be stored in a single large dump. The remainder of waste rock will be used for construction of various infrastructure including roads and the tailings storage facility (TSF). Ore, which makes up the remaining 49.4% of the total tonnage excavated, will be fed to the mill, either directly as run-of-mine (ROM) feed or after being temporarily impounded in one of three low- grade stockpiles. Tailings from the treatment of ore will be impounded in the TSF while the Main Pit is operational. In later years, when mill feed is sourced from the Quarry and/or stockpiles, tailings will be impounded within the depleted Main Pit shell.

The mine will have de-watering systems and an electrical supply system for the electrically powered mining equipment, including large excavators, shovels and trolley-assist haul trucks. Unit operations will consist of drilling, blasting, loading and hauling.

Multiple fleets of load and haul equipment will be employed to ensure the various rock types at Dumont will be mined most productively. Areas where the clay thickness exceeds 7.5 m will be stripped using 90 t and 150 t backhoe excavators while the remaining areas where mining will be conducted on a nominal 5 m bench height will be stripped using 300 t excavators in face shovel configuration. All three of the smaller excavators will be diesel powered and they will load a combined 7% of the 2,080 Mt expit tonnage, or 6% of the total tonnage that includes a further 511 Mt reclaimed from stockpiles. The 600 t excavators, which will operate predominantly on 10 m benches as well as the stockpiles, will be electrically powered and load 22% of the total tonnage. The remaining 72% of total material will be loaded by rope shovels, operating predominantly on 15 m benches and the stockpiles.

The process plant and associated service facilities will process ROM ore delivered to primary crushers to produce nickel concentrate and tailings. The proposed process encompasses:

• Crushing and grinding of the ROM ore;
• Desliming via hydrocycloning;
• Slimes rougher and cleaning flotation;
• Nickel sulphide rougher, scavenger and cleaning flotation;
• Magnetic recovery of sulphide rougher tailings and sulphide cleaner tailings;
• Regrinding of magnetic concentrate; and
• Awaruite recovery circuit, consisting of rougher and cleaner flotation stages.

Concentrate will be thickened, filtered and stockpiled on site prior to being loaded onto railcars or trucks for transport to third-party processing facilities.

The magnetic separation tailings and awaruite rougher tailings will be combined in the coarse tailings thickener. The majority of the thickened coarse tailings will be sent to the TSF, while a small portion will be mixed with the slimes flotation tailings to help settle the material in the slimes tailings thickener. The thickened slimes tailings will also be sent to the TSF in a dedicated pipeline.

The process plant will be built in two phases. Initially, the plant will be designed to process 52.5 kt/d. The expansion will be designed as a duplicate processing plant to increase plant capacity to 105 kt/d. The initial phase will include an allowance for common concentrate thickening facilities.