Magmatic sulphide deposits containing nickel (Ni) and copper (Cu), with or without (±) platinum-group elements (PGMs), account for approximately 60% of the world’s Ni production and are active exploration targets in the United States and elsewhere. On the basis of their principal metal production, magmatic sulphide deposits in mafic rocks can be divided into two major types: those that are sulphide-rich, typically with 10 to 90% sulphide minerals, and have economic value primarily because of their Ni and Cu contents; and those that are sulphide-poor, typically with 0.5 to 5% sulphide minerals, and are exploited principally for PGE.

The Eagle deposit and the Eagle East conduit zone are high-grade magmatic sulphide accumulations containing nickel-copper mineralization and minor amounts of cobalt and PGMs. The economic minerals associated with this deposit are predominately pentlandite and chalcopyrite.

Eagle and Eagle East are part of the same ultramafic intrusive complex and both host high grade primary magmatic nickel copper sulphide mineralization. Mineralization styles are similar at Eagle and Eagle East, consisting of ovoid to pipe-like bodies of mineralized peridotite with concentrations of sulphide mineralization along the base of the intrusion resulting in the accumulation of semi-massive sulphide, and a central core zone of massive sulphide.

Two types of potentially economic mineralization are found in Eagle and Eagle East: semimassive (SMSU) sulphides and massive sulphides (MSU). Disseminated mineralization is also encountered in the peridotite intrusive, however, because it is not economic, the mineralized peridotite with disseminated sulphides has been considered as an intrusive and not a mineralized unit.

Sulphides are deposited as dense droplets in the primary magma due to decreased flow rate in the magma, or a change from laminar to turbulent flow due to changes in the conduit geometry. Multiple pulses occur in the same plumbing system, resulting in three discrete mineralization types which typically have hard contacts. The mineralizing intrusion is Mineralized Peridotite (MPER), which transports sulphides within large volumes of magma, and in this way is able to transport significant quantities of dense sulphides upward through the crust in a diluted form. This results in the conduits between mineralized zones consisting of barren peridotite or weakly mineralized peridotite, such as the upper zone of Eagle East.

Typical mineralization zoning at both Eagle and Eagle East consists of a mineralized peridotite conduit with a core of SMSU and a base of crosscutting MSU that also sills out into the surrounding sediments. The massive sulphide remains liquid for a significant time, so it is able to crosscut other units after emplacement is complete.

Mine production is made up of a combination of ore development through sill drifts or cuts and stope production. The mining method selected for Eagle and Eagle East is the Transverse Sub Level Open Stoping (SLOS) method using a combination of Cemented rock fill (CRF) and nonconsolidated waste rock backfill. This method provides the cost advantages of bulk mining, while maintaining a degree of selectivity and operational flexibility. The majority of the stopes will be mined as transverse bench and fill stopes, with some narrower zones of the orebody mined as longitudinal retreat stopes.

The mining method used at Eagle is suitable to exploit the material at Eagle East, namely a primary/secondary sequence of transverse open stopes with cemented backfill or rockfill. Some longitudinal stopes are used to exploit the narrower sections or the orebody at the extremities.

The Eagle orebody is accessed by the sub-level footwall drives driven off the main decline at 20 m to 25 m vertical intervals. Stopes are designed at 10 m wide and approximately 25 m high (corresponding to the top and bottom sub levels). Stope lengths will vary depending on the width of the orebody, however, due to geotechnical constraints, individual stope panels are limited to a maximum length of 20 m. The same general dimensions will be used for the Eagle East development.

The Humboldt Mill is a former iron ore processing plant that was converted for processing Eagle ore. The ore is transferred from a covered coarse ore storage facility, processed using a conventional three-stage crushing and single-stage ball milling process, and processed through bulk flotation with subsequent separation flotation to produce separate nickel and copper concentrates. Metallurgical recoveries of nickel and copper average 84% and 97% respectively for Eagle Mine ore. Tailings from the plant are deposited sub aqueously in the adjacent former Humboldt iron ore mine open pit, now known as the Humboldt Tailings Disposal Facility (HTDF).

The mill was designed for a throughput of 2,000 tpd and has been shown to be capable of processing up to 2,500 tpd. The Humboldt Mill produces separate nickel and copper concentrates which are transported from the site via rail.

The Eagle process flowsheet uses conventional technologies to produce separate copper and nickel concentrate with a throughput of 2,000 tpd (730,000 tpa). Key elements of the process flowsheet are summarized below:

• Run of mine (ROM) ore in the mine COSA is loaded by front end loader into the road haul trucks to transport ore to the milling facility. There is 10,000 tonnes of storage capacity in the coarse ore storage facility at the mill.
• Initial size reduction of the ore is carried out by a primary jaw crusher to reduce the ore size from nominal minus 450 mm ROM to minus 100 mm.
• Further size reduction of primary crushed ore is carried out in a secondary and tertiary crushing circuit to reduce the ore size from minus 100 mm to nominal minus 10 mm.
• Minus 10 mm tertiary crushed ore is stored in bins and then reclaimed by feeders to feed the grinding circuit.
• Tertiary crushed ore is ground in two, single stage ball mill grinding circuits working in parallel. The ball mills operate in closed circuit with hydrocyclones, targeting a p80 of 100 microns. Sodium carbonate is added to the mills for pH and water chemistry control.
• A bulk copper-nickel concentrate is produced by separating the copper and nickel minerals from gangue material by flotation. The copper nickel bulk concentrate is reground, followed by one or two stages of cleaning to reject further gangue minerals. The bulk cleaner concentrate is then subjected to a final flotation stage where the copper and nickel minerals are separated from one another through the addition of lime. Final concentrate grades are 14% Ni and 3% Cu in the nickel concentrate and 31% Cu and 0.8% Ni in copper concentrate.
• An on-stream analyzer provides real time analysis from 11 streams in the mill and collects a 12 hour shift composite sample for analysis at the onsite analytical laboratory.
• Copper and nickel concentrates are dewatered to 8% to 10% moisture content by independent thickeners and filter press circuits, then loaded into rail cars for shipment. Concentrates are transported by rail to a port or directly to smelter facilities.
• Flotation tailings are thickened and the slurry is pumped to the existing HTDF for subaqueous deposition.
• Facilities are also present for storing, preparing, and distributing reagents used in the process. Reagents include: sodium isopropyl xanthate (SIPX), methyl isobutyl carbinol (MIBC), sodium carbonate, lime, flocculant, and carboxymethylcellulose
(CMC).
• Water from the concentrate dewatering operations, tailings dewatering and the HTDF are recycled for reuse in the process. Plant water stream types include: process water, fresh water, reclaim water, and potable water.