The NorthMet Deposit is a large-tonnage, disseminated accumulation of sulphide in mafic rocks, with rare massive sulphides. Copper to nickel ratios generally range from 3:1 to 4:1. Primary mineralization is probably magmatic, though the possibility of structurally controlled re-mobilization of the mineralization (especially PGEs) has not been excluded. Sulphur source is both local and magmatic (Theriault et al., 2000). Extensive detailed logging has shown no definitive relation between specific rock type and the quantity or grade quality of sulphide mineralization in the Unit 1 mineralized zone or in other units, though the localized noritic to gabbronoritic rocks (related to footwall assimilation) tend to be of poorer PGE grade and higher in sulphur.

The NorthMet Deposit is part of the Duluth Complex in northeastern Minnesota, which is a large, composite, grossly layered, tholeiitic mafic intrusion that was emplaced into comagmatic flood basalts along a portion of the Mesoproterozoic Mid-continent Rift System. NorthMet is one of eleven known copper-nickel deposits that occur along the western edge of the Duluth Complex and within the Partridge River (PRI) and South Kawishiwi (SKI) intrusions. The NorthMet Deposit is hosted within the PRI, which consists of varied troctolitic and (minor) gabbroic rock types that have been subdivided into seven igneous stratigraphic units based on drill core logging.

The metals of interest at NorthMet are copper, nickel, cobalt, platinum, palladium and gold. Minor amounts of rhodium and ruthenium are also present though these are considered to have no economic significance. In general, with the exception of cobalt, the metals have strong positive correlations with copper mineralization. Cobalt is well correlated with nickel and reasonably correlated with copper.

Mineralization occurs in four broadly defined horizons or zones throughout the NorthMet property. Three of these horizons are within basal Unit 1, though they likely will not be discriminated in mining. The upper horizon locally extends upward into the base of Unit 2. The thickness of each of the three Unit 1 enriched horizons varies from 5 ft to more than 200 ft. Unit 1 mineralization is found throughout the base of the NorthMet Deposit. A less extensive (the copper-rich, sulphur-poor Magenta Zone) mineralized zone is found in Units 4, 5 and 6, in the western part of the NorthMet Deposit.

Mineralization occurs in two broad forms. Firstly, sulphides may be disseminated in heterogeneous troctolitic rocks (mainly Unit 1) in which the grain sizes of both silicates and sulphides widely vary. The occurrence and amount of this mineralization within drill holes can be unpredictable over the scale of 20 to 30 ft though mineralization is relatively constant in some horizons (i.e., top of Unit 1). Secondly, economic concentrations of sulphides in the upper units tend to be coarser grained and copper-rich (Units 2 to 7, particularly the Magenta Zone).

Sulphide mineralization consists of chalcopyrite and cubanite, pyrrhotite and pentlandite, with minor bornite, violarite, pyrite, sphalerite, galena, talnakhite, mackinawite and valerite. Sulphide minerals occur mainly as blebs interstitial to plagioclase, olivine and augite grains, but also may occur within plagioclase and augite grains, as intergrowths with silicates, or as fine veinlets. Small globular aggregates of sulphides (less than two centimetres) have been observed in core and in the small test pit on the site. The percentage of sulphide varies from trace to about 5%, but is rarely greater than 3%. Local massive sulphide is present, but rare. Platinum, palladium, and gold are associated with the sulphides as well as in tellurides and bismuthides.

Mining of the NorthMet Deposit and the economic pit cost parameters were based on bulk mining methods 365 days per year. The waste rock and ore would be drilled, blasted, loaded and hauled with conventional drills, trucks and hydraulic shovels.

Ore will be hauled directly to the truck dump/feeder facility for loading on the rail cars. Stockpile ore would be placed in a lined storage area to the east of the truck dump. The direct ship ore would be hauled to the NorthMet processing facility and discharged above the primary crusher. Rehandle of the stockpile ore would be accomplished by a loader and truck hauling to the truck dump/feeder from the stockpile or direct load into railcars.

The original definitive feasibility study (DFS) had envisaged the use of 100 ton trucks, but with the scale of the deposit, much larger trucks were considered. A review was made of various fleet configurations that varied truck capacity and shovel types. The result of that analysis indicated that the 240 ton truck configuration matched with a hydraulic shovel provided the most cost effective method of developing the NorthMet Deposit.

Support equipment included tracked and rubber-tired dozers with graders and small front end loaders. Two large and two small water trucks were envisaged for use to control dust and water the drills for dust suppression. A large rubber tired dozer was included in the fleet of dozers. This was to provide flexibility either at the shovel face or on the dumps without the excessive travel time concerns raised with conventional track dozers. A smaller track dozer was planned for use to manage the tailings facility.

Existing equipment will be reinstated and used for both coarse and fine ore crushing (including gyratory and cone crushers), and in the ore milling circuit (including rod and ball mills). The flotation plant is a new circuit that will be housed in the existing Concentrator building.

In the coarse crushing area, ROM ore (with a top feed size of 48 inch /1,200 mm) is reduced in two stages to 100% passing 3 inch (75 mm) prior to further size reduction in the fine crushing circuit at an average feed rate of 1,666 st/h (1,512 mt/h). ROM ore is delivered by rail from the open pit and dumped sequentially from 100 short ton side tipping rail cars into a 110 short ton dump pocket above the 60 inch (1200 hp/900 kW) gyratory primary crusher. Following primary crushing, the ore gravitates to a second stage of crushing in three parallel 36-inch (540 hp/400 kW) gyratory crushers. The discharge from the secondary crushers is conveyed to a coarse ore bin above the fine crushers, which has a live capacity of approximately 2,200 tons, which is equivalent to approximately 80 minutes of continuous feed.

Fine ore crushing – The coarse crushed product is further reduced in two stages to 8 mm suitable for feed to the milling circuit. Coarse crushed ore is delivered to a coarse ore storage bin that extends the length of the Fine Crushing Building. Since only three fine crushing lines will be reactivated only a portion of the total live storage capacity will be used. From the coarse ore bin material gravitates to three parallel fine crushing lines, each line consisting of a 7 ft (470 hp/350 kW) standard cone tertiary crusher discharging onto two double deck vibrating screens from where oversize discharges to two 7 ft (470 hp/350 kW) short head quaternary cone crushers. The screen undersize material passes directly to the conveyor below, which also collects quaternary crusher products. The final crushed product is conveyed to a fine ore bin in the Concentrator Building, which has a live mill feed storage capacity of approximately 17 hours.

The milling circuits liberate sulphide minerals contained in the ore through a process of particle size reduction. The milling circuit comprises twelve parallel circuits each consisting of a 12 ft diameter and 15 ft long 800 hp (600 kW) rod mill feeding a 1,250 hp (930 kW) ball mill operating in closed circuit with a cyclone, with a circulating load of 250%. Each rod mill receives a proportion of finely crushed ore, approximately 128 st/h (116 mt/h) at P80 of 8 mm, and discharges product to a ball mill, which produces milled product at P80 120 of µm.

The objective of the flotation circuit is to recover a bulk sulphide concentrate containing the base and precious metals whilst rejecting largely siliceous tailings. The concentrate produced is then fed to the POX in the hydrometallurgical plant.

Milled primary cyclone overflow along with flotation regrind cyclone overflow is split to two parallel trains of rougher/scavenger flotation. Rougher concentrate from both trains is combined and undergoes two stages of cleaner flotation to reduce mass and increase sulphide grade ahead of POX. Scavenger concentrate is combined with Cleaner One tailings and is fed to a regrind circuit, which includes one regrind mill operating in closed circuit with a regrind cyclone. The regrind cyclone overflow is directed back to the head of flotation.

Flotation requires a number of reagents and make-up storage tanks and dosing pumps are provided within a nearby dedicated flotation reagents area.

The final cleaner concentrate is mixed with flocculant and thickened, and the resulting underflow is pumped to a fine grinding ISA Mill to produce the POX feed at P80 of 15 µm.