The Maturi, Maturi Southwest, Birch Lake, and Spruce Road deposits are classified as contact-type magmatic nickel–copper–platinum group element deposits which are a broad group of deposits containing nickel, copper, and PGEs occurring as sulfide concentrations associated with a variety of mafic and ultramafic magmatic rocks (Zientek, 2012; Eckstrand and Hulbert 2007).

In all four deposits, mineralization consists of 1% to 5% disseminated chalcopyrite, cubanite, talnakhite, pyrrhotite, and pentlandite in a tabular zone, parallel to the contact. Except at Spruce Road, better grades of copper, nickel and PGEs are associated with more mafic units located near the top of the BMZ, and there is excellent continuity of widths and values from hole to hole and section to section.

Maturi.
Significant Cu–Ni mineralization occurs at the top of the footwall Giants Range Batholith, is hosted within the contact thermal aureole to the Duluth Complex, and is interpreted to be directly derived from the Duluth Complex. Mineralization in the footwall occurs in approximately 85% of the holes drilled to date, but many of the holes without footwall mineralization did not penetrate to sufficient depth to encounter the footwall. The sulfide mineral assemblage in the footwall is the same as in the basal mineralized zone (BMZ), being dominated by chalcopyrite and pyrrhotite with lesser cubanite, pentlandite, bornite, and talnakhite. Pentlandite is the principal nickel mineral, although small amounts of nickel also occur in talnakhite and pyrrhotite. Chalcopyrite, cubanite, talnakhite, and bornite are the principal copper-bearing minerals. A number of localized, Ni-rich massive sulfide bodies have been encountered by drilling at and below the footwall contact. These bodies are as thick as 18.5 ft (5.64 m) and tend to have much higher Ni:Cu ratios than the disseminated mineralization in the BMZ.

Platinum group minerals (PGMs) have been found in various textural positions (Gál et al, 2010) but most commonly occur as finely disseminated grains within sulfide patches. Pyrrhotite and pentlandite are the preferred hosts; however, chalcopyrite can host PGMs. Work by Gál et al. (2010) indicates that these grains are mostly Pt–Pd–bismuth–tellurides (michenerite, moncheite) or Pd–Sn-bearing phases (paolovite) in composition. Rare grains of Ir–arsenides (irarsenite) are enclosed in pyrrhotite.

The largest concentrations of PGMs occur along the grain boundaries of plagioclase and massive sulfide patches or in thin sulfide veinlets. In such places, Ca-alteration of plagioclase is almost always present with some amount of chlorite or serpentine. Grain boundaries of sulfides and biotite or apatite also host PGMs. Most of the Pt–Pd–bismuth–tellurides (michenerite, moncheite, polarite/sobolevskite) and sperrylite are located in such positions.

Birch Lake.
Microscopy on Birch Lake samples by Cabri (2002) identified the major sulfide minerals as chalcopyrite and undefined members of the chalcopyrite family, possibly talnakhite, mooihoekite, putoranite, and/or haycockite. Oxide minerals include chromian spinel, ilmenite, magnetite, and chromite. Native copper and troilite occur locally. Other identified minerals include bornite, chalcocite, and cubanite as well as nickel sulfide minerals heazlewoodite and pentlandite. Trace amounts of altaite, digenite, frobergite, galena, mackinawite, millerite, sphalerite and unidentified PGEbearing minerals, native silver, silver telluride and alloys of silver and gold were identified. Pentlandite contains as much as 2.12% Co. Iron sulfide gangue is pyrrhotite and troilite. PGMs occur as various fine-grained Pd tellurides with other Pt, Os, Ru, Au, Ag, Te, and Bi bearing minerals. Ninety percent of the PGMs are associated with copper sulfides as discrete grains attached to sulfides, as sulfide inclusions, and at the margins between sulfides and gangue silicates (Cabri, 2002). The PGMs locally form halos around, or are included in, interstitial copper sulfides, pyroxenes, secondary amphiboles and biotite. PGMs are also remobilized in chlorite, serpentine, or secondary magnetite.

Spruce Road.
Work by the University of Minnesota (Inco, 1966) on concentrates from Spruce Road on behalf of ACNC found that chalcopyrite, cubanite, pyrrhotite and pentlandite were the primary sulfide minerals. About 70% of the chalcopyrite was present as individual grains or as compound grains with pyrrhotite. Compound grain size was 100 µm to 1,800 µm and averaged about 500 µm. The balance of the chalcopyrite occurs as minute inclusions in olivine corona structures or in pyrrhotite, magnetite, and olivine. Cubanite is not common but, when present, is always associated with chalcopyrite. Pyrrhotite has a similar mode of occurrence to chalcopyrite. Pentlandite occurs as compound grains with pyrrhotite and chalcopyrite or included in pyrrhotite.

SGS Lakefield Research Limited (SGS Lakefield) performed a mineralogical study of core samples from Wallbridge’s drill hole WM-001 (Soever, 2000). SGS Lakefield identified pentlandite, chalcopyrite, cubanite, bornite, mackinawite, violarite, pyrrhotite, pyrite, magnetite, and ilmenite. Soever (2007) notes that chalcopyrite and cubanite were identified as the main copper minerals with particle size ranges of 5 µm to 250 µm and 20 µm to 500 µm, respectively. Nickel was mainly present as pentlandite with grain sizes 2 µm to 250 µm and occurring as exsolution flames in pyrrhotite.

Post-pillar cut-and-fill is a man-entry mining method. It recovers the ore in horizontal slices, starting from a bottom level and advancing upwards. A level will be extracted by developing a horizontal slot1 or room, from footwall to hanging wall, followed by cross-cuts that are perpendicular in both directions from the slot, which are mined on retreat. Unmined pillars will remain between the slots to provide local geomechanical stability. After the slot and cross-cuts have been extracted, a bulkhead will be installed and the mined-out area will be backfilled. Mining will continue with a new level mined immediately above the backfilled level.

Long-hole stoping is a traditional blast hole stoping method where extraction and drilling drifts will be developed within the orebody. A slot raise will be mined between the drilling and extraction drifts to create a void for blasted material. Ore will be drilled from either the drilling (upper) drift or extraction (lower) drift, then loaded with explosives, and blasted towards the slot raise. Broken ore will be mucked both manually and remotely from the extraction drift.

The maximum mining depth will be approximately 4,300 ft below the surface elevation. The underground operation will be accessed via four declines from surface, three to Maturi and one to Maturi Southwest. Maturi Southwest will also be accessed underground from Maturi. All mining access will use ramp systems from the declines. Ventilation raises will connect levels, and tie into ventilation plenums connected by raises to the surface.

The primary crushing plant consisting of a single 60 in. x 89 in. gyratory crusher will be located in a central location underground within the Maturi deposit and will process run-of-mine feed in open circuit. Primary crushed ore will be conveyed to the surface via an overland conveyor and fed to the coarse ore primary crushed stockpile.

The primary grinding circuit will consists of a single 36 ft x 17 ft effective grinding length (EGL) SAG mill and a single 26 ft x 40 ft EGL ball mill. The SAG mill will be operating in closed circuit with a trommel and vibrating screen classification. The screen's oversize stream will recirculate back to the SAG mill feed. The screen's underflow stream will be the SAG circuit's final product feeding the ball mill grinding circuit; no pebble mill is incorporated into the design. The ball mill will be operating in closed circuit with a hydrocyclone classification cluster consisting of 14 x 33 in. diameter cyclones, each cluster fed by a dedicated cyclone feed pump. The cyclone overflow product P80 will be 120 µm feed to the flotation circuit.

The flotation circuit essentially comprises two separate flotation circuits inside a single building producing separate copper and nickel/copper concentrates.

Cyclone overflow pulp will be fed into a single line of six 300 m3 rougher flotation tank cells. Combined rougher concentrate will be pumped to a bank of 8 x 20 in. hydrocyclones where it will be classified. Cyclone underflow product will be reground in a 750 hp vertical mill. Cyclone overflow, at a P80 of 45 µm will then be fed to the first cleaner flotation circuit consisting of single bank of five 70 m3 tank cells.

The first cleaner concentrate will be combined and pumped to a second cleaner flotation consisting of two parallel 15 ft diameter x 46 ft high column flotation cells. First cleaner flotation tails will report to the nickel flotation circuit regrind mill feed box. Second cleaner flotation concentrate will be the final copper concentrate product whilst the second cleaner flotation tails will report back to first cleaner flotation feed. Rougher flotation tailings will be pumped to the nickel flotation circuit.

The first stage of nickel flotation will be the roughing flotation stage, consisting of a single line of seven 300 m3 rougher flotation tank cells with the rougher tailings being final plant tailings. Combined nickel flotation concentrate will report to the nickel flotation circuit regrind mill feed box feeding the nickel regrind circuit classification cyclones. In this feed box the pulp will be combined with the tailings pulp from the first stage of copper cleaner flotation. This combined pulp will be pumped to a bank of eight 20 in. diameter hydrocylones. Cyclone underflow product will be reground in a 750 hp vertical mill.

Cyclone overflow, at a P80 of 45 µm will then be fed to the first cleaner flotation circuit, which will consist of a single bank of six 70 m3 tank cells. Combined first cleaner concentrate will be combined and pumped to the second stage of nickel cleaning flotation, consisting of six 30 m3 tank cells. First cleaner flotation tailings will report back to the feed of nickel rougher flotation. Combined second cleaner flotation concentrate will report to the feed of the third stage of nickel cleaner flotation which will consist of five 15 m3 tank cells. Second cleaner stage tailings will report back to the feed of the first flotation cleaners. The third cleaner stage combined concentrate will also be the final concentrate, whilst the third cleaner stage tailings will report back to the feed of the second cleaner flotation stage.