The Northshore iron deposit is a classic example of the Lake Superior-type BIF deposit. Lake Superior-type BIFs occur worldwide and are exclusively Precambrian in age, deposited from approximately 2,400 Ma to 1,800 Ma.

Magnetite is the principal economic mineral at the Mine and occurs dominantly in thin to thick bands and layers, as medium- to coarse-grained disseminated grains, and as grain aggregates. Magnetic iron content ranges between 22% and 30% in the mineralized stratigraphic subunits within the deposit at the Mine. Local variation in silicate mineralogy and lithologic textures due to contact metamorphism presents unique challenges for grade control relative to deposits hosted in the western Biwabik IF. These changes affect many aspects of the operation including process metallurgy and hardness. Supergene oxidation of magnetite occurs locally along fracture planes but is generally uncommon.

Several geological structures are important at Northshore:
• The sharp contact between the Biwabik IF and the Duluth Complex identified and modeled from drilling data.
• A homocline in the BIF at the contact of the Duluth Complex, striking approximately east-northeast and dipping approximately 7° to the southeast.
• Several high-angle normal faults exhibiting variable displacement.

Emplacement of the Duluth Gabbro is responsible for the variable mineralogy observed at Northshore. Regional metamorphism related to this event locally affected the eastern Biwabik IF, resulting in a metamorphic pyroxene- and/or amphibole-dominant gangue mineralogy in place of more common silicate minerals typical of IFs. Minerals in the iron formation at Northshore include magnetite, chert, quartz, hedenbergite, cummingtonite, actinolite, hornblende, fayalite, ferrohypersthene, diopside, and andradite garnet (Gunderson and Schwartz, 1962). Contact metamorphism also resulted in a local coarsening of magnetite grain size and a decrease in the amount of quartz in the gangue mineralogy. It is believed that the silica present in primary quartz was incorporated into the iron silicate minerals found in the iron formation. Metamorphic grade is strongest to the east and decreases westward with distance from the Duluth Complex. Alteration related to metamorphism is observed to be localized along faults, on dike margins, and in fold axes. The Biwabik IF is interpreted to have experienced minor volume loss in the Eastern Mesabi Range due to loss of water and gases during thermal metamorphism (Ojakangas et al., 2009).

Contact metamorphism of the Biwabik IF at the Peter Mitchell Mine distinguishes the mineralogy from that of the rest of the Mesabi Iron Range. Emplacement of the Duluth Complex on the southeastern margin of the district resulted in re-crystallization, which increased mineral grain size and led to production of iron-rich pyroxenes, amphibole minerals, and minor olivine. Devolatilization near the contact also reduced the thickness of the proximal bedded units and altered any hydroxide minerals present.

Economic mineralization within the mine is hosted entirely within subunits of the Biwabik IF. In the mine area, bedding dips from approximately 5° southeast in the west to 35°southeast near the contact with the Duluth Complex in the east. The entire stratigraphic sequence of the Biwabik IF is present at Northshore, although only subunits of the Upper Cherty member and lesser fractions of adjacent members are mined. The Upper Cherty member averages approximately 160 ft thick, considerably thinner than equivalent stratigraphy in the western Biwabik IF.

The Northshore deposit is mined using conventional surface mining methods. The surface operations include:
• Overburden (glacial till) removal;
• Drilling and blasting (excluding overburden);
• Loading and haulage;
• Crushing and rail loading.

The Mineral Reserve is based on the ongoing annual crude ore production of 16 million long tons (MLT) to 18 MLT producing a total of approximately 5.1 MLT of wet pellets for domestic consumption.

The current LOM plan has mining scheduled for 48 years and mines the known Mineral Reserve. The average strip ratio is approximately 0.8 waste units to 1 crude ore unit (0.8 strip ratio).

The final Northshore pit is a single pit approximately 10.5 mi along strike, up to 1.2 mi wide, and up to 420 ft deep.

The Mine’s operation has a strict crude ore blending requirement to ensure the Plant receives a uniform head grade. The most important blending characteristics of the crude ore are the MagFe, Conc_Fe, and ore hardness (i.e., Grindability). Generally, three crude ore loading points from different subunit groupings (i.e., the Intermediate, High Grade, Footwall Group, and Lower Cherty subunit groupings) are mined at one time to obtain the best blend for the Plant.

Crude ore is hauled to the crushing facility and either direct tipped to the primary crusher or stockpiled in an area adjacent to the primary crusher. Haul trucks are alternated to blend delivery from the multiple crude ore loading points. The crude ore stockpiles are used as an additional source for blending and production efficiency. Crushed crude ore is conveyed to a silo, where it is loaded into 85-ton rail cars for transport to the Plant located 47 mi southeast of the Mine at Silver Bay, Minnesota. Waste rock is hauled to one of the many waste stockpiles within and around the pit.

The major pieces of pit equipment include electric drills, electric rope shovels, haul trucks, front-end loaders (FELs), bulldozers, and graders. Extensive maintenance facilities are available at the mine site to service the mine equipment.

Mine Design
Phase designs for the deposit are largely based on the effective mining width of 200 ft, a minimum BH of 20 ft to allow for increased mining selectivity at the ore-waste contact, and access to the Mineral Reserves. The same bench design parameters used in the final pit design are incorporated into the phase pit designs

The final highwall pit slope is designed at an inter-ramp angle (IRA) of 41°. The bench design consists of 45 ft-high mining benches with a 65° bench face angle (BFA) and alternating 10 ft and 50 ft catch benches (CB). In general, haulage access ramps are developed along the pit footwall slope, which is at less than 8% for the majority of the mining areas.

There are multiple physical mining limits that are applied to the pit optimization and/or the mine plan:
• There is a 600 ft restriction that limits the distance of blasting near the primary crusher.
• The Duluth Gabbro overlies the Biwabik IF in the vicinity of Northshore. The Duluth Gabbro is known to contain elevated levels of sulfide mineralization in some areas. Elevated levels of sulfide minerals in rock present the potential for acid rock drainage and metals leaching when the rock is blasted and stockpiled. Current permits with the MDNR and the Minnesota Pollution Control Agency (MPCA) prohibit the mining and stockpiling of Duluth Gabbro rock by NSM. As a result, a mining limit in the model restricts mining of the Duluth Gabbro.
• Mining limits for crude ore are restricted to within the Northshore-controlled mining leases and owned mineral lands and within the existing Permit to Mine (SLR notes the Permit to Mine boundary limit is shared with or greater in extent than the mineral lease boundary). These leases are with the Mesabi Trust, the State of Minnesota, and Philips-Conoco (formerly Burlington Northern). The Mesabi Trust lease includes the Peters Lease and the Cloquet Lease.

Of additional consideration is the allowance for trespass stripping, which is common among other mines on the Mesabi Range. Trespass stripping allows for mining of waste rock outside of Northshore’s current mineral leases (provided it is still within the Permit to Mine boundary), to expose crude ore to the mineral lease boundary.

The basis of the production schedule is to:
1. Produce a total of approximately 5.1 MLT/y of wet pellets for the LOM:
* This production rate was selected as it represents maintaining the current production assumption throughout the LOM;
* At least 90% of the crude ore used in pellet production must be mined from Mesabi Trust lands (for the first 6.0 MLT of pellets per year).
2. Preserve blending of the crude ore types for as long as possible.
3. Limit total mined tons per period at approximately 32 MLT to balance the mine fleet utilization.

The production schedule is planned yearly throughout the LOM. Scheduling is by mining blocks within the pit phases, with mining blocks sized at approximately 1 MLT per block during the first 20 years of the production schedule, and larger mining blocks (up to 30 MLT) for the remainder of the production schedule.

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Concentrator
The concentrator building contains 17 complete sections and three partial scavenging sections. All 17 sections are similar as per layout, although there are some minor differences in equipment from one section to another. Two products are made in the concentrator: standard concentrate, which targets a pellet silica of 4.80%, and DR-grade concentrate, which targets a pellet silica of 2.00%. The concentrator flowsheet consists of the following unit operations:
• Rod milling – open circuit;
• Cobber magnetic separation;
• Ball milling – closed circuit;
• Rougher magnetic separation;
• Cyclone classification;
• Cyclone overflow screening;
• Finisher hydroseparation;
• Finisher magnetic separation;
• Finisher magnetic concentrate;
• Primary concentrate reverse flotation (primary and secondary);
• Flotation concentrates hydroseparation;
• Flotation concentrate thickening;
• Concentrate collection and vacuum fi ........