The Tilden iron deposit is an example of a Lake Superior-type BIF deposit.
Tilden is dominated by a 100 m-scale, northwest-plunging incline. The hinge line of the incline dips steeply north, plunges 30°NW, and runs down the center of the Main Pit. The hinge line of the incline is mapped locally coincident with the Keweenawan Dike. The Summit Mountain Sill, locally termed the Pillar Intrusive, defines the asymmetry and orientation of the incline. Smaller faults and folds, on a scale of one meter to 20 m, are observed in the Main Pit to follow trends of larger, regional-scale structures. These structures tend to reflect ductile deformation in the Main Pit, where folds with sheared limbs are common (Lukey et al., 2007).
The primary ore mineral at Tilden is hematite, with other minerals including martite (oxidized pseudomorph of magnetite), goethite, and siderite (iron carbonate mineral), as opposed to strictly magnetite.
At Tilden, the Negaunee IF can be divided into five distinct facies:
• Clastic Iron Formation (IFCL) Units: Varying thickness of interbedded slate with laminated chert, iron silicate, and siderite. Clastics have a lower weight recovery (wtrec) due to the presence of interbedded clastic material. They are highly oxidized in the east side of the Main Pit.
• Carbonaceous Iron Formation (IFCB) Units: Alternating thin layers of magnetite, martite (oxidized pseudomorph of magnetite), iron silicate minerals, iron carbonate minerals and chert. Carbonate material is characterized by the presence of siderite (iron carbonate mineral), low phosphorus, and higher wtrec.
• Martite Iron Formation (IFCH) Units: Thicker beds of hematite-martite-chert with intervals of magnete-carbonate. The oxidation level increases in the east and where proximal to intrusive sills.
• Magnetic Iron Formation Units: Magnetite domain consisting of magnete-carbonate and magnete-silicate-chert with variable oxidation. It is defined principally by magnetite content and is generally fresh, with some localized oxidation. At Tilden, it is found within and defines the (now
expended) material of the CDIII Pit.
• Hematite Iron Formation Units: The oxidized equivalent of the Magnetite Iron Formation prominent in both the Empire deposit and in the east side of the Main Pit, is located stratigraphically above the Summit Mountain Sill. It is dominantly composed of hematite and chert interbeds. At Tilden, this unit has locally very high levels of silica and phosphorus in concentrate (consio2 and conphos, respectively).

United Taconite mine
The Thunderbird Mine North (TBN) and Thunderbird Mine South (TBS) deposits are examples of Lake Superior-type banded iron formation (BIF) deposits.
The Thunderbird deposits are overlain by Pleistocene glacial till, outwash, and lacustrine sediment. Overburden thicknesses average approximately 50 ft; however, thicknesses up to 199 ft have been drilled at TBS. Glacial sediment is generally thinnest on the northern portion of the Property and thickens to the south and west.
Magnetite-bearing taconite is currently the principal iron-bearing rock of economic interest on the property. In line with other Superior-type iron formations, magnetite-bearing intervals within the Biwabik IF occur as laterally extensive, stratiform intervals. Economically mineable magnetite occurs exclusively within granular iron-formation (cherty) units of the Biwabik IF.
Current mining operations exploit stratigraphic units of the Upper Cherty (44% of total mining) and Lower Cherty (56%) members. Mineable crude ore intercepts are generally identified by their thickness, crude ore magnetic iron content (MagFe), and concentrate silica content.

The Northshore iron deposit is a classic example of the Lake Superior-type BIF deposit.
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.
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 HibTac deposit is an example of Lake Superior-type BIF deposits.
The Biwabik IF at HibTac consists primarily of carbonates, iron silicates, fine-grained quartz, and iron oxides. These layers are visually distinct, locally separated into slaty beds and cherty beds. The ratio of slaty to cherty beds and distance between these beds are key indicators used during logging, as well as bedding style, texture, color, and magnetic strength. Slaty beds are dark gray in nature, consisting primarily of magnetite in mineralized zones, and range from 0.04 in. to upwards of one inch in thickness. Cherty beds range from gray to green in color depending on the ratio of fine-grained quartz (gray color) to iron silicates (green color). These beds vary in thickness to upwards of twelve inches and may or may not contain disseminated magnetite. Carbonates typically occur as granular, re-crystallized grains of varying size and commonly occur in late-stage quartz-carbonate-filled fractures, which run variably (orientation, length, width, continuity) throughout the iron formation. The Upper Slaty and Lower Slaty members are visually distinctive, as they are dominated by slaty beds; however, these beds rarely contain any notable iron oxide content
The taconite ores mined at HibTac are from several locally recognized, informal subunits of the Lower Cherty member. Waste rock units (Lower Slaty and Upper Slaty members) cap the Lower Cherty and Upper Cherty members and are distinctively fissile and weakly magnetic as compared to the ore units.

The Minorca deposits are examples of Superior-type BIF deposits, specifically the Biwabik Iron Formation (Biwabik IF), which is interpreted to have been deposited in a shallow, tidal marine setting and is characterized as having four main members (from bottom to top): Lower Cherty, Lower Slaty, Upper Cherty, and Upper Slaty. Cherty units generally have a sandy granular texture, are thickly bedded, and are composed of silica and iron oxide minerals. Slaty units are fine grained, thinly bedded, and comprised of iron silicates and iron carbonates, with local chert beds, and they are typically uneconomic. The mineral of economic interest at Minorca is magnetite.

All of iron ore mining operations are open-pit mines.

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

Tilden Mineral Reserves are based on ongoing annual crude ore production of 20 MLT to 22 MLT producing approximately 7.7 MLT of wet hem flux pellets for domestic consumption.

The current LOM plan has mining scheduled for 25 years and mines the known Mineral Reserve. The average stripping ratio is approximately 1.2 waste units to 1 crude ore unit (1.2 stripping ratio).

The final Tilden pit is a single pit approximately 2.5 mi along strike, up to 0.9 mi wide, and up to 1,980 ft deep.

United Taconite mine
United Taconite operations consist of an open pit truck and shovel mine where two stages of crushing occur before the ore is transported by rail, operated by CN, to the plant site.

The Thunderbird Mine North (TBN) and Thunderbird Mine South (TBS) are 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 average crude ore production of approximately 15.4 MLT per year (MLT/y) from TBN and TBS, producing an average of 5.1 MLT/y of wet pellets for domestic consumption.

The current LOM plan has mining for 51 years and mines the known Mineral Reserve. The average strip ratio is 1.1 waste units to 1 crude ore unit (1.1 strip ratio).

Northshore mine
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.

Hibbing Taconite mine
The Hibbing Taconite Property (HibTac) is mined using conventional surface mining methods. The Mine requires large 200-plus ton mining trucks, and some areas of the pit require long hauls. The surface operations include:

• Clearing and grubbing;
• Overburden (glacial till) removal;
• Drilling and blasting (excluding overburden);
• Loading and haulage.

The Mineral Reserve is based on the ongoing annual average ore production of 21.9 MLT from the Group I, II, III, IV, and V pits, producing an average of 5.6 MLT/y of wet pellets for domestic consumption. The HibTac operations have no current expansion plans and are likely to cease operating once the reserves are depleted by 2026.

The current LOM plan has mining scheduled for five years and mines the known Mineral Reserve. The average stripping ratio is 1.0 waste units to 1.0 crude ore units (1.0 stripping ratio).

There are 20 mining pits/phases with varying dimensions, with a maximum depth of approximately 600 ft attained in two of the pits/phases.

Minorca mine
The Laurentian, East 1, and East 2 areas are mined using conventional surface mining methods. The surface operations include:
• Clearing and grubbing;
• Overburden (glacial till) removal;
• Drilling and blasting (excluding overburden);
• Loading and haulage.

The open-pit operation at Minorca has a mining rate of approximately 8.6 million long tons (MLT) of ore per year and produces 2.8 MLT of wet flux iron ore pellets.

The current LOM plan has mining for 14 years and mines the known Mineral Reserve. The average stripping ratio is 0.8 waste units to 1 crude ore unit (0.8 stripping ratio).

The final Laurentian Pit is approximately 1.2 mi long along strike, 0.9 mi wide, and up to 640 ft deep. Crude ore averages approximately 24.4% MagFe. The final East 1 Pit is approximately 0.9 mi along strike, 0.5 mi wide, and up to 310 ft deep. Crude ore within the East 1 Pit averages approximately 22.5% MagFe. The East 2 final pit is approximately 0.7 mi along strike, 0.4 mi wide, and up to 350 ft deep. The East 2 Pit crude ore contains an average of 23.7% MagFe.

TILDEN
Primary crushing, which is operated and maintained by the mining department, is accomplished with a 60 in. x 109 in. Allis-Chalmers gyratory crusher operated to produce a nominal -9 in. crushed product, which is conveyed to the ore storage building ahead of the grinding circuit. Primary grinding is accomplished with eleven, 27 ft-diameter x 14.5 ft-long AG mills, each driven by two synchronous motors that have a combined output of 5,720 hp. Each primary AG mill discharges to a triple-deck screen, producing a -1.5 in. x 0.5 in. product that is used as a grinding media in the pebble mills (excess diverted to the pebble crushers or recirculated back to the primary mill), a -0.5 in. x 2 mm product that is conveyed back to the primary mill, and a -2 mm product that is advanced to the secondary pebble mills. The -2 mm discharge from each AG mill feeds two, 15.5 ft-diameter x 32 ft-long pebble mills, which are operated in closed circuit with a cluster of nine, 15 in.-diameter cyclones to produce a final grind of 80% to 85% passing 25 µm. Caustic soda and slaked lime are added to the water circuit to control pH prior to desliming and flotation.

Fluxstone consisting of dolomite and calcite is received at Tilden via truck and stored in stockpiles. Material is fed from a stockpile via apron feeders and processed in two, 15.5 ft-diameter x 30 ft-long ball mills. The fluxstone slurry is added to the iron concentrate prior to filtering to ensure homogenous mixing.

UNITED TACONITE
Crude ore is blended at the Thunderbird Mine and hauled to the primary crushing station, where it is dumped by 240 ton haul trucks into the 60 in. x 89 in. primary gyratory crusher, followed by secondary crushing in three, 30 in. x 70 in. secondary gyratory crushers located directly beneath the primary crusher. The P80 4 in. product-size material is conveyed to a 20,000 LT, conical surge pile. The surge pile is covered to avoid handling difficulties during extremely cold weather. Crushed ore is reclaimed from the surge pile by apron feeders and a conveyor located in a tunnel beneath the pile and conveyed to rail car loading silos. The material is loaded into rail cars and transported by train to the Fairlane Facility, eight miles away. The average feed rate of the primary crushing station is 3,200 LT/h.
Two additional stages of crushing are provided at the Fairlane Facility. The third stage consists of five Nordberg, seven-foot shorthead crushers operating in parallel and open circuit followed by screens producing a P80 one inch (25.4 mm) product. The P100 0.5 in. (12.7 mm) screen undersize material from the third stage is combined with the screen undersize from the fourth stage to make up the final crusher product to the concentrator. Third-stage screen oversize material feeds the fourth stage of crushing, which comprises eight Nordberg, seven-foot shorthead crushers operating in parallel and in closed circuit with screens, producing the final 85% to 90% passing 0.5 in. product. The average throughput is 50,000 LT/d. Specific power consumption is 3.1 kWh/LT.
The fine crusher product is processed in five separate rod mill – ball mill grinding and magnetic separation lines to produce final magnetite concentrate with a particle size distribution of 76% to 86% passing 325 mesh.

Lines 1 and 2
Grinding lines 1 and 2 have average feed rates of 345 LT/h at 90% operational availability. The two rod mills in lines 1 and 2 are 14 ft-diameter x 20 ft EGL (equivalent grinding length), Nordberg overflow mills operated in open circuit with 2,000 hp motors. Four ball mills (14 ft diameter x 22 ft EGL) are operated in closed-circuit with 26 in.-diameter cyclones.

Lines 3, 4 and 5
The flowsheet for Lines 3, 4, and 5 is similar to lines 1 and 2, with higher average feed rates of 435 LT/h per line at 90% operational availability. The three, 15 ft-diameter x 21 ft EGL rod mills are operated in open circuit and discharge through two 4 ft x 10 ft, 1,200 Gs cobber magnetic separators per line. Cobber tailings are final tailings. The cobber concentrate is advanced to the ball mill grinding circuit, which consists of three, 17 ft-diameter x 42 ft EGL ball mills (one per line) operated in closed circuit with cyclones and screens. The ball mills discharge to twelve, 4 ft by 10 ft rougher magnetic separators (four per line). Rougher tailings are final tailings and are discarded to the tailings hydroseparators.

Fluxstone Grinding Circuit
Fluxstone, a 50%/50% mixture of limestone and dolomite, is ground using a 14 ft-diameter x 20 ft EGL Nordberg overflow ball mill when Mustang flux pellets are being produced.

NORTHSHORE
The Mine and primary and secondary crushing plant are located in Babbitt, Minnesota and the tertiary and quaternary crushing plant is located in Silver Bay, Minnesota. Mine haul trucks dump the crude ore directly into a 60 in. x 89 in. primary gyratory crusher. The primary crushed crude ore falls directly into the four, secondary 30 in. x 70 in. gyratory crushers located directly beneath the primary crusher, and is crushed to -4 in. The -4 in. material is conveyed into rail car loading bins and then loaded into trains and transported to Silver Bay, Minnesota, where the tertiary and quaternary crushing stations, the concentrator, and the pellet plant are located. Upon arriving at Silver Bay, the secondary crushed crude ore (-4 in.) is dumped from the rail cars by automated, two-car dumpers and transported by belt conveyors to the tertiary-quaternary crushing plant storage silos. The crude ore is drawn from the silos and crushed to -0.75 in. in tertiary and quaternary Nordberg 7 ft shorthead cone crushers and then passed over double-drum dry cobbers for primary magnetic separation.

HIBBING TACONITE
The crushing plant consists of two 60 in. x 109 in. Allis Chalmers gyratory crushers that crush run of mine (ROM) ore to minus 10 in., which is then conveyed to the 450,000-ton, crushed-ore stockpile, referred to as the COSP, providing up to five days of crushed ore surge capacity ahead of the concentrator. Crushed ore is reclaimed from the COSP to feed the primary grinding circuit, which consists of nine, 36 ft x 15 ft autogenous grinding (AG) mills, which grind the ore to - 3/16 in.
The screen oversize from all grinding lines is reground in two, 1,250 hp Vertimills operated in closed circuit with cyclones to produce a cyclone overflow of 90% passing 44 microns, which is then subjected to a second stage of finisher magnetic separation.

MINORCA
The primary crusher is a 54 in. x 84 in. gyratory crusher, which crushes the ROM material to P80 6 in. The crushed material is conveyed to a coarse ore stockpile. The coarse ore is reclaimed from the stockpile with vibrating feeders and transported by the conveyor system beneath the stockpile into the secondary crushing plant crusher feed bins. Secondary crushing consists of a bank of Symons 7 ft standard cone crushers. The secondary cone crusher discharge is screened on double-deck screens. The screen oversize material is conveyed to the tertiary, 7 ft Symons short-head cone crushers for fine crushing. The crusher discharge is screened. The screen oversize material feeds conveyors that recycle the material to the tertiary crusher, and the screen is product size with a P100 5/8 in. The crushed product is conveyed to the rod mill feed bin.
The rod mill discharge flows through wet cobber magnetic separators. The cobber non-magnetic tailings flow to the tailings spiral classifier and then to the tailings thickener. The cobber magnetic concentrate is pumped to three parallel ball mills, followed by eight rougher magnetic separators, and the circuit is closed by hydrocyclones.

Tilden Concentrator
The capacity of the Tilden concentrator and pellet plant is 7.7 million long tons per year (MLT/y) of fluxed pellets from both hematite and magnetite crude ore sources. The Plant includes primary crushing, autogenous primary and secondary grinding, selective flocculation and desliming, flotation, filtration, drying, balling (agglomeration), and induration. The concentrator is designed to campaign either hematite ores or magnetite ores but not in combination.
The processing of magnetite-dominant ores at the Tilden concentrator ceased in 2009. Magnetite ore from the Tilden was delivered and processed at the Empire Mine from 2010 through 2016 when the Empire was indefinitely idled. Remaining Mineral Resources and Mineral Reserves at Tilden are processed in hematite-based flotation circuits.
Annual Processing Rate for the LOM: 20.8 MLT/y.

United Taconite Concentrator
Crude material is magnetite taconite with a ROM magnetic iron ........