The TBN and TBS deposits are examples of Lake Superior-type banded iron formation (BIF) deposits.

The iron ore deposit exploited on the Property was originally subdivided into two areas, TBN and TBS. United Taconite still retains the Thunderbird nomenclature in a number of publications and unpublished company reports.

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.

Mineralization
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.

Magnetite formed during diagenesis of precursor iron hydroxides, carbonates, and silicates in the primary iron-formation chemical sediment. Reduction of ferric iron and subsequent ferrous iron mobility within the sedimentary package played a key role in magnetite formation. Units with high primary permeability and porosity display a predilection to formation of magnetite. The high total iron content of the highest magnetite content ores suggests that ferrous iron mobility locally enriched the iron content of the primary iron-formation chemical sediment (Larson, 2010).

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.

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).

Within TBN, a previously mined out and backfilled area known as the Auburn pit will require a modified wall design to incorporate a wall containing both unconsolidated fill and in situ rock. The walls of the Auburn pit will align to an overall wall angle of 31°. In the unconsolidated fill, this is accomplished by the 36° angle of repose and 10 ft CB every bench. In the in situ rock, this is accomplished by the 70° BFA and 45 ft CB every bench. These configurations will allow for a uniform toe across the bench.

The final TBN pit is approximately 4.1 mi long along strike, 0.9 mi wide, and up to 700 ft deep. Primary production includes drilling 12.25 in.-diameter rotary blast holes. Production blast hole depth varies as the pit is transitioning from 35 ft bench heights (BH) to 40 ft BH. Burden and spacing varies depending on the material being drilled. The holes are filled with explosive and blasted. Hydraulic shovels load the broken material into 240 ton payload mining trucks for transport from the pit.

The TBS pit is a currently inactive pit adjacent to the TBN pit. TBS operated for 17 years (from 1976 through 1991), producing 106 MLT of crude ore and 32.6 MLT of pellets. Eveleth Taconite, the previous operator prior to Cliffs acquiring the property, stopped mining in TBS to consolidate mining operations and reduce stripping lead times. The final pit design for TBS is approximately 2.0 mi long, 1.3 mi wide, and up to 640 ft deep. The LOM plan assumes reopening the TBS pit in 2030, which includes time for additional investigation work, dewatering, and re-establishing access for production traffic.

The Thunderbird Mine requires strict crude ore blending requirements to ensure that the Fairlane Facility receives a uniform head grade. The two most important characteristics of the crude ore are magnetic iron content and predicted concentrate silica. Generally, three to four mining areas are mined at one time to obtain the best crude ore blend for the Fairlane Facility. 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.

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

The basis of the production schedule is to:
• Produce a total of approximately 5.1 MLT/y wet pellets for the LOM.
* This production rate was selected as it represents maintaining the current production assumption throughout the LOM.
• Limit yearly concentrate silica to a maximum of 5.2%.
• Preserve blending of the three crude ore types for as long as possible (SLR notes that UC crude ore availability diminishes in the last ten years of the schedule, as it is the uppermost layer stratigraphically and is thus depleted first).
• Limit total mined tons per period at approximately 38 MLT to balance the mine fleet utilization.

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.

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.

Grinding lines 1 and 2 have average feed rates of 345 LT/h at 90% operational availability.

The rod mill discharge flows through two, 4 ft x 10 ft, 1,200 Gauss (Gs) cobber magnetic separators per line. Cobber tailings are final tailings, and cobber magnetic concentrate is advanced to the ball mills. Approximately 35% of cobber feed mass is discarded as tailings. Tailings are treated in spiral classifiers (66 in. diameter and 84 in. diameter). The spiral classifier underflow is discharged as coarse tailings, and the spiral classifier overflow is treated in two, 40 ft-diameter hydroseparators. The overflow of the hydroseparators is further treated in the tailings thickener, which is 300 ft in diameter. The underflow of the hydroseparators is sent di ........