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 United Taconite still retains the Thunderbird nomenclature in a number of publications and unpublished company reports.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 t ........

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, ........

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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 ........