The Tilden iron deposit is an example of a Lake Superior-type BIF deposit. These types of deposits occur worldwide, represent the largest global source of iron ore, and were deposited between 2,700 Ma and 1,800 Ma, formed by chemical precipitation in shallow waters such as continental shelves. Precipitation of iron oxides was due to low atmospheric and ocean oxygen levels resulting in increased iron content in sea water. These deposits are typically characterized by alternating layers of iron oxides and SiO2 -rich material such as cherts or SiO2 -rich sediments.

The Tilden BIF deposit forms the base of the Negaunee IF of the Menominee Group within the MarqueSe Range Super Group. The Tilden BIF is Proterozoic in age and sits on the southern margin of the Marquette trough. It is fault-bounded to the south by Archean gneiss terrane, with the fault contact dipping steeply north and aligning with the south wall of the Main Pit at Tilden.

The Tilden BIF is interbedded with three distinct, syn-sedimentary, mafic intrusive sills: The Summit Mountain Sill, the Suicide Sill, and the Tilden Lake Sill, as well as associated smaller dikes and sills. There is a younger dike series of Keweenawan age (approximately 1.0 Ga) that crosscuts bedding. Alteration is present along all intrusive contacts, with the type and extent of alteration dependent on the thickness of the intrusive and the composition of the iron formation (Lukey et al., 2007). Brittle fractures and late q ........

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.

Mining and processing operations are scheduled 24 hours per day, and mine production is scheduled to directly feed the processing operations.

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.

The Tilden operation has strict crude ore blending requirements to ensure the Plant receives a consistent crude ore feed. The most important characteristics of the crude ore are the crude ore iron grade and predicted concentrate mass recovery, and Conc_Fe, silica, and phosphorus content. Operationally, blending is completed on a shift-by-shift basis. Generally, three to four crude ore loading points are mined simultaneously with dispatch operators issuing real-time adjustments to meet specified crude ore blends for the Plant.

Crude o ........

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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.

Annual Processing Rate for the LOM: 20.8 MLT/y.
Maximum Annualized Processing Rate for the LOM: 22 MLT/y.

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.

Mined ore is directly dumped from haul trucks into a gyr ........