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 quartz veins cut all units.

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).

To date, there has been no formal subdivision of the Negaunee IF at Tilden, and stratigraphy is discussed in relative terms from bottom to top. A stratigraphic section, without formal names or dates, was prepared by SLR.Stragraphically, the upward mineralogical variation is from (martite)-magnete-carbonate-chlorite (“Carbonate”) to (magnetite)-martite (“Martite”) to (martite)-microplaty hemate-goethite (“Hematite”) and represents a transition upwards from dominantly ferrous iron (Fe ) mineralogy to dominantly ferric iron (Fe ) mineralogy (Lukey et al., 2007). Some BIF units were disrupted during turbidite flows that manifest as discontinuous lenses of clastic material. Clastic lithology is most prevalent along the bottom (southern) contact with the Archean gneiss. All BIF units are ferrous iron-dominant and increase in ferric iron content upward (generally northward and westward).

Mineralization
The Tilden Mine is unique among Cliffs’ operations because 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. Tilden is also unique in the world in that the hemate-dominant ore is mined at a low grade, concentrated using a selective-flocculation desliming and flotation process, and pelletized. Although some now-expended areas at Tilden did mine and magnetically recover magnete-dominant ore prior to 2009, remaining Mineral Resources at Tilden are hematite-dominant.

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).

The iron formation facies at Tilden have also been modified by clay-silicate alteration associated with Keweenawan faults in the east of the Main Pit, as well as varying levels of oxidation throughout.

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 ore is hauled to the crushing facility and either direct tipped to the primary crusher or stockpiled. Haul trucks are alternated to blend delivery from the multiple crude ore loading points. Crude ore stockpiles are used as an additional source for blending and production efficiency. Crushed crude ore is conveyed to a covered storage building for stockpiling, prior to being fed to the concentrator. Waste rock and overburden are hauled to one of the many waste stockpiles peripheral to the pit or to the in-pit backfill.

Primary pit equipment includes electric drills, electric rope shovels, haul trucks, front-end loaders (FELs), bulldozers, and graders. Extensive maintenance facilities are available at the Mine to service the mine equipment.

The Tilden final pit design incorporates several design variables including geotechnical parameters (e.g., wall angles and bench configurations), equipment size requirements (e.g., mining height and ramp configuration), and physical mining limits (e.g., property boundaries and existing infrastructure). The following summarizes the design variables and final pit results.

Six separate slope sectors have been identified in thein situ rock. The IRA of the slope sectors varies from approximately 38° to 47°.The bench design consists of 45 V-high mining benches, double benched to a final 90 V BH, with a 48.5° to 66.5° BFA and 35 V to 45 V catch benches (CB).The majority of the final pit’s south wall is an existing final wall located above slope sector 5. It was developed along the footwall of the iron formation and acts as a limit to the new final pit design.

Pit slopes in glacial overburden are designed at an average slope angle of approximately 30°.

Haul roads are incorporated with widths of 120 ft to support two-way traffic and 90 ft for one-way traffic. Ramp gradients are limited to a maximum of 10% to stay within the safe working capabilities of the trucks.

Pit design results are reported using the same topographic surface projection as the pit optimization results (i.e., as per the mine planning block model).

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.

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 ........