Summary:
The HibTac deposit is an example of Lake 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 predominantly composed of chert, magnetite, iron silicates (talc, stilpnomelane), and, in specific geologic units, carbonate (ankerite). Slaty units are fine grained, thinly bedded, and comprised of iron silicates and iron carbonate, with local chert beds, and they are typically uneconomic. The mineral targeted at HibTac is magnetite. Supergene weathering and oxidation has locally altered the primary assemblage to hematite, goethite, and chert, generally increasing in intensity with proximity to isolated occurrences of Cretaceous Coleraine Formation south of the mine and faults or fracture zones. Partial or complete oxidation of magnetite to hematite precludes recovery by magnetic separation, resulting in local degradation of potential ore intervals to waste rock.
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.
Pleistocene Glacial Deposits: Surficial deposits of 0 to 60 ft in thickness unconformably overlie all bedrock units.
Upper Slaty Member: Unit 4-1: This member is generally more than 70 ft thick.
Upper Cherty Member (Composite Subunit 3-1): The Upper Cherty member comprised the majority of “natural” (DSO) ores in the Hibbing area prior to the era of taconite beneficiation.
Lower Slaty Member: Modeled as LS_21, the Lower Slaty member is from 20 ft to 55 ft in thickness and is non-magnetic and dark greenish-gray to black in color.
Lower Cherty Member: The ore-grade intervals are contained with the Lower Cherty member, specifically, the 1-7 through the 1-3. The magnetic iron content ranges from approximately 15% to 18%, with the higher percentages found in the 1-5 and 1-6.
Subunit 1-8: Modeled as LC_18, subunit 1-8 is from 25 ft to 32 ft in thickness and is mostly non-magnetic. It is a variably coarse-grained, medium to thick wavy-bedded ferruginous arenite (granular cherty-silicate ± carbonate taconite).
Subunit 1-7: Modeled as LC_17, the 1-7 ranges from 15 ft to 25 ft in thickness and is moderately to slightly magnetic with medium to thick bedding. In this subunit, the taconite is granular and cherty. Magnetite and moderately thick ferruginous mudstone bands form discontinuous to irregular, gray slaty bands and mottles up to 1.5 in. thick. These are separated by massive ferruginous arenite beds up to eight inches thick that contain moderately abundant, coarse-grained disseminations, diffusions, or patches of magnetite. Minor green silicate minerals are localized along the slaty bands with more abundance. Carbonate mottles are scattered throughout the cherty zones, and minor magnetite-bearing stylolites occur locally. Leached and pitted, blanket-style oxidation zones containing goethite + martite ± maghemite are common proximal to fault zones.
Subunit 1-6: Modeled as LC_16, the 1-6 ranges between 25 ft and 45 ft in thickness and is highly magnetic. It has thick, wavy beds of cherty-silicate taconite. Magnetite laminated with minor ferruginous mudstone and hematite forms gray-black slaty bands up to three inches thick. Intervals of granular chert-grain arenite and/or coarsely crystalline green silicates are two to six inches thick and contain minor magnetite disseminations, which become moderately abundant in the bottom five feet of the unit. Minor magnetite patches are scattered throughout the massive cherty beds. Slaty bands are typically goethite rich where oxidized.
Subunit 1-5: Modeled as LC_15, subunit 1-5 ranges between 45 ft and 85 ft in thickness and is highly magnetic. The unit is a coarse-grained, massive to bedded ferruginous arenite (granular cherty taconite) with minor, wavy to planar bands of magnetite and ferruginous mudstone. Massive cherty layers up to 12 in. thick contain abundant disseminations of coarse-grained, granular magnetite and white or green chert/silicate mineral granules, resulting in a distinctive “salt-and-pepper” texture. The upper eight feet to 15 ft are massive to bedded, with abundant thin (<0.2 in.), slaty magnetite curls, wisps, or diffusions and very few slaty bands. The lower 35 ft to 70 ft contains moderately abundant wavy bands of magnetite laminated with gray, ferruginous mudstone up to one inch thick.
Subunit 1-4: Modeled as LC_14, subunit 1-4 ranges between 9 ft and 11 ft in thickness and is a moderately magnetic, thin-bedded, cherty and slaty taconite that has wavy to even bedding. This is a transitional sequence between subunits 1-5 and 1-3.
Subunit 1-3: Modeled as LC_13, subunit 1-3 ranges from 18 ft to 25 ft thick and is a moderately magnetic, planar-bedded, cherty and slaty taconite. Within the subunit, fissile, gray slaty bands are composed of interlaminated ferruginous mudstone, minor hematite, and magnetite (slaty-silicate taconite) up to 10 in. thick. These are separated by variably coarse-grained, granular cherty-silicate mineral beds up to five inches thick, which contain patchy diffusions or mottles of granular magnetite, mostly white silicate minerals, and characteristic bright red jasper bands or mottles up to 1.5 in. wide. Bedding-parallel quartz + chlorite ± calcite veins up to one inch wide are common and typically exhibit well-developed slickensides or slickensteps on vein margins.
Subunit 8-3: Modeled as LC_83, subunit 8-3 ranges from less than two feet up to 25 ft in thickness.
Subunit 1-2: Modeled as LC_12, subunit 1-2 ranges from 18 ft to 25 ft thick. It is moderately to slightly magnetic, very coarse grained, and composed of massive to bedded ferruginous arenite and local intraformational conglomerate (granular cherty-silicate ± carbonate taconite). It is similar in appearance to the upper portion of subunit 1-5. Subunit 1-2 has massive cherty zones up to 12 in. thick that contain coarse-grained disseminations or diffusions of hematite and/or magnetite. The proportion of hematite increases with depth, and core may have a light reddish-gray tint.
Subunit 1-0: Modeled as LC_10, subunit 1-0 ranges from 18 ft to 25 ft thick and is non-magnetic, oxidized, and is referred to as the “Red Basal” unit.
Summary:
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.
Primary production for all mine pits includes drilling 16.00 in.-diameter rotary blast holes. Production blast hole depth to 40 ft bench heights is drilled. Burden and spacing varies depending on the material being drilled. The holes are filled with explosive and blasted. A combination of front-end loaders (FEL) and electric shovels load the broken material into 240 ton-payload mining trucks for transport from the pit.
The final highwall pit slope is designed at an inter-ramp angle (IRA) of 42.5° for in situ bedrock and 18.4° for surface overburden. The bench design for bedrock consists of double-stacked, 40 ft-high mining benches with a 65° bench face angle (BFA) and a 50 ft catch bench (CB). There are no ramps designed into the final highwall, as the footwall slope is less than 8% for most of the mining areas and can support the development of haulage ramps.
Generally, three groupings of geological subunits are mined at one time to obtain the best blend for the Plant. HibTac mines from three to four ore locations for blending. 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 electric shovels, FELs, haul trucks, drills, bulldozers, and graders. Extensive maintenance facilities are available at the mine site to service mine equipment and the rail fleet.
The basis of the production schedule is to:
• Preserve blending of the three crude ore types for as long as possible, particularly to keep 1-3/1-4 ore percentage below 48%.
• Limit total mined tons per annum in the range of 57 MLT to 60 MLT to balance both stripping requirements and mine equipment fleet utilization in addition to the pellet production.