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.