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Mesabi Range - Hibbing, Northshore, United Taconite, Biwabik, Mount Iron, Hull-Rust-Mahoning, Minorca |
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Minnesota, USA |
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Fe
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The Mesabi Range iron province is located approximately 80 km to the north-west, and parallel to the northern shore of Lake Superior in northern Minnesota, USA. Current and past mines include: Hibbing Taconite, Northshore, United Taconite, Biwabik, Hull-Rust-Mahoning and Mount Iron.
The district has produced 3.6 Gt of iron ore from 525 mines since 1890, of which 2.3 Gt was hematite and/or goethite rich high grade ores grading between 37 and 61% Fe, which formed around 27% of the host Biwabik Iron Formation. Much of this high grade mineralisation had been exhausted by the mid-1950s and production was switched to low grade, largely un-enriched magnetic taconite ores requiring processing that involves a series of grinding and magnetic separation stages to
remove the magnetite from the silica. This processing is necessary, as these deposits are the only source of ore to the US steel mills, which like the mines, are on the Great Lakes, above Niagara Falls, and hence do not have economic access to sea-borne ore imports. Current production from these un-enriched magnetite banded iron formation (taconite) have grades of from <20 to 30% Fe, averaging around 25% Fe. Resources of un-enriched taconite amenable to mining and beneficiation in the late 1960's were estimated at 5 Gt of ore containing 22% Fe.
The major iron formations of the Great Lakes region of North America are hosted within the Paleoproterozoic 2.2 to 1.75 Ga Animikie Group, which was deposited within the Animikie Basin.
The Animikie Basin, part of the Penokean Orogen, was an intracratonic extensional (rift) basin developed over crystalline basement of the Archaean Superior Province. The basement comprises a 2.75 to 2.6 Ga granite-greenstone terrane to the north and a ~3.6 Ga complex of migmatitic gneisses and amphibolites to the south, separated by the generally ENE-WSW trending Great Lakes Tectonic Zone which passes just to the south of Duluth on the SW tip of Lake Superior.
The 700 x 400 km Animikie Basin is elongated parallel to and straddles the Great Lakes Tectonic Zone. Banded iron formation (BIF) has been recognised over a number of intervals (or ranges) around the margins of this basin, 5 of which (including the Mesabi Range) contain sufficient concentrations of iron mineralisation to be economically exploited. The stratigraphic successions have been correlated between each of these 'ranges', although physical continuity between the individual districts has not been demonstrated.
The succession within the Animikie Basin, which unconformably overlies the Archaean basement, is characterised by three Groups:
i). the basal Mille Lacs Group on the north-western side of the basin, and the Chocolay Group on the south-eastern rim,
ii). the ~1878 to 1777 Ma Animikie Group on the north-western margin of the basin, and the lower Menominee and overlying Baraga Group on the south-eastern rim - these units contains the economic BIF units, and
iii). the upper most Paint River Group.
To the south of Lake Superior, the Chocolay, Menominee and Groups together comprise the Marquette Range Supergroup.
The Mille Lacs Group is absent in the iron districts on the north-western margin of the basin and sections of the eastern rim, where the Animikie or Menominee Group sits directly on the Archaean basement. Similarly, the Paint River Group is only locally represented, with the unconformably overlying late Mesoproterozoic (1.10 ±0.01 Ma) Keweenawan basaltic lava flows of the Midcontinent Rift resting directly on the Animikie or Baraga Group.
The rocks of the Animikie Basin form a sequence that is up to 10 km thick and indicate a complete transition from a stable shelf environment to deep water conditions. Irregularities in the basement have influenced the thickness of the sequence. The succession was deformed, metamorphosed and intruded by intermediate to felsic calc-alkaline plutonic rocks of the 1860 ±50 Ma Penokean orogeny.
The three subdivisions listed above, each represents a grossly fining upwards depositional cycle. The Mille Lacs and Chocolay Groups commence with predominantly quartz rich conglomerates and arenites/quartzites. These are overlain by platformal stromatolitic dolomites and shales on the margins of the basin, grading to mafic and intermediate subaqueous volcanogenic rocks, black (carbonaceous) shales and minor chert BIFs towards the axis of the basin.
The Animikie and Menominee Groups, which are largely represented by the major BIF units, were deposited either directly on Archaean basement or on eroded remnants of the Mille Lacs or Chocalay Groups. The major iron formations in different parts of the basin represent either virtually contemporaneous near-strandline shelf sedimentation on either side of the main basin, or deposits formed simultaneously in isolated sub-basins of the main basin. The deposition of iron formation was terminated by the onset of the overlying deep water carbonaceous mudstones, greywacke, siltstone and mafic to felsic volcanogenic rocks that accompanied minor deformation and uplift to form the upper parts of the Animikie Group and the Baraga Group. Locally, deep water turbiditic deposition continued on, to form the Paint River Group. Deposition was terminated by the Penokean orogeny.
The Biwabik Iron Formation constitutes the lower sections of the Animikie Group and hosts the ore in the Mesabi Ranges. It is a Palaeoproterozoic unit outcropping to define a 0.4 to 5 km wide continuous belt over a 200 km strike length. The 60 to 230 m thick Biwabik Iron Formation is underlain by the basal 8 to 110 m thick Pokegama Quartzite (which rests on Archaean granitoid basement), and is in turn overlain by the thick greywacke-shale sequence of the Virginia Formation. These three units together comprise the Animikie Group.
The iron formations of the Biwabik Formation are divided into two styles, namely:
i). cherty rocks which are granular and massive, rich in chert, quartz and iron oxides (dominantly magnetite) with lesser hematite, and
ii). slaty rocks that are fine grained and laminated and composed mainly of iron silicates (minnesotaite and stilpnomelane) and iron carbonates (siderite, ankerite and calcite).
The high grade ores were classified into:
i). High grade blue or brown ore averaging 59 to 61% Fe with <1% alumina and phosphorous, and 2.0 to 3.0% silica, derived from magnetite rich cherty protoliths which have been altered to martite rich ore,
ii). medium grade brown or yellow ore averaging 55 to 56% Fe, with up to 1.5% alumina and 5 to 10% silica, and
iii). low grade yellow or brown ore averaging 50% Fe, >6% alumina and 5 to 10% silica, derived from slaty rocks in the upper part of the Biwabik Formation. The upgraded ore was largely produced by the leaching of up to 40 to 60% of the rock, mainly chert/silica, to form a very porous ore which was compacted to around two thirds of its original thickness.
High grade orebodies had three forms, namely:
i). fissure bodies, which were generally small with dimensions of the order of 15 to 20 m in width, 15 m thick and less than 60 m in length,
ii). larger trough bodies typically as much as 1500 m in length, 330 m wide and 60 to 120 m thick, and
iii). tabular bodies that may be either of the above with steep and sharply defined walls. Approximately 80% of the bodies of the Mesabi Ranges are spatially related to faults.
Ore was also mined from an up to 30 m thick unit of Cretaceous marine sediments formed by reworking of the high grade mineralisation during the Cretaceous. These younger sediments include iron conglomerates with clasts of high grade ore from the Biwabik Formation which resemble canga.
The high grade ores were essentially exhausted by the early 1970's, with mining subsequently from un-enriched magnetite iron formation with approximately 30% Fe, known as taconite.
The taconite is mined from the remaining operations and upgraded to produce magnetite pellets for shipping to steel works on the Great Lakes. Examples of recent productions are as follows:
• US Steel's Minntac operation at Mount Iron produced 19 Mt of taconite pellets in 2003.
• Cleveland Cliff's Hibbing Taconite operation produced 8.25 Mt of taconite pellets in 2005, and from 1976 to 2003 produced
192.6 Mt of pellets.
• Cleveland Cliff's Northshore operation produced 5.3 Mt of taconite pellets in 2005, and from 1955 to 2004 produced 275.7 Mt of pellets.
• Cleveland Cliff's United Taconite operation produced 5.1 Mt of taconite pellets in 2005.
Selected proved + probable reserve figures (Cliffs Natural Resources Annual Report, 2012) include:
Hibbing mine - 316.1 Mt @ 19.1% MagFe;
North Shore - 1063.1 Mt @ 25.0% MagFe;
United Taconite - 386.7 Mt @ 22.2% MagFe.
ArcelorMittal USA operated an iron ore mine through its wholly owned subsidiary ArcelorMittal Minorca and owned a majority stake in Hibbing Taconite Company, which was managed by ArcelorMittal USA from August 12, 2019 (previously managed by Cleveland-Cliffs Inc). The mining operations of ArcelorMittal USA, including all subsidiaries and investments, were sold to Cleveland-Cliffs on December 9, 2020. Reserves and Resources quoted for the ArcelorMittal mines as at 31 December, 2019 (ArcelorMittal, Annual Report, 2020; where Resources are exclusive of Reserves) were:
Hibbing Mine
Proved + Probable Reserve - 131 Mt @ 19.8% Fe;
Measured + Indicated Resource - 146 Mt @ 19.9% Fe;
Inferred Resource - 5 Mt @ 18.1% Fe.
Minorca Mine
Proved + Probable Reserve - 130 Mt @ 23.7% Fe;
Measured + Indicated Resource - 669 Mt @ 22.5% Fe;
Inferred Resource - 21 Mt @ 20.9% Fe.
For more detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 2000.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Selected References:
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Losh, S. and Rague, R., 2018 - Hydrothermal oxidation in the Biwabik Iron Formation, MN, USA: in Mineralium Deposita v.53, pp. 1143-1166.
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Lubben, J.D., Jongewaard, P.K. and Young, M.E., 2011 - Geometallurgy and Ore Processing of the Hibbing Taconite Lake Superior-Type Magnetite Taconite Deposit, Mesabi Iron Range, Minnesota, USA: in Proceedings, Iron Ore 2011 Conference, 11-13 July 2011, Perth, Western Australia, The Australasian Institute of Mining and Metallurgy, Melbourne, pp. 271-280
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Morey G B 1992 - Chemical composition of the eastern Biwabik Iron-formation (Early Proterozoic), Mesabi Range, Minnesota: in Econ. Geol. v87 pp 1649-1658
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Morey G B 1999 - High-grade Iron Ore deposits of the Mesabi Range, Minnesota - product of a continental-scale Proterozoic ground-water flow system: in Econ. Geol. v94 pp 133-142
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Morey G B, Southwick D L 1995 - Allostratigraphic relationships of early Proterozoic Iron-formations in the Lake Superior region: in Econ. Geol. v 90 pp 1983-1993
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Perry E C, Tan F C and Morey G B, 1973 - Geology and stable isotope geochemistry of the Biwabik iron formation, northern Minnesota: in Econ. Geol. v68 pp 1110-1125
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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