Marquette Range - Tilden, Empire, Republic, Humboldt, Mather, Cliffs Shaft, Richmond, Champion

Michigan, USA

Main commodities: Fe
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The Marquette Range iron province is located in northern Michigan, near the city of Marquette on the south shore of Lake Superior in the USA, about 150 km east of the Gogebic Iron Range.   Mineralisation is found over a strike length of around 75 km and width of 5 to 10 km.   Current and past mines include: Tilden, Empire, Humboldt, Mather, Cliffs Shaft, Ohio, Volunter, Richmond, Champion, Greenwood and Republic.

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 Marquete 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 iron formations of the Marquette Range are discontinuously exposed over an interval in excess of 75 km and have been displaced by faulting. There are two main iron formations, the main Negaunee iron formation at the equivalent stratigraphic position as the main iron formations elsewhere in the Animikie Basin, while a second, the Bijiki iron formation occurs in the upper part of the Animikie Formation, unconformably above the Negaunee iron formation. The iron formations of the Marquette Range sequence has been folded into a regional scale westward plunging syncline.

Depositional differences occur along the trend of the range. To the west the Animikie Group equivalents rest directly on the migmatitic gneisses of the ~3.6 Ga basement complex. To the east however, a sequence equivalent to the Chocolay Group is developed, as follows:
Reany Creek Formation - this basal unit is locally absent and rests unconformably on the Archaean basement and comprises a sequence of arkoses, greywackes and quartzite, very similar to the overlying unit described above.
Enchantment Lake Formation - a lenticular unit with a maximum thickness of 150 m, interpreted to unconformably overlie the Reany Creek Formation, commences with a basal conglomerate composed of abundant quartz pebbles in a feldspathic arkose to arkosic matrix, overlain by greywacke, arkose, sericitic quartzite and sericitic slate.
Mesnard Quartzite - a well-sorted quartzite displaying cross bedding and ripple marks, indicative of shallow water deposition.
Kona Dolomite - which is over 800 m thick and is largely composed of silicified dolomite with large cusp stromatolites and includes carbonate cemented quartz arenites and slates,;
Wewe Slate - ferruginous slate, the dominant minerals being quartz, chlorite and sericite.

The sequence above to the east is unconformably overlain by the Menominee Group sequence, which to the west rests directly on Archaean crystalline basement. The Menominee Group comprises, from the base:
Ajibik Quartzite - which occurs across the length of the range and is difficult to distinguish from the Mesnard Quartzite where the two are superposed. It comprises a well sorted, cross bedded quartzite.
Siamo Slate - consists of finely laminated shales interspersed with turbidite layers with locally prominent clastic dykes. They are indicative of deep quiet water conditions interrupted by mass flow deposits and the development of overpressure conditions. The contact of the Siamo and the Negaunee Banded Iron Formation is transitional.
Negaunee Iron Formation - which occurs across the length of the range. It varies from 80 to 300 in thickness to the west, and 135 to 1060 m in the eastern part of the range. The banded iron formation consists of alternating bands of chert and massive hematite-goethite in the oxidised zone, with clastic lenses that are rare to the north but thicken to the south and are composed of quartz, feldspar and chlorite fragments. Where not oxidised, it includes sideritic slates, which may be gruneritic and magnetite rich and grunerite-magnetite-schists which as they are oxidised become hematitic or limonitic, ferruginous slates, ferruginous cherts and jaspilite. The ferruginous cherts and jaspilite are frequently brecciated. Ferruginous chert is abundant in lower and middle parts of the unit, while jaspilite is found at top, just below the contact with the unconformably overlying Goodrich Quartzite. Bedding thickness varies from fine laminations of a few mm to several cm.

The Negaunee Iron Formation is regionally terminated by an unconformity and overlain by the Baraga Group, as follows:
Goodrich Formation - which commences with a polymict conglomerate which contains large, up to 0.5 m, angular, poorly sorted fragments of iron formation in a fine grained matrix and is overlain by a sequence of quartzite up to 60 m thick, which may pass laterally into slates. In the eastern part of the range no further deposition is represented in the Baraga Group, while to the west the Goodrich passes upwards into the Michigame Formation.
Michigame Formation - which is subdivided into:
i). Greenwood Iron Formation - similar to the Bijiki Member summarised below.
ii). Clarksberg Volcanic Member - up to 150 m of mafic tuffs, associated with thick doleritic and gabbroic sills and dykes that may individually be up to 30 m thick.
iii). Lower Slate Member
iv). Bijiki Iron Formation Member - a quartz-carbonate iron formation which may be up to 15 m thick, which is generally leaner than the underlying Negaunee iron formation and has been weathered and leached in the same manner. The composition of the carbonate is nearer ankerite than siderite.
v). Upper Slate Member

The sequence has been modified by metamorphism and by post depositional oxidation and leaching (possibly during the Paleoproterozoic, as well as more recent). Enriched ore is particularly found in the troughs of synclines and where dipping iron formation is intersected by faults or dykes to form pockets of high grade ore. Extensive enriched, oxidised ore has been mined up to 1200 m down plunge to a vertical depth of 900 m below the surface at the Republic mine.

The average primary taconite carries around 26% Fe, while weathered ore is around 31% Fe. Soft, friable, direct shipping ore composed of hematite and goethite and averaging 51.5% Fe have been the principal ores mined. Hard lump and siliceous ores of hematite, magnetite and jaspilite (taconite) are the other main ores exploited.

To 1955, some 276 Mt of ore had been mined from the range, much of which was friable hematite-goethite ore, although production also included progressively more of the un- or poorly-enriched ore that was metallurgically concentrated to produce pellets. For example, in 1955, 5.5 Mt of ore were mined and 6.8 Mt were shipped from eleven mines, three siliceous open pits, and two concentration plants. Three of the mines, Cliffs Shaft, Greenwood, and Champion, produced a very hard lump hematite, used for direct feed in open hearth furnaces. The Republic, Humboldt, and Ohio mines had concentration plants to concentrate low grade iron formation by the removal of silica. Siliceous iron ore was produced from the Tilden, Volunteer and Richmond open pits, a small amount of this ore is required in blast furnace operation. Direct shipping soft red hematite is produced from the other operating mines. Most of the soft hematite ore that year came from the northeastern part of the main iron range.

Various figures are availble on potential resources, including 17.5 Gt @ 26% Fe of un-enriched taconite, with 400 Mt @ 26% of measured, indicated and inferred resources (Klemic, 1970).   Laznicka (1990) quotes a resource of 1173 Mt @ 26% Fe.

The current operations (2006) include:

Tilden - which commenced pellet operations in 1974, and to the end of 2004 had produced 171.6 Mt of pellets, with 7.8 Mt being shipped in 2004 (Cleveland Cliffs website, 2007).
Empire - which commenced pellet operations in 1963, and to the end of 2004 had produced 260.5 Mt of pellets, with 5.4 Mt being shipped in 2004 (Cleveland Cliffs website, 2007).

Selected proved + probable reserve figures (Cliffs Natural Resources Annual Report, 2012) include:
    Empire mine - 22.4 Mt @ 21.0% MagFe;
    Tilden hematite - 625.2 Mt @ 35.8% TotalFe;
    Tilden magnetite - 89.0 Mt @ 29.0% MagFe.

The most recent source geological information used to prepare this summary was dated: 2006.    
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.

  References & Additional Information
   Selected References:
Cannon, W.F.,  1976 - Hard iron ore of the Marquette Range, Michigan: in    Econ. Geol.   v.71 pp. 1012-1028
Miller, R. and Dransfield, M.,  2011 - Airborne Gravity Gradiometry and Magnetics in the Search for Economic Iron Ore Deposits: in   Proceedings, Iron Ore 2011 Conference, 11-13 July 2011, Perth, Western Australia, The Australasian Institute of Mining and Metallurgy, Melbourne,    pp. 109-116

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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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