Cuyuna Iron Range District

Minnesota, USA

Main commodities: Fe Mn
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The Cuyuna Range iron and manganese deposits are located some 100 km SW of the Mesabi Ranges in east-central Minnesota, USA defining a district that is approximately 110 x 32 km of tightly folded iron formation.   Mining commenced in the district in 1911 and finished in 1985.   Historic mines include Croft and Portsmouth.

The iron formations of the Cuyuna Range are hosted by rocks of the Palaeoproterozoic Animikie Group which are best known in the Mesabi Range further to the north-east.

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 Cuyuna Range) contained 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.

Unlike in the Mesabi Range, where the Animikie Group rests directly on the Archaean crystalline basement, the intervening Mille Lacs Group is represented by the following sequence from the base:
Denham Formation - Quartz arenite and siltstone, oxide iron-formation, marble, mafic hypabyssal intrusions and fragmental volcanic rocks metamorphosed to the staurolite grade of the amphibolite facies.
Glen Township Formation - Commencing with conglomerates and quartz sandstones, grading upwards into thin bedded fine grained metasediments with thin dolomite beds, locally developed silicate­carbonate-oxide facies iron formation, carbonaceous shale with sulphides and basalts. Mafic volcanics of the formation have tholeiitic to calc-alkaline affinities and have been dated at 2197 ±39 Ma. Metamorphosed to greenschist to lower amphibolite facies.
Little Falls Formation - Quartz-rich slate, argillite and schist in the northwest segment of the unit and coarse-grained megacrystic garnet-staurolite-schist in the southeast.
Trout Lake Formation - Quartz arenite, siltstone, and chert-rich dolostone.

The overlying Animike Group which hosts the ore deposits, comprises, from the base:
Mahnomen Formation - Claystone, shale, siltstone, and graywacke metamorphosed to the greenschist facies.
Trommald Formation - a 14 to 150 m thick unit composed of carbonate-silicate iron formation and associated manganese oxide deposits. The iron formations have been oxidised and leached near surface to hematite iron formation. Also contains substantial quantities of volcanic and hypabyssal rocks of generally mafic composition. Metamorphosed to the greenschist facies.
Rabbit Lake Formation - Mudstone, slate, greywacke, with lean, siliceous, argillaceous iron formation and associated mafic metavolcanic rocks, all of which have been metamorphosed to greenschist facies. Includes thin beds of carbonate-silicate iron formation.

The sequence in the Cuyuna Range has been deformed into a series of broad, open, eastward-plunging folds with near-vertical axial planes, in contrast to the monoclinal structure of the Mesabi Range.   In addition, the stratigraphic position of the Biwabik Formation in the Mesabi Range is occupied by the Trommald Formation represented by several lenticular units of iron-formation intercalated the underlying Mahnomen Formation and the basal section of the overlying Rabbit Lake Formation.   Stratigraphic relationships imply that the basal contact of the Animikie Group unconformably overlies older folded rocks of the Mille Lacs Group in the Cuyuna North Range.   That unconformity marks the boundary between rocks of the Penokean fold and thrust belt which have undergone two pulses of deformation, while the Animikie basin sequence has only been subjected to one period of deformation.

The main BIF of the Trommald Formation, Unit A, was deposited during two transgressive-regressive cycles in a basin interpreted to have been deposited between a strandline to the west and deeper water to the north and east. It has been divided into seven lithotopes, as follows, from the base:
i). Laminated to very thin bedded, to thick bedded to structurless (upward gradation) epiclastic sandstone, siltstone and shale, >30 m thick, with a matrix of sericite-chamosite-silica as well as calcite and siderite;
ii). Thick bedded to upper structureless mixed epiclastic (quartz arenite) to jaspery chert, 5 to 10 m thick, with a matrix of mainly goethite amd manganese oxides;
iii). Thick bedded to structureless oolitic and pisolitic grainstone iron formation, 18 to 20 m thick, with a matrix of psilomelane, cryptomelane and pyrolusite with lesser goethite and chert;
iv). Thick bedded granular iron formation, with a matrix of manganese oxide and recrystallised grey to white chert, 11 to 30 m thick, including 5 to 7 m containing parallel and irregularly banded oxide rich layers comprising mosaically intergrown layers of hematite or goethite, with or without chert, and intercalated bands of manganite or cryptomelane;
v). Interlaminated thick to thin granular and oxide rich to non-granular banded iron formation, with individual beds of chert, minnesotaite, stilpomelane and siderite with traces of magnetite and chamosite, 20 to 25 m thick;
vi). Thin bedded to laminated, non-granular iron formation with individual beds and laminae similar to the underlying lithotype, predominantly minnesotaite;
vii). Thinly laminated, non-granular ferruginous chert with layers of chert and hematite with lesser siderite and minnesotaite, 7 to 40 m thick.

While Unit A is mineralogically, texturally and chemically similar to other iron formations of the Animikie Group, it locally contains 10 to 100 times greater manganese oxides than the norm of 0.6 to 0.8% in the Biwabik Iron-formation elsewhere in the basin.   Manganese oxides are principally concentrated in the coarser grained parts of Unit A as disseminated grains, thin pods or lenses, and layers as thick as 1.5 m that typically contain about 10% Mn, and locally as much as 20 to 30% Mn.   Two laterally persistent zones about 15 to 18 m thick carry 10 to 50% Mn and coincide in general with stratigraphic positions occupied by oolitic-pisolitic iron-formation.   These zones contain various proportions of psilomelane and cryptomelane, as well as hematite and quartz.   Goethite and manganite are locally abundant representing secondary (supergene) mineralisation formed during a period of intense chemical weathering that modified these rocks in Late Jurassic or Early Cretaceous time.

Between 1904 and 1984, some 106 Mt of iron ore was mined from the Cuyuna Range, much of which contained more than 10% Mn, although no Mn was sepatately recovered due to the metallurgical difficulties involved. The ores from this production averaged 43.3% Fe with 6.5% Mn. Reserves of similar enriched ore contained 13% SiO2, 0.06% P2O5 (Klemic, 1970).

Potential resources of un-enriched iron formation have been estimated as 4400 Mt @ 28% Fe (Klemic, 1970). Other sources suggest resources of 1480 Mt @ 32% Fe. Sections of the iron formation that are manganiferous account for a resource of 20 Mt @ 10.5% Fe.

For detail see the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1993.    
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
 References to this deposit in the PGC Literature Collection:
Cleland J M, Morey G B, McSwiggen P L  1996 - Significance of Tourmaline-rich rocks in the North Range Group of the Cuyuna Iron Range, east-central Minnesota: in    Econ. Geol.   v91 pp 1282-1291
Morey G B, Southwick D L  1993 - Stratigraphic and sedimentological factors controlling the distribution of epigenetic manganese deposits in iron formation of the Emily District, Cuyuna Iron Range, east-central Minnesota: in    Econ. Geol.   v88 pp 104-122

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