Gogebic Iron Range

Michigan, USA

Main commodities: Fe
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The Gogebic Range iron province trends ENE-WSW over an interval of approximately 110 km and 15 km wide that straddles the Michigan-Wisconsin border, 10 to 30 km from the southern shore of Lake Superior in the USA.

Production commenced in 1884 at the Colby mine and reached a peak during the 1890's, with from 3 to 4 Mt of ore being shipped per year from the range. Mining ended during the 1960's when the remaining ore became uneconomic. During the period from 1884 to when operations ceased, 300 Mt of ore are estimated to have been mined.

The major iron formations of the Great Lakes region of North America are hosted within the Paleoproterozoic 2.2 to 1.9 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 Gogebic 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.

In the Gogebic Range the basement comprises a complex of migmatitic gneisses and amphibolites believed to date at ~3.6 Ga, intruded by granitic rocks of the 2750 Ma Puritan batholith.

This basement is overlain by the Chocolay Group, the basal unit of the Marquette Supergroup. The Chocolay Group comprises, from the base, the:
Sunday Quartzite - a basal, crossbedded, reddish vitreous orthoquartzite with several quartz-pebble conglomerate layers, as well as mud chip conglomerates and mud cracked horizons;
Bad River Dolomite - a cherty dolomite, commonly metamorphosed to tremolitic marble. It includes a variable thickness of a distinctive cherty breccia that varies from zero to much of the thickness of the unit and appears to be a reworked residuum of chert nodules and beds left by solution of the dolomite during the hiatus prior to deposition of the unconformably overlying Palms Quartzite.

The Chocolay Group is unconformably overlain by the Menominee Group, which is interpreted to have been deposited on a south-facing passive margin, and comprises from the base:
Palms Formation - which grades from banded argillite near its base to thick-bedded quartzite near the top. At its upper contact, the quartzites grade upward over 1 to 2 m into ferruginous beds of the Ironwood Iron Formation.
Ironwood Iron Formation - the host to the iron mineralisation in the range, this unit is from 100 to 300 m in thickness and generally dips at around 60° to the NW. It contains numerous lithologic types of iron-formation. In the east it comprises five laterally continuous members that are alternating units of thin, even-bedded carbonate iron formation and thick, wavy bedded to lenticular hematite iron-formation. Further west the same lithologies are present, but less coherent due to depositional changes and to the effects of deformation and metamorphism. Locally, a thin (0 to 13 m thick) tuffaceous bed has been mapped, interfingering with more voluminous volcanic rocks that occur both to the east and west. The metadolerite sills cutting the Ironwood Iron-Formation and Bad River Dolomite in the western part of the map area are believed to be coeval with these volcanic rocks and intruded shortly after sedimentation.

The Menominee Group is unconformably overlain by the Baraga Group, the uppermost unit of the Marquette Supergroup. The Baraga Group is represented by the Tyler Formation, a thick greywacke-slate sequence believed to have been deposited in a foreland basin during accretion of island arc terranes to the south at about 1850 Ma. The contact with the underlying Ironwood Iron-Formation may be an unconformity, based on thickness variations in the underlying Ironwood Iron Formation and a conglomerate found locally at the base of the Tyler Formation.

All of the rocks of the Marquette Supergroup have been intruded by mafic dykes and sills and offset by transverse and longitudinal faulting. They are essentially un-metamorphosed to the east, but increases in metamorphic grade toward the west where assemblages include orthopyroxene and garnet in iron formation and large tremolite masses in carbonate rocks of the Bad River Dolomite. This metamorphism was related to Middle Proterozoic thermal events.

Much of the ore exploited consisted of secondary concentrations of iron oxide and hydroxide minerals within the Ironwood Iron Formation, believed to have been formed by deeply circulating meteoric waters which leached silica from the iron formation. Leached and oxidised rocks extend for more than 2000 m below the present surface.

Enriched zones form orebodies with pitch lengths of as much as 5 km and widths up to 120 m that extend up to 450 m parallel to the dip of the iron formation. They generally occur at or near the base of the iron formation along the eastward pitching intersection of dykes and iron formations.

Between the 1880's and the 1960's, more than 300 Mt of ore were produced from the range. Grades are quoted as around 54% Fe, 8.4% SiO2, 0.07% P2O5 (Klemic, 1970)

Analysis of available mapping, drilling and beneficiation tests indicated a remaining resource of 3.7 Gt of un-enriched taconite on the basis of economic and technologic parameters of the mid-1970's (Marsden, 1978). This material was believed to be economically extractable by open-pit mining and amenable to magnetic concentration with recoveries of 30 to 35% Fe
3O4.   Klemic, (1970) quotes a resource of 7.75 Gt @ 26-33% Fe to a depth of 75 M, while Laznicka, (1990) quotes a resource of 2.65 Gt @ 32% Fe.

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

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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