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Menominee Range - East Menominee, Fetch, Iron River-Crystal Falls and Florence districts |
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Michigan, 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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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Menominee Range iron province is located in northern Michigan and Wisconsin, of the order of 100 km south-west of the city of Marquette on the south shore of Lake Superior in the USA.
The Menominee Range district is an administrative, rather than geological entity and includes a number of distinct blocks of deposits that are physically and geologically separated. These are the:
i). East Menominee - which consists of two parallel, ESE trending, steeply south dipping belts of iron formation stretching over an interval of 42 km. The Northern belt is not continuous, with an 8.5 km gap in the centre. Between the commencement of mining in 1874 and 1956, by which time the direct shipping ore had been exhausted, this district had produced 83.2 Mt of ore from 23 mines. In 1957, two mines remained extracting siliceous ore.
ii). Fetch - an east-west trending, tightly folded syncline of sedimentary rocks that include iron formation, covering an area of approximately 25x1.5 km. The iron formation of the district has yielded 0.5 Mt of siliceous ore from four mines, and
iii). Iron River-Crystal Falls and Florence districts - The iron formation of these districts occurs around the margins of a triangular shaped basin that covers an area of approximately 180 sq. km. Between 1882 and 1955 (when some of the mines were still operating), some 166 Mt of iron ore were shipped. Most of the ore produced was soft, red hematite and yellow limonite with a subordinate amount of hard blue hematite from both underground and open pit mines. Only underground mines remained in 1956.
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 Menominee Range) contain sufficiently 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 geology of the Menominee Iron Range varies across the districts that constitute the 'range', as follows.
Throughout the 'range', Mesoarchaean crystalline basement of granite gneiss is unconformably overlain by the Paleoproterozoic Chocolay Group equivalent, comrising, the:
Fern Creek Formation - a diamictic quartzite unit;
Sturgeon Quartzite - a 100 to 600 m thick unit of well-sorted, supermature, quartz arenite that is separated from the underlying Fern Creek Formation by a hiatus and palaeosol;
Randville Dolomite - composed of crystalline dolomite containing biscuit shaped stromatolites, with variable amounts of intercalated sandy dolomite, dolomitic and quartzose slate and phyllite and dolomite conglomerate. This unit is up to 600 m thick in the centre of the basins of the range and has a gradational conformably contact with the underlying Sturgeon Quartzite.
The Chocolay Group is unconformably overlain by the lower Menominee Group, composed of:
Felch Formation - variable siltstone, slate, greywacke and quartzite sequences, with a thin basal conglomerate, which underlie the main iron formation.
Vulcan Iron Formation, (also known as the Amas and Fence River Iron Formations in different parts of the Range) 180 to 230 m thick in the East Menominee district, comprises three units, two iron formations, the Traders and Curry members, separated by the Brier Slate which is composed of quartz, feldspar, magnetite, sericite, chlorite, dolomite, chert, and pyrite. The iron formations are composed of alternating thin layers of chert and iron minerals with a predominance of siderite, hematite, iron silicates, or magnetite. Average iron content for the Traders Member and Curry members are 34% and 31% respectively. The iron formation is faulted, steeply dipping and locally overturned.
The iron formations of the Menominee Group are unconformably overlain by the Baraga Group, as follows:
Michigamme Slate, consist of approximately equal proportions of dark grey, massive, and fine- to medium-grained greywacke and light to dark grey quartzose, micaceous, and graphitic slate with inter-bedded greywacke and minor dolomite, quartzite and some iron formations.
Badwater Greenstone - a thick sequence (up to 9000 m ? thick) of massive chloritised mafic pillow lava flows, tuffs and agglomerates.
The Baraga Group is conformably overlain by the The Paint River Group with an aggregate thickness of at least 1250 m, comprising:
Dunn Creek Slate - a basal sequence of siltstone, slates and greywacke, ranging in thickness between 125 and 250 m.
Riverton Iron Formation, the 30 to 280 m thick consisting dominantly of inter-bedded chert and siderite or chert and hematite or magnetite that average 20 to 30% Fe. Locally, the upper part of the formation is absent due to erosion prior to the deposition of the overlying Hiawatha Greywacke. This unit is intensely faulted and folded and is the primary iron ore host in the Iron River-Crystal Falls district, where the host basin is completely surrounded by greenstones of the upper Baraga Group Badwater Greenstone.
Hiawatha Greywacke, which comprises greywackes, with considerable interbedded slate.
Stambaugh Formation, containing an iron-rich unit, ranging from chloritic mudstone and slate to a laminated cherty siderite-magnetite.
Fortune Lakes Slates, consisting primarily of slate and minor greywacke, represents the uppermost preserved unit of the Penokean Orogen in the Menominee Range.
The mineralisation in the East Menominee and Fetch districts are hosted by the Vulcan Iron Formation and equivalent Amas and Fence River Iron Formations of the Menominee Group. Enriched ore deposits are found in synclinal keels occupied by the lower ferruginous member of the iron formation at the intersection of faults with the footwall slate. Two types of ore have been exploited in this iron formation, i). siliceous, unaltered taconite with sufficient iron to be economically exploited with 31 to 44% Fe and 26 to 44% SiO2, and ii). enriched mineralisation, predominantly composed of hematite, from which the majority of the silica has been leached to prduce ore with more than 60% Fe and <6% SiO2, 0.34% P2O5. Individual orebodies contain up to 9 Mt of ore and persist to depths of around 600 m.
Within the Iron River-Crystal Falls district, the ore is contained within the Riverton Iron Formation of the overlying The Paint River Group. Un-oxidised iron formation in this unit contains 20 to 30% iron. Many of the orebodies in the district are within plunging synclines and are persist to depths of up to 900 m, strike lengths of up to 1600 m and thicknesses of 30 to 90 m. The base of these deposits is usually controlled by either faults of by a decrease in the intensity of oxidation and leaching.
Between 1900 and 1964, some 284.5 Mt of ore was shipped from the Menominee Range.
Remaining oxidised iron formation and un-enriched taconite resources have been estimated as 2350 Mt per 30 vertical metres, for a total potential of 11.5 Gt (Klemic, 1970). Laznicka (1990) quoted a total resource of - 1528 Mt @ 32% Fe.
The most recent source geological information used to prepare this decription was dated: 1990.
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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Klemic, H., 1970 - Iron ore deposits of the United States of America, Puerto Rico, Mexico and Central America: in Survey of World Iron Ore Resources, Occurrence and Appraisal, Department of Economic and Social Affairs, United Nations, New York, pp 411-477.
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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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