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Schefferville |
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Quebec, Canada |
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Fe
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Super Porphyry Cu and Au
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The Schefferville group of iron ore deposit are located in northeastern Quebec and Labrador, Canada and lie within the New Quebec Orogen (Labrador Trough), approximately 200 km north of the Labrador City - Wabush iron mines of the same orogen (see the separate record).
At least 45 hematite-goethite orebodies occur within the 20 km wide Knob Lake Range that runs for 100 km northwest through the Schefferville-Knob Lake area. From 1954 to 1982 the Iron Ore Company of Canada produced approximately 250 Mt of direct shipping iron ore (>50% Fe) from seven open pits within 8 km of Schefferville, shipping up to 10 Mt per year of direct shipping ore to Sept Iles on the Gulf St. Lawrence. About 200 million tonnes of iron resources remain in the 45 deposits.
The New Quebec Orogen (formerly called the Labrador Trough) is an approximately 1000 km long Palaeoproterozoic fold and thrust belt developed along the collision zone between the largely Archaean Rae Province to the northeast and the Archaean Superior Province to the south-west. The 'Schefferville Zone' is in the western part of the central New Quebec Orogen, and preserves the most complete and least deformed portion of the autochthonous succession. In this zone, Superior Province Archaean gneisses are unconformably overlain by the Knob Lake Group, comprising two sedimentary cycles, as follows:
Cycle 1 is a marine-shelf sequence subdivided into the Le Fer siltstone, Denault dolostone, Dolly siltstone-shale and Fleming chert breccia;
Cycle 2 commences with a transgressive quartz-arenite (Wishart Formation), overlain by the Ruth shale and iron formation of the Sokoman Formation, which was deposited on a subsiding marine ramp, and was conformably followed by the flysch sequence of the Menihek Formation.
The Sokoman Formation, and other iron-formations that correlated with it, occur throughout the length of the New Quebec Orogen, forming one of the most extensive iron-formations in the world. The Sokoman Formation has been dated at 1880 ±2 Ma.
The Palaeoproterozoic stratigraphy in the Schefferville area may be summarised as follows, from the base:
Attikamagen Formation - which varies from 30 to 365 m in thickness. It commences with thinly bedded, fine grained greyish green, dark grey to black, or reddish grey argillite and slate, with common lenses of chert, calcareous rocks and arenites. It grades upwards into the Denault dolomite, or into Wishart quartzite in area where dolomite is absent. Beds are intricately drag-folded, and cleavage is well developed parallel to axial planes, perpendicular to axial lines of folds and parallel to bedding planes.
Denault Formation - which is interbedded with the slates of the Attikamagen Formation at its base,
grading upwards into the chert breccia or quartzite of the Fleming Formation. It is primarily composed of dolomite, which weathers buff-grey to brown, mostly in fairly massive beds
which vary in thickness from a few cm to >1 m, some of which are composed of aggregates of dolomite fragments. It has a maximum thickness of 180 m but in many places forms discontinuous lenses that are generally <30 m thick. Leached and altered beds near the iron deposits are rubbly, brown or cream coloured and contain an abundance of chert or
quartz fragments in a soft white siliceous matrix.
Fleming Formation - this unit only occurs over the Dneault dolostones where these are prsent. It has a maximum thickness of about 100 m and is composed of
rectangular fragments of chert and quartz within a matrix of fine chert, ranging from dominantly dolomite matrix near the base, grading upwards into chert and siliceous material.
Wishart Formation - which is the most persistent regional unit. It is comprises thick beds of massive quartzite which are composed of well-rounded fragments of glassy quartz and 10-30% rounded fragments of pink and grey feldspar, well cemented by quartz and minor amounts of hematite and other iron oxides. Individual beds vary from a few cm to >1 m in thickness, although exposures of massive quartzite with no apparent bedding occur most frequently.
Ruth Formation - comprising 3 to 36 m of thinly banded, fissile black, grey-green or maroon ferruginous slate, with lenses of black chert and varying amounts of iron oxides, overlying the Wishart Formation. It is composed of angular fragments of quartz with K-feldspar sparsely distributed through a very fine mass of chlorite, white mica, iron oxides and abundant finely disseminated carbon and opaque material. Much of the slate contains more than 20% Fe.
Sokoman Formation - which hosts more than 80% of the ore in the Schefferville district. Lithologically the iron formation varies in detail in different parts of the district and the thickness of individual members is not consistent. A thinly bedded, slaty facies at the base of the formation consists largely of fine chert with abundant iron silicates and disseminated magnetite and siderite. Thinly-banded oxide facies iron formation occurs above the silicate-carbonate facies in nearly all parts of the district. Jasper bands, which are 1.25 cm or less thick and deep red in colour, or in a few places greenish yellow to grey, are interbedded with hard, blue layers of fine-grained hematite and a minor magnetite. The thin jasper beds grade upwards into both thick massive beds of grey to pinkish chert, and those that are very rich in blue and black iron oxides. These massive beds, which are commonly termed "cherty metallic" iron formation, make up the bulk of the Sokoman Formation. The iron oxides are usually concentrated in layers a few centimetres thick interbedded with leaner cherty beds. Frequently, iron-rich layers and lenses contain more than 50% hematite and magnetite. The upper part of the Sokoman Formation comprises beds of dull green to grey or black massive chert that contains considerable siderite or other ferruginous carbonate. Bedding is discontinuous and the rock as a whole contains much less iron than the lower part of the formation.
Menihek Formation - the Sokoman Formation is conformably overlain by a thin-banded, fissile, dark grey to jet black, sooty argillaceous slate with an unknown total thickness as it is only found in faulted blocks where a tightly folded >300 m thickness is preserved.
In the Schefferville-Knob Lake area (centered on 54° 48' N, 66° 49' W) the Sokoman Formation varies from 120 to 240 m in thickness and is a cherty, Superior-type iron formation, containing peloids and ooliths, commonly having thin and irregular bedding as well as cross-bedding and intraformational conglomerates. It has been stratigraphically divided into three regional, laterally extensive members, namely:
i). a lower 10-30 m thick member of silicate-carbonate (siderite) and shale;
ii). a middle 90-150 m thick member of mainly cherty hematite iron formation containing 30-70% iron oxides; and
iii). an upper 25-60 m thick member with silicate-carbonate.
Igneous activity commenced in the zone during deposition of the lower Knob Lake Group, and reached a climax during the deposition of the Sokoman Formation before ceasing when Menihek Formation sedimentation commenced. Mafic to felsic volcanic rocks occur at the base and top of the Sokoman Formation east of Schefferville, and episodic magmatism of the same episode occurred throughout the entire interval of iron formation deposition. There is a close spatial and temporal relationship between Sokoman deposition and local volcanic activity suggesting a genetic connection.
Three principal types of bedded ore formed in situ from Sokoman iron-formation and Ruth shale.
i). Blue ore, predominantly comprising dark grey-black hematite with lesser amounts of red hematite, martite and brown goethite, derived mainly from the cherty iron oxide member of the Sokoman Formation, forming the richest (mean 54.5% Fe) and most abundant (66%) deposits.
ii). Yellow to brown ore (mean 50.0% Fe and 20% of deposits) comprises mainly goethite and hydrous iron oxides, and dark brown martite, and is derived from the silicate-carbonate member of the Sokoman Formation.
iii). Red ore (mean 48.0% Fe, 14% ofdeposits) consists mainly of red hematite, goethite, soft aluminous silicates and chert, and is derived mainly from the Ruth shale.
The earthy bedded iron deposits are a residual enrichment type within the Sokoman iron formation that formed after two periods of intense folding and faulting, followed by the circulation of meteoric waters in the fractured rocks. The enrichment was largely the result of leaching and loss of silica, producing a strong increase in porosity. This led to a friable, granular and earthy-textured iron ore, wiith siderite and silica minerals being altered to hydrated oxides of goethite and limonite. A second stage of enrichment included the addition of secondary iron and manganese which appear to have been transported in solution and filled pore spaces with limonite-goethite. Secondary manganese minerals, i.e., pyrolusite and manganite, form veinlets and vuggy pockets. The types of iron ores developed in the deposits are directly related to the original mineral facies, with the dominant blue granular ore formed from the oxide facies of the middle iron formation, while the yellowish-brown ore, composed of limonite-goethite, formed from the carbonate-silicate facies, and the red painty hematite ore originating from mixed facies in the argillaceous slaty members.
Rubble ores - >60 m thick that are preserved in minor, down-faulted basins were derived from some of the bedded residual orebodies and contain early Late Cretaceous plant remains. The bedded ores are believed to have formed through the leaching of silica and carbonates, the formation of martite and goethite, and the redistribution of iron oxides by circulating meteoric waters during prolonged and deep chemical weathering in Mesozoic times. The presence of Cretaceous rubble ores suggests, however, that at least some of the bedded residual orebodies may be substantially older.
This summary is paraphrased and quotes from Williams & Schmidt, 2004 and Dupéré (of SGS Geostats Ltd), 2009
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 2004.
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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Seymour, C., Winter, L., O’Driscoll, J. and Butler, R., 2009 - Renewed Exploration in Canada’s Premier Iron Ore District – Labrador West: in Proceedings, Iron Ore 2009 Conference, 27-29 July 2009, Perth, Western Australia, The Australasian Institute of Mining and Metallurgy, Melbourne, pp. 161-170
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Williams G E, Schmidt P W 2004 - Paleomagnetism of the 1.88-Ga Sokoman Formation in the Schefferville-Knob Lake area, Quebec, Canada, and implications for the genesis of iron oxide deposits in the central New Quebec Orogen: in Precambrian Research v128 pp 167-188
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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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