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Lorraine Basin |
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France |
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Fe
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Super Porphyry Cu and Au
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The Lorraine Basin iron ore deposits are located in the 'Gulf of Lorraine and Luxemburg' on the north-eastern margin of the Paris Basin along the north-eastern border of France with Germany and in Luxemburg.
Mining has taken place over a north-south interval of 140 km and width of as much as 25 km, from Nancy in the south to Luxemburg in the north. Industrial scale production commenced in 1860, and reached a peak in 1960 of 62 Mt and 6 Mt per annum in France and Luxemburg respectively before declining substantially.
The Paris Basin is a 500 x 300 km Mesozoic to Tertiary intra-cratonic basin with a maximum thickness of accumulated sediments in the central thickest interval of around 3000 m. The basin is superimposed on Carboniferous and Permian troughs and on a Palaeozoic basement which is generally interpreted as the northern branch of the Variscan thrust belt. The edge of the basin, as defined by the almost circular exposure of Mesozoic outcrops, is related to the presence of successive depocentres near the city of Paris and thinning towards the margins. The margins of the basin are also influenced by Tertiary uplift of basement blocks on the edge of the basin, including the Ardennes massif to the north-east of the 'Gulf of Lorraine and Luxemburg'. The subsidence of the basin was controlled by the reactivation of major late Variscan deep faults or arrays of faults which trend east-west to NE-SW in the east of the basin.
The sequence within the north-eastern section of the Paris Basin may be summarized as follows from the base: i). Permo-Triassic Bunter sandstones; ii). Middle Triassic Muschelkalk carbonates; iii). Upper Triassic Keuper marls and sandstone; iv). Upper Triassic Rhaetian marine sandstones and marl; v). Lower Jurassic Lias marls and sandstones with lesser shales and limestones, including the Lorraine iron formation at the top; vi). Middle Jurassic Dogger basal marls and overlying thick carbonates; vii). Upper Jurassic Malm shales and limestone; and viii). Cretaceous sandstones and carbonates.
There are a series of sub-basins along the basin margin in the controlled by NE-SW trending faults with throws of 90 to 120 m in the 'Lorraine Basin' area, with different thicknesses of the iron formation in each and intervening thinnings and pinchouts between each sub-basin.
The exploited iron formation occurs near the top of the Lower Jurassic Lias system, which comprises, from the base:
i). thin marl; ii). thick limestone; iii). thin marl; iv). ochreous limestone; v). thick marl; vi). sandstone; vii). shales overlain by a thin phosphatic nodule layer; viii). thick marl; ix). the Lorraine iron formation; x). thin conglomerate at the base of the Dogger, followed by thin units of micaceous marl, calcareous sandy beds and a thick limestone.
The iron formation is the type example of 'minette' iron formations which are brownish to dark greenish-brown and are oolitic, composed mainly of siderite and iron silicates (such as chamosites and iron chlorites) and of goethite-siderite and chamosites. Many of the beds within the iron formation contain abundant fine grained clastic material and are transitional to sideritic mudstone or chamositic mudstone or sandstone. Sideritic oolites are commonly nucleated on sand grains. The oolites consist of iron hydroxide or limonite. Associated calcite is generally of detrital origin. Phosphorous is present as calcium phosphate and again usually detrital. In contrast the siderite is chemically deposited.
During the Early and Middle Jurassic, there was a period of gradual marine transgression under uniform rather high temperature in Europe. The sediments deposited during the earliest part of the Jurassic were apparently sandy and included much coarse material. However, during the latter part of the Early Jurassic and during the Middle Jurassic, erosion slowed and regression began, and the sediments deposited became more argillaceous and the littoral deposits were rich in iron precipitates. Late Jurassic time was a period in which calcareous sediments were laid down, and in which large iron deposits were not formed.
The iron formation has been interpreted to have formed during the period of regression over an eroded continent where abundant argillaceous sediments were deposited at the outlets of rivers. Wave erosion of the argillaceous zone then reworked argillaceous deposits into a tidal flat zone. Iron was extracted from ferrous clays in the exposed argillaceous sediments and transported into rivers by acid runoff as hydrosols of ferric oxides. On coming into contact with sea water, the resultant change in pH caused the Fe to be precipitated as ooliths that were modified and cemented during diagenesis.
The formation usually contains more than 20% silica which is in chamosite, iron silicates or to a lesser extent as quartz grains, but not as chert. Iron contents are generally <40% and lime ranges from 5 to 20%.
The iron formation is closely associated with carbonaceous shale, mudstone and sandy shales deposited in marine or shallow basins. Cyclic sedimentation with repetition of lithologies calcareous and sideritic siltstone and limestone with goethitic (or chamositic) oolites is characterisitic.
The characteristics of the iron formation varies from mine to mine, although the formations can be grouped into two categories, namely:
i). Siliceous facies - mainly of Toarcian age in Luxemburg and the Ardennes, where the ooliths are generally cemented by chlorite and silica with a CaO:SiO2 of <1.4, generally >32% Fe (ranging from 29 to 36%) and average of 0.7% P.
ii). Carbonate facies - mostly of Aalenian age (younger than Toarcian) in the Lorraine area (further south), in which the ooliths are generally cemented by carbonate. The a CaO:SiO2 of >1.4, generally >25% Fe (ranging from 22 to 30%) and average of 0.55% P,
The iron formations (comprising one or more iron bearing units) within the various sub-basins of the 'Lorraine Basin' vary from 10 to 60 m in thickness and have an average dip of 3°SW. Where best developed in one of these sub-basins, there are up to 12 iron bearing units, although only 2 or 3 are exploitable (i.e., >2 m thick with >25% mean Fe) with thicknesses of generally 3 to 4 m each.
The approximate total production from the 'Lorraine Basin' is estimated at 1 Gt @ 30% Fe,
from a total resource of the order of 3.7 Gt @ 30% Fe.
The most recent source geological information used to prepare this decription 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.
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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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