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A Classification of Iron Formations Based On Canadian Mineralogist Vol. 18, pp.215-222 (1980) A CLASSIFICATIONOF IRON FORMATIONSBASED ON DEPOSITIONALENVIRONMENTS GORDON A. GROSS GeologicalSurvey of Canada,601 Booth Steet, Ottawa, Ontario KIA 088 d6p6t ABsrRAcr profondes.La classificationdes milieux de des pr6cipit6schimiques de mindraux de fer et de environne- groups sediments are rec- silice repose sur les critdres suivants: Two of ferruginous plate-formeconti- (1) precipilated iron-forma- ment du bassin(milieu n6ritique, ognized: chemicallv proximit6 d'un centre banded chert and nentaleou fond oc6anique); tions composed mainly of thinly ou d'un sys- (2) commonly consisting volcanique, d'une zone d'extension iron minerals; ironstones de lithologies s6dimen' oolitic chamosite-siderite-goethite beds with ap- tdme de failles; association of distinctes; minEralogie,struc- preciable clay and detrital constituents. Both groups taire et volcanique et facies lithologiquede la for' form under a wide range of depositional environ- tures s6dimentaires fer. ments and have distinctive lithological and min- mation de (Traduit Par la R6daction) eralogical facies, Two principal types of siliceous are recognized, Lake Superior and iron-formation s6dimentferrugineux Algoma, based on the characteristics of their dep- Mots-clAs:formation de fer, Sup6rieur,type Algoma, ositional basins and the kinds of associated rock. argileux, type du lac oc6anique,arc volcanique. The Lake Superior type was deposited with quartz- milieu de d6p0t, cr6te ite, dolomite and black shale in continental-shelf marge continenlale. environments, and the Algoma type with volcanic and greywacke rock assemblages along volcanic INrnooucrron arcs, rift zones and deep-seatedfault and fracture svstems. Factors pertinent to the classification of Ferruginous sedimentary rocks were classified depositional environments for chemical precipita- in two major categories, ironstones and iron tion of iron and silica in iron formations include formations. in Volume I of the Geological Survey neritic. continental-shelf and deep-oceanbasin envi- of Canada's Economic Geology ,Serieson the proximity to volcanic centres, rift zones, ronments: iron depositsof Canada (Gross 1965). During fault systems; type of associated sedimentary and preparation of this volume in 1959 it was volcanic rock; mineralogy, sedimentary features and the iron-formations lithological facies of the iron formation. recognized that the siliceous occu; as two major types and can be classified Keyworils: iron formation, ironstone, Lake Superior on the basis of associatedtypes of rock, lithologi- type, Algoma 1yps, dspositional environment' cal features and interpretation of their deposi- oceanic ridge, volcanic arc, continental shelf. tional environment (Fig. 1). Since the intro- duction of this classification, more detailed work Sorvruernr on the petrography, stratigraphy, sedimentation, tectoniJ setting ana depositional environment of On distinguedeux groupesde s6dimentsferru- iron for.mations and associated rocks has de- gineux: (1) pr6cipit6s chimiques, contenant surtout monstrated an even wider contrast in conditions des cherts finement lamin6s et des min6raux de of occurrence of iron formation (Gross L973) ' (2) en argile et en 6l6ments fer: s6diments enrichis An attempt is made in this paper to outline the par des lits oolithiques Dr d6tritiques, caract6ris6s among.different sedimentary envi- sid6rite et goethite. Les deux groupes relationships chamosite, in a general tectonic r€sultent d'environnements de d6p6t trEs vari6s et ronments as concerved the montrent plusieurs facies lithologiques et min6ralo- framework and to update the basis for gigues. De plus, on distingue deux types principaux classification of iron formations. parmi les formations de fer siliceuses, selon le caractdre du bassin s6dimentaire et des roches asso- DeposlrtoNel. ENVTRoNMENTS ci6es. ks exemples du type "Lac Sup6rieur", in- avec quartzite, dolomie et shale noir, terstratifi6s The well-known Lake-Superior-type iron for- proviennent de la plate-forme continentale; ceux du distributed in Proterozoic rocks, type sont associ6s aux roches volcani- mations, widely "Algoma" continental-shelf ques et aux grauwackes le long d'arcs volcaniques, were deposited in near-shore zones de rift, et r6seaux de failles et de cassures environments and are associatedwith dolomite, 215 2t6 THE CANADIAN MINERALOGIST IRON-FORMATION SILIQEOUS oxidefacies silicatefacies carbonatefacies sulphidefacies = :] A' oo -U) A< 6(D :an .,(D='O- ENVIRONMENTOF DEPOSITION oj. il;o IRON.FORMATIONS O- 3(D AND IRONSTONES or:J JO' rI oo (r ,frdo \\ \\ % t. 'u'F t\ sulphidefacies t \' t.. t.-- tttb""1:Ju1"u , -1i.lt1fii: - oxidefacieb .O ALUMINOUS IRONSTONE Ftc. l. Diagram showing major types of chemically precipitated iron formation and ironstone with their sedimentary facies and depositional environment. Examples are from (a) Michipicoten, Ontario, (b) Moose Mountain, Ontario, (c) Temagami, Ontario, (d) Knob Lake, I-abrador and Qu6bec, (e) Iron River, Michigan, (f) Gunflint and Biwabik iron formations, Ontario and Minnesota, (g) Wabana, Newfoundland, (h) Clear Hills, Alberta (after Gross 1965). quartzite, black shale and minor amounts of fusive and exhalative hydrothermal processes tuffaceous and other volcanic rocks (Gross related to volcanic centres (Gross 1965). Oxide, 1965). The Algoma-type iron formations, found silicate and carbonate lithological facies are som- in all ages of rock, are consistently associated mon to both these groups, and polymetallic with greywacke sedimentary units and volcanic sulfide facies, bearing copper, zinc, lead, silver, rocks; they apparently formed close to volcanic gold, iron and manganese,are commonly asso- centres and were produced by fumarolic, ef- ciated with the other Algoma-type facies near A CLASSIFICATION OF IRON FORMATIONS 2r7 2 z- !E o =:: oo 33 z a o6 @ o6 tl, cu o o a E o , ;,1 1 q : i; o rd 9 -; o a z g; ic :.:-j o {d ;sr^:?: ol ,( !o oO !c Ya i6 xE g H k q q o o a o o w o q o CO a o e o k o r) o .9 F N lri 218 THE CANADIAN MINERALOGIST centresof volcanism (Gross 1965, 1967, James Skibi Lake in Ontario, deposited some distance 1954, Stanton 1960, Moorhouse 1965, Goodwin from effusive volcanic centres. Except for their 196r, 1965). smaller size, they compare directly with Kursk Banded siliceous sedimentaryunits and Krivoy Rog iron formations listed with the 'occurferruginous most commonly in Lake-Superior- and Lake-Superior-type sequences.Some of the iron Algoma-type depositional environments. Exten- formations in deep water and turbiditic sedimen- sive iron-formation deposition also took place in tary rocks may have formed by deposition of marine rift basins, along fault scarps, in grabens iron-bearing minerals and silica derived from and along contemporary ocean ridges (Gross ocean-ridge systemsrather than from volcanism 1973, Piper et al. 1975, Bischoff 1969). Iron associatedwith island-arc systems (Piper et al. formations recognized as of classical Lake Su- 1975, Bostriim 1970). perior type were formed on continental shelves, Iron formations deposited in grabens and as- relatively near the shore (James 1954, Gross sociated with rift systemsrange in age from the 1965). It is now realized that many other ex- Cambrian-Precambrian Snake River beds in the amples of iron formation classifiedat one time Rapitan Group in the Yukon to the recent and as Lake Superior type have associatedsedi- contemporary sedimentation in the Red Sea mentary rocks and features indicative of depo- grabensand along the East Pacific Rise (Gross sition in deeper water far offshore on the con- 1973, James 1969, Piper et al.. L975). tinental shelf or on slopes adjacent to or in Faults, rifts and graben systems along the deep marine basins (Alexandrov 1973, Plaksen- margins of ancient cratons and continents have ko et al. 1973). On the other hand, Algoma- provided suitable environments for the deposi- type iron-formation deposition extends from the tion of iron formations, thick sequencesof vol- multifacies development found near volcanic canic rocks and greywacke-turbidite sediments, centres to the predominant oxide facies extend- as exemplified around the Ungava craton (Gross ing far into the greylvacke and turbidite sedi- 1968, 1970, Dimroth et al. 1970) and in other ments. These sediments were deposited at con- parts of the Canadian Shield (Goodwin 1973). siderabledistances from the volcanic centresand Iron formations deposited on the continental the sources of the hydrothermal effusions that side of these marginal tectonic systems display provided the iron, silicon, manganeseand other typical features of Lake Superior type, and those elementspresent in these rocks (Gross 1970, on the seawardside of tectonic systemsare more 1973, Goodwin 1973, Alexandrov 1973). typical of Algoma type. Iron formations depo- Figure 2 shows important examples of dif- sited within tectonic belts are usually more ferent types of iron formation and the order closely identified with the Algoma type. De- in which they occur ( I ) in relation to near- pending on the relative position of the tectonic shore and deeperwater shelf conditions and (2) belt with respect to the continental margin, the with respectto their proximity to volcariic cen- iron formation present in grabens and fault- tres or to rift and graben tectonic systems.The scarp basins near continental margins may have Lake Superior and Labrador Trough formations distinctive lithologies, as found in the Snake have associated dolomite and quartzite, indi- River
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  • Colloidal Origin of Microbands in Banded Iron Formations
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