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

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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 iron formation (Eisbacher 1978). Those cating mature sedimentdevelopment on a marine iron formations in basins at the outer edge of continental shelf, whereasthe Krivoy Rog iron a continentalshelf may resemblecommon Algo- formations have a high proportion of volcanic ma-type lithologies,as shown in the Orissa and rocks, turbidite sedimentand deeperwater fea- Krivoy Rog iron formations. tures (Gross 1968, Plaksenkoet aI. 1973, Alex- The very wide range of sedimentary environ- androv 1973, Z,ajac 1974). The depositional ments in which precipitation of iron formation environmentof the Hamersleyiron formation in has taken place is indicated in Figure 2. To a Australia is not consideredto be an anomalous greater or lesserextent, volcanic rocks are known type under this classification scheme, although to occur in, or in immediate associationwith, it has distinctivesedimentary features indicative all the various environmental'typesof iron for- of depositionunder stabletectonic conditionsin mation. There appearsto be no age restriction the basin. Theseconditions led to the formation for the occurrence and development of any of of sequencesof microbandingthat can be cor- thesetypes of iron formation, with the exception relatedover distanceof two hundredkilometres of the near-shore Lake Superior type, found '1973).a (Trendall only in Proterozoic rocks. The explanation for Examplesof Algoma-type iron formation are this exception is believed to relate to special shown in relative order from those near volcanic worldwide tectonic features at that time and centres to others such as Lake St. Joseph and not to unique factors influencing sedimentation A CLASSIFICATION OF IRON FORMATIONS 2L9

of silica and iron-bearing minerals, such as at- factors that contributed to the formation and mospheric changes or biogenic developments geological history of an iron-formation bed (Cloud 1973, LaBeree 1973). (Gross 1965, 1973,Goodwin 1962, 1973, James Classification of the iron formations is based & Sims 1973.Semenenko 1973). It has become on descriptivedata for the various lithofacies evident through the study of genetic models of iron-rich sediment present, their mineralogy, of the iron formations and iron-ore deposits sedimentaryfeatures, associated rocks and their that the banded siliceousiron-formation group regional tectonic setting at the time of deposi- of ferruginous sedimentsoccurs in a wide var- tion and subsequenttectonic history. Table I iety of sedimentary-basinenvironments, ranging , intlicates various kinds of data collected in the from stable epeirogenicto metastableorogenic study and classificationof an iron range. Syn- tectonic conditions. The thickness and lateral thesis and study of the descriptivedata lead to extent of an iron formation depend directly on interpretation of the various factors listed in the the tectonicstability of the sedimentarybasin as left margin of Table 2. The nature and char- well as on other sedimentaryprocesses operating acter of the primary depositionalbasin and the in the basin.The accumulationof siliceousiron- physical and chemical environment for deposi- rich muds and ooze is controlled by the intro- tion and precipitationof the iron-formation sedi- duction or restriction of clastic sedimentand by ment are deducedfrom the studv of thesebasin- biochemicalactivity in the basin environment. environment factors. Fumarolic and hydrothermal emanationsdis- chargednear volcanic centres,along deep-seated Dtscussrou fracture zones and from areas of high thermal gradient in the earth's crust are believed to be Numerous geologicalmodels have been out- the principal sources of iron, silica and other lined in the literature to account for significant metallic constituents in the iron formations

IABtI I. ROCK DESCRIPTIONSIN THE CLASSIFICATIONOF IRON-FORIIATIONS

BASIN TYPES LAKE SUPEBIOB TYPE ALGOMA TYPE Mlcnlplc9rsn TYPICAL Lake Supetior Mary River BASINS Gunllint Labrador Trough Lak€ Belo Horizonte Temagami Sturgeon Transvaal Melvill6 Peninsula Hamerslsy Cerro BoliYar Skibi Lako Snake River Otissa Kapico Fed Sea Mud Kursk East Pacilic Riso Krivoy Rog Lakg St. Joseph EPEIROGENIC oBocE$-a TECTONIC Cratonic shelf Volcanac arca Ocean ridges ENVIRONMENT S lopo Exl stve volcanism,Fracture 9Ystems Rilts, Graben

ASSOCIATED ROCKS Black shale.Chert C hert Cherl, Conglomerale Sedimenta r y Ouarlzite.Dolomile Greywacke.Turbidites Mudstones,EYaporites Volcanic Tuff s acidic + docreasing VoI canoc lasl acs basic Pyroclastics Inirusive Basic dykes.sills

IRON.FORMATION LITHOFACIES Oxide xxx xx Silicate ttt x CarDonale x x Sulphids tX Clast ic x carbon shale mud wtcko xlr sand Mirod tacies xxr MINERALOGY METAMORPHISM low high STRUCTURAT STYLE 22O rHE cANADIAN MrNERALocrsr

TA B I.E 2. A CLASSIFICAT]ONFOR DEPOSITIONAL BASI'{S OF SILlCEOUSIROI{-FOBHATIOIiIS

BASIN TYPES LAKE SUPERIOR TYPE ALGOMA TYPE TYPICAL Lak6 Supgliot illctrl9l9elsrt SAStNS GuntI i nt Mart Rivor Labrador Trough Balo Horizonto famagami Sturgeon Lake Tranavaal MolYlllo Poninsula Hamerslet Csrro Bolivar Skibi Lako Snake River Orissa Kapico Rod Sea Mud Kurs h Ris€ Krivoy Rog Lako St. Josoph Eaat Pacific

TECTONIC EPEIROGENIC QBQqENIL ENVIRONMENT Cratonic ahelt Volconic 6rca Ocoan ridggs Ertruaiye volcaniSm, Fracture gtstomg Ritts. Grabon TECTONtc Subsidence Deep tracture systems Tonaion tracturaa ACTIVITY Uplitr Thrust taulte Ritt8. Block taults BASIN ENVIRONMENT MARINE ngritic a a shelt a a a slope a a O deoD oceln a a STABILITY St abl s Msta stabl e

ENERGY LEVEL Hiqh Low CIRCULATION Restrictod Limiled Thermal clrculation,Tutbidity currsntg

DEPTH Shal low OsoD Varied

BASIN SIZE Largo Small Vari.ble

AGE SEDIMENTARY FEATURES bodding Macro,Cross-bedding Micro-laminated Micro-, Macro-laminated Itrogslar terture oit I ltic dofo?mation Broccia.Scout,Slumps Slumps,Cronulated bod8 Braccia,Scout ASSOCIATEO voLcANtc a CENTRES closo aa di9tant a a

(Gross 1973, Goodwin 1973, Bostriim 1970, and ooze (Gross 1965, 1,972). The restriction .Piper et al. 1975). Whether the constituents of other types of sedimentation and the tectonic of the iron-rich beds are derived directly from stability of the basin environments are dominant differentiation of magma sources or by leaching factors in determining the size, location, distri- of elements from crustal rocks by circulation of bution and development of facies of the iron saline solutions has not been clearly demon- formation. The final characteristicsof the iron- strated. Both processesmay be significant con- silica beds are determined by the broad range tributors of the constituents. Research suggests of conditions and processesoperating in a sedi- that sirculation of a saline solution of seawater mentary basin, no two iron-formations having composition would be quite capable of leaching been depositedin identical environments.Varia- metal and silica constituents from enclosing tions in chemical composition of the different rocks in sufficient volumes and in the propor- iron-formations and their sedimentary facies tion now found in the iron formations (Hajash have been under study for some time and are r97s). reported in other papers (James 1966, Gross Iron and silica, contributed to depositional 1965.Gross & Mcleod 1980). basins from hydrothermal effusive sources, are Figure 2 shows some of the major features deposited in beds of iron formation thpough recognized in the different tectonic environ- direct chemical precipitation, by flocculation ments for iron-formation deposition. It should of colloidal and fine particulate iron oxide and be noted that a proportionately greater number silica and by the sedimentary reworking and of iron-formation beds occur in the zone be- concentration of the silica-iron-bearins muds tween volcanic centres and the cratonic shelf A CLASSIFICATION OF IRON FORMATIONS 22r zone than can be indicated in Figure 2. This Gnoss, G.A. (f965): Geology of iron deposits in area is common territory between the environ- Canada. I. General geology and evaluation of Econ, Geol. Rep. ment models originally designated for Lake iron deposits. Geol. Surv. Can. 22. Superior and Algoma types of iron formation. Research is continuing on the possibility of (1967)t Geology of iron deposits in Can- differentiating iron sediments related to (1) ada. III. Iron ranges of the Labrador peosyn- thrust-fault systems and (2) graben and ocean- Grenville regions of Canada. Geol, Surv. Can. ridge systems. Documentation of the distribution Econ, Geol. Rep,22. of iron formations in relation to ancient global- (1968): Geology of iron deposits in Can' plate is of scale fracture systems and tectonics ada. III. Iron ranges of the Labrador geosyn- special significance in the understanding and cline, Geol. Surv. Can. Econ. Geol. Rep. 22. classification of iron-formation occurrences. (1970): Nature and occurrence of iron ore deposits. Iron ore deposits of Canada and the RBrunsNcrs West Indies. /n Survey of World Iron Ore Re- sources. United Nations Publ. 13-31, 237-269. Ar.rxlNonov, E.A. (1973): The Precambrian banded iron-formations of the Soviet Union. Econ. Geol, (1972): Primary features in cherty iron- 68,1035-1062. formation. Sed, Geol. 7, 24L-261,

Brscnorr, J.L. (1969): Red Sea geothermal brine (1973): The depositional environment of deposits: their mineralogy, chemistry, and genesis. principal types of Precambrian iron-formation. In l{ot Brines and Recent Heavy Metal Deposits /r Genesis of Precambrian fron and Manganese (M.P. in the Red Sea: a Geochemical and Geophysical Deposits. Proc. Kiev Symp. 1970 Semenen- Account (E.T. Degens & D.A. Ross, eds.). ko, ed.), Unesco Earth Sci. 9, 15-21, Springer-Verlag, New York, & McLEoo, C.R. (1980): A preliminary Bosrndu, K. (1970): Submarine volcanism as a assessmentof the chemical composition of iron source for iron, Earth Planet, Sci, Lett, 9, 348- formations in Canada. Can. Mineral. 18,223'229. 354. HArAsH, A, (1975): Hydrothermal processesalong CLouD, P. (1973): Paleoecological significance of mid-ocean ridges: an experimental investigation. the banded iron-formation, Econ. Geol. 68, lL35- Contr, Mineral. Petrology 53, 205-226. 1143. Jeuss, H.L. (1954): Sedimentary facies of iron- Econ. Geol. 49, 235-293. DnvlRoru, E., Bmlon, W,R.A., BrncnnoN, R. & formation. JAcrsoN, G.D. (1970): The filling of the Circurn- (1966): Data of geochemistry (6th ed.). geosyncline. Ungava /n Symposium on Basins and W. Chemistry of iron-rich sedimentary rocks. (A.J. Geosynclines of the Canadian Shield Baer, U.S. Geol, Surv. Prof. Pap. 440-W. ed,) Geol, Surv. Can. Pap.7A40. (1969): Comparison between Red Sea de- Ersuecnrr, G.H. (1978): Re-definition and subdivi- posits and older ironstone and iron-formation. In sion of the Rapitan Group, Mackenzie mountains. Hot Brines and Recent Heavy Metal Deposits in Geol. Surv, Can, Pap.77-35. the Red Sea: a Geochemical and Geophysical Account (E.T. Degens & D,A. Ross, eds.), GooowrN, A.M. (1961): Some aspects of Archean Springer-Verlag, New York. structure and mineralization. Econ. Geol, 56, 897-9t5. & SIrvrs,P.K. (1973): Precambrian iron- formations of the world. Econ. GeoI. 68, 913- (1962): Structure, stratigrapdy and origin 1174. of iron formations, Michipicoten Area, Algoma District, Ontario, Canada. Geol, Soc. Amer. Bull. LeBsncs, G.L. (1973): Possible biological origin 73, 561-585. of Precambrian iron-formations. Econ. GeoI. 68, 1098-1109. (1965): Mineralized volcanic complexes in the Porcupine - Kirkland Lake - Noranda region, Moonnousr, W.W. (1965): Stratigraphic position Canada. Econ. Geol. 60, 995-971. of sulphides in the Archaean. Can. Inst' Mining M etall. T rans. 68, 261-264. (1973): Archaean volcanogenic iron-formation of the Canadian shield, /z Genesis of Pre- Prps& D.2., Vnnn, H.H., BenrneNo, W.G. & CHASE, cambrian Iron and Manganese Deposits. Proc. R.L. (1975): An iron-rich deposit from the Kiev Symp. 1970 (M.P. Semenenko, ed.), Llnesco Northeast Pacific. Earth Planet. Sci. Lett. 26, Earth Sci. g, 23-32. tt4-120. 222 THE CANADIAN MINERALOGIST

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