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FeatureBanded ironformations andpalaeoenvironment:a problem inpetrogenesisBanded iron formations, or ‘BIFs’, are finely lami-nated deposits of alternating iron/silica-rich layers, ofrestricted mineralogy, and seemingly the products ofquiet, deep-water deposition. Terrigenous content isabsent, suggesting formation far offshore, yet alsounder conditions shallow enough for periodic turbu-lence (Fig. 1). Ahead of any environmental consid-eration, depositional models must first explain the ex-traordinary lateral continuity of these deposits, inwhich centimetre-scale lamination can be traced con-tinuously over more than 50 000 km2.

David PageDepartment of

Mineralogy, The

Natural History

Museum, London

The mineralogy, chemistry and stratigraphy of Precambrian banded ironformations have been extensively documented, yet the way such formationsare produced remains a subject for debate. Differentiation by varving,microbial precipitation and secondary alteration are all seen as possiblemechanisms, but discussion returns to the lack of any modern analogue.Nothing like banded iron forms anywhere in the world today or has doneduring the entire Phanerozoic. Where do we begin with such enigmatic rocks?

It is difficult to imagine any physical sedimentaryprocess that could produce fine-scale layering oversuch vast areas, and for this reason banded-iron for-mations are generally considered to be chemical pre-cipitates. Where structures of apparent sedimentaryorigin occur within the banding, they are mostlyviewed as tectonic or diagenetic artefacts. This is notalways so, as the current structures in Fig. 1 show,but such features are rare, and once banded iron for-mations are fully developed, unperturbed laminatesare the usual result.

In decreasing scale, they comprise three elements:‘macrobands’ – large (stratigraphic) divisions, com-posed of alternating iron/silica-rich ‘mesobands’(usually < 10 cm thick, about 15 of which can beseen in Fig. 2), divided into submillimetre-scale‘microbands’ (example arrowed, bottom right). Theselast demonstrate well-defined regularity, and this in-terval has led to their interpretation as annual incre-mental layers (or varves).

The classification of banded iron formations recog-nizes two generalized types – thin ‘Algoma-type’ de-posits associated with Archaean greenstone belts,and a massive ‘Superior-type’ characterized by greatlateral extent, restricted to the Proterozoic. The Ham-ersley Basin, 900 km north of Perth in Western Aus-tralia, is the largest expanse of Superior-type deposits,containing 6000 billion m3 of banded iron formation.This area alone supplies 10% of global iron-ore pro-duction annually, and comprises the type example forearly Proterozoic banded-iron sedimentation. In con-trast, the Goldfields region of the Yilgarn Block,500 km to the south, is an Archaean greenstoneterrane, and Algoma-type deposits form part of thishighly deformed metasedimentary complex. Thesebanded iron formations are relatively unknown, be-ing of little more than ancillary interest to their gold-bearing hosts, but they display all the principal litho-logical features of their Proterozoic relatives, in asequence some 500 million years older.

Precambrian environments

The terrestrial environment was strongly reducingthroughout the Archaean – early Proterozoic, and

Fig. 1. Bifurcatingoscillation (wave)ripples in Nunngarrabanded ironformation, YouanmiGreenstone Belt,Eastern Goldfields,Western Australia(about 3000 millionyears old).

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this would have controlled the availability of ionicspecies for chemical reaction. In the case of iron for-mation, an atmosphere/hydrosphere with a low levelof ambient oxygen would allow long residence timesfor ferrous ions in solution (the oxidized ferric form ofiron is insoluble in water). Such reactive specieswould be oxidized at source in the modern, oxygen-rich environment, but this persistence in the oceanseems central to the deposition of iron formations.Based on the iron reserve described earlier for theHamersley Basin, more than 1012 tonnes of iron weredeposited across an area of only 100 000 km2. Aver-aged over a projected 25-million-year period for depo-sition of this sequence, this equates to almost500 000 tonnes of banded iron formation a year.

Reducing conditions may be necessary to accountfor deposition on the scale of Superior-type bandediron formations, but if such conditions allowed theaccumulation of iron in the early oceans, precipita-tion of iron oxides, or their precursors, requires anequally significant oxidizing agent. The source of thisoxygen is not certain, but may have resulted fromlocalized biological activity (see later). Although theavailability of silica is less of a problem than that ofiron, its abundance, in the absence of continentalinput (or organisms that sequester this element), sug-gests that a hydrothermal origin may be common toboth. The conditions of precipitation and pore-waterchemistry of these deposits have yet to be resolved,but there is little doubt deposition was closely associ-ated with the oxidation state of the Precambrianoceans.

Microstructure

There is much uncertainty over the nature of bandediron formations. They are (variably) seen as the prod-ucts of physical sedimentation, chemical precipita-tion, diagenetic solution and solid-state recrystalliza-tion. While they remain to be defined as eitherprimary or secondary deposits, there is little prospectof understanding their origins.

The sample in Fig. 2 is Algoma-type banded ironformation from the Nullagine area of the Yilgarn,with features that illustrate this problem on a smallerscale. The undulating topography defined by the

thick chert band (at the base of the sample) visiblycontrols the geometry of the layers above, which be-come lensoid before a return to more regular, rhyth-mic deposition. The initial impression is one ofcompaction, reflecting lateral mass-movement (i.e.post-depositional, pre-lithification). However, closerinspection reveals magnetite layers running throughthe lenses, continuous with the surrounding iron-rich groundmass (arrow, Fig. 2). Such continuitywould not be preserved by fluid migration of the sur-rounding siliceous element. Physical sedimentationor precipitation might produce these features, but thiswould require extremely steady deposition of the indi-vidual iron and silica phases, however derived. Whatis the relationship between these apparent accumula-tion surfaces and the form of the overlying strata?Are these facies variations, or is the banding meta-morphic, and such ‘pseudo-structures’ simply theproduct of recrystallization?

A thin-section view of the microbands (Fig. 3)shows hematite, magnetite and chert as the majormineral components, with martite and fibrousgoethite visible near the specimen margins. Theseminor ferrous minerals may be oxidation and/or hy-dration products, but the magnetite is concentratedin fine, wavy laminations and infills, interstratifiedwith the chert bands. The uniform grain size of thismagnetite, euhedral and coarser-grained than thehematite or chert, suggests it may be recrystallized,but no cross-cutting grain boundaries are visible.Any diagenetic change might only have involved de-hydration or loss of volatiles; it is difficult to see whatcircumstances could cause oxidation of the principalmineral species on a scale vast enough to impart afoliation to an entire iron formation. If the magnetite

Fig. 2. Unnamed bandediron formation from Oroyatenement (locality as forFig. 1). Hematitic chert(light), magnetite (dark).Note magnetite microband(arrowed).

Fig. 3. Accretionaryconcentrations ofmagnetite (opaque), withhematite andmicrocrystalline chert asseen in cross-polarizedreflected light.

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is accepted as an original depositional component,this compositional variation may indicate physical in-fluence during deposition and not be the result of al-teration. Whatever mechanism operated to segregatethe silica and iron-rich phases must have occurredprior to deposition (any precipitate would have to bedifferentiated by this point).

Secondary processes

The banding in banded iron formations is similar tothat seen in some massive sulphide deposits, and theunparalleled lateral extent makes a secondary expla-nation attractive. In 1983, Trendall suggested thatthe lamination could be the result of post-depositionalalteration, primary depositional continuity being de-stroyed (by massive silica redistribution) during dia-genesis. The proximity of iron-enriched ‘supergene’ore-bodies to many banded iron formations lends sup-port to this idea and any such remobilization hasimportance for the perceived mode of deposition. Thisactivity is well documented in the ore deposit minedat Mt Tom Price, in the western Pilbara, 500 km tothe north, with iron-enrichment up to 60% (cf. 20–40% for other Hamersley banded iron formations). Alarge volume of silica is removed and, run to comple-tion, this process would eventually result in the com-plete loss of all primary mineral phases.

The hydrothermal activity outlined by Trendallwould have to operate on a truly massive scale toaccount for repositories as large as the HamersleyBasin, and in all occurrences of banded iron forma-tions, to be considered a formative mechanism. Howcan silica mobility of this order occur during diagen-esis? Doubtless the crystalline species now presentreflect migration of metastable, probably hydrous,phases towards more stable assemblages, but the re-equilibration and recrystallization of higher-grademetamorphic processes are not involved here. Giventhe distribution of Archaean – early Proterozoicbanded iron formations within a variety of geologicalsettings, a tectonic role in their structure also seemsunlikely. The low-grade ‘shield’ deposits of the Ham-ersley, highly deformed examples from the Ukraineand the Grenvillian of Canada, and the greenstonecomplexes of the Transvaal and the Yilgarn all pro-duce typical banded iron formations.

This question of primary or secondary origin iscentral, but if bulk solution and redeposition has oc-curred in banded iron formations, it has conservedthe mineral texture on a granular level – i.e. it ismetasomatic, preserving microstructural andminerogenic differentiation – in a highly induratedrock type with no obvious solution pathways. Re-placement and recrystallization are common in Pre-cambrian rocks, but as a causal agent in band forma-tion are inconsistent with the textural detail.

Qualitatively, the Yilgarn banded iron formations arehard to distinguish from those of the Hamersley Ba-sin, and the latter are regarded as late diagenetic orlower greenschist facies at most. The conclusionseems to be that layering in banded iron formations isprimary (or at least synsedimentary) in origin, butthat there is only so much to say of palaeoenviron-ment on the basis of fine lamination and currentstratification, other than steady deposition fromsuspension.

The influence of life

The major episode of banded-iron production be-tween 3800 and 1700 million years ago indicateshighly stable environmental conditions throughout asubstantial period of early Earth history. The absenceof multicellular organisms during this time allowedthese deposits to be preserved, but microbial life mayhave played a direct role in their production, supply-ing oxygen to reduced iron species in the oceans.There is no physical (fossil) evidence for biologicalinvolvement in banded-iron genesis, but it is interest-ing to ask whether any process other than oxygenicphotosynthesis – e.g. the photolysis of water – couldhave supplied oxygen in the quantities necessary toaccount for sequences as great as the Hamersley Ba-sin. Indirect evidence of biogenic activity in bandediron formations exists within the Isua supracrustalbelt in Greenland, for which carbon isotopic data in-dicate the emergence of living systems in the Hadean,a period usually considered prebiotic. The disappear-ance of banded iron formations after about 1300 mil-lion years ago may indicate the point at which bio-logically produced oxygen exceeded the supply ofreduced compounds in the oceans, and mark thechange in the redox state of the Earth’s atmosphere.

Uniformitarianism

Banded iron formation is not alone in generating de-bate over conditions of formation or depositional en-vironment. Other temporally restricted lithologies –e.g. the Mesozoic ‘Plattenkalke’, source of the per-fectly preserved Archaeopteryx fossils – are similarlyobscure in their origins. In the study of geology,physical processes seen operating at the Earth’s sur-face today are taken to have operated in a similarmanner in the past. Our understanding of the forma-tion of the rock record relies on this ‘uniformitarian’approach. Where modern analogues are absent, thatrecord becomes much less clear. Whether derivedfrom a combination of sedimentary sources, or fromdiagenetic modification of an unknown precursor,banded iron formations, and their strongly reducingenvironment of formation, are unique to the Precam-brian. Can we expect familiar answers under such

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In his famous address to the Geological Society ofAmerica in 1957, H. H. Read concluded that ‘thereare granites and granites’. This is equally true forophiolites, slices of oceanic lithosphere producedby sea-floor spreading and preserved by obductionduring plate collision. Although they form insimilar ways, it is clear that there are differenttypes of ophiolite which originate under differentconditions. Compared to the ‘classic’ ophiolites ofOman, many, such as those in the Alps, lack asheeted dyke complex and were for a long timeconsidered abnormal. Analogues for this type havenow been found forming today and they occurwhen the rate of spreading is slow.

Excursion guide 14An excursion in the French–Italian Alps: the‘Atlantic’-type ophiolite at Mont Chenaillet

Ian WinterfloodGeology Department,

Arnold School, Blackpool

Fig. 1. The location of theMont Chenaillet areawithin the wider region.

circumstances – does uniformitarianism properly ap-ply here?

Suggestions for further reading

1 : 250 000 Geological Series – Explanatory Notes,Youanmi Western Australia Sheet SH/50–4 Interna-tional Index. Australian Government PublishingService, Canberra, 1983.

Schopf, J.W. (ed.) 1983. Earth’s Earliest Biosphere: ItsOrigin and Evolution. Princeton University Press,Princeton.

Mojzsis, S.J., Arrhenius, G., McKeegan, K.D.,Harrison, T.M., Nutman, A.P. & Friend, C.R.L.1996. Evidence of life on Earth before 3,800 mil-lion years ago, Nature, v.384, pp.55–59.

Trendall, A.F. 1983. The Hamersley Basin. In: Iron-Formation: Facts and Problems,. Developments in Pre-cambrian Geology 6 (Trendall, A. F. & Morris, R. C.,eds).

Two types of ophiolite are now recognized: (1) the‘Pacific’ type, where rapid spreading produces a ‘clas-sic’ ophiolite, and (2) the ‘Atlantic’ type, spreadingslowly, and lacking the dyke complex. A superbly pre-served and easily accessible example of the Atlantictype is found in the Alps at Mont Chenaillet, on theFrench–Italian border near Montgenèvre. Excellentfield guides (in French) and maps of the area have

been published and a way-marked geological trail es-tablished.

Ophiolites in the AlpsDespite the problems identified by Mason in GeologyToday 16 years ago, ophiolites are generally acceptedas fragments of oceanic lithosphere which have beenobducted (pushed up rather than being subducted,pushed down) during a collision at a destructive platemargin. When found in a mountain belt, they areevidence for the former presence of an ocean or atleast an ocean basin. A good example is the ophiolitic

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