Neoproterozoic banded iron formations

ISSN 0024-4902, Lithology and Mineral Resources, 2009, Vol. 44, No. 1, pp. 78–86. © Pleiades Publishing, Inc., 2009.Original Russian Text © A.V. Ilyin, 2009, published in Litologiya i Poleznye Iskopaemye, 2009, No. 1, pp. 87–95.

78

Ferruginous quartzites commonly identified in theEnglish literature as banded iron formation (BIF), themost significant iron ore formation, represent the rawmaterial base for ferrous metallurgy in many countries.It has long been noted that BIFs belong to deep hori-zons of the Precambrian. Accumulation of iron ores,like many other mineral resources, periodically pro-ceeded in the Earth’s history (Strakhov, 1947, 1949;Kholodov and Butuzova, 2004a, 2004b). Moreover, anincommensurably greater amount of Fe is concentratedin Precambrian BIFs than in Phanerozoic deposits.

BIFs have only been dated by the methods of abso-lute geochronology. Progress in this branch of geologyachieved at the latest 1990s–earliest 2000s, particularlyupgrading of the isotopic dating (Rb/Sr,

207

Pb/

208

Pb,

206

Pb/

238

U,

207

Pb/

235

U

, Re–O, Lu–Hf, and others), hasmade it possible to reliably establish the age of BIFs.The lower age limit for BIFs is determined on the basisof clastic zircon enclosed in the quartzites or adjacentrocks in the section; the upper limit, on the basis of zir-con, monazite, baddeleyite, and other minerals in intru-sions.

Two age families of the BIF are distinguished on theglobal scale based on numerous dates. The first family,which includes all deposits and iron ore basins (KrivoiRog, KMA, Hamersley, Lake Superior, and many oth-ers) that are well known and mined, spans a great inter-val from 3.2 to 1.89 Ga (Klein and Beukes, 1997.). Thesecond family, which was revealed lately, is muchsmaller but widespread on the Earth. This family is

clearly expressed in time and falls into the interval from0.85 to 0.7 Ga. According to the International Strati-graphic Scale (2004), the first family should be referredto the Archean–Paleoproterozoic; the second family, tothe Neoproterozoic, more exactly, to the Cryogenicperiod of the Neoproterozoic that lasted from 0.85 to0.63 Ga (Fig. 1). In addition to the time range of depos-its, basins, ore-bearing formations, the figure also illus-trates their scale, i. e., the amount of Fe hosted in themrelative to the Hamersley and KMA basins accepted asthe maximum. We also used data from (Klein and Beu-kes, 1993).

The age interval of the oldest BIF family can be con-siderably narrower than that mentioned above. Quartz-ites of Africa (Witwatersrand) and South America (SanFrancisco Craton) dated by the Rb/Sr method based onbulk samples (Klein and Beukes, 1993) are probablynot older than 2.8 Ga, and the Gun Flint BIF in the LakeSuperior province was dated by the U/Pb zirconmethod at

1898 ± 1

Ma (Young, 2002); i. e., the ageinterval is narrowed to 2.8–1.89 Ga. In any case, the“peak” of the oldest iron accumulation falls within theArchean–Proterozoic boundary.

It is worth mentioning that the Erzin BIF basin dis-covered in Tuva by A.V. Ilyin and V.M. Moralev (Ilyinand Moralev, 1956) was later traced into Mongolia. Thepresent paper is devoted to the memory of ValeriiMikhailovich Moralev.

Neoproterozoic Banded Iron Formations

A. V. Ilyin

Geological Institute, Russian Academy of Sciences, Pyzhevskii per. 7, Moscow, 119017 Russiae-mail: [email protected]

Received October 29, 2007

Abstract

—Two epochs of the formation of ferruginous quartzites—Archean–Paleoproterozoic (3.2–1.8 Ga)and Neoproterozoic (0.85–0.7 Ga)—are distinguished in the Precambrian. They are incommensurable in scale:the Paleoproterozoic Kursk Group of the Kursk Magnetic Anomaly (KMA) extends over 1500 km, whereas theextension of Neoproterozoic banded iron formations (BIF) beds does not exceed a few tens of kilometers. Theirthickness is up to 200 m and not more than 10 m, respectively. The oldest BIFs are located in old platforms,whereas Neoproterozoic BIFs are mainly confined to Phanerozoic orogenic (mobile) zones. NeoproterozoicBIFs universally associate with glacial deposits and their beds include glacial dropstones. In places, they under-lie tillites of the Laplandian (Marinoan) glaciation (635 Ma), but they are more often sandwiched between gla-ciogenic sequences of the Laplandian and preceding Sturtian or Rapitan glaciation (730–750 Ma). Neoprotero-zoic BIFs are rather diverse in terms of lithology due to variation in the grade of metamorphism from place toplace from low grades of the greenschist facies up to the granulite facies. Correspondingly, the ore componentis mainly represented by hematite or magnetite. The REE distribution and (Co + Ni + Cu) index suggest aninfluence of hydrothermal sources of Fe, although it was subordinate to the continental washout. Iron was accu-mulated in seawater during glaciations, whereas iron mineralization took place at the earliest stages of postgla-cial transgressions.

DOI:

10.1134/S0024490209010064

LITHOLOGY AND MINERAL RESOURCES

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NEOPROTEROZOIC BANDED IRON FORMATIONS 79

Following the ideas predominating half a centuryago, the Erzin iron ore basin was attributed to theArchean without a bit of doubt (Ilyin and Moralev,1958). Quite recently, however, rocks hosting BIFs ofthe basin have been dated by the

206

Pb/

238

U

zirconmethod (16 samples) at

767 ± 15

Ma (Kozakov et al.,2005; Salnikova et al., 2001). The Neoproterozoic ageof the Erzin BIF also follows from correlation withother regions (Jiafu et al., 1987; Klein and Beukes,1993; Young, 2002).

DISTRIBUTION OF BIFS

Neoproterozoic BIFs are widespread in all conti-nents. Figure 2 demonstrates only the formations reli-ably dated by different authors (Jiafu et al., 1987; Kleinand Beukes, 1993; Young, 2002, and others) at theNeoproterozoic based on either isotopic geochronologydata or the position in the stratigraphic section amongdefinitely Neoproterozoic rocks. Figure 2 does notshow many other BIF deposits, the age of whichremains unclear, e.g., deposits in the southeastern partof eastern Sayan (Butov, 1996), western Kazakhstan,central Mongolia, northern and southern ranges of theTien Shan located in Russia, Central Asia, and China,respectively (Bushinskii, 1966), northern India (LesserHimalayas), Scandinavia (the eastern boundary ofCaledonides), northwestern Africa (Taudeni Synec-lise), southern edge of the San Francisco Craton in Bra-zil (Dorr, 1973), and others. Within Russia, the majorityof Neoproterozoic BIF deposits are located in thesouthern Far East, e.g., the Bureya, Khangai, and Omo-lon massifs, as well as the Malyi Khingan region(Arkhipov, 2007). The age of BIFs in all regions listedabove is Neoproterozoic rather than Archean–Pale-oproterozoic.

In contrast to Archean–Paleoproterozoic BIFs,which are located exclusively within old platforms(Russian, North American, and others) and shields(Ukrainian, Baltic, Aldan, and others), Neoproterozoicquartzites occur in Phanerozoic mobile zones or oro-gens (Pan African, Brazilian–African, Sayan–Baikal,Damar, and others).

Neoproterozoic BIFs are widespread in the YangtzePlatform, which is referred to by Chinese tectonists as“flexible,” in contrast to the “rigid” North China Plat-form (Ilyin, 1983). The Yangtze Platform differs fromold platform structures by the absence of the Archeanand Paleoproterozoic basement. In essence, the formerstructure represents a vast Sinian carbonate platformwith abundant Sinian (Neoproterozoic)–Early Paleo-zoic carbonate rocks.

The Yangtze Platform is the largest reservoir ofNeoproterozoic BIFs that make up the northern andsouthern belts. The first belt, which is confined to thesouthern edge of the Jiannan Uplift, extends from theJiangxi Province to Hunan and further to the GuangxiProvince. The second belt is located along the north-

western edge of the Catasian Uplift and is traced fromthe Fujian to Guangdong Province. In terms of paleo-geography, the uplifts represented a lowland within avast marine basin of the platform (Jiafu et al., 1987).

542

630

850

1000

11

109

8

76

54

123

Eon Era SystemAge,

Ediacaran

Cryogenic

Tonian

Neo

prot

eroz

oic

Mes

opro

tero

zoic

Pale

opro

tero

zoic

Neo

arch

ean

Mes

oarc

hean

Pale

oarc

hean

Eoa

rche

an

Ar

ch

ea

nP

ro

te

ro

zo

ic

1600

2500

2800

3200

3600

Ma

Fig. 1.

Distribution of BIFs in geological time. (1–3)Neoproterozoic BIFs: (1) Erzin basin, (2) Rapitan Group,(3) Yangtze Platform basin; (4–11) Paleoproterozoic–Archean BIFs (basins): (4) Krivoi Rog, (5) Lake Superior,(6) Hamersley, (7) KMA, (8) Yilgarn, (9) Venezuela,(10) Zimbabwe, (11) Witwatersrand.

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The northern and southern belts extend over about1300 km. They enclose several major deposits underoperation, which represent the main source of iron oresin China. The BIF beds tens of meters thick extend overtens of kilometers (up to 60 km) and a substantial depthinterval.

In the considerably smaller Erzin basin, the Mugurdeposit has proven BIF reserves of approximately 300 Mtwith the cutoff Fe grade of 37%. The deposit enclosestwo contiguous BIF beds (~10 m thick) extending over60 km (Ilyin and Moralev, 1956). The basin is likely toextend southwestward beneath the sand cover of theBorig Del basin from Tuva to Mongolia, where BIFsare also found on the northern slopes of Khangai (Ilyinand Moralev, 1958). Thus, the Erzin basin extends overseveral hundreds of kilometers.

BIFs of the Rapitan Group (Klein and Beukes,1993) are located in the Mackenzie Mountains in north-western Canada (the Yukon Province and Northwestern

Territories). In terms of the area, thickness, and exten-sion of beds, they are similar to BIFs of the Erzin basin.

Other iron ore deposits, suites, and formationsshown in Fig. 2 are considerably smaller and poorlystudied.

LITHOSTRATIGRAPHIC CHARACTERISTICSOF THE NEOPROTEROZOIC BIF-HOSTING

ROCKS

Regions of Neoproterozoic BIF distribution arediverse in terms of the character of primary rocks,metamorphism grade, and rock succession in sections.All regions have the following common and significantfeatures: (1) the section includes one or two glaciogenicformations, as well as tillites, dropstones, and glacialmixtites among the BIFs; (2) a thick dolomitesequence, cap dolomite according to P. Hoffman (Hoff-man et al., 1997) and “crowning” dolomite according toN.M. Chumakov (Chumakov, 2004), is widespread

13

12

1110

9

1516

1718

6

7

8

19 20

3

4

5

1

2

Fig. 2.

Location of Neoproterozoic BIFs. (1) Tindir Formation (Alaska, United States); (2) Rapitan Group (Canada); (3) KingstonPeak Formation (California, United States); (4) Jacodigo Group (Brazil); (5) Urucun Group (Brazil); (6) Yara Group (Togo);(7, 8) Damara orogen (Namibia): (7) Chuos Group, (8) Damara Group; (9) Jazir deposit (Oman); (10) Yamata deposit (Buryat, Russia);(11) Buren manifestation (Tuva); (12) Erzin basin (Tuva–Mongolia); (13) Malyi Khingan ore-bearing formation (Russia); (14) LakeKhanka ore-bearing formation (Russia); (15–18) BIF of the Yangtze Platform: (15, 16) northern belt, (17, 18) southern belt; (19) Yud-namutana Group (Australia); (20) deposits of the eastern part of the Adelaide “geosyncline.”


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above the upper (youngest) glacial formation corre-sponding to the last Neoproterozoic (Laplandian) glaci-ation.

The rock section under consideration (Sinian rocks,according to Chinese geologists) was most thoroughlystudied with isotopic dating on the Yangtze Platform(Barfod et al., 2002; Yin et al., 2005, and others). Theyattracted interest as early as the latest 19th centurywhen old fossil glacial deposits (Nantuo tillites) werefound first by von Richthofen and later by two Ameri-can expeditions headed by G. Wallcott and V. Black-vailder discovered (Ilyin, 1983). The tillites are under-lain by sandstones and shales of the Liantuo Formation.The overlying section includes the following forma-tions (Fig. 3):

1. The Chongan Formation corresponding to theSturtian glaciation consists of varvites—varved claysof glacial lakes or “boulder clays” (according to LiSyguan)—with angular boulders (up to 20 cm across)characterized by “glacial” striation on the surface. Thethickness is 110 m.

2. The Fulu interglacial formation composed ofsandstones and shales hosting keratophyre lava andseveral BIF beds. Manganese ore beds occur at the top.

The sequence includes individual dropstones com-posed of different allogenic rocks drifted to the basinfloor from melting icebergs and pack ice. The quartziteshave a very sharp contact with the barren rocks. Rockmetamorphism corresponds to the greenschist facies.The thickness is 80 m.

3. The Nantuo Formation, which represents the mostintense Marinoan glaciation of the Neoproterozoic, issimilar in composition to the Chongan Formation. Thethickness is 100 m.

4. The Doushantuo Formation represents the typicalcap dolomite, according to P. Hoffman. The basal sec-tion is composed of a thick carbonate (dolomitic at thelower part and calcareous in the upper part) sequence.The upper part of the sequence is attributed to theLower Silurian (Ilyin, 1973) like the Tamda Group ofthe Malyi Karatau Ridge in southern Kazakhstan(Bushinskii, 1966). The formation occurs in all regionsabove the uppermost glacial sequence. Moreover, nei-ther dolomite nor limestone is found at lower horizonsof the Sinian section.

Data on the age of two lower formations in the sec-tion are scanty and contradictory. The lower glacial for-mation is estimated at 750 (Chumakov, 2004) or 720 Ma

P P P

P P P P P

Datingmethod

599584

207

Pb/Pb

208

Lu/Hf

Age

,M

a

Thi

ckne

ss,

m

Section

Form

atio

n

Era

Dou

shan

tuo

Nan

tuo

> 250

100

80

110

741

Fulu

Cho

ngan

Ne

op

ro

te

ro

zo

ic

1

3

5

7

2

4

6

8

Fig. 3.

Position of the Fulu BIF of the Yangtze Platform between two glacial formations (Nantuo and Chongan). (1) BIF; (2) phos-phorite; (3) basalt; (4) tillite and diamictite; (5) fluvioglacial sandstone and gravelstone; (6) varved clay; (7) sericite and chloriteschists; (8) dolomite (“cap dolomite”, after P. Hoffman (Hoffman et al., 1997) or “crowning dolomite,” after N.M. Chumakov (Chu-makov, 2004)).

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(Jiafu et al., 1987). BIFs of the Fulu Formation weredated at 741 Ma by the U/Pb method. Much more reli-able are the U/Pb zircon dates for tillites of the NantuoFormation (635.23

±

0.57 Ma, Condon et al., 2005) anddolomites of the Doushantuo Formation (628.3

±

0.54 Ma,Barford et al., 2002). The second date indicates the ini-tiation of postglacial transgression. Phosphorites of theWeng’an deposit (Guizhou Province) at the base ofDoushantuo dolomites were dated at 584.3

±

26 Ma bythe Lu/Hf method and 599.3

±

4.2 Ma by the U/Pbmethod (Barford et al., 2002). Tillites of the Ghaub For-mation (Namibia), which were correlated by P. Hoff-man with the Nantuo tillites, were found to be of iden-tical age (635.51

±

0.54 Ma, Hoffman et al., 2004). Til-lites of the Marinoan glaciation in the Ediacaranstratotype (southeastern Australia) exhibit the sameRe/Os age (635.1

±

1.0 Ma, Kendall et al., 2006). Thus,the BIFs under consideration are certainly older than635 Ma and they belong to the Cryogenic period of theNeoproterozoic.

In Canada, Rapitan BIFs occur between two glacialsequences and above upper tillites. Host rocks are rep-resented by glacial mixtites (Klein and Beukes, 1993)of sand and clay sizes enriched in gravelstones andboulder fragments. Volcanics and carbonate rocks arealmost absent and they appear only as cap dolomitesthat are not so widespread as in deposits of China, Mon-golia, Africa, North America, and South America. Aspointed out above, all rocks of the Rapitan group arecharacterized by moderate dislocations and greenschistfacies of metamorphism. Their U/Pb zircon dates rangefrom 755

±

14 to 730

±

17 Ma.The Erzin iron ore basin has the following specific

features:1. The section is composed of the lower metaterrig-

enous and upper carbonate (conformably overlying)complexes. A small stratigraphic hiatus is likely to sep-arate the complexes (Ilyin and Moralev, 1956). Terrig-enous rocks of the lower complex are metamorphosedup to the amphibolite–granulite facies.

2. The carbonaceous complex, which is widelydeveloped in southern Tuva and northern Mongolia,represents a typical “cap dolomite.” Its base includesone or two horizons of typical rocks referred to as “per-forated shales” (Ilyin, 1973)—stratified varved clays(varvites) of periglacial basins with angular fragmentsof different rocks delivered to the bottom from meltingicebergs. The fragments were loosely connected withthe lithified clays. Therefore, they drop out at present-day exposures and angular holes are left at their previ-ous sites.

3. In the Mugur BIF deposit area 200–300 m aboveglacial deposits, dolomites and black graphitic marblesinclude several horizons of organic-rich phosphoriteswith a small amount of phosphoric anhydrite (10–15%)(Yudin, 1965). In the eastern Lake Khubsugul regionnearly at the same stratigraphic level, cap dolomitescomprise phosphorites of the large Khubsugul basin

(Ilyin, 1973). BIFs of the Mugur deposit occur 100–200 m below glacial deposits.

4. BIFs occur in the uppermost part of the lowercomplex and are associated with graphitic shales and“red” muscovite schists with powdery pyrite dissemi-nation.

The absolute age was estimated at 767

±

15 Ma forBIFs and ~600 Ma for phosphorites. The first date wasobtained by the U/Pb zircon method (Salnikova et al.,2001) based on garnet–biotite plagiogneiss underlyingthe quartzites (Fig. 4a), whereas the second date wasbased on the

87

Sr/

86

Sr ratio in Khubsugul phosphorites.In other words, BIFs belong to the Cryogenic intervalof the Neoproterozoic, whereas phosphorites belong tothe Ediacaran.

Cooccurrence of phosphorites and NeoproterozoicBIFs and their closeness in the stratigraphic section arealso characteristic of the Malyi Khingan Ridge, Khan-kai region, Buryatia, and other regions (Bushinskii,1966).

Three isotopic dates are available for the Neoprot-erozoic BIFs—767 Ma (Tuva), 755 Ma (Canada), and741 Ma (southern China). All other regions are charac-terized by their close relation to glacial deposits. Judg-ing from the age of host rocks, most deposits and basinswere probably formed within the interval of 730–770 Ma,i.e., in the Cryogenic period of the Neoproterozoic.Further studies will likely refine these dates, but it isalready evident now that the BIFs are much older thancap dolomites (630 Ma) and Ediacaran phosphorites. Atthe same time, Neoproterozoic BIFs, phosphorites, andcap dolomites are cognate formations, because theirorigin and accumulation were related to the Neoprot-erozoic glaciation, the subsequent deglaciation, andpostglacial transgression. They are also marked by acommon domain.

CHARACTERISTICS OF BANDED IRON FORMATIONS

Unlike the Archean–Proterozoic BIFs, Neoprotero-zoic quartzites are much thinner and extended, weaklymetamorphosed in most cases, and distinguished byspecific geochemical criteria. The Archean–Protero-zoic BIFs exhibit a monotonous composition through-out the entire ore sequence. For instance, the sequencein the Hamersley basin is entirely composed of magne-tite quartzites with slight variations in the Fe contentand relative thickness of alternating magnetite andquartzite interbeds. The Neoproterozoic 10-m-thickBIF sequence is mainly composed of interbeds of strat-ified mixtites, sandstones, and shales. This feature isespecially typical of the Rapitan Formation. A similarcombination of barren and ore-bearing interbeds is alsotypical of deposits in China. The exception is providedby beds in the Erzin basin, where the 10-m-thick BIFbed is devoid of any other rocks (Fig. 4).


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In some regions of the Rapitan basin, ore interbedsare thicker than barren interbeds: the ore interbeds areup to 1 m thick, while the thickness of barren interbedsdoes not exceed 10 cm. An inverse relationship with alesser thickness of ore interbeds is observed in otherregions. Thus, the BIF undergoes considerable changesalong the strike, first of all, in the Fe content. Ore inter-beds include varved–stratified hematite lutites (Kleinand Beukes, 1993), which are composed of hematitecemented by quartz and nodular lutites with ovoidhematite nodules composed of hematite and orientedalong the bedding. Nodules of the millimeter dimen-sion often merge together. The interbeds also includesand- and gravel-sized dropstones (

n

mm to 2–3 cmthick) found universally along the strike.

BIFs of the Yangtze basin are similar in lithology tothe Canadian variety, but they differ by greater thick-ness and extension of beds. Moreover, they are charac-terized by a higher grade of metamorphism.

The Erzin BIF, which is localized in a tectonicallyactive zone of conjunction of the Precambrian Tuva–Mongolia Massif and Caledonian structures, differsfrom the BIFs in other regions by a higher grade ofmetamorphism (amphibolite facies). It was thoughtpreviously that their transformation was caused byinjection phenomena (Ilyin and Moralev, 1956). Theyexhibit a clear banded structure related to the alterna-tion of interbeds of magnetite and fine quartz grains.They are divided into coarse- and thin-banded varieties.In some places, the thin-banded varieties comprise

C C C

P P PC C C

P P P P P

P P P P P

C C C C

a

Pb/U age767 ± 15 Ma

Sang

ilen

Gro

upM

ugur

For

mat

ion

Tesk

hem

For

mat

ion

b

Section

Thi

ckne

ss

,m

1.60

4.00

4.00

2.00

1

2

3

4

5

6

7

8

9

10

11

12

13

14

Fig. 4.

(a) Position of BIFs in the Neoproterozoic section of the Erzin iron ore basin (Tuva–Mongolia); (b) detailed section of theupper bed in the Mugur BIF deposit. (1) Limestone and marble; (2) graphite schist; (3) phosphorite; (4) BIF; (5) garnet–biotite pla-gioclase gneiss; (6) amphibolite; (7) quartzite; (8) cordierite and staurolite gneiss; (9) thin horizons of BIF and barren quartzites atthe top of magnetite quartzite bed; (10–12) BIF: (10) massive, (11) thin-banded, (12) vaguely banded; (13) massive amphibole–magnetite quartzite; (14) tillite.

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cummingtonite and pass into amphibole–magnetiterocks. Laminae are up to 2 cm thick in the coarse-banded varieties and less than 0.7 mm thick in the thin-banded varieties. The average ratio of magnetite andquartz laminae is 1 : 1.

The density of quartzites in the Mugur deposit is3.53 g/cm

3

and the bulk Fe content is 39–40% (less than37%in some rocks). Thus, Neoproterozoic BIFs differlittle in terms of the Fe content from the Archean–Pro-terozoic varieties (table).

Manganese mineralization (braunite–hausmannite)associates with iron minerals in some regions. TheMnO content in deposits of the southern Far East ofRussia varies within 2–10% and never falls below 2%.Manganese deposits are also characteristic of the Cryo-genic period. The deposits are large in the Yangtze Plat-form (Condon et al., 2005), southeastern Australia(Kendall et al., 2006), and Namibia in western Africa(Hoffman et al., 2004). Association with manganesemineralization is not characteristic of NeoproterozoicBIFs.

The Neoproterozoic BIFs show the following fea-tures of trace element distribution:

1. High content of phosphorus (1.24%

ê

2

é

5

inTuva) compared to 0.2% in the Hamersley basin. The

ê

2

é

5

content in the Neoproterozoic BIFs of otherregions is approximately 1% or more.

2. The

Σ

REE content in hematite or magnetite of oreinterbeds is appreciably lower than in barren interbeds(12–13 and 100–120 ppm, respectively).

3. The Eu anomaly is either absent or poorlyexpressed.

4. Extremely low U concentration (0.05–0.06 ppm),which is considerably lower as compared to theArchean–Proterozoic BIFs in Krivoi Rog.

5. The (Co + Ni + Cu) index (Klein and Beukes,1993) calculated for the Rapitan BIFs is 140 ppm,which indicates an insignificant role of hydrothermalsources of Fe.

6. All rocks of ore horizons exhibit

δ

13

C

carb

valuesvarying from –3 to

–4

‰ PDB.

ORIGIN OF BIFS AND SOME FEATURESOF NEOPROTEROZOIC METALLOGENY

Neoproterozoic metallogeny, particularly of theEdiacaran and Cryogenic periods, was mostly deter-mined by glaciations. Some of them, for instance, theGreat Laplandian or Varanger glaciation, which is alsoknown as the Marinoan glaciation (635 Ma), was ofglobal character (Hoffman et al., 1997 and others). Atthat time, ice covered not only continents, but alsooceans. Thus, it isolated hydrosphere from atmosphere.Under such conditions, dissolved ferrous iron accumu-lated in seawater and precipitated when the ice thawed.In the course of thawing, huge ice masses gave rise topack ice and icebergs with fragments of different rocks,

which settled on the bottom (dropstones) and “contam-inated” the Fe-rich sediments. Alkalinity of WorldOcean water increased in the course of deglaciationowing to interaction between the carbon dioxide-richatmosphere and intensely weathered rocks of ice bedsexposed after thawing. This process promoted anintense influx of Ca, Na, K, and nutrients that providedthe bloom of biota. Iron was also present among ele-ments washed off from continents. In regions withNeoproterozoic rocks without volcanics (RapitanGroup), such a washout was probably the main sourceof Fe. This is suggested by the REE distribution and(Co + Ni + Cu) index in the BIFs (Klein and Beukes,1993).

The wide development of cap dolomites was relatedto increase in water alkalinity, acid rains, and intensifi-cation of reactions leading to the weathering of rocks inthe bed of thawed ice.

Formation of Neoproterozoic BIFs in the Cryogenicperiod was replaced by mass phosphogenesis in theEdicaran period. Thus, the Late Proterozoic is charac-terized by the successive generation of BIFs and phos-phorites.

Time difference between the two episodes was100 Ma in China and much less in Tuva and Mongolia.Some geochemical features of these rock categoriesindicate a certain similarity between them. Forinstance,

Σ

REE is 10–15 ppm in the Khubsugul phos-phorites and 10–12 ppm in the Rapitan BIFs (HREEsare scarce in both cases). Both phosphorites and BIFslack uranium. Thus, the Late Neoproterozoic stands outas a geological time boundary, which marked the onsetof large-scale genesis of phosphorites and served as ter-minal stage of large-scale accumulation of BIFs(Kholodov and Butuzova, 2001, 2004a, 2004b). In thehistorical–geological aspect, the boundary correspondsto termination of the Rodinia breakup and formation ofseveral oceans from a single Panthalassa, for instance,the Iapetus, the origin of which is dated at 608 Ma (Yinet al., 2005).

CONCLUSIONS

Two epochs of accumulation of BIFs are distin-guished in the Precambrian: Archean–Paleoproterozoicand Neoproterozoic. The first epoch showed up in dif-ferent old platforms as diachronous episodes andspanned the time interval from 3.2 to 1.89 Ga. The sec-ond epoch was short-lived and did not extend beyondthe Cryogenic period of the Neoproterozoic (0.8–0.63 Ma). It is likely that the first epoch spanned alesser time interval (from 2.8 to 1.89 Ga) and showedup more intensely than the second epoch. This wasrelated to oxygenation of the atmosphere, which pro-moted the transfer of Fe

+2

dissolved in water into Fe

+3

and, correspondingly, the precipitation of iron hydrox-ides. The second epoch was related to Neoproterozoicglaciations (more precisely, interglacial epochs) with


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intense washout from continents and transport of Fe tosea basins. Hydrothermal sources of Fe are also estab-lished for some basins. The oldest and NeoproterozoicBIFs are compositionally similar, except for a highP

2

O

5

content in the Neoproterozoic BIFs and certaindifferences in the content of trace elements. In bothcases, precipitation of Fe and accumulation of BIFsproceeded at the earliest stages of postglacial transgres-sions. The transgression caused by ice thawing in thecourse of the global Marinoan glaciation, the last glaci-ation in the Neoproterozoic, fostered the generation ofcap dolomites (628 Ma) at the base. Similar dates (600–550 Ma) were obtained for “old” phosphorites, whichaccompany the cap dolomites in all phosphate-bearingbasins and, thus, belong to the Ediacaran interval of theNeoproterozoic.

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Chemical composition of Neoproterozoic BIFs (%)

Sample no. SiO

2

TiO2 A12O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 H2O(–) H2O(+)

1 35.68 0.03 0.12 50.25 0.5 0.09 0.33 5.99 0.01 0.006 0.91 0.04 5.18

2 36.98 0.27 1.96 34.32 18.56 0.43 0.51 1.54 0.02 0.03 1.24 0.1 4.7

3 38.24 0.17 1.20 48.04 1.03 0.07 0.81 6.01 0.02 0.05 1.33 0.06 3.85

Note: (Sample 1) Snake River deposit, Rapitan Group, Canada (Klein and Beukes, 1993); (Sample 2) Mugur deposit, Tuva (Ilyin andMoralev, 1958); (Sample 3) Siadong deposit, Jiangxi Province, China (Jiafu et al., 1987).

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Neoproterozoic banded iron formations