geochemistry, petrology and origin of neoproterozoic

Precambrian Research 101 (2000) 49–67

Geochemistry, petrology and origin of Neoproterozoicironstones in the eastern part of the Adelaide Geosyncline,

South Australia

B.G. Lottermoser a,*, P.M. Ashley b

a School of Earth Sciences, James Cook Uni6ersity, P.O. Box 6811, Cairns, Qld 4870, Australiab Di6ision of Earth Sciences, Uni6ersity of New England, Armidale, NSW 2351, Australia

Received 31 March 1999; accepted 19 November 1999

Abstract

The eastern part of the Adelaide Geosyncline contains well preserved glaciomarine sequences of the Sturtianglaciation (:750–700 Ma) including calcareous or dolomitic siltstone, manganiferous siltstone, dolostone anddiamictite units and the associated Braemar ironstone facies. The ironstone facies occurs as matrix to diamictites andas massive to laminated ironstones and comprises abundant Fe oxides (hematite, magnetite) and quartz, minorsilicates (muscovite, chlorite, biotite, plagioclase, tourmaline), carbonate and apatite, and detrital mineral grains andlithic clasts. Micro-textures indicate that magnetite and hematite are of metamorphic origin. They are intergrown withsilicates and carbonates, with the mineral assemblage indicative of greenschist facies (biotite grade) metamorphism.Chemical compositions of ironstones vary greatly and reflect changes from silica-, alumina-poor ironstones formed bypredominantly chemical precipitation processes to silica-, alumina-rich examples with a significant detrital component.Silica-, alumina-poor ironstones are characterised by low concentrations of transition metals and large ion lithophileand high field strength elements and display REE signatures of modern coastal seawater. The Braemar faciesaccumulated in a marine basin along the border of a continental glaciated highland and a low-lying weatheredlandmass. Wet-based glaciers originated from the Palaeoproterozoic to Mesoproterozoic metamorphic basement anddebouched into a fault-controlled depocentre, the Baratta Trough. The intimate association of dolostones, mangani-ferous siltstones, ironstones and diamictites can be explained by a transgressive event during a postglacial period.Hydrothermal exhalations added significant amounts of Fe and other metals to Neoproterozoic seawater. Melting offloating ice led to an influx of clastic detritus and deposition of glaciomarine sediments from wet-based glaciers andto oxygenation of ferriferous (9manganiferous), carbonate and CO2 charged coastal waters. Release of CO2 to theatmosphere from the oxygenated waters resulted in the precipitation of carbonate as dolostones and oxygenation offerriferous (9manganiferous) waters led to the precipitation of Fe3+ oxides as laminated ironstones and as matrixof diamictic ironstones. Further increases in Eh conditions led to the precipitation of Mn oxides or carbonates andtheir incorporation in clastic sediments. Thus the Braemar ironstone facies is the result of chemical precipitation ofdissolved Fe (and Mn) during a postglacial, transgressive period and formed in a near-coastal environment under

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B.G. Lottermoser, P.M. Ashley / Precambrian Research 101 (2000) 49–6750

significant terrestrial influences. © 2000 Elsevier Science B.V. All rights reserved.

Keywords: Ironstones; Geochemistry; Glaciation; Adelaide Geosyncline; Neoproterozoic; South Australia

1. Introduction

Glaciogenic and iron-rich rocks are intimatelyassociated in Neoproterozoic sequences. Examplesare known from North and South America (e.g.Young, 1976; Yeo, 1983; Urban et al., 1992; Kleinand Beukes, 1993; Graf et al., 1994), Africa (e.g.Breitkopf, 1986; Buhn et al., 1992), China (Ruiand Piper, 1997), and South Australia (Whitten,1970). In some cases, the iron-rich rocks arepresent as iron-rich clastic sediments (e.g. SouthAustralia; Whitten, 1970; China; Rui and Piper,1997), whereas others occur as ironstones (e.g.Klein and Beukes, 1993). However, despite nu-merous research efforts, the reason for the com-mon association of iron- and carbonate-rich andglaciogenic rocks in the Neoproterozoic, the ap-parent glaciation in low-latitude environmentsduring that time, and the source of chemicalcomponents and genesis of ironstones remaincontroversial.

Neoproterozoic sedimentary rocks of the Ade-laide Geosyncline in South Australia and far west-ern New South Wales host well preservedglaciomarine sequences and associated ferrugi-nous units (Preiss, 1987; Preiss et al., 1993; Fig.1). These ferruginous rocks are rich in magnetiteand/or hematite, iron-bearing silicates and car-bonates. The purpose of this paper is to describethe geochemical composition of Braemar iron-stones, to establish the genetic processes responsi-ble for their formation and thepalaeoenvironment of deposition, and to discusstheir genesis in light of other models forNeoproterozoic glaciogenic and iron-rich rockoccurrences.

2. Geology

The Adelaide Geosyncline in South Australia isa major, deeply subsident Neoproterozoic toCambrian sedimentary basin which overlies

Palaeoproterozoic to Mesoproterozoic metamor-phic basement rocks. It contains one of the mostcomplete and well preserved Neoproterozoic suc-cessions and displays evidence of two major glaci-ations during the Neoproterozoic, the Sturtian(:750–700 Ma) and Marinoan glaciation (:650–600 Ma; Preiss, 1987; Preiss et al., 1993). Thewidespread Sturtian glaciation event is manifest inthe Umberatana Group (Preiss et al., 1998), andparticularly in the great thicknesses ofglaciomarine sedimentary rocks deposited in thefault-controlled Baratta Trough, extending fromthe central Flinders Ranges to the Yunta-Olaryregion in eastern South Australia (Sumartojo andGostin, 1976; Preiss, 1987; Preiss et al., 1993; Fig.1). Much of the glaciogenic sedimentation in theUmberatana Group is characterised by diamictite,laminated siltstone and orthoquartzite, but inplaces there are distinctive intercalated dolomiticand ferruginous units (Preiss, 1987; Preiss et al.,1993). In this paper, diamictite is a nongeneticterm referring to poorly sorted siliciclastic sedi-mentary rocks containing a wide range of classsizes in an abundant fine-grained matrix in whichthe clasts are dispersed so that most of them arenot in contact (cf. Panahi and Young, 1997).

The section in the Barratta Trough comprisesthe diamictite- and quartzite-dominated PualcoTillite (3300 m) and overlying siltstone- and sand-stone-dominated Benda Siltstone (260 m; Preiss etal., 1993). Both units pass laterally into the lentic-ular, ferruginous Braemar ironstone facies in theYunta-Olary region and the thinner HolowilenaIronstone (130 m) in the central Flinders Ranges(Preiss, 1987; Preiss et al., 1993). In places, thePualco Tillite passes vertically into the Braemarironstone facies (also known as the Hoof HeartedFormation). The latter, although locally lenticu-lar, is widespread in the Yunta-Olary region and apossible equivalent is present southwest of BrokenHill in western New South Wales (Preiss, 1987).In the Yunta area, four to six lenticular ironstoneunits grade into the host diamictites and siltstones

B.G. Lottermoser, P.M. Ashley / Precambrian Research 101 (2000) 49–67 51

with decreasing iron minerals, but also containdolostone beds and quartzites (Whitten, 1970;Figs. 2 and 3). Throughout the Yunta-Olary re-gion, ironstone occurrences crop out prominently(Fig. 4A) and are interbedded with diamictites,carbonate-rich rocks, quartzites (in part withheavy mineral lamination), siltstones and man-ganiferous siltstones. The distribution of the iron-stone and associated ferruginous siltstones anddiamictites is especially notable on aeromagnetic

images. The ironstones are particularly prominentat Razorback Ridge south of Yunta (Fig. 2),where the thickest iron-rich sub-units have beenevaluated as a potential iron ore resource (Whit-ten, 1970). Although at some locations, there isonly one prominently ferruginous horizon, atmany locations, there are several zones (e.g. 3 or4), separated by tens to hundreds of metres ofother strata, e.g. in the Bimbowrie Hill region andat Razorback Ridge (Fig. 3).

Fig. 1. Location of the Adelaide Geosyncline in South Australia showing the inferred distribution of Sturtian ferruginous facies ofthe Umberatana Group in the Baratta Trough (modified from Preiss et al., 1993).


Fig. 2. Outcrop of ferruginous facies (Braemar ironstone facies) and associated Sturtian glaciogenic rocks (Pualco Tillite) in theeastern part of the Adelaide Geosyncline (modified from Rogers, 1978; Forbes, 1991).

3. Sampling and methods of analysis

Thirty-nine samples (BR1–12, BR29–47,BR49–56) were taken from surface outcrop andincluded laminated and diamictic ironstones, silt-stones, diamictites and carbonate-rich rockswhich were representative of the rock types and

occurrences in the Olary-Yunta region. In addi-tion, 17 samples (BR13–28, BR48) were collectedfrom the exploration adit dump at RazorbackRidge. Thin and polished thin sections and blockswere prepared and subsequently investigated byoptical microscopy. Twenty-seven laminated iron-stone, clastic and carbonate sediment samples


were crushed and pulverised in a chrome steel ringmill. Major and trace elements were analysed byX-ray fluorescence on duplicate fused discs andpressed powder pellets at the Division of EarthSciences, University of New England (UNE). Se-lected rare earth elements (REE, La to Lu;LREE: light REE, La to Sm; HREE: heavy REE,Tb to Lu) and additional elements (As, Au, Hf,Sb, Sc, Ta, Th, U, W) were determined on ninesamples by instrumental thermal neutron activa-tion analysis at Becquerel Laboratories, Sydney.REE concentrations exceeded the detection limitsby several orders of magnitude. In addition, dataon geochemical reference materials were within10% of the accepted values. Oxygen and carbon

isotope mass spectrometry on 15 rock sampleswas conducted at the Centre for Isotope Studies,CSIRO, Sydney, following conventional CO2 gen-eration using phosphoric acid. Electron mi-croprobe analyses were performed on garnets,carbonates and chlorites of ironstone and siltstonesamples at UNE.

4. Petrography and mineralogy

The Braemar ironstone facies consists of lentic-ular laminated and diamictic ironstones interbed-ded in calcareous or dolomitic siltstone includingseveral thin quartzite and dolostone units (Fig. 3).

Fig. 3. Stratigraphic succession of the Adelaide Geosyncline in the southeastern Nackara Arc (a, b) (modified from Preiss, 1987;Preiss et al., 1998) and (c) Braemar ironstone facies in the Razorback Ridge area (modified from Whitten, 1970).


Fig. 4. (A) Typical outcrop of the Braemar ironstone facies. Ironstone is intercalated with carbonate-bearing siltstone and minordiamictite and dolostone. Near Bimbowrie Hill, AMG: 420 700 mE, 6 455 650 mN. (B) Diamictic ironstone with recrystallisedcarbonate-rich siltstone, quartz and carbonate clasts (sample BR24). Razorback Ridge, AMG: 379 740 mE, 6 352 770 mN. (C)Laminated ironstone. Darker laminae are rich in magnetite and hematite, and lighter laminae in siliciclastic and carbonatecomponents (sample BR6). Field of view approximately 30 mm long, note scale bar in millimetres. Iron Peak, AMG: 384 100 mE,6 353 900 mN. (D) Laminated ironstone with interbedded lighter coloured siltstone displaying cross-laminations and soft-sedimentdeformation (sample BR28). Razorback Ridge, AMG: 379 740 mE, 6 352 770 mN. (E) Laminated ironstone with interbedded lightercoloured siltstone displaying soft-sediment deformation (sample BR16). Razorback Ridge, AMG: 379 740 mE, 6 352 770 mN.

The Braemar ironstone facies is made up of twotypes, diamictic and laminated ironstones, whichare substantially different in macroscopic appear-ance (Fig. 4B–E) but apart from the clasts, identi-

cal compositionally.The mineralogy of the laminated ironstones and

the matrix of diamictic ironstones is simple: com-mon facies are fine-grained (typically B0.05 mm)


and composed of magnetite, hematite and quartzwith minor muscovite, chlorite, biotite, carbonate,

apatite, plagioclase and tourmaline. Associatedsiltstones contain abundant quartz, biotite, car-

Fig. 4. (Continued)


Fig. 4. (Continued)

bonate, plagioclase, muscovite, chlorite, variableamounts of magnetite and hematite and traces ofclinozoisite/epidote, tourmaline, zircon and pyrite(altered to goethite due to supergene oxidation).Detrital mineral grains and lithic clasts occur inlaminated and diamictic ironstones and siltstones.They are angular to subrounded and include scat-tered detrital grains of quartz, carbonate, plagio-clase, K-feldspar, muscovite and tourmaline,foliated sediments, siltstones, quartzites, andquartzofeldspathic and quartz-carbonate rocks.Detrital feldspars have been variably replaced bycarbonate, muscovite and traces of chlorite andbiotite, and biotite has been retrogressed to chlor-ite and traces of rutile.

Diamictic ironstones are massive and clastsrange in size from 10 mm to 1.2 m but are mostcommonly between 25 mm and 150 cm (Fig. 4B).A few striated boulders were noted by Whitten(1970). Angularity and nature of the detritalgrains and lithic clasts are similar to those foundin the associated laminated ironstones andsiltstones.

Laminated ironstones are usually inequigranu-lar with grain sizes ranging from B0.1 to 5 mm.Lamination is generally well developed and rangesfrom B0.5 mm to 1 cm in thickness (Fig. 4C–E).The laminae are defined by the relative abundance

of magnetite and hematite, ranging from :80 to:20%. Magnetite grains display varying degreesof martitization and larger subhedra are up to 0.1mm in diameter. Rare pressure shadows of chlor-ite and/or biotite are well developed adjacent tomagnetite–hematite porphyroblasts. However,much hematite is not weathering-related, becausegrains display a preferred orientation oblique tocompositional laminations, and late dilationalveins contain magnetite, quartz, carbonate,goethite and platy hematite. Therefore hematiteis, in part, syn- or pre-tectonic. Locally, nearlypure layers of Fe-oxides (:80%) are present,with magnetite, hematite and quartz forming ametamorphic granoblastic aggregate.

There are large variations in modal proportionsof the major rock-forming minerals quartz, Feoxides, carbonate and silicates within single rockspecimens thereby forming fine layers of iron-stones and siltstones. Fe oxide-rich laminae dis-play sharp or gradational bases with associatedsiltstone layers. The Fe-oxide beds are commonlygraded with magnetite decreasing and abundancesof quartz and silicates increasing. Other sedimen-tary structures include cross-laminations (Fig.4D), microfaulting and piercement of laminae,and micro- to meso-scale folding of laminae (Fig.


4D–E) which may be the result of soft-sedimentdeformation.

The Braemar ironstone facies has undergoneregional metamorphism and deformation. Therocks display interlocking aggregates of mineralgrains and rare porphyroblastic Fe oxide andcarbonate grains. Slaty cleavage is commonly

defined by the preferred orientation of layer sili-cates and hematite plates. The subhedral shape ofthe magnetite crystals, the presence of rare por-phyroblastic magnetite grains, together with theoccurrence of magnetite/hematite-bearing veinsand foliated hematite, indicate that the magnetiteand some hematite are of metamorphic originand not detrital. The Fe oxides are intergrownwith silicates and carbonates, with the mineralassemblages indicative of greenschist facies (bi-otite grade) metamorphism.

Carbonates in the ironstones and associatedferruginous siltstones are ferroan dolom-ite (Fe0.01–0.10Mn0.00–0.03Ca0.48–0.53Mg0.37–0.46CO3)and ferroan calcite (Fe0.01–0.06Mn0.00–0.01Ca0.92–

0.99Mg0.00–0.02CO3) in composition and chlorite istypically ripidolite (Si 2.61–2.73 atoms per for-mula unit and atomic Fe/Fe+Mg, 0.27–0.63).Calculations using chlorite compositions on theAl(IV)–T plot of Cathelineau (1988) indicatechlorite growth at :360–400°C. In the Bim-bowrie Hill region (Fig. 2), the Braemar iron-stone facies is associated with manganiferoussiltstone units :1 m thick. These are composedof variable amounts of fine-grained (B0.05mm)granoblastic carbonate, garnet, magnetite, quartz,plagioclase, muscovite and phlogopite (Holm,1995). Garnet is typically spessartine (py2.6–3.2

alm4.2–9.0spess82.l–87.2gross1.4–2.2uvar0–0.1a-ndra3.5–

11.4) in composition, with carbonates includingcalcite, ankerite and manganoan magnesian sider-ite.

5. Geochemistry

5.1. Major and trace elements

The major oxide components of the laminatedironstones are SiO2 and Fe2O3. All ironstonesconsist of \70 wt.% SiO2+Fe2O3 (all Fe asFe3+) with Fe2O3 ranging between 22.94 and78.91 wt.% (N=20) (Table 1 and Fig. 5). Minorelement contents of the ironstones show somevariations, with Al2O3 ranging from 0.28 to 10.64wt.%, CaO from 0.10 to 5.82 wt.%, K2O from0.03 to 3.43 wt.%, MgO from 0.02 to 3.76 wt.%,Na2O from 0.10 to 3.11 wt.% and LOI from 0.20

Fig. 5. Ternary plot of (a) Si�Fe�Al, (b) Si�Fe�(Ca+Mg),and (c) Al�(Ca+Mg) (Na+K) for ironstones (�; N : 20) andclastic sediments (; N : 6).

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Table 1Representative geochemical analyses of Mn-rich sediment (sample R74203; Holm, 1995), dolostone (sample BR30), siltstones (samples BR38, BR45), aluminousironstones (samples BR15, BR36), and ironstones (samples BR8, BR13, BR40, BR52, BR53)a

BR45 BR15 BR36 BR8 BR13 BR40 BR52 BR53BR30Sample BR38R74203ca-qz-pl-Mineralogy Qz-pl-Kfs-qz-bio-ca- Mt-hm-qz-Feox-qz-ca- Feox-qz-ca-Mt-hm-go-Feox-qz-ca- Mt-hm-qz-Mt-hm-qz-

ca-chl/biochl-bio-cabio-ms-tm- bio-qz-capl-ms-chl- pl-chl-bio-lithic clasts- chl-bio-cachl-ms-pl-lithic clastsap-tm goap-plgo-tm-zir bio-mt-ca-m

s-chlMt Mulga Razorback BraemarBimbowrieOultalpa Iron Peak RazorbackRazorback OultalpaBimbowrieLocation Braemar

NW NWS RidgeRidge Hillridge6 454 240 6 353 900Northing 6 352 7706 459 900 6 440 620 6 326 240 6 325 1506 352 610 6 440 620 6 443 250 6 352 770422 790 384 100 379 740Easting 411 760421 800 371 820 371 820379 090 411 760 433 700 379 740

47.86 40.54 28.12 28.68 14.8118.58 33.2965.65 23.5467.1340.10SiO2

TiO2 0.68 0.16 0.83 0.70 0.57 0.48 0.260.22 0.370.33 0.18Al2O3 10.19 3.16 11.92 10.06 7.82 6.46 2.92 3.97 2.742.80 3.16Fe2O3 7.28 4.96 5.08 13.50 25.52 37.20 66.77 66.2062.34 49.7678.91MnO 6.29 0.36 0.13 0.04 0.15 0.30 0.04 0.06 0.230.11 0.18MgO 2.80 2.2213.26 2.023.15 1.281.56 0.883.76 3.27 1.59CaO 14.12 22.94 3.34 1.63 3.68 3.49 0.28 0.64 1.980.33 3.86Na2O 2.44 1.661.65 0.112.24 1.323.49 0.050.26 1.29 0.80K2O 2.19 0.15 2.60 1.82 2.55 1.77 0.240.82 0.190.32 1.37P2O5 0.17 0.02 0.19 0.37 0.46 0.68 0.23 0.24 0.940.15 0.64S 0.01 0.020.00 0.010.01 0.000.01 0.010.01 0.01 0.03

5.16LOI 12.57 34.49 2.63 1.20 6.43 3.99 0.20 0.49 0.20 2.11Total 98.80 99.73 100.6799.26 100.04 99.06 99.49 100.54 99.66 101.00 100.53

As 2.5na B0.5 1.8 B0.5 3.9 10.4na B0.5 na 1.1na B5 B5 B5 B5 B5 B5 B5Au na na B5

Ba 271353 145 43 201 61 18029 527 530 44444 24 20 29 3014 279Cu 13188

1416 10 13 10 10 8 8B1 15 15Ga2.7 1.6 2 0.7 1.9 1.1Hf na na 8.2 na 3.89 3 8 6 67 415Nb 16132

1756 19 5 6 B5 7 144 16 14Ni8 7 10 6 9Pb B512 3 17 13 7

101 37 16 96 5109 9115Rb 1081235B0.2na 0.4 B0.2 0.5 0.6 0.7 2.2na 0.4 naSb

10.4 5 5.3 5.2 6.3 8.9Sc na na 12.5 na 12146 24 33 7 56104 7781 91Sr 152 525

0.9na 0.9 B0.5 0.8 B0.5 0.7 B0.5na 1.3 naTa8.9 4.2 6.5 2.4 6.3 4.7Th na na 16 na 9.6

c1 B1 B1 B1 B1B1 B1naU na na2.88776 60 80 88 85 52 1132 54 56V

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Table 1 (Continued)

BR36 BR8 BR13 BR40Sample BR52R74203 BR53BR30 BR38 BR45 BR15Feox-qz-ca-Qz-pl-Kfs- Feox-qz-ca-ca-qz-pl- qz-bio-ca-Mineralogy Mt-hm-qz-Mt-hm-qz- Mt-hm-qz- Mt-hm-go- Feox-qz-ca-

pl-ms-chl- chl-bio-calithic clasts chl-bio-cabio-ms-tm- bio-qz-cachl-ms-pl- pl-chl-bio- ca-chl/biolithic clasts-ap-tmgo-tm-zir gobio-mt-ca-m ap-pl

s-chlMt MulgaOultalpa RazorbackRazorback BimbowrieBimbowrieLocation Iron Peak Razorback BraemarOultalpa Braemar

RidgeNW NWridge HillS Ridge6 454 240 6 353 900 6 352 770 6 440 620 6 326 240 6 325 150Northing 6 459 900 6 352 610 6 440 620 6 443 250 6 352 770422 790 384 100 379 740 411 760 371 820 371 820379 740Easting 421 800 433 700411 760379 090

B1 1.3 B1 B1 B1W 9.7na na B1 na B133 28 26 13 3124 49Y 33271525

61 42 52 33 38 101 17 3720 46Zn 3192 51 73 26 65130 40157Zr 14226638

21.8na 22.9 10.6 5.41 6.21 13.4 15.2na 33.2 naLana 43.4 49.1 22.9 11.4 13.4 29.7 33.8Ce nana 69.9

29.6 12.4 7.12 7.59 15.422.9 19.9naNd na na 33.75.71 2.52 1.73 1.74 3.03Sm 4.76na na 6.54 na 4.741.37 0.62 0.44 0.56 0.890.99 1.23Eu na1.5nana

0.75na 1.11 0.62 0.45 0.37 0.66 1.06na 1.06 naTb1.46 0.96 0.75 0.53 0.951.03 1.54naHo na1.38na

2.62na 3.45 2.60 2.39 1.27 2.82 4.04na 3.14 naYbna 0.41 0.46 0.35 0.36 0.17 0.40 0.58Lu nana 0.38

2.52 2.65 1.97 2.24 2.78 2.013.19 2.89(La/Sm)cn

5.52 5.17 3.15 1.56 3.79 1.12 2.729.08(La/Lu)cn

1.64 1.20(Tb/Lu)cn 0.85 1.48 3.48 1.241.89 1.24115.16 53.57 30.05 31.84 67.25 82.1198.64150.8�REE

a Major elements given in wt%, trace elements in ppm, Au in ppb. Abbreviations: na, not analysed; ap, apatite; bio, biotite; ca, carbonate; chl, chlorite; Feox, Feoxides, i.e. magnetite and/or hematite; go, goethite; hm, hematite; Kfs, K-feldspar; mt, magnetite; ms, muscovite; pl, plagioclase; qz, quartz; tm, tourmaline; zir, zircon.Reference to sample locations is given in northings (N) and eastings (E) of the Australian Mapping Grid (AMG). Sample numbers refer to samples stored in theDivision of Earth Sciences, University of New England. Chondrite normalised ratios (La/Sm)cn, (La/Lu)cn, and (Tb/Lu)cn are calculated using chondrite values givenby Boynton (1984).


to 7.4 wt.%. Such variations reflect differentmodal contents of magnetite and/or hematite,quartz, plagioclase, carbonate, biotite, chloriteand muscovite in the analysed samples.

Ironstones with higher Si contents tend to havehigher A1 and Ca+Mg values (Fig. 5a, b). Theseelement trends indicate the addition of plagioclaseand carbonate. The associated clastic sedimentshave lower Fe and higher Si, Al and Ca+Mgvalues, and similar Na+K contents compared tothe ironstones (Fig. 5a–c). There is also variationin the Na+K content, reflecting the abundanceof biotite, chlorite and muscovite in both iron-stones and clastic sediments (Fig. 5c).

Clastic-dominated sediments have lower Fe2O3,P2O5 and V contents than the ironstones andAl2O3, TiO2, Na2O, K2O, Hf, LREE, Nb, Pb, Sc,Ta, Th, U and Zr are somewhat more abundant(samples BR38, BR45; Table 1). Such increasedelement concentrations compared to the associ-ated ironstones are due to more plagioclase, K-feldspar, biotite, muscovite, chlorite, and lithicclasts within the analysed samples. The siltstonesand sandstones have trace element abundancessimilar to the average upper crust with the excep-tion of lower Nb, Zr, Ba and Sr values (cf. Taylorand McLennan, 1981). The compositions of man-ganiferous siltstones are quite similar to the clasticsediments, except for higher MnO, Ni, V and Zncontents (sample R74203, Table 1). The analysisof a dolostone indicates that clastic sedimentsexhibit higher trace element contents with theexception of lower Sr values (sample BR30, Table1).

Trace element constituents of the ironstonesshow large scale variations and appear to belargely dependent on the type and quantity of theminerals present. The ironstones are depleted inmost transition metals (Sc, V), high field strengthelements (Nb, U, Th, Zr, Hf, Pb, LREE), andlarge ion lithophile elements (Ba, Sr, Rb) whencompared to the average upper continental crust(cf. Taylor and McLennan, 1981). Only the Ni, Yand HREE concentrations are similar to averageupper crustal abundances. Such low trace elementconcentrations could either reflect their removalduring metamorphism, which is most unlikely asmany of these elements are regarded as immobile,

or may have important implications for thesource(s) of these elements and the depositionalenvironment of the Braemar facies.

For the Braemar ironstones, a correlation ma-trix of log-transformed data (N=20) shows thatthere are significant positive correlations (r\+0.6) of Al with Ti, Ca, Mg, K, Ga, Hf, Rb, Sc,Ta, Th and Zr, and of Si with Ti, Ca, Hf, Sc, Sr,Ta, Th, Zr, La, Ce and the �REE content. Thesecorrelations reflect increasing sedimentary inputsof siliciclastic material to chemical sediments (cf.Ewers and Morris, 1981; Klein and Beukes, 1993;Manikyamba and Naqvi, 1995). In contrast, Feexhibits weak positive correlations with few ele-ments, including As, Cu, Sb, V and Zn (r= +0.4– +0.6) pointing to a hydrothermal source ofthese metals. Thus the chemical compositions ofBraemar ironstones reflect variations from iron-stones formed by predominantly chemical precipi-tation processes to examples with a significantdetrital component.

5.2. Rare earth elements

Laminated ironstones possess �REE concentra-tions (�REE: La+Ce+Nd+Sm+Eu+Tb+Ho+Yb+Lu) ranging from 30.05 to 115.16 ppmand chondrite normalised (La/Sm)cn ratios of1.97–2.89, (La/Lu)cn ratios of 1.56–6.23, and (Tb/Lu)cn ratios of 0.85–1.73 (Table 1). Fig. 6 illus-trates the REE patterns of Braemar ironstonesnormalised to the North American Shale Com-posite (NASC; Gromet et al., 1984). All iron-stones display REE patterns with variable LREEdepletions, modest negative Ce anomalies and noEu anomalies with the exception of sample BR40,which exhibits a distinctly positive Eu anomaly.

A correlation matrix of log-transformed iron-stone data (siliceous, aluminous and silica-, alu-mina-poor laminated ironstones; N=9) revealsthat correlations of REE with most elements areinsignificant (rB+0.7). However, La, Ce and the�REE content show correlations with Si (+0.6)and all REE and also the �REE content showslight positive correlations with Mn (+0.4– +0.8), P (+0.6– +0.8), Ca (+0.5– +0.6), Ba (+0.1– +0.6), Sc (+0.5– +0.8), Sr (+0.7– +0.8),Th (+0.4– +0.6) and Y (+0.5– +0.9). Correla-


Fig. 6. NASC normalised REE patterns for (a) silica-, alu-mina-poor ironstones (BR8, BR13, BR25, BR35, BR40, BR52,BR53), and (b) siliceous, aluminous ironstones (BR15, BR36),and clastic sediment (BR38). NASC values taken from Grometet al. (1984).

BR13, BR35, BR40, BR52, BR53; Table 1). Suchsamples are moderately depleted in LREE andvariably depleted or enriched in HREE comparedto the NASC (Fig. 6a). Siliceous, aluminous iron-stones display REE patterns only very slightlydepleted in LREE compared to the NASC (Fig.6b). They also have �REE contents and (La/Sm)cn, (La/Lu)cn and (Tb/Lu)cn ratios similar tothe clastic sediment sample BR38 (Table 1 andFig. 6b). The clastic sediment BR38 shows arelatively flat REE pattern, nearly identical tothat of the NASC.

The strong similarities of the REE patterns ofthe Braemar siltstone and siliceous, aluminousironstones with the NASC REE distribution isconsistent with these sediments gaining their REEfrom detrital sources. However, silica-, alumina-poor ironstones display a different REE geochem-istry indicating that the REE were gained duringchemical precipitation.

5.3. Carbon and oxygen isotopes

Sheet-like dolostones are commonly associatedwith Neoproterozoic glaciogenic rocks (Kennedy,1996; Hoffman et al., 1998) and such dolostonescap the Braemar facies (Fig. 3). In addition, car-bonate occurs as ferroan dolomite and ferroancalcite within siltstones and ironstones. Sedimen-tological and stable isotope data of Adelaideandolostones have been interpreted to reflect apalaeoenvironment whereby carbonate sedimenta-tion occurred during a postglacial marine trans-gression in deep waters (below storm wave base;Kennedy, 1996).

Ironstone and siltstone samples for stable iso-tope analyses were selected from several siteswithin the Yunta-Olary region and 15 sampleswere analysed (Table 2). The Braemar facies hasundergone diagenesis and metamorphism and theobserved carbonate within these rocks has clearlyrecrystallised during metamorphism. However,dolostones are an integral part of the sedimentarysequence and there is no petrographic evidencefor major carbonate mobilisation or veining, andtherefore the carbonate within the Braemar faciesis regarded as sedimentary in origin.

tions of REE with A1 (+0.1– +0.4), Ti (+0.2–+0.4) and Fe (−0.3– −0.5) are much lower.Such element correlations suggest that the REEwithin the ironstones are largely incorporated intoaccessory apatite and carbonate.

The ironstones have been subdivided accordingto their SiO2 and A12O3 contents and individualREE distributions into two different suites. Iron-stones are here called siliceous, aluminous ifSiO2\40 wt.% and A12O3\6 wt.% (samplesBR15, BR36; Table 1) and silica-, alumina-poor ifSiO2B40 wt.% and A12O3B6 wt.% (samplesBR8, BR13, BR35, BR40, BR52, BR53; Table 1).Samples from the same locality can have differentSiO2 and A12O3 contents and REE distributions.Silica-, alumina-poor ironstones have the lowest�REE concentrations and the lowest (La/Sm)cn,(La/LU)cn and (Tb/LU)cn ratios (samples BR8,


Carbon isotope values vary greatly (d13CPDB−5.5– +0.9‰), however, d13CPDB isotopic signa-tures are nearly all negative, whereas oxygenisotope values range from d18OSMOW+10.6 to+29.5‰ (Table 2). The distinctly negative d13CPDB

values of Braemar facies samples are in agreementwith the pronounced negative d13CPDB values ofmarine carbonates in Neoproterozoic successions(cf. Kaufman et al., 1991; Kaufman and Knoll,1995). Negative d13CPDB excursions occur duringthe otherwise enriched Neoproterozoic isotopicvalues and are coincident with major glaciations(cf. Kaufman et al., 1991; Kaufman and Knoll,1995; Hoffman et al., 1998).

Carbon isotopic values are also in agreementwith those obtained by Williams (1979) andKennedy (1996) in Australian Neoproterozoic capdolostones. Kennedy (1996) detected a distinctd13CPDB depletion upsection in several successionsof widely separated Neoproterozoic basins andsuggested that more negative d13CPDB values cor-relate with greater paleobathymetry within themarine depositional basin. Samples of this studycannot be related to a distinct stratigraphicprofile, however, samples taken in the Olary re-gion close to the unconformity with the

Palaeoproterozoic to Mesoproterozoic metamor-phic basement possess slightly higher d18OSMOW

(+15.0– +29.5‰) and d13CPDB values(d13CPDB−5.0– +0.9‰) than those in theBraemar area (d18OSMOW+10.6– +28.3‰;d13CPDB−5.5– −2.2‰) (Table 2). Lowerd13CPDB values in samples from the Braemar areaimply that the Barratta Trough deepened to thesouth-southwest, which is in agreement withpalaeogeographic reconstructions (cf. Preiss,1987).

6. Sources of chemical components

6.1. Origin of Al, Fe, Mn and Si

The patterns of element abundances preservedin ancient chemical sediments can be used toconstrain the influence of seawater, hydrothermal,biogenic and detrital sources on the sedimentcomposition (e.g. Dymek and Klein, 1988; Won-der et al., 1988; Derry and Jacobsen, 1990). Purechemical sediments are enriched in Mn and Fe,but addition of detrital or volcanic materialcauses their dilution and enrichment of Ti, A1

Table 2Carbonate C and O isotopic data from the Braemar ironstone facies

LocationSample Mineral d18OSMOW‰ d13CPDB

‰

Braemar areaIron Peak Ferroan dolomiteBR7 +20.9 −3.7Iron PeakBR10 Calcite −5.5+21.5

+19.6BR15 Ferroan dolomite −2.4Razorback RidgeRazorback RidgeBR16 Ferroan dolomite +16.7 −3.7

BR30 +20.0Razorback Ridge −3.3Ferroan dolomiteBR48 +10.6Razorback Ridge −4.0Ferroan dolomite

−2.2+28.3Ferroan dolomiteBR55 Braemar

Olary area−3.3+18.5Ferroan dolomiteBR36 Bimbowrie Hill

Outalpa Ferroan dolomiteBR41 +23.4 −0.8Ferroan dolomiteOutalpa +23.5 +0.9BR42

Mt Mulga Ferroan dolomiteBR44 +15.0 −5.0R74205 Bimbowrie Hill Calcite +27.6 −0.2

−3.5+24.9Ferroan dolomiteR74199 Bimbowrie HillBimbowrie Hill Ferroan dolomiteR74203 +27.9 −2.0Bimbowrie HillR74210 Ferroan dolomite +29.5 −0.8


Fig. 7. Composition of Braemar ironstones and associatedclastic sediments in terms of Fe/Ti versus Al/Al+Fe+Mn.Curve represents mixing of East Pacific Rise sediment withterrigenous and pelagic sediment (modified from Barrett, 1981;Wonder et al., 1988).

6.2. Origin of REE

The Neoproterozoic Braemar ironstone pos-sesses REE patterns (i.e. weak LREE and Cedepletions and with one exception no clear, posi-tive Eu anomaly; Fig. 6a, b), which are broadlysimilar to other Neoproterozoic ironstones includ-ing the Urucum formation of Bolivia and Braziland the Rapitan formation of Canada (Fryer,1977; Derry and Jacobsen, 1990; Klein andBeukes, 1993; Graf et al., 1994). Klein and Beukes(1993) concluded that the Rapitan ironstone has aREE signature similar to seawater and the chemi-cal sediments gained their REE from this source.

The positive correlation of La and Ce with Sisuggests that the siliceous, aluminous ironstonesof the Braemar facies obtained much of theirLREE from detrital sources. However, silica-, alu-mina-poor ironstones of the Braemar facies haveREE patterns unlike the NASC (Fig. 6a). Overall,the REE pattern shapes of silica-, alumina-poorironstones are more like those of modern coastalseawaters (cf. Elderfield et al., 1990). The REEconcentrations and patterns of coastal seawatersare intermediate between those for rivers and forocean waters, reflecting the influence of continen-tal drainage (Elderfield et al., 1990). Similarly,Graf et al. (1994) suggest that the NeoproterozoicUrucum ironstone in South America formed in amixture of river and ocean water. It is thus possi-ble that the Braemar ironstones obtained theirREE from detrital sources and coastal seawater.

7. Palaeoenvironment

Braemar siltstones and ironstones display lowconcentrations of large ion lithophile elementsand high field strength elements compared to theupper continental crust (Taylor and McLennan,1981) suggesting the absence of felsic or basicvolcanic debris in the source region. In addition,the REE composition of an analysed siltstone(sample BR38, Table 1) is largely identical to thatof the early Proterozoic upper continental crust(Condie, 1991). Thus during Neoproterozoictimes, detrital materials were delivered from anexposed upper continental crust characterised by

and Zr (cf. Bonatti, 1975). Proposed methods fordistinguishing between seawater, hydrothermal,biogenic and detrital sources are based on differ-ences in the mineralogical, chemical and isotopiccomposition. Geochemical differences can be il-lustrated using a series of discrimination dia-grams. However, discrimination diagrams have tobe applied with caution as some of them (e.g.Fe�Mn�(Co+Co+Ni)10×diagram) canprovide misleading information on the origin ofmetalliferous sediments (cf. Lottermoser, 1991).

For the Braemar ironstone facies positive corre-lations of Al and Si with Ti, Ca, Mg, K, Ga, Hf,Rb, Sc, Sr, Ta, Th, Zr, La and Ce indicate thatthese elements clearly derived from detritalsources. Addition of alumina-rich detrital mate-rial to a chemical sediment decreases the Fe/Tiratio and increases the proportion of A1 withrespect to the hydrothermal/hydrogenous ele-ments, Fe and Mn (cf. Barrett, 1981). This trendis illustrated in the Fe/Ti versus Al/(Al+Fe+Mn) diagram (Fig. 7) whereby pure chemical sed-iments, mixed chemical–detrital sediments,terrigenous sediment and pelagic clay plot on acompositional curve. The Braemar ironstones andclastic sediments plot on the chemical–detritalmixing curve and ironstones cluster toward thechemical sediment end.


no volcanicity to the depositional environmentof the Braemar ironstone.

Deposition of the Pualco Tillite diamictitesduring the Sturtian glacial maximum was re-stricted to the marine Baratta Trough and themain depocentre was in the Braemar area(Preiss, 1987). The diamictites have been inter-preted as glaciomarine sediments, deposited fromwet-based glaciers originating from thePalaeoproterozoic to Mesoproterozoic Willyamabasement (Curnamona Cratonic Nucleus) anddebouching into a marine basin (Preiss et al.,1993). The lack of local detritus in the basaldiamictite and the generally regionally planar de-positional surface probably imply deposition ofthe Pualco Tillite from an extensive floating ice-sheet (Preiss, 1987). Reworking of the diamic-tites, possibly by water currents, is indicated byinterbedded quartzites, or the quartzites mayhave been derived from a different, more maturesediment source than the associated diamictites.

The Pualco Tillite rests on a slightly irregularerosional surface developed on the Burra Group,or locally on crystalline basement, over thewhole Olary province (Preiss, 1987). It containsbasement-derived material near Olary, probablyshed from the exposed Willyama Inliers. An800-m long slide block of granite in the PualcoTillite adjacent to the MacDonald Fault andlenticular granite conglomerates suggest an ac-tively rising fault scarp in the Olary region andit is thus interpreted that the diamictites andconglomerates may have been deposited in deepglacial valleys of a highland terrain (Preiss,1987). Faulting is less obvious at the other mar-gins of the Barratta Trough, which may havebeen a halfgraben. The preservation of pre-glacial regoliths in parts of the southern andcentral Flinders Ranges, as well as possibly onthe Stuart Shelf, suggests that the lowlands tothe west were not severely glaciated (Preiss,1987). These sedimentological data imply thatthe Braemar ironstones accumulated in a basinalong the border of a continental glaciated high-land to the northeast and a low-lying weatheredlandmass to the west.

8. Genesis

8.1. Formation of the Braemar facies

The role of glaciation in the formation ofNeoproterozoic ironstones has been emphasisedby a number of authors (Yeo, 1983; Urban etal., 1992; Klein and Beukes, 1993; Graf et al.,1994). The occurrence of ironstones in severalNeoproterozoic glacial deposits inspired the hy-pothesis that during glaciation, the underlyingstagnant seawater was cut off from oxygen sup-ply and was rendered anoxic by organic matterdecomposition. Build-up of dissolved Fe oc-curred in the Proterozoic oceans during glacialperiods and deposition of Fe followed duringtransgressive interglacial periods (e.g. Urban etal., 1992; Klein and Beukes, 1993). In fact, the‘snowball-type Earth’ theory suggests the pres-ence of floating pack ice over most of the oceansurface at middle to high latitudes as well asequatorial glaciation (Kirschvink, 1992). Oxy-genation of ferriferous waters after glaciationwould drive the precipitation of ferric oxide inoxic and highly oversaturated surface waters (cf.Kaufman et al., 1991; Hoffman et al., 1998).

Previous authors have assigned the origin ofthe Braemar ironstones to chemical precipitationin a lacustrine environment (Preiss, 1987) or tophysical accumulation of detrital Fe oxides(Whitten, 1970; Preiss, 1987). However, ourcompositional data show that the Braemar facieswas generated by the intermixing of chemicalprecipitates and terrigenous debris on a conti-nental margin. Evaporation of waters in a playa-lake complex (cf. Eugster and Chou, 1973) didnot produce Braemar ironstones as indicated bythe palaeogeographic environment.

The possibility that ferriferous exhalationshave influenced the formation of the Braemarironstone is supported for the following reason.While the rocks exhibit exceptionally low con-centrations of As, Cu, Pb and Zn compared toupper crustal abundances values (Taylor andMcLennan, 1981), they exhibit positive correla-tions of Fe with As, Cu, Sb, V and Zn, elementstypical of ironstones formed under hydrothermalinfluences (e.g. Dymek and Klein, 1988; Duhiget al., 1992; Lottermoser and Ashley, 1996; Ash-


ley et al., 1998). Thus the Fe of the Braemarironstone originally derived from hydrothermalexhalations.

The Braemar ironstone facies consists of lentic-ular laminated and diamictic ironstones and di-amictic ironstones are interbedded with Braemarfacies devoid of, and also with, dropstones (Fig.3c). Thus Fe deposition was clearly associatedwith ice-melting and clastic sedimentation andoccurred during openwater ocean circulation andmelting of icebergs. The presence of Fe oxide-richdiamictites indicates that refrigeration prevailedduring Braemar facies deposition, and was proba-bly maintained throughout deposition of theBenda Siltstone as indicated by scattered drop-stones brought in by floating icebergs (cf. Preiss,1987). Thus the Braemar facies, including thedolostones, was deposited during the waningstages of the Pualco glaciation, during a climatechange from deep refrigeration to slightly warmertemperatures. When the sea ice retreated, oxy-genation of the water column of ferriferous (+manganiferous), carbonate and CO2 chargedwaters occurred (cf. Kaufman et al., 1991; Hoff-man et al., 1998). These waters precipitated car-bonate upon release of CO2 to the atmosphere (cf.Kaufman et al., 1991; Hoffman et al., 1998) andhence the dolostones of the Braemar facies areinterpreted as inorganic cold water deposits.

During glacial periods build-up of dissolved,reduced hydrothermal Fe occurred. A subsequenttransgressive event during an interglacial periodand associated melting of floating ice led to theoxidation of coastal seawater and the precipita-tion of dissolved Fe. Coprecipitation and adsorp-tion of REE from the water column caused acoastal seawater REE signature of the chemicalsediments. Intercalations of Fe-poor siltstonesand diamictites between the ferruginous facies(Fig. 3c) would indicate an episodic decrease in Feprecipitation and dominating clastic sedimenta-tion. These siliceous, aluminous sediments ob-tained a detrital REE signature. Climatecontrolled regressions and transgressions of thesea ice are the most likely reason for the presenceof intercalated non-ferruginous clastic sedimentsin diamictic and laminated ironstones. Redox po-tential differences kept Mn in solution, however,

increasing oxidising conditions led to the precipi-tation of Mn oxides or carbonates and their in-corporation in clastic sediments and the resultingformation of manganiferous siltstones (cf. Urbanet al., 1992; Manikyamba and Naqvi, 1995).

8.2. Geotectonic setting

The palaeolatitude of Neoproterozoic iron-stones and associated glacial sediments has beencontroversial. Meert and Van der Voo (1994)argued, using palaeomagnetic data, thatNeoproterozoic glaciations did not occur below25° latitude. However, recent palaeomagnetic dataindicate that the formation of Neoproterozoicironstones and associated glacial sediments oc-curred in low-latitude environments. The Mari-noan glaciation in South Australia (650–600 Ma),including permafrost, grounded glaciers andmarine glacial deposition, occurred near thepalaeoequator (Schmidt and Williams, 1995).Similarly, the formation of the Rapitan ironstoneand associated glacial sediments (ca. 725 Ma) ofnorthwestern Canada occurred in a low-latitudeenvironment (Park, 1997). Young (1992, 1995)proposed that the Sturtian glaciation in the Ade-laide Geosyncline corresponds to the Rapitanglaciation in northwestern Canada. Such strati-graphic correlations would imply that the iron-stones of the Sturtian Braemar facies (ca.750–700 Ma) were deposited in a low-latitudeenvironment. However, further geochronologicaland palaeomagnetic studies will be required toestablish whether the Sturtian and Rapitan glacia-tions are diachronous or synchronous. In addi-tion, Rui and Piper (1997) stated that thesedimentary cycles recognised in the Neoprotero-zoic successions of Australia and Canada wereprobably driven by global eustatic changes and donot imply close proximity of the two regions.

9. Conclusions

The Braemar ironstone facies represents theconsolidated product of Neoproterozoic Fe-richchemical sediments. They occur as matrix cementto diamictites and as massive to laminated iron-


stones. Associated rock types include diamictites,dolostones, quartzites and siltstones. Mineralogi-cally the ironstone facies comprises major quartzand Fe oxides (hematite, magnetite), minor silicates(muscovite, chlorite, biotite, plagioclase, tourma-line), carbonate and apatite, and detrital mineralgrains and lithic clasts.

Elements of detrital derivation (Si, A1, Ti, Ca,Mg, K, Ga, Hf, Rb, Sc, Sr, Ta, Th, Zr, La, Ce)exhibit positive correlations as a result of compet-ing clastic and chemical sedimentation. Weak pos-itive correlations of Fe with As, Cu, V and Znindicate that these elements derived from vol-canogenic, ferriferous exhalations. The REE geo-chemistry appears to be controlled by clasticcontributions for siliceous, aluminous ironstones,whereas the average REE signature of silica-,alumina-poor ironstones is very similar to that ofpresent-day, coastal seawater. Thus chemical com-ponents of the Braemar ironstone derived from theerosional drainage of weathering solutions from theexposed Proterozoic continental crust, hydrother-mal exhalations and coastal seawater.

The similarities between the Braemar ironstonefacies and other Neoproterozoic ironstones includ-ing the association with glaciogenic rocks, mangan-iferous sediments and dolostones, the lack oftransition metal enrichments and the REE signa-tures of modern coastal seawater suggest thatNeoproterozoic ironstones share a common gene-sis: they are likely the result of chemical precipita-tion during interglacial/postglacial periods andformed in near-coastal environments under signifi-cant terrestrial influences.

Acknowledgements

The research was supported by the AustralianResearch Council. B.C. McKelvey, O.H. Holm andD.J. Whitford are gratefully acknowledged fortheir help with various aspects of the projectincluding sampling, sample preparation, data ac-quisition and stimulating discussions. The Breedingfamily of Braemar station is thanked for theirhospitality during a field visit in 1995. N.J. Beukesand an anonymous reviewer are thanked for com-menting on the manuscript.

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