Directory UMM :Data Elmu:jurnal:P:Precambrian Research:Vol101.Issue1.2000:

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Geochemistry, petrology and origin of Neoproterozoic

ironstones 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 Uni}

6ersity,P.O.Box6811,Cairns,Qld4870,Australia

b_Di

6ision of Earth Sciences,Uni6ersity of New England,Armidale,NSW2351,Australia

Received 31 March 1999; accepted 19 November 1999

Abstract

The eastern part of the Adelaide Geosyncline contains well preserved glaciomarine sequences of the Sturtian glaciation (:750 – 700 Ma) including calcareous or dolomitic siltstone, manganiferous siltstone, dolostone and diamictite units and the associated Braemar ironstone facies. The ironstone facies occurs as matrix to diamictites and as massive to laminated ironstones and comprises abundant Fe oxides (hematite, magnetite) and quartz, minor silicates (muscovite, chlorite, biotite, plagioclase, tourmaline), carbonate and apatite, and detrital mineral grains and lithic clasts. Micro-textures indicate that magnetite and hematite are of metamorphic origin. They are intergrown with silicates 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 by predominantly 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 lithophile and high field strength elements and display REE signatures of modern coastal seawater. The Braemar facies accumulated in a marine basin along the border of a continental glaciated highland and a low-lying weathered landmass. Wet-based glaciers originated from the Palaeoproterozoic to Mesoproterozoic metamorphic basement and debouched 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 of floating ice led to an influx of clastic detritus and deposition of glaciomarine sediments from wet-based glaciers and to oxygenation of ferriferous (9manganiferous), carbonate and CO2charged coastal waters. Release of CO2to the

atmosphere from the oxygenated waters resulted in the precipitation of carbonate as dolostones and oxygenation of ferriferous (9manganiferous) waters led to the precipitation of Fe3+_{oxides as laminated ironstones and as matrix}

of diamictic ironstones. Further increases in Eh conditions led to the precipitation of Mn oxides or carbonates and their incorporation in clastic sediments. Thus the Braemar ironstone facies is the result of chemical precipitation of dissolved Fe (and Mn) during a postglacial, transgressive period and formed in a near-coastal environment under

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* Corresponding author. Fax: +61-7-40421284.

E-mail address:bernd.lottermoser@jcu.edu.au (B.G. Lottermoser)

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B.G.Lottermoser,P.M.Ashley/Precambrian Research101 (2000) 49 – 67 50

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

1. Introduction

Glaciogenic and iron-rich rocks are intimately associated in Neoproterozoic sequences. Examples are known from North and South America (e.g. Young, 1976; Yeo, 1983; Urban et al., 1992; Klein and Beukes, 1993; Graf et al., 1994), Africa (e.g. Breitkopf, 1986; Bu¨hn et al., 1992), China (Rui and Piper, 1997), and South Australia (Whitten, 1970). In some cases, the iron-rich rocks are present as iron-rich clastic sediments (e.g. South Australia; 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 and glaciogenic rocks in the Neoproterozoic, the ap-parent glaciation in low-latitude environments during that time, and the source of chemical components and genesis of ironstones remain controversial.

Neoproterozoic sedimentary rocks of the Ade-laide Geosyncline in South Australia and far west-ern New South Wales host well preserved glaciomarine sequences and associated ferrugi-nous units (Preiss, 1987; Preiss et al., 1993; Fig. 1). These ferruginous rocks are rich in magnetite

and/or hematite, iron-bearing silicates and

car-bonates. The purpose of this paper is to describe the geochemical composition of Braemar iron-stones, to establish the genetic processes

responsi-ble for their formation and the

palaeoenvironment of deposition, and to discuss their genesis in light of other models for Neoproterozoic glaciogenic and iron-rich rock occurrences.

2. Geology

The Adelaide Geosyncline in South Australia is a major, deeply subsident Neoproterozoic to

Cambrian sedimentary basin which overlies

Palaeoproterozoic to Mesoproterozoic metamor-phic basement rocks. It contains one of the most complete 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). The widespread Sturtian glaciation event is manifest in the Umberatana Group (Preiss et al., 1998), and

particularly in the great thicknesses of

glaciomarine sedimentary rocks deposited in the fault-controlled Baratta Trough, extending from the central Flinders Ranges to the Yunta-Olary region in eastern South Australia (Sumartojo and Gostin, 1976; Preiss, 1987; Preiss et al., 1993; Fig. 1). Much of the glaciogenic sedimentation in the Umberatana Group is characterised by diamictite, laminated siltstone and orthoquartzite, but in places there are distinctive intercalated dolomitic and ferruginous units (Preiss, 1987; Preiss et al., 1993). In this paper, diamictite is a nongenetic term referring to poorly sorted siliciclastic sedi-mentary rocks containing a wide range of class sizes in an abundant fine-grained matrix in which the clasts are dispersed so that most of them are not in contact (cf. Panahi and Young, 1997).

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with decreasing iron minerals, but also contain dolostone 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 with heavy mineral lamination), siltstones and man-ganiferous siltstones. The distribution of the iron-stone and associated ferruginous siltiron-stones and diamictites is especially notable on aeromagnetic

images. The ironstones are particularly prominent at Razorback Ridge south of Yunta (Fig. 2), where the thickest iron-rich sub-units have been evaluated as a potential iron ore resource (Whit-ten, 1970). Although at some locations, there is only one prominently ferruginous horizon, at many locations, there are several zones (e.g. 3 or 4), separated by tens to hundreds of metres of other strata, e.g. in the Bimbowrie Hill region and at Razorback Ridge (Fig. 3).

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Fig. 2. Outcrop of ferruginous facies (Braemar ironstone facies) and associated Sturtian glaciogenic rocks (Pualco Tillite) in the eastern 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 and included laminated and diamictic ironstones,

silt-stones, diamictites and carbonate-rich rocks

which were representative of the rock types and

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were crushed and pulverised in a chrome steel ring mill. Major and trace elements were analysed by X-ray fluorescence on duplicate fused discs and pressed powder pellets at the Division of Earth Sciences, 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 nine samples by instrumental thermal neutron activa-tion analysis at Becquerel Laboratories, Sydney. REE concentrations exceeded the detection limits by several orders of magnitude. In addition, data on geochemical reference materials were within 10% of the accepted values. Oxygen and carbon

isotope mass spectrometry on 15 rock samples was 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 siltstone samples 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 including several thin quartzite and dolostone units (Fig. 3).

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Fig. 4. (A) Typical outcrop of the Braemar ironstone facies. Ironstone is intercalated with carbonate-bearing siltstone and minor diamictite and dolostone. Near Bimbowrie Hill, AMG: 420 700 mE, 6 455 650 mN. (B) Diamictic ironstone with recrystallised carbonate-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 carbonate components (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-sediment deformation (sample BR28). Razorback Ridge, AMG: 379 740 mE, 6 352 770 mN. (E) Laminated ironstone with interbedded lighter coloured 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 two types, diamictic and laminated ironstones, which are 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:

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and composed of magnetite, hematite and quartz with minor muscovite, chlorite, biotite, carbonate,

apatite, plagioclase and tourmaline. Associated siltstones contain abundant quartz, biotite,

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Fig. 4. (Continued)

bonate, plagioclase, muscovite, chlorite, variable amounts of magnetite and hematite and traces of

clinozoisite/epidote, tourmaline, zircon and pyrite

(altered to goethite due to supergene oxidation). Detrital mineral grains and lithic clasts occur in laminated 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, and quartzofeldspathic and quartz-carbonate rocks. Detrital feldspars have been variably replaced by carbonate, muscovite and traces of chlorite and biotite, and biotite has been retrogressed to chlor-ite and traces of rutile.

Diamictic ironstones are massive and clasts range in size from 10 mm to 1.2 m but are most commonly between 25 mm and 150 cm (Fig. 4B). A few striated boulders were noted by Whitten (1970). Angularity and nature of the detrital grains and lithic clasts are similar to those found

in the associated laminated ironstones and

siltstones.

Laminated ironstones are usually

inequigranu-lar with grain sizes ranging from B0.1 to 5 mm.

Lamination is generally well developed and ranges

from 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 degrees

of martitization and larger subhedra are up to 0.1 mm in diameter. Rare pressure shadows of

chlor-ite and/or biotite are well developed adjacent to

magnetite – hematite porphyroblasts. However,

much hematite is not weathering-related, because grains display a preferred orientation oblique to compositional laminations, and late dilational

veins contain magnetite, quartz, carbonate,

goethite and platy hematite. Therefore hematite is, in part, syn- or pre-tectonic. Locally, nearly

pure layers of Fe-oxides (:80%) are present,

with magnetite, hematite and quartz forming a metamorphic granoblastic aggregate.

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

The Braemar ironstone facies has undergone regional metamorphism and deformation. The rocks display interlocking aggregates of mineral grains and rare porphyroblastic Fe oxide and carbonate grains. Slaty cleavage is commonly

defined by the preferred orientation of layer sili-cates and hematite plates. The subhedral shape of the magnetite crystals, the presence of rare por-phyroblastic magnetite grains, together with the

occurrence of magnetite/hematite-bearing veins

and foliated hematite, indicate that the magnetite and some hematite are of metamorphic origin and not detrital. The Fe oxides are intergrown with silicates and carbonates, with the mineral assemblages indicative of greenschist facies (bi-otite grade) metamorphism.

Carbonates in the ironstones and associated

ferruginous 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 is

typically 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 the Al(IV) – T plot of Cathelineau (1988) indicate

chlorite growth at :360 – 400°C. In the

Bim-bowrie Hill region (Fig. 2), the Braemar iron-stone facies is associated with manganiferous

siltstone units :1 m thick. These are composed

of 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 including calcite, ankerite and manganoan magnesian sider-ite.

5. Geochemistry

5.1. Major and trace elements

The major oxide components of the laminated

ironstones are SiO2 and Fe2O3. All ironstones

consist of \70 wt.% SiO2+Fe2O3 (all Fe as

Fe3+_{) with Fe}

2O3 ranging between 22.94 and

78.91 wt.% (N=20) (Table 1 and Fig. 5). Minor

element contents of the ironstones show some

variations, with Al2O3ranging from 0.28 to 10.64

wt.%, CaO from 0.10 to 5.82 wt.%, K2O from

0.03 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

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B

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

BR45 BR15 BR36 BR8 BR13 BR40 BR52 BR53

BR30

Sample R74203 BR38

ca-qz-pl-Mineralogy qz-bio-ca- Qz-pl-Kfs- Feox-qz-ca- Feox-qz-ca- Mt-hm-qz- Mt-hm-qz- Mt-hm-go- Feox-qz-ca- Mt-hm-qz-ca-chl/bio chl-bio-ca

bio-ms-tm- bio-qz-ca

pl-ms-chl- lithic clasts- chl-ms-pl- chl-bio-ca pl-chl-bio-lithic clasts

ap-tm ap-pl go

go-tm-zir bio-mt-ca-m s-chl

Mt Mulga Razorback Bimbowrie Braemar

Oultalpa Iron Peak Razorback

Razorback Oultalpa

Bimbowrie

Location Braemar

NW S Ridge Hill Ridge NW

ridge

6 454 240 6 353 900

Northing 6 459 900 6 352 610 6 440 620 6 443 250 6 352 770 6 352 770 6 440 620 6 326 240 6 325 150 422 790 384 100 379 740

Easting 421 800 379 090 411 760 433 700 379 740 411 760 371 820 371 820

47.86 40.54 28.12 28.68 14.81

18.58 67.13 65.65 33.29 23.54

40.10 SiO2

TiO2 0.68 0.16 0.83 0.70 0.57 0.48 0.22 0.33 0.18 0.37 0.26

Al2O3 10.19 3.16 11.92 10.06 7.82 6.46 2.92 3.97 2.80 3.16 2.74

Fe2O3 7.28 4.96 5.08 13.50 25.52 37.20 66.77 62.34 78.91 49.76 66.20

MnO 6.29 0.36 0.13 0.04 0.15 0.30 0.04 0.06 0.11 0.18 0.23

MgO 2.80 13.26 3.15 1.56 3.76 3.27 0.88 1.28 2.02 2.22 1.59

CaO 14.12 22.94 3.34 1.63 3.68 3.49 0.28 0.64 0.33 3.86 1.98

Na2O 2.44 1.65 2.24 3.49 0.26 1.29 0.05 1.32 0.11 1.66 0.80

K2O 2.19 0.15 2.60 1.82 2.55 1.77 0.82 0.32 1.37 0.19 0.24

P2O5 0.17 0.02 0.19 0.37 0.46 0.68 0.23 0.24 0.15 0.64 0.94

S 0.01 0.00 0.01 0.01 0.01 0.01 0.01 0.00 0.01 0.02 0.03

5.16

LOI 12.57 34.49 2.63 1.20 6.43 3.99 0.20 0.49 0.20 2.11

Total 98.80 99.73 99.26 100.04 99.06 99.49 100.54 99.66 101.00 100.53 100.67

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B

BR36 BR8 BR13 BR40

Sample R74203 BR30 BR38 BR45 BR15 BR52 BR53

Feox-qz-ca-Qz-pl-Kfs-

Feox-qz-ca-ca-qz-pl-

qz-bio-ca-Mineralogy Mt-hm-qz- Mt-hm-qz- Mt-hm-go- Feox-qz-ca-

Mt-hm-qz-pl-ms-chl- chl-bio-ca

lithic clasts lithic clasts- chl-ms-pl- bio-ms-tm- chl-bio-ca bio-qz-ca pl-chl-bio- ca-chl/bio ap-tm

go-tm-zir bio-mt-ca-m ap-pl go

s-chl Mt Mulga

Oultalpa Razorback

Razorback Bimbowrie

Bimbowrie

Location Iron Peak Razorback Oultalpa Braemar Braemar

Ridge

NW NW

ridge S Ridge Hill

6 454 240 6 353 900 6 352 770 6 440 620 6 326 240 6 325 150 Northing 6 459 900 6 352 610 6 440 620 6 443 250 6 352 770

422 790 384 100 379 740 411 760 371 820 371 820 379 740

Easting 421 800 379 090 411 760 433 700

B1 1.3 B1 B1 B1

29.6 12.4 7.12 7.59 15.4

22.9 19.9

na

Nd na na 33.7

5.71 2.52 1.73 1.74 3.03

Sm na na 6.54 na 4.74 4.76

1.37 0.62 0.44 0.56 0.89

0.99 1.23

Eu na na 1.5 na

0.75

na na 1.06 na 1.11 0.62 0.45 0.37 0.66 1.06

Tb

1.46 0.96 0.75 0.53 0.95

1.03 1.54

2.52 2.65 1.97 2.24 2.78 2.01

3.19 2.89

(La/Sm)cn

5.52 5.17 3.15 1.56 3.79 1.12 2.72

9.08 (La/Lu)cn

1.64 1.20

(Tb/Lu)cn 1.89 1.24 0.85 1.48 3.48 1.24

115.16 53.57 30.05 31.84 67.25 82.11

98.64 150.8

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, Fe}

oxides, 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 the Division of Earth Sciences, University of New England. Chondrite normalised ratios (La/Sm)cn, (La/Lu)cn, and (Tb/Lu)cnare calculated using chondrite values given

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to 7.4 wt.%. Such variations reflect different

modal contents of magnetite and/or hematite,

quartz, plagioclase, carbonate, biotite, chlorite and muscovite in the analysed samples.

Ironstones with higher Si contents tend to have

higher A1 and Ca+Mg values (Fig. 5a, b). These

element trends indicate the addition of plagioclase and carbonate. The associated clastic sediments

have lower Fe and higher Si, Al and Ca+Mg

values, and similar Na+K contents compared to

the ironstones (Fig. 5a – c). There is also variation

in the Na+K content, reflecting the abundance

of 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 and

Al2O3, TiO2, Na2O, K2O, Hf, LREE, Nb, Pb, Sc,

Ta, Th, U and Zr are somewhat more abundant (samples BR38, BR45; Table 1). Such increased element concentrations compared to the associ-ated ironstones are due to more plagioclase, K-feldspar, biotite, muscovite, chlorite, and lithic clasts within the analysed samples. The siltstones and sandstones have trace element abundances similar to the average upper crust with the excep-tion of lower Nb, Zr, Ba and Sr values (cf. Taylor and McLennan, 1981). The compositions of man-ganiferous siltstones are quite similar to the clastic sediments, except for higher MnO, Ni, V and Zn contents (sample R74203, Table 1). The analysis of a dolostone indicates that clastic sediments exhibit higher trace element contents with the exception of lower Sr values (sample BR30, Table 1).

Trace element constituents of the ironstones show large scale variations and appear to be largely dependent on the type and quantity of the minerals present. The ironstones are depleted in most transition metals (Sc, V), high field strength elements (Nb, U, Th, Zr, Hf, Pb, LREE), and large ion lithophile elements (Ba, Sr, Rb) when compared to the average upper continental crust (cf. Taylor and McLennan, 1981). Only the Ni, Y and HREE concentrations are similar to average upper crustal abundances. Such low trace element concentrations could either reflect their removal during metamorphism, which is most unlikely as many of these elements are regarded as immobile,

or may have important implications for the source(s) of these elements and the depositional environment of the Braemar facies.

For the Braemar ironstones, a correlation

ma-trix of log-transformed data (N=20) shows that

there 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. These

correlations reflect increasing sedimentary inputs of siliciclastic material to chemical sediments (cf. Ewers and Morris, 1981; Klein and Beukes, 1993; Manikyamba and Naqvi, 1995). In contrast, Fe exhibits weak positive correlations with few

ele-ments, including As, Cu, Sb, V and Zn (r= +

0.4 –+0.6) pointing to a hydrothermal source of

these metals. Thus the chemical compositions of Braemar ironstones reflect variations from iron-stones formed by predominantly chemical precipi-tation processes to examples with a significant detrital component.

5.2. Rare earth elements

Laminated ironstones possessREE

concentra-tions (REE: La+Ce+Nd+Sm+Eu+Tb+

Ho+Yb+Lu) ranging from 30.05 to 115.16 ppm

and chondrite normalised (La/Sm)cn ratios of

1.97 – 2.89, (La/Lu)cnratios 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 ironstones normalised to the North American Shale Com-posite (NASC; Gromet et al., 1984). All iron-stones display REE patterns with variable LREE depletions, modest negative Ce anomalies and no Eu 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) reveals

that correlations of REE with most elements are

insignificant (rB+0.7). However, La, Ce and the

REE content show correlations with Si (+0.6)

and all REE and also the REE content show

slight 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),

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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 Gromet et al. (1984).

BR13, BR35, BR40, BR52, BR53; Table 1). Such samples are moderately depleted in LREE and variably depleted or enriched in HREE compared to the NASC (Fig. 6a). Siliceous, aluminous iron-stones display REE patterns only very slightly depleted 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 to

the clastic sediment sample BR38 (Table 1 and Fig. 6b). The clastic sediment BR38 shows a relatively flat REE pattern, nearly identical to that of the NASC.

The strong similarities of the REE patterns of the Braemar siltstone and siliceous, aluminous ironstones with the NASC REE distribution is consistent with these sediments gaining their REE from detrital sources. However, silica-, alumina-poor ironstones display a different REE geochem-istry indicating that the REE were gained during chemical precipitation.

5.3. Carbon and oxygen isotopes

Sheet-like dolostones are commonly associated with Neoproterozoic glaciogenic rocks (Kennedy, 1996; Hoffman et al., 1998) and such dolostones cap the Braemar facies (Fig. 3). In addition, car-bonate occurs as ferroan dolomite and ferroan calcite within siltstones and ironstones. Sedimen-tological and stable isotope data of Adelaidean dolostones have been interpreted to reflect a palaeoenvironment 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 sites within the Yunta-Olary region and 15 samples were analysed (Table 2). The Braemar facies has undergone diagenesis and metamorphism and the observed carbonate within these rocks has clearly recrystallised during metamorphism. However, dolostones are an integral part of the sedimentary sequence and there is no petrographic evidence for major carbonate mobilisation or veining, and therefore the carbonate within the Braemar facies is 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 REE within the ironstones are largely incorporated into accessory apatite and carbonate.

The ironstones have been subdivided according

to their SiO2 and A12O3 contents and individual

REE distributions into two different suites. Iron-stones are here called siliceous, aluminous if

SiO2\40 wt.% and A12O3\6 wt.% (samples

BR15, BR36; Table 1) and silica-, alumina-poor if

SiO2B40 wt.% and A12O3B6 wt.% (samples

BR8, BR13, BR35, BR40, BR52, BR53; Table 1). Samples from the same locality can have different

SiO2 and A12O3 contents and REE distributions.

Silica-, alumina-poor ironstones have the lowest

REE concentrations and the lowest (La/Sm)cn,

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Carbon isotope values vary greatly (d13_C

PDB−

5.5 –+0.9‰), however, d13_C

PDB isotopic

signa-tures are nearly all negative, whereas oxygen

isotope values range fromd18_O

SMOW+10.6 to+

29.5‰ (Table 2). The distinctly negative d13_C

PDB values of Braemar facies samples are in agreement

with the pronounced negative d13

CPDB values of

marine carbonates in Neoproterozoic successions (cf. Kaufman et al., 1991; Kaufman and Knoll,

1995). Negative d13_C

PDB excursions occur during

the otherwise enriched Neoproterozoic isotopic values 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 agreement with those obtained by Williams (1979) and Kennedy (1996) in Australian Neoproterozoic cap dolostones. Kennedy (1996) detected a distinct d13_C

PDBdepletion upsection in several successions

of widely separated Neoproterozoic basins and

suggested that more negatived13

CPDBvalues

cor-relate with greater paleobathymetry within the marine depositional basin. Samples of this study cannot be related to a distinct stratigraphic profile, however, samples taken in the Olary

re-gion close to the unconformity with the

Palaeoproterozoic to Mesoproterozoic

metamor-phic basement possess slightly higher d18_O

SMOW

PDBvalues in samples from the Braemar area

imply that the Barratta Trough deepened to the south-southwest, which is in agreement with

palaeogeographic reconstructions (cf. Preiss,

1987).

6. Sources of chemical components

6.1. Origin of Al,Fe, Mn and Si

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

Table 2

Carbonate C and O isotopic data from the Braemar ironstone facies

Location

Sample Mineral d18_O

SMOW‰ d13CPDB

‰

Braemar area

Iron Peak Ferroan dolomite

BR7 +20.9 −3.7

Iron Peak

BR10 Calcite +21.5 −5.5

+19.6

BR15 Razorback Ridge Ferroan dolomite −2.4

Razorback Ridge

BR16 Ferroan dolomite +16.7 −3.7

BR30 Razorback Ridge Ferroan dolomite +20.0 −3.3

BR48 Razorback Ridge Ferroan dolomite +10.6 −4.0

−2.2

Mt Mulga Ferroan dolomite

BR44 +15.0 −5.0

R74205 Bimbowrie Hill Calcite +27.6 −0.2

−3.5

+24.9 Ferroan dolomite

R74199 Bimbowrie Hill

Bimbowrie Hill Ferroan dolomite

R74203 +27.9 −2.0

Bimbowrie Hill

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Fig. 7. Composition of Braemar ironstones and associated clastic sediments in terms of Fe/Ti versus Al/Al+Fe+Mn. Curve represents mixing of East Pacific Rise sediment with terrigenous 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 Ce depletions and with one exception no clear, posi-tive Eu anomaly; Fig. 6a, b), which are broadly similar to other Neoproterozoic ironstones includ-ing the Urucum formation of Bolivia and Brazil and the Rapitan formation of Canada (Fryer, 1977; Derry and Jacobsen, 1990; Klein and Beukes, 1993; Graf et al., 1994). Klein and Beukes (1993) concluded that the Rapitan ironstone has a REE signature similar to seawater and the chemi-cal sediments gained their REE from this source. The positive correlation of La and Ce with Si suggests that the siliceous, aluminous ironstones of the Braemar facies obtained much of their LREE from detrital sources. However, silica-, alu-mina-poor ironstones of the Braemar facies have REE patterns unlike the NASC (Fig. 6a). Overall, the REE pattern shapes of silica-, alumina-poor ironstones are more like those of modern coastal seawaters (cf. Elderfield et al., 1990). The REE concentrations and patterns of coastal seawaters are intermediate between those for rivers and for ocean waters, reflecting the influence of continen-tal drainage (Elderfield et al., 1990). Similarly, Graf et al. (1994) suggest that the Neoproterozoic Urucum ironstone in South America formed in a mixture of river and ocean water. It is thus possi-ble that the Braemar ironstones obtained their REE from detrital sources and coastal seawater.

7. Palaeoenvironment

Braemar siltstones and ironstones display low concentrations of large ion lithophile elements and high field strength elements compared to the upper continental crust (Taylor and McLennan, 1981) suggesting the absence of felsic or basic volcanic debris in the source region. In addition, the REE composition of an analysed siltstone (sample BR38, Table 1) is largely identical to that of the early Proterozoic upper continental crust (Condie, 1991). Thus during Neoproterozoic times, detrital materials were delivered from an exposed upper continental crust characterised by and Zr (cf. Bonatti, 1975). Proposed methods for

distinguishing between seawater, hydrothermal, biogenic and detrital sources are based on differ-ences in the mineralogical, chemical and isotopic composition. Geochemical differences can be il-lustrated using a series of discrimination dia-grams. However, discrimination diagrams have to be applied with caution as some of them (e.g.

FeMn(Co+Co+Ni)10×diagram) can

provide misleading information on the origin of metalliferous 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 that these elements clearly derived from detrital sources. Addition of alumina-rich detrital

mate-rial to a chemical sediment decreases the Fe/Ti

ratio and increases the proportion of A1 with

respect to the hydrothermal/hydrogenous

ele-ments, Fe and Mn (cf. Barrett, 1981). This trend

is illustrated in the Fe/Ti versus Al/(Al+Fe+

Mn) diagram (Fig. 7) whereby pure chemical

sed-iments, mixed chemical – detrital sediments,

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no volcanicity to the depositional environment of the Braemar ironstone.

Deposition of the Pualco Tillite diamictites during the Sturtian glacial maximum was re-stricted to the marine Baratta Trough and the main depocentre was in the Braemar area (Preiss, 1987). The diamictites have been inter-preted as glaciomarine sediments, deposited from

wet-based glaciers originating from the

Palaeoproterozoic to Mesoproterozoic Willyama basement (Curnamona Cratonic Nucleus) and debouching into a marine basin (Preiss et al., 1993). The lack of local detritus in the basal diamictite and the generally regionally planar de-positional surface probably imply deposition of the Pualco Tillite from an extensive floating ice-sheet (Preiss, 1987). Reworking of the diamic-tites, possibly by water currents, is indicated by interbedded quartzites, or the quartzites may have been derived from a different, more mature sediment source than the associated diamictites.

The Pualco Tillite rests on a slightly irregular erosional surface developed on the Burra Group, or locally on crystalline basement, over the whole Olary province (Preiss, 1987). It contains basement-derived material near Olary, probably shed from the exposed Willyama Inliers. An 800-m long slide block of granite in the Pualco Tillite adjacent to the MacDonald Fault and lenticular granite conglomerates suggest an ac-tively rising fault scarp in the Olary region and it is thus interpreted that the diamictites and conglomerates may have been deposited in deep glacial valleys of a highland terrain (Preiss, 1987). Faulting is less obvious at the other mar-gins of the Barratta Trough, which may have been a halfgraben. The preservation of pre-glacial regoliths in parts of the southern and central Flinders Ranges, as well as possibly on the Stuart Shelf, suggests that the lowlands to the west were not severely glaciated (Preiss, 1987). These sedimentological data imply that the Braemar ironstones accumulated in a basin along the border of a continental glaciated high-land to the northeast and a low-lying weathered landmass to the west.

8. Genesis

8.1. Formation of the Braemar facies

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

Previous authors have assigned the origin of the Braemar ironstones to chemical precipitation in a lacustrine environment (Preiss, 1987) or to physical accumulation of detrital Fe oxides (Whitten, 1970; Preiss, 1987). However, our compositional data show that the Braemar facies was generated by the intermixing of chemical precipitates and terrigenous debris on a conti-nental margin. Evaporation of waters in a playa-lake complex (cf. Eugster and Chou, 1973) did not produce Braemar ironstones as indicated by the palaeogeographic environment.

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Ash-ley et al., 1998). Thus the Fe of the Braemar ironstone originally derived from hydrothermal exhalations.

The Braemar ironstone facies consists of lentic-ular laminated and diamictic ironstones and di-amictic ironstones are interbedded with Braemar facies devoid of, and also with, dropstones (Fig. 3c). Thus Fe deposition was clearly associated with ice-melting and clastic sedimentation and occurred during openwater ocean circulation and melting of icebergs. The presence of Fe oxide-rich diamictites indicates that refrigeration prevailed during Braemar facies deposition, and was proba-bly maintained throughout deposition of the Benda Siltstone as indicated by scattered drop-stones brought in by floating icebergs (cf. Preiss, 1987). Thus the Braemar facies, including the dolostones, was deposited during the waning stages of the Pualco glaciation, during a climate change from deep refrigeration to slightly warmer temperatures. When the sea ice retreated,

oxy-genation of the water column of ferriferous (+

manganiferous), carbonate and CO2 charged

waters occurred (cf. Kaufman et al., 1991; Hoff-man et al., 1998). These waters precipitated

car-bonate upon release of CO2to the atmosphere (cf.

Kaufman et al., 1991; Hoffman et al., 1998) and hence the dolostones of the Braemar facies are interpreted as inorganic cold water deposits.

During glacial periods build-up of dissolved, reduced hydrothermal Fe occurred. A subsequent transgressive event during an interglacial period and associated melting of floating ice led to the oxidation of coastal seawater and the precipita-tion of dissolved Fe. Coprecipitaprecipita-tion and adsorp-tion of REE from the water column caused a coastal seawater REE signature of the chemical sediments. Intercalations of Fe-poor siltstones and diamictites between the ferruginous facies (Fig. 3c) would indicate an episodic decrease in Fe precipitation and dominating clastic sedimenta-tion. These siliceous, aluminous sediments

ob-tained a detrital REE signature. Climate

controlled regressions and transgressions of the sea ice are the most likely reason for the presence of intercalated non-ferruginous clastic sediments in 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 resulting formation of manganiferous siltstones (cf. Urban et al., 1992; Manikyamba and Naqvi, 1995).

8.2. Geotectonic setting

The palaeolatitude of Neoproterozoic iron-stones and associated glacial sediments has been controversial. Meert and Van der Voo (1994)

argued, using palaeomagnetic data, that

Neoproterozoic glaciations did not occur below 25° latitude. However, recent palaeomagnetic data indicate that the formation of Neoproterozoic ironstones and associated glacial sediments oc-curred in low-latitude environments. The Mari-noan glaciation in South Australia (650 – 600 Ma),

including permafrost, grounded glaciers and

marine glacial deposition, occurred near the palaeoequator (Schmidt and Williams, 1995). Similarly, the formation of the Rapitan ironstone and associated glacial sediments (ca. 725 Ma) of northwestern Canada occurred in a low-latitude environment (Park, 1997). Young (1992, 1995) proposed that the Sturtian glaciation in the Ade-laide Geosyncline corresponds to the Rapitan glaciation 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-latitude environment. However, further geochronological and palaeomagnetic studies will be required to establish whether the Sturtian and Rapitan glacia-tions are diachronous or synchronous. In addi-tion, Rui and Piper (1997) stated that the sedimentary cycles recognised in the Neoprotero-zoic successions of Australia and Canada were probably driven by global eustatic changes and do not imply close proximity of the two regions.

9. Conclusions

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iron-B.G.Lottermoser,P.M.Ashley/Precambrian Research101 (2000) 49 – 67 66

stones. Associated rock types include diamictites, dolostones, quartzites and siltstones. Mineralogi-cally the ironstone facies comprises major quartz and Fe oxides (hematite, magnetite), minor silicates (muscovite, chlorite, biotite, plagioclase, tourma-line), carbonate and apatite, and detrital mineral grains 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 Zn indicate that these elements derived from vol-canogenic, ferriferous exhalations. The REE geo-chemistry appears to be controlled by clastic contributions for siliceous, aluminous ironstones, whereas the average REE signature of silica-, alumina-poor ironstones is very similar to that of present-day, coastal seawater. Thus chemical com-ponents of the Braemar ironstone derived from the erosional drainage of weathering solutions from the exposed Proterozoic continental crust, hydrother-mal exhalations and coastal seawater.

The similarities between the Braemar ironstone facies and other Neoproterozoic ironstones includ-ing the association with glaciogenic rocks, mangan-iferous sediments and dolostones, the lack of transition metal enrichments and the REE signa-tures of modern coastal seawater suggest that Neoproterozoic ironstones share a common gene-sis: they are likely the result of chemical

precipita-tion during interglacial/postglacial periods and

formed in near-coastal environments under signifi-cant terrestrial influences.

Acknowledgements

The research was supported by the Australian Research Council. B.C. McKelvey, O.H. Holm and D.J. Whitford are gratefully acknowledged for their help with various aspects of the project including sampling, sample preparation, data ac-quisition and stimulating discussions. The Breeding family of Braemar station is thanked for their hospitality during a field visit in 1995. N.J. Beukes and an anonymous reviewer are thanked for com-menting on the manuscript.

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