Geochemistry, petrology and origin of Neoproterozoic ironstones in the eastern part of the Adelaide

19 Pages·2000·1.13 MB·English

Precambrian Research 101 (2000) 49–67
www.elsevier.com:locate:precamres
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
aSchoolofEarthSciences, JamesCookUni6ersity, P.O.Box6811, Cairns, Qld4870,Australia
bDi6isionofEarthSciences, Uni6ersityofNewEngland, Armidale, NSW2351,Australia
Received 31March1999; accepted19November1999
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
diamictiteunitsandtheassociatedBraemarironstonefacies.Theironstonefaciesoccursasmatrixtodiamictitesand
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
lithicclasts.Micro-texturesindicatethatmagnetiteandhematiteareofmetamorphicorigin.Theyareintergrownwith
silicates and carbonates, with the mineral assemblage indicative of greenschist facies (biotite grade) metamorphism.
Chemicalcompositionsofironstonesvarygreatlyandreflectchangesfromsilica-,alumina-poorironstonesformedby
predominantlychemicalprecipitationprocessestosilica-,alumina-richexampleswithasignificantdetritalcomponent.
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.
HydrothermalexhalationsaddedsignificantamountsofFeandothermetalstoNeoproterozoicseawater.Meltingof
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 CO charged coastal waters. Release of CO to the
2 2
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(cid:27) 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
*Corresponding author. Fax: (cid:27)61-7-40421284.
E-mail address:[email protected] (B.G. Lottermoser)
0301-9268:00:$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S0301-9268(99)00098-4
50 B.G. Lottermoser, P.M. Ashley:Precambrian Research101(2000)49–67
significant terrestrial influences. © 2000 Elsevier Science B.V. All rights reserved.
Keywords:Ironstones; Geochemistry; Glaciation; Adelaide Geosyncline; Neoproterozoic; South Australia
1. Introduction Palaeoproterozoic to Mesoproterozoic metamor-
phic basement rocks. It contains one of the most
Glaciogenic and iron-rich rocks are intimately complete and well preserved Neoproterozoic suc-
associatedinNeoproterozoicsequences.Examples cessions and displays evidence of two major glaci-
are known from North and South America (e.g. ations during the Neoproterozoic, the Sturtian
Young,1976;Yeo,1983;Urbanetal.,1992;Klein (:750–700 Ma) and Marinoan glaciation (:
and Beukes, 1993; Graf et al., 1994), Africa (e.g. 650–600Ma;Preiss,1987;Preissetal.,1993).The
Breitkopf, 1986; Bu¨hn et al., 1992), China (Rui widespreadSturtianglaciationeventismanifestin
and Piper, 1997), and South Australia (Whitten, the Umberatana Group (Preiss et al., 1998), and
1970). In some cases, the iron-rich rocks are particularly in the great thicknesses of
present as iron-rich clastic sediments (e.g. South glaciomarine sedimentary rocks deposited in the
Australia; Whitten, 1970; China; Rui and Piper, fault-controlled Baratta Trough, extending from
1997), whereas others occur as ironstones (e.g. the central Flinders Ranges to the Yunta-Olary
Klein and Beukes, 1993). However, despite nu- region in eastern South Australia (Sumartojo and
merous research efforts, the reason for the com- Gostin, 1976; Preiss, 1987; Preiss et al., 1993; Fig.
mon association of iron- and carbonate-rich and 1). Much of the glaciogenic sedimentation in the
glaciogenic rocks in the Neoproterozoic, the ap- Umberatana Groupischaracterised bydiamictite,
parent glaciation in low-latitude environments laminated siltstone and orthoquartzite, but in
during that time, and the source of chemical places there are distinctive intercalated dolomitic
components and genesis of ironstones remain and ferruginous units (Preiss, 1987; Preiss et al.,
controversial. 1993). In this paper, diamictite is a nongenetic
Neoproterozoic sedimentary rocks of the Ade- term referring to poorly sorted siliciclastic sedi-
laideGeosynclineinSouthAustraliaandfarwest- mentary rocks containing a wide range of class
ern New South Wales host well preserved sizes in an abundant fine-grained matrix in which
glaciomarine sequences and associated ferrugi- the clasts are dispersed so that most of them are
nous units (Preiss, 1987; Preiss et al., 1993; Fig. not in contact (cf. Panahi and Young, 1997).
1). These ferruginous rocks are rich in magnetite The section in the Barratta Trough comprises
and:or hematite, iron-bearing silicates and car- the diamictite- and quartzite-dominated Pualco
bonates. The purpose of this paper is to describe Tillite (3300 m) and overlying siltstone- and sand-
the geochemical composition of Braemar iron- stone-dominated Benda Siltstone (260 m; Preiss et
stones, to establish the genetic processes responsi- al., 1993). Both units pass laterally into the lentic-
ble for their formation and the ular, ferruginous Braemar ironstone facies in the
palaeoenvironment of deposition, and to discuss Yunta-Olary region and the thinner Holowilena
their genesis in light of other models for Ironstone (130 m) in the central Flinders Ranges
Neoproterozoic glaciogenic and iron-rich rock (Preiss, 1987; Preiss et al., 1993). In places, the
occurrences. Pualco Tillite passes vertically into the Braemar
ironstone facies (also known as the Hoof Hearted
Formation). The latter, although locally lenticu-
2. Geology lar,iswidespreadintheYunta-Olaryregionanda
possibleequivalentispresentsouthwestofBroken
The Adelaide Geosyncline in South Australia is Hill in western New South Wales (Preiss, 1987).
a major, deeply subsident Neoproterozoic to In the Yunta area, four to six lenticular ironstone
Cambrian sedimentary basin which overlies units grade into the host diamictites and siltstones
B.G. Lottermoser, P.M. Ashley:Precambrian Research101(2000)49–67 51
with decreasing iron minerals, but also contain images. The ironstones are particularly prominent
dolostone beds and quartzites (Whitten, 1970; at Razorback Ridge south of Yunta (Fig. 2),
Figs. 2 and 3). Throughout the Yunta-Olary re- where the thickest iron-rich sub-units have been
gion, ironstone occurrences crop out prominently evaluated as a potential iron ore resource (Whit-
(Fig. 4A) and are interbedded with diamictites, ten, 1970). Although at some locations, there is
carbonate-rich rocks, quartzites (in part with only one prominently ferruginous horizon, at
heavy mineral lamination), siltstones and man- many locations, there are several zones (e.g. 3 or
ganiferous siltstones. The distribution of the iron- 4), separated by tens to hundreds of metres of
stone and associated ferruginous siltstones and otherstrata,e.g.intheBimbowrieHillregionand
diamictites is especially notable on aeromagnetic at Razorback Ridge (Fig. 3).
Fig.1.LocationoftheAdelaideGeosynclineinSouthAustraliashowingtheinferreddistributionofSturtianferruginousfaciesof
the Umberatana Group in the Baratta Trough (modified from Preiss et al., 1993).
52 B.G. Lottermoser, P.M. Ashley:Precambrian Research101(2000)49–67
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 occurrences in the Olary-Yunta region. In addi-
tion, 17 samples (BR13–28, BR48) were collected
Thirty-nine samples (BR1–12, BR29–47, from the exploration adit dump at Razorback
BR49–56) were taken from surface outcrop and Ridge. Thin and polished thin sections and blocks
included laminated and diamictic ironstones, silt- were prepared and subsequently investigated by
stones, diamictites and carbonate-rich rocks optical microscopy. Twenty-seven laminated iron-
which were representative of the rock types and stone, clastic and carbonate sediment samples
B.G. Lottermoser, P.M. Ashley:Precambrian Research101(2000)49–67 53
werecrushedandpulverisedinachromesteelring isotope mass spectrometry on 15 rock samples
mill. Major and trace elements were analysed by was conducted at the Centre for Isotope Studies,
X-ray fluorescence on duplicate fused discs and CSIRO, Sydney, following conventional CO gen-
2
pressed powder pellets at the Division of Earth eration using phosphoric acid. Electron mi-
Sciences, University of New England (UNE). Se- croprobe analyses were performed on garnets,
lected rare earth elements (REE, La to Lu; carbonatesandchloritesofironstoneandsiltstone
LREE: light REE, La to Sm; HREE: heavy REE, samples at UNE.
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- 4. Petrography and mineralogy
tion analysis at Becquerel Laboratories, Sydney.
REE concentrations exceeded the detection limits The Braemar ironstone facies consists of lentic-
by several orders of magnitude. In addition, data ular laminated and diamictic ironstones interbed-
on geochemical reference materials were within ded in calcareous or dolomitic siltstone including
10% of the accepted values. Oxygen and carbon several 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).
54 B.G. Lottermoser, P.M. Ashley:Precambrian Research101(2000)49–67
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: 420700 mE, 6455650 mN. (B) Diamictic ironstone with recrystallised
carbonate-rich siltstone, quartz and carbonate clasts (sample BR24). Razorback Ridge, AMG: 379740 mE, 6352770 mN. (C)
Laminated ironstone. Darker laminae are rich in magnetite and hematite, and lighter laminae in siliciclastic and carbonate
components(sampleBR6).Fieldofviewapproximately30mmlong,notescalebarinmillimetres.IronPeak,AMG:384100mE,
6353900 mN. (D) Laminated ironstone with interbedded lighter coloured siltstone displaying cross-laminations and soft-sediment
deformation(sampleBR28).RazorbackRidge,AMG:379740mE,6352770mN.(E)Laminatedironstonewithinterbeddedlighter
coloured siltstone displaying soft-sediment deformation (sample BR16). Razorback Ridge, AMG: 379740 mE, 6352770 mN.
The Braemar ironstone facies is made up of two cal compositionally.
types, diamictic and laminated ironstones, which Themineralogyofthelaminatedironstonesand
are substantially different in macroscopic appear- the matrix of diamictic ironstones is simple: com-
ance(Fig.4B–E)butapartfromtheclasts,identi- mon facies are fine-grained (typically B0.05 mm)
B.G. Lottermoser, P.M. Ashley:Precambrian Research101(2000)49–67 55
and composed of magnetite, hematite and quartz apatite, plagioclase and tourmaline. Associated
with minor muscovite, chlorite, biotite, carbonate, siltstones contain abundant quartz, biotite, car-
Fig. 4. (Continued)
56 B.G. Lottermoser, P.M. Ashley:Precambrian Research101(2000)49–67
Fig. 4. (Continued)
bonate, plagioclase, muscovite, chlorite, variable of magnetite and hematite, ranging from :80 to
amounts of magnetite and hematite and traces of :20%. Magnetite grains display varying degrees
clinozoisite:epidote, tourmaline, zircon and pyrite of martitization and larger subhedra are up to 0.1
(altered to goethite due to supergene oxidation). mm in diameter. Rare pressure shadows of chlor-
Detrital mineral grains and lithic clasts occur in ite and:or biotite are well developed adjacent to
laminated and diamictic ironstones and siltstones. magnetite–hematite porphyroblasts. However,
They are angular to subrounded and include scat- much hematite is not weathering-related, because
tered detrital grains of quartz, carbonate, plagio-
grains display a preferred orientation oblique to
clase, K-feldspar, muscovite and tourmaline,
compositional laminations, and late dilational
foliated sediments, siltstones, quartzites, and
veins contain magnetite, quartz, carbonate,
quartzofeldspathic and quartz-carbonate rocks.
goethite and platy hematite. Therefore hematite
Detrital feldspars have been variably replaced by
is, in part, syn- or pre-tectonic. Locally, nearly
carbonate, muscovite and traces of chlorite and
pure layers of Fe-oxides (:80%) are present,
biotite, and biotite has been retrogressed to chlor-
with magnetite, hematite and quartz forming a
ite and traces of rutile.
metamorphic granoblastic aggregate.
Diamictic ironstones are massive and clasts
There are large variations in modal proportions
range in size from 10 mm to 1.2 m but are most
of the major rock-forming minerals quartz, Fe
commonly between 25 mm and 150 cm (Fig. 4B).
oxides, carbonate and silicates within single rock
A few striated boulders were noted by Whitten
specimens thereby forming fine layers of iron-
(1970). Angularity and nature of the detrital
grains and lithic clasts are similar to those found stones and siltstones. Fe oxide-rich laminae dis-
in the associated laminated ironstones and play sharp or gradational bases with associated
siltstones. siltstone layers. The Fe-oxide beds are commonly
Laminated ironstones are usually inequigranu- graded with magnetite decreasing and abundances
lar with grain sizes ranging from B0.1 to 5 mm. of quartz and silicates increasing. Other sedimen-
Laminationisgenerallywelldevelopedandranges tary structures include cross-laminations (Fig.
from B0.5 mm to 1 cm in thickness (Fig. 4C–E). 4D), microfaulting and piercement of laminae,
Thelaminaearedefinedbytherelativeabundance and micro- to meso-scale folding of laminae (Fig.
B.G. Lottermoser, P.M. Ashley:Precambrian Research101(2000)49–67 57
4D–E) which may be the result of soft-sediment defined by the preferred orientation of layer sili-
deformation. cates and hematite plates. The subhedral shape of
The Braemar ironstone facies has undergone the magnetite crystals, the presence of rare por-
regional metamorphism and deformation. The phyroblastic magnetite grains, together with the
rocks display interlocking aggregates of mineral occurrence of magnetite:hematite-bearing veins
grains and rare porphyroblastic Fe oxide and and foliated hematite, indicate that the magnetite
carbonate grains. Slaty cleavage is commonly 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 (Fe Mn Ca Mg CO )
0.01–0.10 0.00–0.03 0.48–0.53 0.37–0.46 3
and ferroan calcite (Fe Mn Ca
0.01–0.06 0.00–0.01 0.92–
0.99Mg CO ) in composition and chlorite is
0.00–0.02 3
typically ripidolite (Si 2.61–2.73 atoms per for-
mula unit and atomic Fe:Fe(cid:27)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 (py
2.6–3.2
alm spess gross uvar a-ndra
4.2–9.0 82.l–87.2 1.4–2.2 0–0.1 3.5–
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 SiO and Fe O . All ironstones
2 2 3
consist of \70 wt.% SiO (cid:27)Fe O (all Fe as
2 2 3
Fe3(cid:27)) with Fe O ranging between 22.94 and
2 3
78.91 wt.% (N(cid:30)20) (Table 1 and Fig. 5). Minor
element contents of the ironstones show some
variations, with Al O ranging from 0.28 to 10.64
2 3
wt.%, CaO from 0.10 to 5.82 wt.%, K O from
Fig. 5. Ternary plot of (a) Si(cid:3)Fe(cid:3)Al, (b) Si(cid:3)Fe(cid:3)(Ca(cid:27)Mg), 2
and(c)Al(cid:3)(Ca(cid:27)Mg)(Na(cid:27)K)forironstones((cid:14);N:20)and 0.03 to 3.43 wt.%, MgO from 0.02 to 3.76 wt.%,
clastic sediments ((cid:9); N: 6). Na O from 0.10 to 3.11 wt.% and LOI from 0.20
2
5
Table 1 8
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
Sample R74203 BR30 BR38 BR45 BR15 BR36 BR8 BR13 BR40 BR52 BR53
Mineralogy ca-qz-pl- 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-
lithic clasts pl-ms-chl- lithic clasts- chl-ms-pl- bio-ms-tm- chl-bio-ca chl-bio-ca bio-qz-ca pl-chl-bio- ca-chl:bio
go-tm-zir bio-mt-ca-m ap-tm ap-pl go
s-chl
Location Bimbowrie Razorback Oultalpa Mt Mulga Razorback Bimbowrie Iron Peak Razorback Oultalpa Braemar Braemar
B
ridge NW S Ridge Hill Ridge NW .G
Northing 6459900 6352610 6440620 6443250 6352770 6454240 6353900 6352770 6440620 6326240 6325150 .
L
Easting 421800 379090 411760 433700 379740 422790 384100 379740 411760 371820 371820 o
tte
r
m
SiO2 40.10 18.58 67.13 65.65 47.86 40.54 28.12 28.68 14.81 33.29 23.54 os
TiO2 0.68 0.16 0.83 0.70 0.57 0.48 0.22 0.33 0.18 0.37 0.26 er,
AFel2OO3 107..1298 34..1966 115..9028 1103..0560 275..8522 367..4260 662..9727 632..9374 728..8901 439..1766 662..7240 P.M
2 3 .
MnO 6.29 0.36 0.13 0.04 0.15 0.30 0.04 0.06 0.11 0.18 0.23 A
MgO 2.80 13.26 3.15 1.56 3.76 3.27 0.88 1.28 2.02 2.22 1.59 sh
CaO 14.12 22.94 3.34 1.63 3.68 3.49 0.28 0.64 0.33 3.86 1.98 ley
:
Na2O 2.44 1.65 2.24 3.49 0.26 1.29 0.05 1.32 0.11 1.66 0.80 Pr
KO 2.19 0.15 2.60 1.82 2.55 1.77 0.82 0.32 1.37 0.19 0.24 e
2 ca
PO 0.17 0.02 0.19 0.37 0.46 0.68 0.23 0.24 0.15 0.64 0.94 m
2 5 b
S 0.01 0.00 0.01 0.01 0.01 0.01 0.01 0.00 0.01 0.02 0.03 r
ia
LOI 12.57 34.49 2.63 1.20 6.43 3.99 0.20 0.49 0.20 5.16 2.11 n
Total 98.80 99.73 99.26 100.04 99.06 99.49 100.54 99.66 101.00 100.53 100.67 Re
s
e
a
r
c
As na na B0.5 na 1.1 2.5 B0.5 1.8 B0.5 3.9 10.4 h
Au na na B5 na B5 B5 B5 B5 B5 B5 B5 10
1
Ba 353 29 527 530 444 271 145 43 201 61 180 (2
Cu 9 8 18 13 14 44 24 20 29 30 27 00
0
Ga 16 B1 15 15 14 10 13 10 10 8 8 )
4
Hf na na 8.2 na 3.8 2.7 1.6 2 0.7 1.9 1.1 9
–
Nb 15 2 13 16 7 9 3 8 6 6 4 67
Ni 56 4 16 14 17 19 5 6 B5 7 14
Pb 12 3 17 13 7 8 7 10 6 9 B5
Rb 115 5 123 108 109 101 37 16 96 5 9
Sb na na 0.4 na B0.2 0.4 B0.2 0.5 0.6 0.7 2.2
Sc na na 12.5 na 12 10.4 5 5.3 5.2 6.3 8.9
Sr 152 525 81 91 104 146 24 33 7 56 77
Ta na na 1.3 na 0.9 0.9 B0.5 0.8 B0.5 0.7 B0.5
Th na na 16 na 9.6 8.9 4.2 6.5 2.4 6.3 4.7
U na na 2.8 na B1 c1 B1 B1 B1 B1 B1
V 76 2 54 56 87 60 80 88 85 52 113