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 magnetitehematite-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 Fe
0.01 – 0.10
Mn
0.00 – 0.03
Ca
0.48 – 0.53
Mg
0.37 – 0.46
CO
3
and ferroan calcite Fe
0.01 – 0.06
Mn
0.00 – 0.01
Ca
0.92 –
0.99Mg
0.00 – 0.02
CO
3
in composition and chlorite is typically ripidolite Si 2.61 – 2.73 atoms per for-
mula unit and atomic FeFe + Mg, 0.27 – 0.63. Calculations using chlorite compositions on the
AlIV – 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 B 0.05mm
granoblastic carbonate, garnet, magnetite, quartz, plagioclase, muscovite and phlogopite Holm,
1995. Garnet is typically spessartine py
2.6 – 3.2
alm
4.2 – 9.0
spess
82.l – 87.2
gross
1.4 – 2.2
uvar
0 – 0.1
a-ndra
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
2
and Fe
2
O
3
. All ironstones consist of \ 70 wt. SiO
2
+ Fe
2
O
3
all Fe as Fe
3 +
with Fe
2
O
3
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 Al
2
O
3
ranging from 0.28 to 10.64 wt., CaO from 0.10 to 5.82 wt., K
2
O from 0.03 to 3.43 wt., MgO from 0.02 to 3.76 wt.,
Na
2
O from 0.10 to 3.11 wt. and LOI from 0.20
Fig. 5. Ternary plot of a SiFeAl, b SiFeCa+Mg, and c AlCa+Mg Na+K for ironstones ; N: 20 and
clastic sediments ; N: 6.
B .G
. Lottermoser
, P
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Research
101 2000
49 –
67
Table 1 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
BR38 R74203
ca-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-chlbio
chl-bio-ca bio-ms-tm-
bio-qz-ca pl-ms-chl-
pl-chl-bio- lithic clasts-
chl-bio-ca chl-ms-pl-
lithic clasts ap-tm
go ap-pl
go-tm-zir bio-mt-ca-m
s-chl Mt Mulga
Razorback Braemar
Bimbowrie Oultalpa
Iron Peak Razorback
Razorback Oultalpa
Bimbowrie Location
Braemar NW
NW S
Ridge Ridge
Hill ridge
6 454 240 6 353 900
Northing 6 352 770
6 459 900 6 440 620
6 326 240 6 325 150
6 352 610 6 440 620
6 443 250 6 352 770
422 790 384 100
379 740 Easting
411 760 421 800
371 820 371 820
379 090 411 760
433 700 379 740
47.86 40.54
28.12 28.68
14.81 18.58
33.29 65.65
23.54 67.13
40.10 SiO
2
TiO
2
0.68 0.16
0.83 0.70
0.57 0.48
0.26 0.22
0.37 0.33
0.18 Al
2
O
3
10.19 3.16
11.92 10.06
7.82 6.46
2.92 3.97
2.74 2.80
3.16 Fe
2
O
3
7.28 4.96
5.08 13.50
25.52 37.20
66.77 66.20
62.34 49.76
78.91 MnO
6.29 0.36
0.13 0.04
0.15 0.30
0.04 0.06
0.23 0.11
0.18 MgO
2.80 2.22
13.26 2.02
3.15 1.28
1.56 0.88
3.76 3.27
1.59 CaO
14.12 22.94
3.34 1.63
3.68 3.49
0.28 0.64
1.98 0.33
3.86 Na
2
O 2.44
1.66 1.65
0.11 2.24
1.32 3.49
0.05 0.26
1.29 0.80
K
2
O 2.19
0.15 2.60
1.82 2.55
1.77 0.24
0.82 0.19
0.32 1.37
P
2
O
5
0.17 0.02
0.19 0.37
0.46 0.68
0.23 0.24
0.94 0.15
0.64 S
0.01 0.02
0.00 0.01
0.01 0.00
0.01 0.01
0.01 0.01
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
100.67 99.26
100.04 99.06
99.49 100.54
99.66 101.00
100.53 As
2.5 na
B 0.5
1.8 B
0.5 3.9
10.4 na
B 0.5
na 1.1
na B
5 B
5 B
5 B
5 B
5 B
5 B
5 Au
na na
B 5
Ba 271
353 145
43 201
61 180
29 527
530 444
44 24
20 29
30 14
27 9
Cu 13
18 8
14 16
10 13
10 10
8 8
B 1
15 15
Ga 2.7
1.6 2
0.7 1.9
1.1 Hf
na na
8.2 na
3.8 9
3 8
6 6
7 4
15 Nb
16 13
2 17
56 19
5 6
B 5
7 14
4 16
14 Ni
8 7
10 6
9 Pb
B 5
12 3
17 13
7 101
37 16
96 5
109 9
115 Rb
108 123
5 B
0.2 na
0.4 B
0.2 0.5
0.6 0.7
2.2 na
0.4 na
Sb 10.4
5 5.3
5.2 6.3
8.9 Sc
na na
12.5 na
12 146
24 33
7 56
104 77
81 91
Sr 152
525 0.9
na 0.9
B 0.5
0.8 B
0.5 0.7
B 0.5
na 1.3
na Ta
8.9 4.2
6.5 2.4
6.3 4.7
Th na
na 16
na 9.6
c1 B
1 B
1 B
1 B
1 B
1 B
1 na
U na
na 2.8
87 76
60 80
88 85
52 113
2 54
56 V
B .G
. Lottermoser
, P
.M .
Ashley Precambrian
Research
101 2000
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67
59 Table 1 Continued
BR36 BR8
BR13 BR40
Sample BR52
R74203 BR53
BR30 BR38
BR45 BR15
Feox-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-ca lithic clasts
chl-bio-ca bio-ms-tm-
bio-qz-ca chl-ms-pl-
pl-chl-bio- ca-chlbio
lithic clasts- ap-tm
go-tm-zir go
bio-mt-ca-m ap-pl
s-chl Mt Mulga
Oultalpa Razorback
Razorback Bimbowrie
Bimbowrie Location
Iron Peak Razorback
Braemar Oultalpa
Braemar Ridge
NW NW
ridge Hill
S Ridge
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
421 800 433 700
411 760 379 090
B 1
1.3 B
1 B
1 B
1 W
9.7 na
na B
1 na
B 1
33 28
26 13
31 24
49 Y
33 27
15 25
61 42
52 33
38 101
17 37
20 46
Zn 31
92 51
73 26
65 130
40 157
Zr 142
266 38
21.8 na
22.9 10.6
5.41 6.21
13.4 15.2
na 33.2
na La
na 43.4
49.1 22.9
11.4 13.4
29.7 33.8
Ce na
na 69.9
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
4.76 na
na 6.54
na 4.74
1.37 0.62
0.44 0.56
0.89 0.99
1.23 Eu
na 1.5
na na
0.75 na
1.11 0.62
0.45 0.37
0.66 1.06
na 1.06
na Tb
1.46 0.96
0.75 0.53
0.95 1.03
1.54 na
Ho na
1.38 na
2.62 na
3.45 2.60
2.39 1.27
2.82 4.04
na 3.14
na Yb
na 0.41
0.46 0.35
0.36 0.17
0.40 0.58
Lu na
na 0.38
2.52 2.65
1.97 2.24
2.78 2.01
3.19 2.89
LaSm
cn
5.52 5.17
3.15 1.56
3.79 1.12
2.72 9.08
LaLu
cn
1.64 1.20
TbLu
cn
0.85 1.48
3.48 1.24
1.89 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 andor 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 LaSm
cn
, LaLu
cn
, and TbLu
cn
are calculated using chondrite values given by Boynton 1984.
to 7.4 wt.. Such variations reflect different modal contents of magnetite andor 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 Fe
2
O
3
, P
2
O
5
and V contents than the ironstones and Al
2
O
3
, TiO
2
, Na
2
O, K
2
O, 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 sources 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 possess REE concentra-
tions REE: La+Ce+Nd+Sm+Eu+Tb+ Ho + Yb + Lu ranging from 30.05 to 115.16 ppm
and chondrite normalised LaSm
cn
ratios of 1.97 – 2.89, LaLu
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 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 r B + 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,
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 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,
LaLu
cn
and TbLu
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 SiO
2
and A1
2
O
3
contents and individual REE distributions into two different suites. Iron-
stones are here called siliceous, aluminous if SiO
2
\ 40 wt. and A1
2
O
3
\ 6 wt. samples
BR15, BR36; Table 1 and silica-, alumina-poor if SiO
2
B 40 wt. and A1
2
O
3
B 6 wt. samples
BR8, BR13, BR35, BR40, BR52, BR53; Table 1. Samples from the same locality can have different
SiO
2
and A1
2
O
3
contents and REE distributions. Silica-, alumina-poor ironstones have the lowest
REE concentrations and the lowest LaSm
cn
, LaLU
cn
and TbLU
cn
ratios samples BR8,
Carbon isotope values vary greatly d
13
C
PDB
− 5.5 – + 0.9‰, however, d
13
C
PDB
isotopic signa- tures are nearly all negative, whereas oxygen
isotope values range from d
18
O
SMOW
+ 10.6 to +
29.5‰ Table 2. The distinctly negative d
13
C
PDB
values of Braemar facies samples are in agreement with the pronounced negative d
13
C
PDB
values of marine carbonates in Neoproterozoic successions
cf. Kaufman et al., 1991; Kaufman and Knoll, 1995. Negative d
13
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
d
13
C
PDB
depletion upsection in several successions of widely separated Neoproterozoic basins and
suggested that more negative d
13
C
PDB
values 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 d
18
O
SMOW
+ 15.0 – + 29.5‰ and
d
13
C
PDB
values d
13
C
PDB
− 5.0 – + 0.9‰
than those
in the
Braemar area
d
18
O
SMOW
+ 10.6 – + 28.3‰;
d
13
C
PDB
− 5.5 – − 2.2‰
Table 2.
Lower d
13
C
PDB
values 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
