Fig. 7. Composition of Braemar ironstones and associated clastic sediments in terms of FeTi versus AlAl + 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.
FeMnCo+Co+Ni10×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 FeTi ratio and increases the proportion of A1 with
respect to the hydrothermalhydrogenous ele- ments, Fe and Mn cf. Barrett, 1981. This trend
is illustrated in the FeTi versus AlAl + Fe + Mn diagram Fig. 7 whereby pure chemical sed-
iments,
mixed chemical – detrital
sediments, terrigenous sediment and pelagic clay plot on a
compositional curve. The Braemar ironstones and clastic sediments plot on the chemical – detrital
mixing curve and ironstones cluster toward the chemical sediment end.
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
