School of Engineering and Science - Jacobs University

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PRESERVATION OF PRIMARY REE PATTERNS WITHOUT CE ANOMALY
Figure 1. Simplified stratigraphic position of the Mooidraai and
Hotazel formations of the Transvaal Supergroup, South Africa.
Its stratigraphic position is indicated in Figure 1.
Deposition of iron formations and intercalated
manganiferous oxides and carbonates of the Hotazel
Formation followed the extrusion of the Ongeluk
basaltic andesites that define the base of the Voëlwater
Subgroup. The Hotazel Formation is best known as host
to the enormous resource of manganese ores that
constitute the Kalahari Manganese Field. Conformably
above the Hotazel Formation follow shallow marine
limestones and dolomites of the Mooidraai Formation.
Carbonate slump breccias and sideritic carbonate units
are restricted to the lower part of the Mooidraai
Formation, whereas microbiolaminated and stromatolitic
carbonates predominate the upper part. The contact of
the Mooidraai Formation with the discordantly overlying
Mapedi shales and quartzites of the Olifantshoek
Formation is erosional. The Mooidraai carbonates did
neither experience significant metamorphism nor
deformation. A detailed description of the geological
setting of the Mooidraai Formation can be found in
Beukes (1983) and Swart (1999).
The petrography of the limestones and of the
silicified dolomites has been described by Tsikos et al.
(2001) and Bau et al. (1999), respectively. Note that the
dolomite samples examined here originate from the
stromatolitic upper part of the Moodraai Formation (Bau
et al., 1999); the limestone samples, on the other hand,
are thought to represent the complete succession (Tsikos
et al., 2001). The geology and the C-O-Sr isotope
geochemistry of the Hotazel and Mooidraai formations
are discussed by Schneiderhan et al. (2006).
Unfortunately, the respective absolute ages of the
Hotazel and the Mooidraai Formation are not very well
constrained. No radiometric age is available for the
Hotazel Formation and the Pb-Pb carbonate “age” for
the Mooidraai dolomites discussed here is 2394 +/- 26
Ma (Bau et al., 1999). Since the reliability of Pb-Pb
carbonate ages is a matter of concern, the correct
depositional age of the Hotazel and Mooidraai sediments
is still unclear. The currently most widely accepted age
for the Voëlwater Subgroup is bracketed by an illconstrained
2222 Ma age for the underlying Ongeluk
lava (the problems associated with the data on which
the Ongeluk age is based have been addressed by Bau
et al., 1999) and by the 2060 Ma ages of the Bushveld
and Molopo Farms igneous complexes (e.g., Walraven
et al., 1990, Walraven and Hattingh, 1993; Reichardt,
1994; Buick et al., 2001). If the stratigraphic correlation
between the Ongeluk and the Hekpoort extrusives is
correct, a recent Re-Os age of 2316 +/- 7 Ma for pyrite
in a carbonaceous shale from the base of the Timeball
Hill Formation (Hannah et al., 2004) below the
Hekpoort basalt narrows the depositional age of the
Hotazel and Mooidraai formations down to between
~2.32 and ~2.06 Ga. However, the “normal” 13 C values
(Figure 2) of the Mooidraai carbonates (Bau et al., 1999;
Tsikos et al., 2001) demonstrate that they formed
before the Lomagundi Event, i.e. before the
worldwide positive excursion of 13 C values of marine
carbonate deposited between ~2.25 Ga and ~2.07 Ga
ago (Aharon, 2005, and references therein). Hence, the
current best estimate suggests that the Hotazel
iron and manganese formations and the Mooidraai
carbonates formed between ~2.32 and ~2.25 Ga ago.
The Hotazel Formation and the Mooidraai Formation,
therefore, represent a well-preserved Mid-
Paleoproterozoic succession of shallow marine chemical
sediments.
Results
The chemical compositions of limestone and silicified
dolomite from the Mooidraai Formation have been
presented by Tsikos et al. (2001) and Bau et al. (1999),
respectively, and will only be summarized here (see
these publications for data). Additional data can be
found in Swart (1999) and Schneiderhan et al. (2006),
but since REY data are incomplete or not available, we
will not consider these in the present contribution which
focusses on the REY distribution.
The Mooidraai limestone samples (Figure 2) are only
slightly silicified (CaO: 34.07 to 52.56%; MgO: 0.60 to
1.94%; SiO 2 : 1.39 to 13.50%) and show low MnO (0.14
to 1.30%) but high Fe 2 O 3 (2.42 to 23.92%). The most Feenriched
carbonates occur in the bottom and top parts
of the Mooidraai Formation (Tsikos et al., 2001). Low
Al 2 O 3 contents (0.10 to 0.53%) indicate that the amount
of detrital aluminosilicates in the chemical sediment is
negligible. Ba and Sr concentrations range from 9 ppm
to 105 ppm, and from 789 ppm to 1524 ppm,
respectively. Concentrations of individual REY vary
within a factor of six between samples; Nd
concentrations, for example, range from 0.26 ppm to
1.69 ppm. REY SN patterns (‘ SN ’: indicates normalization
to post-Archean Australian Shale, PAAS, from McLennan,
1989; note that Tsikos et al. (2001) do not report Y data)
are strongly HREE-enriched (Figure 3).
The Mooidraai dolomite (Figure 2) discussed here is
strongly silicified (CaO: 15.68 to 17.74%; MgO: 10.04 to
11.59%; SiO 2 : 39.3 to 46.5%). MnO and Fe 2 O 3 range from
0.25 to 0.30% and from 1.26 to 1.44%, respectively,
SOUTH AFRICAN JOURNAL OF GEOLOGY