Variations in carbonate mineralogy and chemistry of the Griquatown and Kuruman Iron Formations
1. Variations in carbonate mineralogy and
mineral chemistry of the Griquatown
and upper Kuruman Iron Formations
and their possible controls
G.A. Ballantyne (B.Sc. Geology and Chemistry)
Supervisor: Professor H. Tsikos
Rhodes University
Department of Geology
PRIMOR
2. Background
• Mn increases stratigraphically upwards in BIFs of
the Transvaal Supergroup (Ghaap Group)
(Rafuza, 2015)
• Hosts of manganese are exclusively carbonate
minerals such as ankerite Ca(Fe, Mg,Mn)(CO3)2
and siderite (Fe(Mg, Mn)CO3) .
• BIFs in general are very Mn poor. Thus an
increase of Mn in BIF is potentially very
significant with respect to atmosphere-ocean
oxygenation and the formation of manganese
deposits around the Great Oxidation Event.
3. The Transvaal Supergroup Stratigraphy
2.39 Ga
2.45 Ga
2.56 Ga
2.22 Ga
u/c (?)
u/c
POSTMASBURGGROUPGHAAPGROUP
Hotazel Fm
(BIF + Mn)
Ongeluk Fm
(andesite)
Mooidraai Fm
(carbonate)
Makganyene Fm
(diamictite)
Griquatown &
Kuruman Fms
(BIF)
Cambellrand
Subgroup
(carbonate)
500m
4. Log of Lo Core
Transition to Kuruman
GriquatownFmKurumanFm
5. Aims
• Examine the distribution of siderite and ankerite in the
selected samples;
• Do the two carbonates coexist on a fine scale at all?
• When they do, are they associated with similar non-
carbonate mineralogy?
• What are their textural relationships, if/when they coexist?
• Are they chemically comparable in every instance (e.g. do
they both show predictable Mg/Mn ratio variation across
stratigraphy).
• Ultimately, I wish to constrain whether:
• The two carbonates are entirely diagenetic or possibly
primary in origin;
• If primary, whether an active carbon cycle was in operation
during the formation of BIF in the Palaeoproterozoic ocean.
6. Methods
Primarily a petrographic study which I made use of
the following:
• Transmitted light microscopy
• EPMA
• XRF
• Trace elements (ICP-MS)
• XRF and trace elements analysed from powdered
bulk rock.
7. Mineralogy
Mineral Group Mineral
Carbonates Ankerite
Siderite
Calcite
Oxides Magnetite
Hematite
Silicates Greenalite
Minnesotaite
Stilpnomelane
Riebeckite
Chert (Quartz)
Sulphides Pyrite
13. Overview of suboxic Fe-Mn diagenesis
• If the carbonates form entirely via diagenetic processes, Mn &
Fe will enter the carbonate structure when they are reduced
by bacteria in the sediment that are able to utilize manganese
and iron oxides.
• Mn will be reduced first because thermodynamically it is the
better e- acceptor making it a more favourable species for
bacteria.
• Diagenetic model predicts that if Mn increases in carbonates
there would be a subsequent decrease in Fe in carbonates i.e.
Mn and Fe should anticorrelate.
• However, the data of this study suggests differently with Mg
and Mn summed versus Fe to be the actual anticorrelation.
15. Conclusion
• A new mechanism of carbonate deposition should be adopted
in modelling of carbonate fraction within Kuruman and
Griquatown IFs.
• Primary precipitation of carbonates out of the water column
seems like a plausible alternative as opposed to a diagenetic
mechanism.
• Precipitation of carbonates out of the water column would
then have recorded a unique chemical signal in terms of the
stratified water column in which they formed and would then
be incorporated into the sediment on arrival from the water
column.
16. References
Bau, M. and Dulski, P. (1999) Comparing yttrium and rare earths in
hydrothermal fluids from the Mid-Atlantic Ridge: implications
for Y and REE behaviour during near-vent mixing and for the
Y/Ho ratio of Proterozoic seawater. Chem. Geol. 155, 77–90.
Rafuza, S. (2015) – Carbonate Petrography and Geochemistry of
BIF of the Transvaal Supergroup: evaluating the potential
of Iron Carbonates as proxies for Palaeoproterozoic Ocean
Chemistry. MSc. Thesis. Rhodes University, Grahamstown,
South Africa.
Tsikos, H. (2015) – Sedimentary Iron Deposits. Lecture slides.
Rhodes University, Grahamstown, South Africa
