Geosciences

1. Jean-Clément BEYEME ZOGO
STRATIGRAPHY OF THE HOTAZEL IRON FORMATION
IN THE KALAHARI MANGANESE FIELD AND
THE EVIDENCE OF LATE MICROBIAL ACTIVITY

2. INTRODUCTION
BACKGROUND
• Kalahari Manganese Field
• Hotazel Iron Formation
RESULTS
• Core Logging
• Petrography
• SEM-EDS Spectrum
• SEM Mapping
DISCUSSION
• Comparative Description
CONCLUSION

3. INTRODUCTION
The Kalahari Manganese Field is situated in extremely
well-preserved 2.2 Ga sedimentary and volcanic rocks
of the Transvaal Supergroup, near Hotazel in the
Northern Cape Province of South Africain- in the
Western margin of the 3.1 Ga Kaapvaal Craton:
• Representing by far the largest known land based
manganese deposit on Earth (≈ 4 billion tons);
• World ‘s top producer of manganese ore metal
tonnage;
• Hosting a world renowned, unique and varied
assemblage of aesthetic collectors’s mineral
specimens.
Manganese beds occur along the stratigraphy of Hotazel
Iron Formation with BIFs and it consists of :
• Iron formation bands alternating with
• Three manganese beds
• Two iron ores formed at different time and by
different processes ?
In the attempt to evaluate the potential iron ore of
Hotazel Iron Formation, several boreholes were
accessed - and detailed core logging led to the
discovery of ferruginous clastic textured mudstones/
shales in boreholes from the Northern part of KMF.
• Petrography: Presence of alteration minerals and
unknown forms of micro-organismes
• Hematite Clast Dating: 2.0 -1.9 Ga (not discussed);
• EDS Spectrum and Mapping show that these
structure are composed of Fe, Ca, Mn ± Si, Al, Ti .
• Microbial activities oxidation of primary iron?

4. BACKGROUND: KALAHARY GEOLOGY
The Kalahari Manganese Field consists of rocks of the
Transvaal Supergroup in the Griqualand West Region
located in the Northern Cape Province.
The Kalahari Manganese Field is covered by the Kalahari
sand and the red-beds of Elim-Olifantshoek Groups
followed by the Beaumont Formation.
The Kalahari Manganese Field was affected by a series of
deformation which led to the formation of three
metamorphic belts namely:
• Magondi-Limpopo Belt (2,050 - 1,950 Million years)
• Kheis Belt (1,280 – 1,180 Million years)
• Namaqualand Belt (1,040 – 1,030 Million years)
Both Kheis and Namaqualand affected the ore deposits of
the KMF
Specially the Kheis, major thrust fault system called the
Blackridge Thrust caused duplication of the Transvaal
beds
In some areas, rocks of the Transvaal Supergroup have
been eroded – and young strata form the
Gamagara/Mapedi Formation are in contact with the
Hotazel Iron Formation.

5. BACKGROUND: HOTAZEL IRON FORMATION
The Hotazel Iron Formation occurs on top of
the Ongeluk Lava and is overlain by the
Mooidraai Formation. In broad stratigraphy
terms, the Hotazel Iron Formation consists of:
• A banded iron formation with
• Interbbeds of manganese ore
But the detailed stratigraphy of the Hotazel
varies from one locality to another.
In the Southern part of the KMF:
• The banded iron formations consist of
siderite chert magnetite rhythmite iron
formations;
• The manganese beds are composed of
minnesotaite ribbon and bandlutite;
• The transition between the iron and
manganese beds is marked by pisolitic
kutnahorite hematite lutite and hematite
braunite lutite.
In the northern part of the KMF:
• Iron formations have been oxidized into
hematite rhythmite
• The transitional zones into MnFe ores

6. RESULTS: CORE LOGGING
In the Northern part of the Kalahari Manganese
Field, the lithostratigraphy of the Transvaal
Supergroup has been significantly affected and
the Hotazel’s stratigraphy as well:
• The Mooidraai Formation has been eroded;
• The Gamagara/Mapedi Formation is resting
unconformably on top of the Hotazel;
• The banded iron formations has been
oxidized into ore;
• Breccia/conglomeratic and oxidized iron
formations;
• Ferruginous clastic textured mudstones were
identified on drillcores; Intercepting oxidized
breccia iron formations;
• Presence of pisoliths and old hematite in the
ferruginous mudstones;
Ferruginous clastic textured mudstones have
been interpreted as paleosols in similar
environments.

7. RESULTS: PETROGRAPHY
Euhedral Hematite Carbonate Manganese Detrital Zircon Alteration Minerals
Stromatolite Radial Hematite Pisoliths Pisoliths/Ooids

8. RESULTS: PETROGRAPHY
Ooids or Pisolith? Spherical forms in voids/hematite Radial or needle like hematite Aggregates of unknown forms
Aggregated no Cement Multiple forms: Rim + Membrane Surrouding Zircon and Hematite Different sizes and Forms
Several forms like microbial activity are found in the voids – surrounded by hematite (massive or specularitic);
No specific form: spherical; elongated, flatten; plein with or without inner core – sometimes surrounding hematite;

9. RESULTS: SEM EDS SPECTRUM
EDS Chemistry Al Si Ca Ti Mn Fe
Rim (1) 1.92 3.31 3.05 0.71 1.53 57.65
Membrane (2) 7.88 11.74 9.91 0.46 5.48 27.75
Ratio (1)/(2) 0.24 0.28 0.31 1.57 0.28 2.08
• It is notorious to determine the mineralogical
composition - Exception in the case of hematite and
zircon – Presence alteration minerals as chlorite.

10. RESULTS: SEM MAPPING
Mostly composed of iron, and calcium with more less manganese while the surrounding is made of silicon,
aluminum and potassium

11. RESULTS: SEM MAPPING
• The outer rim is more iron rich than the membrane – While the membrane is richer calcium and other
elements

12. DISCUSSION: Comparative Description
• Ooids or Ooliths : are small coated spherical or
ellipsoidal sedimentary grains (2mm diameter)
usually composed of calcium carbonate but
sometimes made-up of iron or phosphate base
minerals – They form by chemical precipitation of
cryptocrystalline iron hydroxides on available
grains on the seafloor, from seawater enriched
with Fe, Al and Si – The enrichment can be a
result of hydrothermal fluids, volcanic ash falls
in shallow basins or rapid weathering of fresh
volcanic rocks;
• Pisoids: are similar to ooids but larger than 2mm;
• Iron ooliths: Iron ooliths consist of a central
nucleus which is an iron mineral, quartz grain,
broken fossil (calcite or phosphate) or a heavy
mineral (zircon, rutile, lenxocence etc.) and
concentric layers around the nucleus, composed
mainly of iron minerals.
• Two types are found: chamosite or goethite type
of ooliths.
• Goethite ooliths are more abundant and have
greater economical importance.
• Pisooid or Pisolith is a concentric sedimentary
grain, >2mm in diameter.
A PPL image showing abundant mollusc
bioclast, peloids, and rounded intraclasts in a
sparry calcite cement. Intraclasts include
burrows infilled by calcite. Field of view 10 mm.
A PPL image showing grain-supported ooliths
and bioclasts with sparry calcite cement and
interparticle porosity. Field of view 10 mm.
A XPL image showing an oolith with a micrite
nucleus surrounded by a cortex with radiating
calcite. The grain is surrounded by sparry
calcite cement. Field of view 1 mm.
https://wwwf.imperial.ac.uk/earthscie
nceandengineering/rocklibrary/viewgl
ossrecord.php?gID=00000000253

13. • Three major mechanisms are currently implicated in
the oxidation of FeII to FeIII:
1. abiotic O2 precipitation,
2. UV driven photochemical Fe2+ oxidation and;
3. Microbial transformation.
• In addition to the low Archaean O2 levels, recent
experimental studies have rejected photochemical Fe2+
oxidation as a potential major contributor to BIF
formation (Konhauser et al., 2002; Potsh et al.,
2011).
• Instead, it has been suggested through microbial culture
studies and biogeochemical extrapolations that BIF
deposition in the stratified Precambrian iron-rich
oceans was entirely possible under the activity of the
iron-oxidizing bacteria, especially with the
involvement of anoxygenic photoferrotrophs
(Kappler et al., 2005).
• Morphologically, some of the elongated Hotazel’s
organisms or forms look similar to oxidizing bacteria.
• Most scientific believe that the amount of atmospheric
oxygen was insignificant up until ≈ 2.4 Ga , when the
Great Oxidation Event (GOE) occurred .
• Pisolith and hematite clasts from the ferruginous
mudstone/shale were dated and the results show ages
variations between 1.9 and 2.0 Ga.
DISCUSSION: Comparative Description
A micrograph of an iron-oxidizing bacteria,
Acidimicrobium ferrooxidans.Credit: DOE,
Joint Genome Institute –
http://www.astrobio.net/news-exclusive/rock-
bands-spin-an-oxygen-record/

14. • Hotazel Iron Formation is part of the Postmasburg Group
and hosts most manganese beds of the Transvaal
Supergroup in the Kalahari Manganese Field.
• Transvaal Supergroup is represented by several
kilometres of thick succession of sedimentary and
volcanic rocks that were deposited in a shallow sea
environments.
• Tectonic events that affected the rocks of the Transvaal
Supergroup after its deposition, took place in the same
environmental settings.
• The presence of red-beds or paleosols has been used as
evidence for highly oxygenated atmosphere and possible
hot and humid climatic conditions.
• Hotazel micro-organisms vary from 1 to 20μm – very
small compare to oolith and pisolith – and show different
forms and shapes- without inner core as bacteria.
• Their occurrence is restricted in the empty or pore
spaces- Surrounded by hematite (massive or needle).
Justification for their preservation.
• Presence of Euhedral hematite grains (old magnetite).
• Hotazel’s micro-organisms might have formed in
response to oxidized primary iron minerals.
CONCLUSION