Jean-Clément BEYEME ZOGO

STRATIGRAPHY OF THE HOTAZEL IRON FORMATION IN THE KALAHARI MANGANESE FIELD AND

THE EVIDENCE OF LATE MICROBIAL ACTIVITY

Paleoproterozoic Mineralization Research Group – Geology Department

INTRODUCTION

BACKGROUND

• Kalahari Manganese Field

• Hotazel Iron Formation

RESULTS

• Core Logging

• Petrography

• SEM-EDS Spectrum

• SEM Mapping

DISCUSSION

• Comparative Description

CONCLUSION

INTRODUCTIONThe 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?

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.

BACKGROUND: HOTAZEL IRON FORMATIONThe 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

RESULTS: CORE LOGGINGIn 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.

RESULTS: PETROGRAPHY

Euhedral Hematite Carbonate Manganese Detrital Zircon Alteration Minerals

Stromatolite Radial Hematite Pisoliths Pisoliths/Ooids

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;

RESULTS: SEM EDS SPECTRUM

EDS Chemistry Al Si Ca Ti Mn FeRim (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 27.75Ratio (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.

RESULTS: SEM MAPPING

Mostly composed of iron, and calcium with more less manganese while the surrounding is made of silicon, aluminum and potassium

RESULTS: SEM MAPPING

• The outer rim is more iron rich than the membrane – While the membrane is richer calcium and other elements

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/earthscienceandengineering/rocklibrary/viewglossrecord.php?gID=00000000253

• 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/

• 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