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Variations in carbonate mineralogy and mineral chemistry of the Griquatown and upper Kuruman Iron Formations and their possible controls By: Gordon Allan Ballantyne Supervisor: Professor Harialos Tsikos Thesis submitted in partial fulfilment of the requirements for degree of Bachelor of Science in Honours in the Department of Geology, Rhodes University, Grahamstown

Hons Thesis

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Variations in carbonate mineralogy and

mineral chemistry of the Griquatown

and upper Kuruman Iron Formations

and their possible controls

By: Gordon Allan Ballantyne

Supervisor: Professor Harialos Tsikos

Thesis submitted in partial fulfilment of the requirements for degree of Bachelor of Science in

Honours in the Department of Geology, Rhodes University, Grahamstown

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Declaration

I declare that this thesis, titled “Variations in carbonate mineralogy and mineral chemistry of

the Griquatown and upper Kuruman Iron Formations and their possible controls” is my own

work, and sources of information from publications and other references is adequately cited.

The submission of this thesis is in compliance for the fulfilment of the Bachelor of Science in

Honours degree in the Department of Geology at Rhodes University, Grahamstown, South

Africa.

__________________________ __________________________

Name of candidate Signature

Signed on ________ day of __________________ 2016.

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“That's the thing about rocks--they don't break easily. When I held them,

I wanted to be like them-strong and steady, weathered but not broken.”

― Ellen Dreyer, The Glow Stone

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Table of Contents Acknowledgements ................................................................................................................. viii

Abstract ...................................................................................................................................... x

List of abbreviations .................................................................................................................. xi

Chapter 1. Introduction ............................................................................................................. 1

1.1 Regional Geology – The Transvaal Basin ......................................................................... 1

1.2 Geology of the Griqualand West Basin .......................................................................... 2

1.2.1 Ghaap Group (Griquatown and Kuruman iron formations) ........................................ 4

1.3 Aims and objectives ............................................................................................................. 5

2. Methodology .......................................................................................................................... 6

2.1 Sampling strategy............................................................................................................. 6

3. Petrography ........................................................................................................................... 8

Sample: Lo 01 (127.75m) ....................................................................................................... 9

Sample Lo 01 (127.75 m) ..................................................................................................... 10

Sample Lo 03 (145.70 m) ..................................................................................................... 11

Sample Lo 12 (249.50 m) ..................................................................................................... 12

Sample Lo 12 (249.50 m) ..................................................................................................... 13

Sample Lo 15 (284.70 m) ..................................................................................................... 14

Sample Lo 18 (320.90 m) ..................................................................................................... 15

Sample Lo 20 (345.45) ......................................................................................................... 16

Backscattered images .......................................................................................................... 17

Evidence for a low diagenetic effect on the Griqualand West Basin .................................. 19

4. Geochemistry ....................................................................................................................... 20

4.1 Introduction ................................................................................................................... 20

4.2 Sampling strategy and analytical methods .................................................................... 20

4.3 Results ............................................................................................................................ 22

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4.3.1 XRF .......................................................................................................................... 22

4.3.2 Traces ...................................................................................................................... 24

4.3.3 EPMA data ............................................................................................................... 27

4.3.3.1 Ankerite ................................................................................................................ 27

4.3.3.2 Siderite ................................................................................................................. 28

4.3.4 Ratio relationships .................................................................................................. 33

4.3.5 Summary ................................................................................................................. 37

5. Discussion ............................................................................................................................. 39

5.1 BIF research -the road thus far ...................................................................................... 39

5.2 Anoxygenic phototrophic Fe(II)-oxidation – a possible mechanism for BIF deposition?

.............................................................................................................................................. 40

.............................................................................................................................................. 41

5.3 A new look at the formation of the Griqualand West BIFs ........................................... 41

5.3.1 Accommodating the geochemical data from this study ......................................... 41

5.4 Implications of this study ............................................................................................... 45

6.Conclusion ............................................................................................................................. 47

6.1 Significances of this study .............................................................................................. 47

6.2 Proposed future research .............................................................................................. 47

7. References ........................................................................................................................... 48

Appendices .................................................................................................................................. i

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Table of Figures

FIGURE 1. DISTRIBUTION AND GROSS STRATIGRAPHIC SUBDIVISION OF THE TRANSVAAL SUPERGROUP IN THE STRUCTURAL BASINS OF

GRIQUALAND WEST AND TRANSVAAL. FROM BEUKES (1983). ................................................................................... 2

FIGURE 2. LOG OF THE LO SECTION EXAMINED IN THIS STUDY. RED TRIANGLES INDICATE POINTS OF SAMPLING. LOGS WERE PROVIDED

COURTESY OF PAUL OONK (2016), PHD STUDENT, RHODES UNIVERSITY, GEOLOGY DEPARTMENT.................................... 6

FIGURE 3. GREEN STAR INDICATES THE LOCALITY OF THE LO DRILL CORE IN CLOSE PROXIMITY TO THE KALAHARI MANGANESE FIELD

WITH THE LOCAL REGIONAL GEOLOGY. FROM TSIKOS (2015). ..................................................................................... 7

FIGURE 4. SAMPLE LO 01 IN THE UPPER STRATIGRAPHY OF THE EXAMINED SECTION. OCCURRING MINERALS ARE: MAGNETITE, CHERT,

STILPNOMELANE AND CARBONATE. SCALE BAR AT THE TOP OF THE IMAGE...................................................................... 9

FIGURE 5. SAMPLE LO 01 FROM THE UPPER STRATIGRAPHY OF THE EXAMINED SECTION. SAME THIN SECTION AS PREVIOUS FIGURE

(FIGURE 4) NOT HOW QUICKLY THE MODAL ABUNDANCES OF THE MINERALS CHANGE. SCALE BAR AT THE TOP OF THE IMAGE. 10

FIGURE 6. SAMPLE LO 03 FROM NEAR THE TOP OF THE EXAMINED STRATIGRAPHY. THIN SECTION CONTAINS CARBONATE, MAGNETITE,

STILPNOMELANE, QUARTZ AND HEMATITE. THIS WAS THE ONLY HEMATITE OBSERVED THROUGHOUT THE ENTIRE EXAMINED

SECTION. SCALE BAR AT THE TOP OF THE IMAGE. ..................................................................................................... 11

FIGURE 7. SAMPLE LO 12 IS SAMPLED AROUND MID-WAY IN THE STRATIGRAPHY OF THE EXAMINED SECTION. NOTE THE 'SEA' OF

CHERT. OTHER MINERALS INCLUDE MAGNETITE AND CARBONATE. SCALE BAR AT THE TOP OF THE IMAGE. .......................... 12

FIGURE 8. SAMPLE LO 12 FROM AROUND MID-WAY IN THE STRATIGRAPHY. NOTE HOW THERE IS AN FE-SILICATE AND OXIDE RICH

BAND COMPARED TO A FE-SILICATE AND OXIDE POOR BAND LYING IN SUCCESSION TO ONE ANOTHER. MINERALS INCLUDE:

MAGNETITE, CARBONATE, CHERT, RIEBECKITE AND STILPNOMELANE. SCALE BAR AT THE TOP OF IMAGE.............................. 13

FIGURE 9. SAMPLE LO 15 IS FROM AROUND MID-WAY IN THE EEXAMINED STRATIGRAPHY. NOTE THE CHERT RICH MATRIX AND

SEEMINGLY HOW THE CARBONATES ARE 'FLOATING' ON IT. SCALE BAR IS AT THE TOP OF IMAGE. ....................................... 14

FIGURE 10. SAMPLE LO 18 IS STARTING TO RANSITION INTO THE KURUMAN IF. THE RIEBECKITE IS VERY WHISPERY AND FIBROUS.

OTHER MINERALS INCLUDE: CARBONATE, CHERT AND STILPNOMELANE. SCALE BAR AT THE TOP OF IMAGE. ......................... 15

FIGURE 11. SAMPLE LO 20 IS NEAR THE BASE OF THE STRATIGRAPHY IN THE EXAMINED SECTION. NOTICE HOW COARSE THE RIEBECKITE

HAS BECOME. THERE IS ALSO A HIGH ABUNDANCE OF CALCITE HERE. OTHER MINERALS INCLUDE: CARBONATE AND MAGNETITE.

SCALE BAR IS AT THE TOP OF IMAGE. .................................................................................................................... 16

FIGURE 12. (ON THE NEXT PAGE) MINERAL OCCURRENCES AND TEXTURAL RELATIONSHIPS OF THE GRIQUATOWN AND KURUMAN IRON

FORMATION. SCALE AT THE BOTTOM OF IMAGES. ................................................................................................... 17

FIGURE 13. MAJOR OXIDE CONCENTRATIONS IN WT% FOR FE2O3, MN3O4, MGO AND CAO. NOTE THE SPIKE IN MN3O4 NEAR THE

TOP OF THE STRATIGRAPHY OF THE EXAMINED SECTION. ........................................................................................... 23

FIGURE 14. REE DIAGRAM NORMALISED AGAINST PAAS. THE PROFILE SHOWS A RELATIVELY FLAT SLOPE WITH A POSITIVE EU

ANOMALY THAT VARIES THROUGHOUT THE STRATIGRAPHY........................................................................................ 24

FIGURE 15. MN3O4 FROM XRF ANALYSIS PLOTTED AGAINST NI, CU AND CO TO SEE IF ANY OF THESE TRACE METALS ARE BEHAVING IN

A SIMILAR FASHION TO MN. ............................................................................................................................... 25

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FIGURE 16. BA AND ZR APPEAR AS IF THEY ARE BEHAVING IN A SIMILAR FASHION TO MN. HOWEVER, BA LIKES ENTERING

CARBONATES SO THIS COULD HAVE TO DO WITH THE CARBONATE FRACTION AND ZR IS A KNOWN DETRITAL ELEMENT. .......... 26

FIGURE 17. EPMA MAJOR OXIDE (FEO, MNO, MGO, CAO) COMPOSITIONAL VARIATIONS FOR ANKERITE AGAINST STRATIGRAPHY.

NOTE THE SPIKE OF INCREASED ABUNDANCE IN MN UP STRATIGRAPHY........................................................................ 30

FIGURE 18. EPMA SIDERITE MAJOR OXIDE (FEO, MNO, MGO, CAO) COMPOSITIONAL VARIATIONS AGAINST STRATIGRAPHY. NOTE

THE SPIKE OF INCREASED ABUNDANCE IN MN UP STRATIGRAPHY. ............................................................................... 32

FIGURE 19. FEO VERSUS MNO RELATIONSHIPS FOR ANKERITE A), AND SIDERITE B). ............................................................... 34

FIGURE 20. UNEXPECTED ANTI-CORRELATION BETWEEN MGO AND MNO SUMMED VERSUS FEO FOR ANKERITE A), AND SIDERITE B).

NOTE HOW MUCH BETTER THE R2 ARE IN THIS RELATIONSHIP COMPARED TO THE MNO VERSUS FEO RELATIONSHIP. ........... 35

FIGURE 21. JUXTAPOSITION OF ANKERITE AND SIDERITE MG + MN : FE RATIO PROFILES AGAINST STRATIGRAPHIC HEIGHT WITH BULK

ROCK MN:LOI FROM XRF ANALYSIS. DUE TO THE RELATIVELY LOW RESOLUTION OF THE DATA IN THIS STUDY THERE ARE ONLY A

FEW REFERENCE POINTS FOR THE EPMA DATA, IF THERE WERE MORE THE LIKELY HOOD OF THE THREE PROFILES LOOKING

SIMILAR WOULD BE GREATER. ............................................................................................................................. 37

FIGURE 22. DIRECT MICROBIAL FE (II) OXIDATION VIA ANOXYGENIC FE(II)-OXIDIZING PHOTOTROPHY (MODIFIED FROM POSTH ET AL.,

2010A). ........................................................................................................................................................ 41

FIGURE 23. RED BARS REPRESENT AVERAGE VALUES FOR THE EON WHICH THEY REPRESENT, NAMELY: ARCHEAN, MID-PROTEROZOIC

AND NEOPROTEROZOIC-PHANEROZOIC. C. THE PRESENCE OF SIGNIFICANT MO ENRICHMENTS IN THE ARCHAEAN (ARROW)

SUGGESTS THE PRESENCE OF OXIDATIVE PROCESSES AT LEAST AS FAR BACK AS 2.5 GYR AGO. FROM LYONS ET AL., (2014). .. 44

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List of Tables

TABLE 1. STRATIGRAPHY OF THE GRIQUALAND WEST TRANSVAAL SEQUENCE. NOTE THAT THE SHADED BOXES REFER TO THE

STRATIGRAPHY THAT WAS EXAMINED IN THIS STUDY. MODIFIED AFTER (TSIKOS, 1999; RAFUZA, 2015; FRYER, 2016) ......... 3

TABLE 2. IDENTIFIED MINERAL GROUPS IN THE GRIQUATOWN AND UPPER KURUMAN BIFS ARE PRESENTED IN THE TABLE BELOW.

RELATIVE ABUNDANCES ARE DESIGNATED BELOW (FORMAT ADAPTED FROM RAFUZA, 2015). ........................................... 8

TABLE 3. PARAGENETIC SEQUENCE AND EXPECTED GRADE OF DIAGENESIS OF BIF MINERAL ASSEMBLAGES EXPECTED FOR THE

RESPECTIVE METAMORPHIC GRADES. MODIFIED AFTER KLEIN (1983) AND RAFUZA (2015) ........................................... 19

TABLE 4. ANKERITE MAJOR-OXIDE CONCENTRATIONS WITH STRATIGRAPHIC HEIGHT FOR THE LO DRILL CORE. TOTALS ARE CALCULATED

BY THE SUM OF MAJOR ELEMENT CONCENTRATIONS EXCLUDING CO2. ........................................................................ 29

TABLE 5 SIDERITE MAJOR-OXIDE CONCENTRATIONS WITH STRATIGRAPHIC HEIGHT FOR THE COLLECTED LO DRILL CORE SAMPLES.

TOTALS ARE CALCULATED BY THE SUM OF MAJOR ELEMENT CONCENTRATIONS EXCLUDING CO2. ...................................... 31

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Acknowledgements

I would firstly like to thank my parent Allan and Leanne Ballantyne for their unconditional

love and support that they have bestowed upon me my entire life. It is thanks to them that I

have had all the essentials I have ever needed through their continued dedication as parents.

Thank you for always giving me courage and support when I am down and for instilling the

values I have today, I would not be the person I am if it wasn’t for parents like you.

I would like to thank South32 from the Hotazel Manganese mines for making the Lo drill core

available for this project. Your continued support of research will inspire a new generation of

researchers.

Thank you to the honours class of 2016 that has displayed an absolutely impeccable meaning

of fellowship throughout the year. It has been a real honour getting to know each one of you

personally. I will forever cherish the fond memories that were made and the fun times that

were had on the honours field trip.

I would also like to thank my best friend John de Bruyn who continually demonstrates what

it is to be a compassionate and considerate human being. Your support throughout the year

both in and out of the academic realm has been truly astounding. I hope that our friendship

will continue to grow and one-day lead to bigger endeavours in the working world.

Andrea, Chris and Thulani are thanked as part of the technical team for processing my

samples and making thin sections. As well as Deon for his expertise and guidance on the

probe. Your contribution to this thesis is great appreciated.

All the lecturing staff of the geology department are thanked for their continuous efforts

throughout my undergraduate years as well as honours year. None of the research and

knowledge instilled in us would be possible without you.

Lastly I would like to thank my supervisor, Professor Hari Tsikos for his contribution to this

thesis both physically and emotionally. Your passion and enthusiasm is something that has

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always inspired me. I would also like to thank you for your willingness and time that you have

given me and this project. It has been a privilege to be part of the PRIMOR team. I wish you,

your current and future research candidates all the best with the future research that lies

ahead.

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Abstract

Analytical techniques have improved immensely over the past few decades which

have allowed us to answer complex questions that were once not considered possible. Such

techniques have allowed the growth of research along with the ability to challenge current

theories. In this thesis rocks of the Transvaal Supergroup were examined from the Griqualand

West Basin in the Northern Cape province, South Africa with special focus on the Griquatown

and upper Kuruman iron formations. The aim of this study is to establish whether Mn and Fe

produce an antithetic relationship or not when entering the structure of carbonates (i.e

ankerite and siderite) as well as the mechanism of formation – diagenetic or primary?

Geochemical analyses were conducted via XRF on bulk rock powder samples, ICP-MS for trace

elements and finally electron probe micro-analyser (EPMA) to target specific carbonate

grains. The purpose of these analyses was to gain a better understanding of the high and

somewhat anomalous increase in manganese abundance that is apparent towards the top of

the stratigraphy with respect to the Mn/Fe ratio and attempt to gain insight on the formation

mechanism with regard to carbonates. There was no antithetic relationship between Mn and

Fe via, EPMA analyses, however it was found that the antithetic behaviour of these elements

when entering carbonates is best described by Mg summed with Mn versus Fe i.e.

Mg + Mn/Fe. The rare earth elements displayed a ‘flat’ profile suggesting a seawater

environment with a positive Eu excursions suggesting a hydrothermal source, however, these

excursions were not constantly of the same magnitude suggesting another component/s was

at play with mixing taking place. It appears that the atmosphere was not oxidative at the time

of formation of the studied BIFs according to the trace elements. Ba and Co did show slight

correlations with respect to Mn however, this could be attributed to other processes taking

place. It is proposed that the formation of carbonates is not a diagenetic one due to the

behaviour of the trace metals, i.e. Mn and Fe were in the sediment as oxides but rather

formed as carbonates via a primary process of precipitation out of the water column.

Keywords: banded iron formation, BIF, Northern Cape, South Africa, Griquatown, Kuruman,

carbonate, ankerite, siderite, water column, diagenetic.

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List of abbreviations

1. BIF – banded iron formation

2. Calcite – Cal

3. Carbonate – Cb

4. GOE – great oxidation event

5. Hematite – Hem

6. IF – iron formation

7. Magnetite – Mag

8. Quartz – Qtz

9. Riebeckite – Rbk

10. Stilpnomelane – Stp

11. PAAS – post Archean average shale

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Chapter 1. Introduction

1.1 Regional Geology – The Transvaal Basin

It is believed that the 2.65 to 2.05 Transvaal Supergroup, which is essentially a sequence

perched on a platform, consists of a package of chemical sedimentary rocks that developed

over a large part of the Kaapvaal Craton which is said to form the core of Southern Africa

(Beukes 1983, Moore et al, 2001; McCarthy and Rubidge, 2005). The Transvaal Supergroup is

developed in two spatially adjacent basins (Figure 1), the Transvaal Basin which is located in

the central part of the Kaapvaal Craton and the Griqualand West Basin which is located along

the western edge of the craton (Beukes and Gutzmer, 2008; Moore et al, 2001). The former

basin confines the Bushveld Complex towards the east, whereas the latter basin is confined

at the western Kaapvaal margin which extends sub-surface beneath younger Kalahari

deposits into southern Botswana (Beukes and Gutzmer, 2008; Moore et al, 2001). The focus

of this thesis will comprise of banded iron formations (BIFs) from the Griqualand West Basin

with particular emphasis on carbonate mineralogy with relation to manganese. From a

petrographic point of view, the stratigraphy that will be examined is of the Griquatown and

upper Kuruman iron formations. According to Knoll and Beukes (2009) the Griqualand West

Basin has been severely eroded and most likely covered the entire Kaapvaal Craton at the

time of formation and in some areas had a stratigraphic thickness up to 11Km.

The stratigraphy of the Transvaal Supergroup is diverse in a textural as well as chemical sense.

It contains a large variety of lithologies that demonstrate complex lateral and vertical facies

disparities across the basin (Fryer, 2016). An analysis of these facies changes show that the

individual formations of the Transvaal Supergroup were deposited in environments that

ranged from deep-water basinal settings right through into very shallow platform settings

above normal wave base (Klein and Beukes, 1989; Beukes and Klein, 1990; Klein, 2005; Beukes

and Gutzmer, 2008). This thesis as a whole draws on the idea of a shallowing up system as

well as plausible cycles of transgression and regression.

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This thesis will give particular attention and will only deal with the rocks of the Griqualand

West basin that host the Transvaal Supergroup in the Northern Cape Province, with specific

emphasis on its lower portion that hosts the Griquatown and Kuruman iron formations of the

Asbesheuwel Subgroup.

1.2 Geology of the Griqualand West Basin

As mentioned earlier, the Griqualand West basin is located along the western edge of the

Kaapvaal Craton. In Table 1 below, the stratigraphy of the basin is shown in a relatively simple

format.

Figure 1. Distribution and gross stratigraphic subdivision of the Transvaal Supergroup in the structural basins of Griqualand West and Transvaal. From Beukes (1983).

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Table 1. Stratigraphy of the Griqualand West Transvaal Sequence. Note that the shaded boxes refer to the stratigraphy that was examined in this study. Modified after (Tsikos, 1999; Rafuza, 2015;

Fryer, 2016)

Two major groups can be found within the Griqualand West sequence namely: The Ghaap

group, which is located in the lower stratigraphy, and the Postmasburg group which is located

in the upper stratigraphy (table 1 above). The Ghaap group will be discussed in the coming

sections especially with respect to the Asbesheuwels Subgroup with special emphasis placed

on the Griquatown and Kuruman iron formations.

Supergroup Group Subgroup Formation Lithology Approx.

thickness (m)

Tran

svaa

l

Postmasburg

Voëlwater Mooidraai

Carbonate ± Chert

300

Hotazel BIFs, Mn Ore 250

Ongeluk Andesite 500

Makganyene Diamictite 50 - 100

Ghaap

Koegas (2.415 Ga)

Nelani

Siliciclastic, BIF + Mn Ore

240 - 600

Rooinekke

Naragas

Kwakwas

Doradale

Pannetjie

Asbesheuwels

Griquatown Clastic

Textured BIF 200 - 300

Kuruman Microbanded BIF

150 - 750

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1.2.1 Ghaap Group (Griquatown and Kuruman iron formations)

Since the early 70’s a considerable amount of work and research has gone into the research

of the Griqualand West Basin with regard to the sedimentology, stratigraphy, depositional

palaeo-environment and geochemistry of the Ghaap group which is located in the lower

stratigraphy of the Transvaal Supergroup (see table 1 above) (Beukes, 1983, 1987), Klein and

Beukes (1989), and Beukes and Klein (1990). This work was conducted in order to clarify

stratigraphic transitions between the lower Campbellrand Subgroup and Asbesheuwels

Subgroup as well as stratigraphic transitions between the Kuruman and Griquatown iron

formations that form part of the Asbesheuwels Subgroup. Beukes and Klein (1990) have

thoroughly described the transitions from the Kuruman to the overlying Griquatown iron

formation. The base of the transition is comprised of the Riries member, which following on

the relatively chert-rich underlying Groenwater member, is a chert-poor greenalite-siderite

rhythmite.

Beukes (1983, 1984) and Beukes and Klein (1990) reported in their research that the

Asbesheuwels subgroup (table 1 above) is divided into two texturally different iron

formations which form the subject of this thesis, namely, the lower microbanded 150-750m

thick Kuruman iron formation, and the clastic-textured orthochemical and allochemical 200-

300m thick Griquatown iron formation. However, the Kuruman and Griquatown iron

formations are hard to distinguish geochemically from one another even though they have

blatant textural differences between them and their genesis is therefore interpreted to be

under broadly similar palaeo-environmental conditions (Beukes and Klein, 1990).

Beukes and Klein (1989) and Klein and Beukes (1990) have described the development of the

Ghaap Group as an evolving depositional system. Drowning of the basin later led to the

deposition of the conformably overlying Kuruman iron formation, which is comprised of finely

laminated BIF which are mostly rich in magnetite and Fe-silicate minerals (see petrography

section). Following this, shallowing continued of the basin and ultimately logged the transition

to the Griquatown iron formation (Beukes and Klein, 1990).

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1.3 Aims and objectives

This study will be focussing on the Lo drill core from the Griqualand West basin of the

Transvaal Supergroup in the Northern Cape, South Africa. Essentially this Lo drill core

represents the Griquatown and upper most Kuruman iron formations of the Asbesheuwels

Subgroup. The main aim will be to examine the distribution of carbonates i.e. siderite and

ankerite in the selected samples and compare to previous work done to establish whether

results are reproducible basin wide. This will be achieved through petrographic XRF, ICP-MS,

and microprobe analysis. Ultimately, the author wishes to constrain whether the two

carbonates are entirely diagenetic or possibly primary in origin and if they are primary in

origin ascertain whether an active carbon cycle was in operation during the formation of BIF

in the Palaeoproterozoic ocean.

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2. Methodology

2.1 Sampling strategy

A single drill core was sourced from London farm, Kalahari Manganese field, Hotazel

Manganese mines, South32 for this study (see log below in Figure 2). This specific drill core is

designated as ‘Lo’. It was sourced within 10s of kms of other drill cores from past and ongoing

studies.

Griq

uato

wn

Fm

Ku

rum

an

Fm

Transition to Kuruman

Figure 2. Log of the Lo section examined in this study. Red triangles indicate points of sampling. Logs were provided courtesy of Paul Oonk (2016), PhD student, Rhodes University, Geology Department.

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The green star in Figure 3 below shows the location of where the Lo drill core originates from.

Regional geology is also shown.

Figure 3. Green star indicates the locality of the Lo drill core in close proximity to the Kalahari Manganese Field with the local regional geology. From Tsikos (2015).

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3. Petrography

According to Rafuza (2015), many of the Archean-Palaeoproterozoic iron formations studied

around the world through the decades, are suggested to have undergone only very low grade

metamorphism. These include those of the Transvaal Supergroup and Hamersley Group of

Australia. In areas where BIF is preserved it most commonly contains mineral assemblages

that barely enter the low grade greenschist facies of metamorphism which are typical of late

(burial) diagenesis (Klein 1983, 2005). Table 2 below documents the minerals identified in the

examined section.

Table 2. Identified mineral groups in the Griquatown and upper Kuruman BIFs are presented in the

table below. Relative abundances are designated below (format adapted from Rafuza, 2015).

Key: XXX: Synonymous to abundant component (>20%)

XX: Common component (>5%)

X: Trace component

This chapter deals primarily with the examination of the mineral assemblages in a

petrographic context studied under transmitted light.

Mineral Group Mineral Formula Griquatown Kuruman

Carbonates Ankerite [Ca(Fe2+,Mg,Mn)(CO3)2] XXX XX

Siderite (Fe2+(CO3) XXX XX

Calcite CaCO3 X X

Oxides Magnetite [Fe2+ Fe23+ O4] XX XXX

Hematite [Fe23+O3] X X

Silicates Greenalite [(Fe2+,Mg)6Si4O10(OH)8] X X

Minnesotaite (Fe2+,Mg)2Si4O10(OH)2 X XX

Stilpnomelane [K0.6(Mg,Fe2+,Fe3+)6Si8Al (O,OH)27.2-4H20] XXX XX

Riebeckite Na2(Fe2+3Fe3+

2)Si8O22(OH)2 XXX XXX

Chert (Quartz) SiO2 XXX XXX

Sulphides Pyrite FeS2 X X

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Sample: Lo 01 (127.75m)

Sample Lo 1 contains magnetite, carbonate, chert and stilpnomelane. The relative modal

abundance of these minerals are: magnetite ~35%, carbonate ~25%, chert ~25% and

stilpnomelane ~15%. This sample is relatively fine grained. The magnetite grains are sub-

hedral to euhedral in shape and appear to be randomly distributed with no particular

orientation. They range in size from 200 to 400 µm. The carbonate fraction in this sample is

very fine grained (~100 µm) and appears to be associated with the Fe-silicates and oxides

which is made up of stilpnomelane and magnetite respectively. Stilpnomelane occurs as

reddish brown masses. All the mineralogy appears to ‘float’ on a chert matrix. See Figure 4

below.

Mag

Stp

Cb

Chert

Figure 4. Sample Lo 01 in the upper stratigraphy of the examined section. Occurring minerals are: magnetite, chert, stilpnomelane and carbonate. Scale bar at the top

of the image.

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Sample Lo 01 (127.75 m)

This is the same sample as the previous one. Note the variability in the same sample between

two consecutive bands. Sample Lo 1 contains: carbonate, chert, magnetite and trace amounts

of Stilpnomelane. The relative modal abundances of these minerals are: carbonate ~50%,

chert ~47%, magnetite ~2%, and stilpnomelane ≪1%. This sample contains a much higher

carbonate fraction (~double the amount of carbonate) and very little Fe- silicates and oxides.

The carbonates range in size (~100 to 200 µm) and appear to be sub-hedral in shape.

Magnetite grains have a similar size range to that of the carbonates (~100 to 200 µm) and

appear to be euhedral in shape. Stilpnomelane occurs in trace amounts and plays an

insignificant role with respect to the mineralogy in this band of the thin section. This is another

example of where all the mineralogy appears to be ‘floating’ on a chert rich matrix. See Figure

5 below.

Cb

Mag

Chert

Stp

Figure 5. Sample Lo 01 from the upper stratigraphy of the examined section. Same thin section as previous figure (Figure 4) not how quickly the modal abundances of

the minerals change. Scale bar at the top of the image.

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Sample Lo 03 (145.70 m)

Sample Lo 03 contains stilpnomelane, magnetite, carbonate, quartz and hematite. The

relative modal abundances of these minerals are: stilpnomelane ~45%. magnetite ~25%,

carbonate ~15%, quartz 14% and hematite <<1%. Stilpnomelane occurs as relatively well

defined grain boundaries, it has coherent anhedral grains that are yellowish red in colour and

the grain range in size from ~50 to 250 µm. Magnetite grains exhibit a sub-hedral grain shape

and appear to be randomly orientated. The carbonates in this sample are relatively large (~50

to 20 µm), but not as abundant as the previous sample before this (Lo 01). This sample has

quartz in it rather than chert which means it is slightly coarser grained. This was the only thin

section in the entire section studied to contain hematite, however, only in trace amounts. The

hematite occurs as bright red grains but look more like specks in thin section. See Figure 6

below.

Stp

Mag

Hem

Cb

Qtz

Figure 6. Sample Lo 03 from near the top of the examined stratigraphy. Thin section contains carbonate, magnetite, stilpnomelane, quartz and hematite. This was the only hematite observed throughout the entire examined section. Scale

bar at the top of the image.

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Sample Lo 12 (249.50 m)

Sample Lo 12 contains chert, carbonate and magnetite. The relative modal abundances of

these minerals are: chert ~95% carbonate ~3%, magnetite ~2%. This sample contains a

considerably large amount of chert making it a matrix rich rock. Note that this thin section

has no Fe-silicates and a minimal amount of oxides. The carbonates appear to be sub-hedral

in shape and range from 100 to 200 µm in size. These grains appear to be at random and do

not have any preferred orientation. The magnetite grains seem to be sub to euhedral in shape

in have a size of ~100 µm. See Figure 7 below.

Chert

CB

Mag

Figure 7. Sample Lo 12 is sampled around mid-way in the stratigraphy of the examined section. Note the 'sea' of chert. Other minerals include magnetite and

carbonate. Scale bar at the top of the image.

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Sample Lo 12 (249.50 m)

This is the same sample as the previous one. Note the variability in the same sample between

two consecutive bands. This sample (Lo 12) contains: chert, magnetite, carbonate,

stilpnomelane, riebeckite. The relative modal abundances of these minerals are: chert ~35%,

magnetite ~30%, carbonate ~25%, stilpnomelane ~8%, riebeckite ~2%. Here banding can

be seen. The oxides and Fe-silicates are all together i.e. magnetite and stilpnomelane as well

as riebeckite with some chert, while the carbonates and remaining chert are together in the

next consecutive band. The magnetite occurs as sub-hedral grains and ranges from 50 to 400

µm in size. Stilpnomelane occurs in what appears to be aggregated masses and appears to be

closely associated with the magnetite. Riebeckite grains appear to be bladed in crystal habit,

are highly pleochroic and appear to be overprinting stilpnomelane. Average riebeckite size is

~200 µm. See Figure 8 below.

Mag Stp

Rbk

Chert

Cb

Figure 8. Sample Lo 12 from around mid-way in the stratigraphy. Note how there is an Fe-silicate and oxide rich band compared to a Fe-silicate and oxide poor

band lying in succession to one another. Minerals include: magnetite, carbonate, chert, riebeckite and stilpnomelane. Scale bar at the top of image.

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Sample Lo 15 (284.70 m)

Sample Lo 15 contains chert, quartz and carbonate. The relative modal abundances of these

minerals are: chert ~43%, quartz ~18%, carbonate ~38%, magnetite <<1%. With a

considerable amount of chert, this section of rock is matrix supported. A small pocket of

quartz is present with grains averaging ~30 to 40 µm. Carbonate grains are large relative to

the surrounding mineralogy with ankerite grains averaging ~200 µm and are euhedral in

shape. It appears that the ankerite is being replaced/altered by another mineral. EPMA

analyses conducted on these carbonates (both ankerite and siderite) suggest that no

replacement or alteration has taken place and the analyses returned a good reading for

carbonate. One small grain of anhedral magnetite can be seen, it is approximately 10 µm in

size. See Figure 9 below.

Qtz

Cb

Chert

Figure 9. Sample Lo 15 is from around mid-way in the eexamined stratigraphy. Note the chert rich matrix and seemingly how the carbonates are 'floating' on it.

Scale bar is at the top of image.

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Sample Lo 18 (320.90 m)

Sample Lo 18 contains chert, quartz and carbonate. The relative modal abundances of these

minerals are: chert ~65%, riebeckite ~21%, carbonate ~13%, stilpnomelane <<1%. With a

considerable amount of chert, this section of rock is matrix supported. The dominant chert

matrix gives the appearance that all the other minerals are ‘floating’ on the matrix. Riebeckite

in this section of the rock appears to be whispery/fibrous almost having the texture of a

feather. Riebeckite crystals range from ~100 to 200 µm and are a deep blue colour in PPL.

Carbonates are euhedral in shape and are ~100 µm in size. Some of the carbonates appear

to be zoned. There is a small amount of stilpnomelane that appears to occur in an aggregated

mass, however, there is hardly enough to affect the modal abundances. See Figure 10 below.

Chert

Stp

Rbk

Cb

Figure 10. Sample Lo 18 is starting to ransition into the Kuruman IF. The riebeckite is very whispery and fibrous. Other minerals include: carbonate, chert and

stilpnomelane. Scale bar at the top of image.

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Sample Lo 20 (345.45)

Sample Lo 20 contains riebeckite, calcite, carbonate and magnetite. The relative modal

abundances of these minerals are: riebeckite ~55%, carbonate ~25%, calcite ~15%,

magnetite ~15%. Riebeckite displays a blade like crystal habit that is randomly orientated.

The riebeckite seems to be overprinting all the other mineralogy except the magnetite.

Riebeckite is strongly pleochroic and has a high birefringence. The riebeckite crystals range

from ~100 to 600 µm. This sample has a large amount of calcite, which is relatively rare for

this package of rocks. Due to the chemical similarity of carbonates i.e. ankerite and siderite

relative to calcite it makes it difficult to tell them apart, therefore this was done using EPMA

analysis. Magnetite consists of relatively finer grains (~50 to 200 µm) and are euhedral in

shape. See Figure 11 below.

Rbk

Mag

Cb

Cal

Figure 11. Sample Lo 20 is near the base of the stratigraphy in the examined section. Notice how coarse the riebeckite has become. There is also a high

abundance of calcite here. Other minerals include: carbonate and magnetite. Scale bar is at the top of image.

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Backscattered images

A) Major contrast between ankerite and magnetite

B) Large ankerite (darker grains) with smaller siderites (bright grains) showing

coexistence of the two carbonates

C) Ankerite grains which have a euhedral shape with calcite which is anhedral in shape.

Riebeckite it present as whispery fibres.

D) Coexistence of ankerite and siderite with what looks like could be a replacement

texture?

E) Ankerite with minnesotaite, notice the very characteristic bow-tie texture of the

minnesotaite.

F) Ankerite banding in the presence of minnesotaite.

Figure 12. (on the next page) Mineral occurrences and textural relationships of the Griquatown and Kuruman iron formation. Scale at the bottom of images.

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A B

C D

E F

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Evidence for a low diagenetic effect on the Griqualand West Basin

In the context of this study the degree of diagenesis and/or metamorphism that has taken

place is important. The less diagenesis and/or metamorphism that has taken place the more

confidently the original primary environment can be deciphered. According to Rafuza (2015),

iron silicates are particularly useful as indicators of metamorphic grade, as opposed to Fe-

oxides and carbonates. Klein (1983) studied various iron formations around the world which

had been exposed to various degrees of metamorphism and was able to determine the

paragenetic sequence of these iron formations and listed them in order of increasing grades

of metamorphism (Table 3). The prograde metamorphism of iron-formations produces

sequentially Fe-amphiboles, then Fe-pyroxenes, and finally (at highest grade) Fe-olivine-

containing assemblages. Such metamorphic reactions are isochemical except for

decarbonation and dehydration (Klein, 2005).

According to Klein’s (2005) paragenetic sequence as well as the observed mineralogy studied

in this thesis, these BIFs have undergone very little metamorphism.

Low

Siderite

Riebeckite

Greenalite

"Fe3O4∙H2O" magnetite

Grade of MetamorphismMedium High

Diagenetic Biotite

Zone

Garnet

Zone

Staurolite-Kyanite

and Kyanite ZoneSiliminite Zone

Early Late

Chert Quartz

Stilpnomelane

Talc - Minnesotaite

Dolomite - Ankerite

Calcite

Table 3. Paragenetic sequence and expected grade of diagenesis of BIF mineral assemblages expected for the respective metamorphic grades. Modified after Klein (1983) and Rafuza (2015)

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4. Geochemistry

4.1 Introduction

Tsikos et al., (2010) suggested that older BIFs in the lower Transvaal Supergroup (i.e. the

Griquatown and Kuruman iron formations) may very well record a progressive enrichment in

contained Mn as a precursor signal to the major Mn anomaly in the Hotazel strata.

Work conducted by Fryer (2016) and Rafuza (2015) was carried out based on the above

suggestion of Tsikos et al. (2010) of Mn abundances in the Griquatown and upper Kuruman

BIF from drillcore intersections similar to the Lo core studied here. The results from these two

studies both indeed show a Mn enrichment recorded in the upper Griquatown BIF, although

Mn is hosted entirely within the carbonate fraction of the rock. This study effectively aims to

further contribute to the work of Fryer (2015) and Rafuza (2015) by understanding the

significance of the Mn signal, in light of prevailing models that interpret BIF carbonates as

entirely diagenetic in origin. Furthermore, to whether geochemical data is reproducible

throughout the Griqualand West basin. For these reasons, the majority of samples that were

examined petrographically in the previous chapter were also analysed geochemically via

three methods. The first being major element analyses by X-ray fluorescence (XRF) of bulk

rock sample powders (at Stellenbosch University). Secondly, trace elements were analysed

via ICP-MS where they were scrutinised on REE plots and depth profiles. Lastly, microprobe

analyses of the two carbonate species (conducted in house using the Electron Probe

Microanalyser) namely ankerite and siderite; Interpretations will be made by focussing on the

key mineral-specific elemental oxide abundances together with the trace elements and their

distribution across stratigraphy, together with the petrographic and mineralogical

observations.

4.2 Sampling strategy and analytical methods

8 carbon coated thin sections were chosen from the section studied here in order to gain

analytical results in a broad sense for the examined stratigraphy in this thesis. These thin

sections were used in the EPMA analysis. As an additional aid to using the microscope in

determining the mineralogical make-up of the carbonate fraction in the chosen thin sections,

a combination of EPMA data along with accompanying back-scattered images was used to

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identify coexisting carbonates from one another. As mentioned earlier, the goal of this

exercise was to capture and cross-examine any stratigraphic signal in manganese distribution

with respect to ankerite and siderite or both if they are in coexistence with one another as it

was with the findings of Fryer (2016) and Rafuza (2015). Rafuza (2015) outlined four common

obstacles that hindered the achievement of optimum data towards the above goal, these

obstacles were also present in this study and can be listed as the following:

occasionally substandard polishing and coating some samples;

the very fine-grained nature of the carbonate grains, especially with regards to

siderite;

mixed analytical data, particularly through the detection of Si related to the

groundmass surrounding carbonate grains;

the occurrence of only a single carbonate (specifically ankerite) in many sections.

EPMA data as well as those of XRF and ICP-MS were used with objective of revealing any

broad trends in manganese distribution across stratigraphy. With respect to the EPMA, XRF

and ICP-MS data, these were all used and in conjunction with one another and an attempt

was made from all angles in as much detail as possible with the data on hand to establish the

stratigraphic and mineral-specific behaviour of manganese and other carbonate associated

components (FeO, MgO and CaO) in the examined thin sections and bulk-rock powders.

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4.3 Results

4.3.1 XRF

The method of X-ray fluorescence (XRF) was employed in order to establish quantitatively the

wt% via Fe ore fusion of the manganese present as well as the other carbonate associated

components (i.e. Fe2O3, MgO and CaO from the bulk-rock powders obtained from the Lo

drillcore. This analysis was conducted by Stellenbosch University in Stellenbosch, South Africa.

Table 4 provides the major elements weight percents (as oxides) for the XRF analysis.

From the plots of Figure 13, it is evident that Fe2O3, MgO and CaO all show an overall increase

upwards in stratigraphy. Mn3O4 values range from 0.07-5.04 wt% with an average of 0.69

wt%. Interestingly, Mn3O4 specifically shows relatively low values at the base of the examined

section (averaging around 0.4 wt%) and appears to continue to gradually increase up section,

finally reaching a value of circa 1 wt% near the top. A distinct spike of Mn3O4 is present at

circa 128m depth below surface. Both Fryer (2016) and Rafuza (2015) report this spike in

Mn3O4 and record a two-prong excursion towards high Mn3O4 separated by a plateau of lower

values. This is difficult to see here because of the relatively low resolution of the data

collected.

Fe2O3 exhibits a similar profile to Mn3O4 at the start in the lower stratigraphy, increasing

gradually upwards until about halfway through the examined section. The values then appear

to decrease temporarily and then increase again towards the top. Values of the Fe2O3 XRF

analysis range from 18.01-52.70 wt%, with an average of 36.65 wt%. Highest Fe2O3 value

(52.70 wt%) occurs circa midway through the section at circa 237 m. The MgO values of the

XRF analysis range from 1.52-8.56 wt%, with an average of 3.56 wt%. The stratigraphic profile

shows a characteristic “zig zag” pattern of sharply fluctuating values over short stratigraphic

intervals, with distinct MgO minima towards the base of the examined section. Like the Mn3O4

profile, MgO also records an increase stratigraphically upwards, however, this is broadly

speaking and appears to be only a very slight increase. The highest MgO (8.56 wt%) is

recorded circa three quarters of the way up the stratigraphy at circa 176m. CaO appears to

be pointing to two-prong excursion towards high CaO separated by a plateau of lowers values.

However, CaO does not display any obvious stratigraphic trend but varies rather greatly on

either side of the plateau. CaO values range from 0.35-15.96 wt%, with an average of 4.07

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wt%. Scrutinising the data in Figure 13 it would appear that overall the stratigraphic patterns

of increasing Mn3O4, MgO and broadly speaking Fe2O3 suggest a broad modal increase up

section in the carbonate component, which according to Rafuza (2015) would account for the

increased manganese in the rocks.

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15 25 35 45 55

Dep

th (

m)

Fe2O3(wt%)

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0 2 4 6

Dep

th (

m)

Mn3O4(wt%)

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1 3 5 7 9

Dep

th (

m)

MgO(wt%)

110

160

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260

310

360

0 5 10 15 20

Dep

th (

m)

CaO(wt%)

Figure 13. Major oxide concentrations in wt% for Fe2O3, Mn3O4, MgO and CaO. Note the spike in Mn3O4 near the top of the stratigraphy of the examined section.

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4.3.2 Traces

In addition to the XRF data presented in the foregoing section, trace elements were also

analysed from the bulk rock powders using ICP-MS at Stellenbosch University, Western Cape

province, South Africa. Trace elements are present in concentrations of <0.1% and are

expressed in ppm. Trace elements are also useful in understanding the environment of

formation and this case Rare Earth Element (REE) plots were used to further understand the

environment of the Griquatown and upper Kuruman BIFs. REE diagrams are a useful way of

displaying data because the REE behave geochemically in a similarly. In Figure 14 below are

the REE plots for the rocks examined in the section in this study.

All REE were normalised to PAAS, PAAS values from Cai (2010), in order to establish how

enriched or depleted a certain element is. In this case Figure 14 above shows that Eu has a

positive anomaly, therefore it is enriched relative to the other REEs. This particular profile is

also relatively flat and highly suggestive of a seawater environment. After quantifying Eu by a

ratio of where it is and where it should be, it was found on average to be 1.16 times higher

than it should be while the highest individual case was 2.4 times higher. A positive Eu anomaly

of this sort is indicative of a hydrothermal origin. It would be expected that the Eu anomaly

be constant throughout the stratigraphy if it were a single component system, however, this

0,01

0,10

1,00

10,00

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

PAAS-Normalised REE diagram

Figure 14. REE diagram normalised against PAAS. The profile shows a relatively flat slope with a positive Eu anomaly that varies throughout the stratigraphy.

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is not the case. Therefore, it is evident that a pure hydrothermal origin is not at play here and

the system has an additional component/s and depending on how they are mixed then

determines the observed variations in the Eu anomaly.

Trace metals were also used in order to try and establish a relationship with manganese. This

was done by comparison using depth profiles against Mn3O4 from the XRF bulk rock powder

analysis. Geochemical analyses conducted on manganese nodules in the modern ocean will

almost certainly display an enrichment in trace metals such as Ni, Cu and Co. These

enrichments are caused when manganese oxides precipitate and essentially behave as

‘garbage bins’ by absorbing a lot of other trace metals as well as adsorbing trace metals to

110

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0 2 4 6

Dep

th (

m)

Mn3O4(wt%)

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360

0 5 10 15 20D

epth

(m

)Ni(ppm)

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360

0 100 200 300 400

Dep

th (

m)

Cu(ppm)

110

160

210

260

310

360

0 5 10

Dep

th (

m)

Co(ppm)

Figure 15. Mn3O4 from XRF analysis plotted against Ni, Cu and Co to see if any of these trace metals are behaving in a similar fashion to Mn.

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the surface via ionic bonding. Figure 15 above shows Mn3O4 and the concentrations of Ni, Cu

and Co stratigraphically.

From figure 15 above it is evident that Ni and Cu have no relationship with Mn which shows

that they weren’t acting in concert. The only trace metal to show an increase up stratigraphy

with Mn is Co. Another mechanism is possibly at play here because it is highly improbable

that Co gets adsorbed to the surface otherwise similar patterns would be observed with Ni

and Cu as is observed in the modern ocean. Below in figure 16 are the trace elements Ba and

Zr.

From Figure 16 above it is also apparent that there is an increase in Ba and Zr up stratigraphy.

It is apparent that Ba is behaving very similarly to Co (Figures 16 & 17). Ba is a trace element

that is commonly found in carbonates thus, it is likely to be associated with the carbonate

fraction of the rocks and therefore could have an association with Mn which is not yet well

documented and understood. Zr on the other hand is a known detrital trace element that also

seems to be increasing up stratigraphy like Mn. This increase of Zr is more likely to be

suggesting an increased detrital fraction and therefore possible shallowing of the ocean

environment. The other above mentioned traces (Co and Ba) could also therefore be

associated with an increased detrital fraction and not necessarily be associated with the Mn.

110

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0 20 40 60 80D

epth

(m

)

Zr(ppm)

110

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310

360

0 50 100

Dep

th (

m)

Ba(ppm)

Figure 16. Ba and Zr appear as if they are behaving in a similar fashion to Mn. However, Ba likes entering carbonates so this could have to do with the

carbonate fraction and Zr is a known detrital element.

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4.3.3 EPMA data

The trace data presented in the foregoing section is now followed by EPMA data in which

individual ankerite and siderite grains were targeted in order to understand their mineral

chemistry through the stratigraphy. This was done with the intention of establishing

distinctive geochemical patterns, if any, which when used in conjunction with the XRF data

and preceding petrography may help clarify the potential cogenetic origin of the carbonates

and confine their formation to a specific environment. Due to ankerite’s relatively high

abundance in the examined section compared to siderite, ankerite will be dealt with first then

siderite will follow immediately thereafter. Data that was considered pure for analysis were

averaged for the appropriate elemental oxide (i.e. MnO, FeO, MgO and CaO). It must be

reiterated that the data for this study is of relatively low resolution compared to previous

work done (e.g. Fryer, 2016; Rafuza, 2015).

4.3.3.1 Ankerite

EPMA data for ankerite are shown in Table 4 and plotted in Figure 17. Data which is presented

is averaged from multiple analyses of ankerite grains in a given sample. Values for FeO exhibit

a profile with generally higher but variable values over most of the section, however,

stratigraphically these values suddenly decrease at the top. FeO ranges from 14.73-24.10

wt%, with an average of 20.64 wt%. MgO values vary highly throughout the stratigraphy with

a “zig zag” pattern, ranging from 6.43-12.95, with an average of 8.34 wt%. CaO has a much

smaller range of 28.14-30.13 wt%, with an average of 28.98 wt%.

MnO in ankerite overall reveals a relatively contrasting profile when compared to the other

carbonate oxide components with values increasing generally with stratigraphic height. In

absolute terms of the data, the MnO content of the data ranges substantially, from 0.48-3.98

wt%, with an average of 1.49 wt%. However, with this overall increasing upward trend there

is a bulge of MnO in ankerite near the base of the examined section which then decreases

higher up in the stratigraphy and then increases again to a maximum at the top of the

examined section. This sort of behaviour suggests that MnO in ankerite does not become

progressively richer up section because stratigraphically at lower levels high values are

recorded. The study of Rafuza (2015) which had a much higher resolution data set suggest

that high Mn “spikes” may be masked in bulk rock or in this case XRF geochemical data (see

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by comparison the XRF profile for Mn in the previous section) if the modal abundance of such

ankerite in respective samples is relatively low.

4.3.3.2 Siderite

EPMA data for siderite are shown in Table 5 and plotted in Figure 18, as was done with the

ankerite data above. Data which is presented is averaged from multiple analyses of siderite

grains in a given sample. With regard to the profiles for siderite, they are not as richly

populated with data as compared to those with ankerite, due to siderite been

characteristically finer grained and less abundant in the examined rocks as was also noted by

Rafuza (2015). This made obtaining a large population of data for siderite difficult and

therefore practically not possible.

It appears that certain parallels can be drawn of the profiles for siderite chemistry of major

oxide stratigraphic variations to those presented for ankerite. Values for MgO and CaO depict

like for ankerite a similar variable stratigraphic pattern. It may however be argued that

siderite starts off quite calcic in the lower stratigraphy, decreases in calcic content when

moving up stratigraphy then ends off very calcic at the top again. Ankerite on the other hand

starts off with a low calcic content and ends with a relatively low calcic content. In absolute

terms of the data, Mgo ranges from 5.27-6.79 wt%, with an average of 6.26 wt%; while CaO

ranges from 0.43-1.00 wt%, with an average of 0.66 wt%.

FeO values exhibit a profile which appear to have high values at the lower sections of the

stratigraphy as well as at the mid sections which then gradually curve away towards lower

values at the top of the stratigraphy. In absolute terms, the FeO content ranges from 50.01-

58.27 wt%, with an average of 54.85 wt%. With regard to MnO, it displays a similar profile to

ankerite, however, it starts at moderate values at the base of the stratigraphy which then

curves towards lower values at the mid sections and then curves back out again to a maximum

at the top of the stratigraphy. The most likely cause of this profile is due to the lack of

measurable siderite grains in the basal part of the section. Therefore, the siderite grains do

not resolve a high MnO peak as it does for the corresponding ankerite profile. In terms of

absolute values for siderite there appears to be a substantially large range from 0.84-6.05

wt%, with an average of 2.29 wt%.

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Table 4. Ankerite major-oxide concentrations with stratigraphic height for the Lo drill core. Totals are calculated by the sum of major element concentrations excluding CO2.

Ankerite major oxide concentrations (wt%)

Sample Stratigraphic

height (m) MgO MnO FeO CaO Totals

Lo 01 127,75 8,06 3,98 18,62 28,14 58,80

Lo 03 145,7 12,95 1,14 14,73 30,13 58,95

Lo 07 191,15 6,78 0,48 24,10 29,10 60,45

Lo 12 249,5 8,25 0,64 21,72 29,27 59,88

Lo 15 284,7 9,42 1,60 18,46 28,21 57,69

Lo 18 320,9 6,43 1,76 23,24 28,59 60,02

Lo 20 345,45 7,66 1,58 20,87 29,79 59,90

Lo 21 355,6 7,14 0,71 23,39 28,58 59,82

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110

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14 19 24 29

Dep

th (

m)

FeO(wt%)

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0 1 2 3 4

Dep

th (

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MnO(wt%)

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6 8 10 12 14

Dep

th (

m)

MgO(wt%)

110

160

210

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360

28 29 30 31

Dep

th (

m)

CaO(wt%)

Figure 17. EPMA major oxide (FeO, MnO, MgO, CaO) compositional variations for ankerite against stratigraphy. Note the spike of increased abundance in Mn up stratigraphy.

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Table 5 Siderite major-oxide concentrations with stratigraphic height for the collected Lo drill core samples. Totals are calculated by the sum of major element concentrations excluding CO2.

Siderite major oxide concentrations (wt%)

Sample Stratigraphic

(m) MgO MnO FeO CaO Totals

Lo 01 127,75 6,30 6,05 50,01 0,78 63,14

Lo 03 145,7 6,79 1,81 55,78 0,73 65,11

Lo 07 191,15 5,27 0,84 58,27 0,46 64,84

Lo 12 249,5 6,30 0,99 56,75 0,55 64,59

Lo 15 284,7 6,55 1,37 53,29 0,43 61,65

Lo 20 345,45 6,34 2,64 54,98 1,00 64,96

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48 50 52 54 56 58 60

Dep

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m)

FeO(wt%)

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Dep

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m)

CaO(wt%)

Figure 18. EPMA siderite major oxide (FeO, MnO, MgO, CaO) compositional variations against stratigraphy. Note the spike of increased abundance in Mn up stratigraphy.

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4.3.4 Ratio relationships

From the XRF data presented in the preceding sections it is evident that it is useful in terms

of the studied intersection (i.e. the Griquatown and upper Kuruman BIFs) to provide a simple

technique in terms of analysing the behaviour of Mn in a stratigraphic sense. It appears that

Mn is hosted exclusively in the carbonate fraction of the rocks, as was also suggested by

Rafuza (2015) where speciation analyses were conducted on the rocks. If this is the case, then

it is assumed that the XRF data obtained here with respect to manganese comes from the

carbonate fraction of the rocks and may provide a meaningful record of Mn distribution in

the examined section of this study. The further use of microprobe application on the

individual carbonates, i.e. ankerite and siderite, is able to quantify and give further support

to the XRF results on a more targeted and precise mineral specific level. For these data (i.e

XRF and mineral chemical) to have any relevance they must be plotted in such a way that they

are assessed fully. This can be achieved through the use of ratio diagrams where the relative

abundances for Mn and Fe contained in the carbonates (ankerite and siderite) as well as bulk

carbonate from XRF is depicted. This may become evident through the respective binary plots

of Figure 19 as both ankerite and siderite display a broadly antithetic behaviour between Mn

and Fe. Ratio diagrams are also depicted in a stratigraphic ratio profile form in Figure 21.

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From the above binary plots, it is evident that MnO and FeO are behaving in a dissimilar way

as the Mn/Fe ratio for ankerite hardly displays any relationship and therefore do not anti-

correlate with one another. Siderite on the other hand shows more evidence of an antithetic

relationship however it is not strong enough to draw any concrete conclusions. This will be

discussed in greater detail in the discussion. Further possible relationships were explored in

order to establish which of the major oxide species best display an antithetic relationship. It

is apparent that the best antithetic behaviour can be observed by the sum of MnO and MgO

versus FeO. Figure 20 below shows a binary plot of this antithetic relationship.

R² = 0,1313

0

1

1

2

2

3

3

4

4

5

14 16 18 20 22 24 26

Mn

O(w

t%)

FeO(wt%)

R² = 0,753

0

1

2

3

4

5

6

7

49 51 53 55 57 59

Mn

O(w

t%)

FeO(wt%)

A

B

Figure 19. FeO versus MnO relationships for ankerite A), and siderite B).

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Figure 20 above shows an unusual antithetic relationship. This has implications in terms of

the correlation between MnO and FeO suggesting that they do not actually anti-correlate and

that the actual anti-correlation is between the MnO and MgO summed versus FeO.

R² = 0,83

5

6

7

8

9

10

11

12

13

49 50 51 52 53 54 55 56 57 58 59

MgO

+ M

nO

(wt%

)

FeO(wt%)

R² = 0,9749

6

7

8

9

10

11

12

13

14

15

14 16 18 20 22 24 26

MgO

+ M

nO

(wt%

)

FeO(wt%)

A

B

Figure 20. Unexpected anti-correlation between Mgo and MnO summed versus FeO for ankerite A), and siderite B). Note how much better the R2 are in this relationship compared to the MnO

versus FeO relationship.

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The (Mg + Mn)/Fe ratio profiles against stratigraphic height for both ankerite and siderite

show a common resemblance in Figure 21. From scrutinising these profiles, it is evident that

this relationship implies that with regard to the anti-correlation of MgO and Mn summed

relative to FeO in the carbonates, both ankerite and siderite record very similar signals in a

stratigraphic sense. Being very cautious, an open interpretation could suggest that the

carbonates are behaving in a certain manner whereby Mg is coupled with Mn in a similar

fashion, while Fe behaves in a passive manner. This behaviour is observed in both ankerite

and siderite and thus suggests co-genesis of the two carbonates. The similarity of the mineral

specific stratigraphic (Mg + Mn)/Fe pattern with that of the bulk rock obtained from XRF data,

show a spike at the top of the examined intersection which strengthens the case of the above

proposal that a common origin is possible for the two carbonates.

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4.3.5 Summary

A few preliminary conclusions can be drawn from the results presented above based on the

combination of XRF, ICP-MS and EPMA analytical data with regard to the carbonate fraction

in the examined BIF of the Griquatown and upper Kuruman iron formations:

XRF analyses seem to suggest that there is an increase in MnO stratigraphically

upwards in the examined section as recorded in the bulk rock. This is characterised

110

160

210

260

310

360

0 0,5 1 1,5

Dep

th (

m)

Ank (Mg + Mn)/Fe vs depth(m)

110

160

210

260

310

360

0 0,1 0,2 0,3

Dep

th (

m)

Sid (Mg+Mn)/Fe vs depth(m)

110

160

210

260

310

360

0 0,1 0,2

Dep

th (

m)

Mn/LOI vs depth(m)

Figure 21. Juxtaposition of ankerite and siderite Mg + Mn : Fe ratio profiles against stratigraphic height with bulk rock Mn:LOI from XRF analysis. Due to the relatively low resolution of the data in this study there are only a few reference points for the EPMA data, if there were more the likely

hood of the three profiles looking similar would be greater.

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by two distinctive maxima Figure 6 at circa 128 and 159 m below the surface, which

are separated by a small plateau of relatively low values.

Trace metals suggest that there were no/very little oxidative processes happening at

the time of formation of the Kuruman and Griquatown BIF.

Trace data suggests a marine environment with a REE plot depicting a seawater

profile. Ni and Cu remain stagnant and show no relationship to Mn.

It is apparent that according to the EPMA data on a mineral specific level, ankerite

and siderite simultaneously both display an overall and progressive increase of Mg

and Mn summed relative to Fe stratigraphically upwards which is in agreement with

Rafuza (2015). A similar sort of profile is depicted by the XRF profile for the bulk rock.

A stronger antithetic relationship is evident between Mg and Mn summed versus Fe

which suggests that Mg and Mn are coupled while Fe behaves in a passive manner.

Both EPMA mineral specific data as well as XRF bulk rock data record relatively high

Mg and Mn summed to Fe ratios that is observed at the top of the stratigraphy in the

examined section.

Generally speaking the lowest (Mg + Mn)/Fe ratios and MnO values can be found the

base of the examined section.

In broad terms it appears that the geochemical results from the rocks in the examined

section of this study are reproduced in the examined section of Rafuza (2015) and

further suggests that geochemical results may be reproducible basin wide. However,

caution must be taken here as the results of this study are of a much lower resolution

and may not be fully representative of the entire section.

In conclusion of the above summary, a preliminary conclusion may cautiously be drawn from

the above geochemical parameters. With regard to ankerite and siderite in the Griquatown

and upper Kuruman iron formations, their profiles are similar which indicates that Mn, Fe and

Mg are behaving in a similar way in the carbonates therefore they are probably forming at

the same time. Trace metals suggest that oxide deposition did not take place due to the

general dearth of trace metals in the examined section

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5. Discussion

5.1 BIF research -the road thus far

Since BIF was first arbitrarily assigned by James (1954) to describe thinly laminated or bedded

formations on which this current thesis based, much has been learned about these

Neoarchean and Palaeoproterozoic deposits and is still a keen topic of research for scholars.

Since early research began into BIF, genetic modelling has been at the forefront and

subsequently involved a large variety of diverse processes and consequent evolutionary

changes of the early Earth’s oceanic and atmospheric compositions. A number of authors

when writing about BIFs in general state in their opening line that ocean chemistry as well as

atmospheric conditions that we thrive in today were vastly different during the time of BIF

formation (e.g. Crowe et al., 2008; Lyons et al., 2014 amongst others). Widespread anoxia

would have dominated the atmosphere as well as most of the ocean. To this affect it is safe

to say that BIFs worldwide have played a pivotal role in the evolution of the Earth’s

atmosphere.

With respect to studies conducted on BIFs and especially to those studied in this thesis from

the Asbesheuwles Subgroup, constrains on diagenesis and metamorphism is crucial when

trying to reconstruct the primary depositional environment of the circa 2.4 Ga old rocks from

the Griquatown and Kuruman iron formations which are also prime candidates for the pre-

great oxidation event (GOE). As mentioned earlier these rocks were subject to mostly upper

diagenetic to very low grade metamorphism. This makes it highly probable that any primary

chemical signatures that were recorded in the primary environment of formation are likely to

be unaltered which makes geochemical analysis on these rocks a vital tool of evaluation.

The dominant processes responsible for the primary formation of BIF are still a matter of

much debate and contention, although increasing evidence points towards photoferrotrophy

as a plausible oxidative mechanism, however, considering BIF mineralogy alone presents

evidence that for primary BIF formation some form of Fe(II) oxidation was necessary (Crowe

et al., 2008; Posth et al, 2010a; Rafuza, 2015). The question that now arises is: what sort of

process or mechanism is responsible for the deposition of the initial precipitates of iron

required to form these deposits of such large magnitude?

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5.2 Anoxygenic phototrophic Fe(II)-oxidation – a possible mechanism for BIF

deposition?

In more recent years, biotic mechanisms have gained popularity amongst many researchers

(Llirós et al., 2015; Posth et al., 2010a; Crowe et al., 2008; amongst others). Other models have

been proposed in the past such as the oxygenic photosynthesis model by Cloud (1968) which

models microbes in BIF genesis; as well as the UV photo-oxidation model (Cairns-Smith, 1978)

as an abiotic means of BIF formation. Essentially the anoxygenic phototrophic Fe(II)-oxidation

model is a combination of the oxygenic photosynthesis model and the UV photo-oxidation

model.

In this model (Figure 22), sun light (UV) rather than free oxygen produced by cyanobacteria

and/or eukaryotes in the photic zone may have been responsible for coupling the carbon and

iron cycles via a photosynthesis mechanism. Kappler et al., (2005) reported that ferrous iron

served as the electron donor for these photothrophs which convert CO2 into biomass by using

light energy via the following reaction (Figure 22):

4Fe2+ + CO2 + 11H2O → [CH2O] + 4Fe(OH)3 + 8H2O+

Scientific data has been made available through experimental studies (modern ocean

analogues as well as lab studies) which lends support to this model (e.g. Llirós et al., 2015;

Kappler et al., 2005 amongst others). Such experiments were able to demonstrate that in

order to account for the large expansion of these deposits as seen in Superior type formations

that the organisms responsible would have been proficient in oxidising appreciable amounts

of ferrous Fe. Kappler et al. (2005) were also able to demonstrate that such phototrophs

through growth experiments could effectively oxidise Fe(II) up to a few 100 m’s of depth in

the water column.

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5.3 A new look at the formation of the Griqualand West BIFs

5.3.1 Accommodating the geochemical data from this study

Depending on what school of thought one has when it comes to the genesis of the Griquatown

and Kuruman BIFs, it will impact on the way that the geochemical data is viewed. If these

rocks are entirely diagenetic in origin in terms of the examined carbonates in a chemical and

petrographic sense with regard to the overprinting of diagenetic textures as well as the

proposed paragenetic scheme by Klein (1983), then the preceding sections could be perceived

to adequately explain their mode of origin and mineral chemical variations.

The anomalous elevated manganese in the Griquatown BIF is a phenomenon that needs to

be adequately addressed and a conceivable explanation must be derived as this is an atypical

feature of BIF worldwide. The general lack of manganese in BIF may have been due to an

effective process such as the recycling of transient Mn oxides/hydroxides by ferrous iron

within the upper parts of the primary water column, or to no oxidation of Mn at all and its

resultant progressive enrichment in solution relative to iron (Tsikos et al., 2010). Either of

these processes taking place in the water column would have led to the development of a

distinct spike of high Mg + Mn/Fe. There is currently quite a broad range of work being done

by fellow postgraduate researchers in the Rhodes University Geology Department

surrounding this topic.

Figure 22. Direct microbial Fe (II) oxidation via anoxygenic Fe(II)-oxidizing phototrophy (modified from Posth et al., 2010a).

4Fe2+

+ CO2 + 11H2O → [CH2O] + 4Fe(OH)3 + 8H2O+

ℎ𝑣

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If the diagenetic model is correct, carbonates (i.e. ankerite and siderite) would form entirely

via a diagenetic process and the two redox sensitive species (Mn and Fe) would enter the

carbonate structure when they are reduced. A key mechanism here would be that there were

some bacteria in the sediment that could utilise the manganese and iron oxides. Presumably

these oxides would have been mixed with organic carbon which would have precipitated out

of the water column. As soon as the bacteria have reduced the manganese and iron oxides,

these reduced species would be incorporated into carbonate and a Mn bearing ankerite or

siderite would be formed. On the other hand, what complicates matters is that there is Mg

and Ca also entering the structures of the carbonates and this process is not yet adequately

understood but it is presumably through the pore fluids, if the process was diagenetic.

Therefore, when the carbonate forming reactions happen assuming that they all form

diagenetically, a carbonate mineral which is in equilibrium with the fluid chemistry will be

formed. Ca is invariant and therefore no further consideration of this species is necessary.

Because Mn and Fe are the redox sensitive species it would be reasonable to assume that if

one species is increasing in carbonate, the other should be decreasing, in other words they

would anti-correlate with one another every time because they are the redox sensitive

species and perhaps the two of them together would anti-correlate with Mg. If Mn and Fe do

not anti-correlate at least their sum should anti-correlate with Mg because then essentially

it’s the redox sensitive versus the redox non-sensitive.

However, the results of this study have brought to light an interesting relationship that was

previously unknown. According to Figure 20 under the results section suggests the actual anti-

correlation is not of the Mn and Fe redox species but rather of Mn summed with Mg versus

Fe and thus suggesting that Fe is passive. Therefore, it appears as if Mg is coupled with Mn in

some way. The question is in what fashion could Mg and Mn be behaving in if the Mn is redox

sensitive and the Mg is not? One way to suggest a possible solution to this problem is to

propose that they are sourced from a similar kind of source in that both Mg and Mn are

already at the same 2+ oxidation state which would then allow for the carbonate to draw in

both Mg and Mn. This relationship of Mg summed with Mn versus Fe was a relationship that

was found in both ankerite and siderite via the EPMA data and suggests that these two

carbonates were probably co-precipitating or both forming together which indicates that they

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are not alien to one another. If the carbonates only had two species to contend with, say Fe

and Mn, then an anti-correlation would most certainly be definite.

Trace element data also proved useful in this study. The REE plot in Figure 14 displayed a

rather flat profile with the occurrence of a positive Eu anomaly throughout the stratigraphy.

The flat profile is indicative of a seawater environment while the positive Eu anomaly is

indicative of hydrothermal fluids. These hydrothermal fluids invariably break down K-feldspar

which release Eu which are carried into the water column. When the hydrothermal fluids

precipitate the Eu that was released from the K-spar will be recorded in the sediment. Having

determined that the BIFs of the Griquatown and upper Kuruman iron formations have a

hydrothermal component the Eu anomaly should be constant throughout the stratigraphy,

however, this is not the case. In this instance it is not purely hydrothermal in original but has

another component/s as well and depending on how they are mixed results in variations in

the Eu anomaly.

Manganese nodules in the modern ocean contain trace metals such as Ni, Cu, Co. These

elements were plotted and compared to manganese from the XRF bulk rock powder analysis.

It is apparent that there is no relationship with respect to manganese, however, Co does

increase up stratigraphy in a similar fashion to Mn (Figure 15). In the modern ocean,

manganese nodules are metal rich because when manganese oxides precipitate they

scavenge all other metals around them and incorporate them into their structure. This is not

observed in the Griquatown and Kuruman BIFs, therefore the presence of oxides in the

sediment is slim and most likely not to have been an operating mechanism at the time. Other

trace metals Figure 16 such as Ba and Zr also increase in concentration up stratigraphy. Zr is

a known detrital element and could suggest shallowing, therefore these other elements (Co

and Ba) could also be representative of a detrital fraction. At the same time Ba is fond of

entering the carbonate structure therefore this increase could be a direct result of Ba entering

carbonates. The results of this study alone cannot ascertain what these trace elements

represent at the time of formation. A suggestion can however be made that these trace

elements are not all necessarily linked by one process because if the environment changes

several other things might change along with it but not necessarily in the same way or under

the same forcings. For example, there may be two changes happening concomitantly that are

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not related to one another but respond together to the same cause; so if there is shallowing

happening more detritus could be introduced into the system and there may be more Mn

deposition taking place. Essentially it doesn’t matter if they are of the same source, as a result

they often occur together.

Lyons et al., (2014) report trace elements as records of ocean redox evolution. Figure 23

shows a diagram of molybdenum concentrations throughout time. Every time there is a spike

in the concentration, according to Lyons et al., (2014) an oxidative process has taken place.

The arrow in Figure 23 below indicates an oxidative process at 2.5 Ga, just before the

formation of the Kuruman and Griquatown BIFs. It could therefore be possible that the trace

metals in this study that showed an increase in concentration up stratigraphy (e.g. Ba and Co)

could be responding to a much smaller or responding early to the onset of the next oxidative

process.

Figure 23. Red bars represent average values for the Eon which they represent, namely: Archean, mid-Proterozoic and Neoproterozoic-Phanerozoic. c. The presence of significant Mo enrichments in the Archaean (arrow) suggests the presence of oxidative processes at least as

far back as 2.5 Gyr ago. From Lyons et al., (2014).

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5.4 Implications of this study

As a result of the data presented in the foregoing section as well as the conclusions drawn in

the discussion above, it is rather imminent that an alternative mechanism be adopted in the

modelling of the carbonate fraction of the Griquatown and upper Kuruman BIFs. The

geochemical data presented as a whole can best be used to describe a scenario of primary

precipitation of carbonate mineral particle species directly from a chemically heterogeneous

but stratified water column in an ocean environment. This also includes subsequent re-

crystallisation of such particles during diagenesis with a limited degree of chemical change.

To put things simply, primary carbonate precipitation directly out of the water column as a

model, is proposed instead of a diagenetic origin for the BIF carbonates.

Certain characteristic features stand out that suggest that the carbonate fraction is not of

diagenetic origin. These features include the increase of Mn in the carbonates

stratigraphically upwards as well as the relative low trace metal abundance in the

stratigraphy. Therefore, primary precipitation of carbonates out of the water column seem

like a plausible alternative as opposed to a diagenetic mechanism. If such a process was

indeed at play in the water column at the time of BIF formation it can be envisaged that a

strong chemocline caused by a strong vertical chemistry gradient between the relative

abundances of Mn(II) and Fe(II) with depth would have developed. Such a model is

geochemically favourable against such a strongly stratified water column and can be used in

simple yet elegant explanations. Primary carbonate particles that form in the water column

will have contrasting mineral chemical signatures with respect to Mg + Mn/Fe due to the area

in terms of the stratified water column they formed in, this includes cycles of transgression

and regression. 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.

Although there is still much contention and debate regarding the formation of BIFs around

the world, if this primary water column model were to be accepted and hold true, it would be

a great scientific breakthrough with regard to the origin of BIF and especially carbonate in

general. It would mean that we are one step closer in understanding the early

Palaeoproterozoic Earth and the systems which governed. In pre-GOE BIF settings such as

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the Griquatown and upper Kuruman iron formations oxidation of Mn was not attained but

through an active biological redox cycling of Mn as well as carbon and Fe, Mn was able to

enter into carbonates that formed in the water column as Mn2+. This would have required a

continuous supply of organic matter and high valence Fe from the photic zone. Such a process

in the past would have been equivalent to a present day biological pump, the only difference

being that the ancient equivalent of a modern day biological pump would have made use of

an electron acceptor and Fe oxy-hydroxide would have been the ideal candidate. Carbonates

would effectively have acted as carbon sinks and much of the organically derived carbon

would have consequently been transferred into BIF. Summing up, it can be said that with

regard to the BIFs studied in this thesis, when it comes to the chemical signature of the

carbonates, they may well record primary water column processes especially with respect to

the long term redox behaviour of manganese during BIF genesis. At the end of the day no

matter how we define BIFs and their formation, the ‘how, when and why’ behind the Earth’s

dynamic and complex rock record will continue to motivate a generation of researchers.

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6.Conclusion

6.1 Significances of this study

With regards to the Griquatown and upper Kuruman BIF sections that were examined in this

study, a conclusion was reached that ultimately implicates the mechanism in which

carbonates are formed. After thorough examination of geochemical data from these BIFs it

appears that a diagenetic component to carbonate formation constitute a direct clash with

the results of this study. Absence of trace metals through the greater part of the stratigraphy

suggests formation as an oxide in the sediment is improbable. The author therefore argues

and is in agreement of recent research that the carbonates that were studied in this thesis,

as well as the rest from the Griqualand West Basin, originally formed from a primary

precipitation directly out of a well-stratified water column in which chemical signatures were

adopted from the water column, which was characterised by strong chemocline gradients due

to the dissolved Mg + Mn/Fe ratio. The chemical signature of the water column would then

have been successfully recorded in the precursor BIF sediment.

The exact reason to the increase in Mn up stratigraphy cannot be determined from the results

of this study alone, however, trace element data could give a little insight into why this could

be the case. Zr is a very well-known detrital elements and behaves similarly to Mn, this

increase in Mn could therefore be associated with a shallowing environment therefore

increasing the Mn content coupled with a strong Mn gradient in the water column with time.

This sort of enrichment can be explained through simple Rayleigh fractionation processes or

a slight oxygen ‘whiff’ during BIF formation.

6.2 Proposed future research

One way in which a more conclusive study could be done is to include isotope data in order

to gain more of an understanding in how these deposits were formed. Secondly a laboratory

experiment could be set up in which a study is done on how the Mg +Mn / Fe anti-correlation

works and how these elements enter the carbonate structure.

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basin of the Transvaal Supergroup, South Africa, in: Iron-Formations: facts and

problems, eds., Trendall, A.F., and Morris, R.C.: developments in Precambrian

Geology 6, Elsevier Sci. Pbl., p. 131-209.

Beukes, N.J. (1984) Sedimentology of the Kuruman and Griquatown iron-formations,

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distribution and applications. Journal of Sedimentology IAS Special Publication Series.

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. M.Sc. Thesis unpublished. Rhodes University,

Grahamstown, South Africa, 138pp.

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Tsikos, H., (1999) Petrographic and geochemical constraints on the origin and post-

depositional history of the Hotazel iron-manganese deposits, Kalahari Manganese

Field, 104 South Africa. Ph.D. Thesis Unpublished. Rhodes University, Grahamstown,

South Africa, 217pp.

Tsikos, H., Matthews, A., Erel, Y., and Moore, J.M., (2010) Iron isotopes constrain

biogeochemical redox cycling of iron and manganese in a Palaeoproterozoic stratified

basin, Earth and Planetary Science Letters, Vol. 298, pp. 125-134.

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Appendices

Analytical Methods

EPMA

Quantitative mineral chemical analyses were obtained by using four wavelength dispersive

spectrometers on a JEOL JXA-8230 electron probe micro-analyzer at Rhodes University. The

beam was generated by a Tungsten cathode; 15 kV accelerating potential, 15 nA current, and

1 µm beam size was applied. All elements except Ba and Sr were measured on K-alpha peaks.

Barium and Sr measured on L-alpha. Counting times were 10 seconds on the peak, and 10

total on the background, for all elements. Commercial “SPI” standards were used for intensity

calibration. The standards were Dolomite (Ca), Diopside (Mg), Plagioclase (Na, Si, Al),

Hematite (Fe), Galena (S), SrTiO3 (Sr), Rhodonite (Mn), Orthoclase (K), Benitoite (Ba).

Calibration acquisitions were peaked on the standards, while unknown acquisitions were

peaked on the samples before each point analysis. The data was collected with JEOL software.

An automated ZAF matrix algorithm was applied to correct for differential matrix effects.

Oxygen was calculated by stoichiometry.

Acknowledgements:

I would like to thank Rhodes University for access to the Electron Microprobe (the purchase

of which was partially funded by NRF National Equipment Program grant UID 74464).

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Major oxide concentrations (wt%) raw results for XRF

Sample name Fe2O3 Mn3O4 CaO MgO LOI Depth (m)

(wt%) (wt%) (wt%) (wt%) (wt%)

Lo1 49,00 1,03 5,39 2,54 8,48 118,60

Lo2 25,54 5,04 12,39 5,44 25,93 127,75

Lo2b 31,38 0,24 2,52 1,52 4,55 136,47

Lo3 36,79 0,73 3,74 4,02 16,36 142,05

Lo3b 28,92 0,58 9,60 4,54 17,31 145,70

Lo4 23,96 0,45 5,34 4,19 14,90 151,08

Lo5 33,05 3,00 5,67 4,41 20,99 158,75

Lo5b 36,41 0,55 0,80 3,32 10,65 167,55

Lo6 18,01 1,06 15,96 8,56 25,13 175,80

Lo7 41,82 0,41 1,95 3,53 15,43 178,60

Lo7b 18,29 0,23 1,66 2,05 6,36 191,15

Lo8 43,72 1,24 7,27 4,55 26,92 197,70

Lo8b 41,79 0,49 2,16 3,59 9,61 201,00

Lo9 42,27 0,68 2,62 4,73 13,42 212,95

Lo10 33,35 0,29 0,35 2,82 6,14 224,60

Lo11 52,70 0,39 0,95 4,08 11,79 236,50

Lo12 33,90 0,26 1,60 2,27 6,90 249,50

Lo13 40,85 0,13 0,44 2,20 2,79 260,80

Lo14 38,57 0,36 0,78 3,15 6,56 271,70

Lo15 44,36 0,36 1,07 3,64 7,50 284,70

Lo15b 49,84 0,07 0,99 2,91 6,27 293,05

Lo16 41,02 0,08 1,48 3,08 5,88 296,45

Lo16b 43,10 0,66 8,35 3,88 14,88 303,18

Lo16c 33,04 0,70 3,91 3,62 14,92 306,56

Lo17 46,83 0,73 2,57 3,52 8,93 309,00

Lo18 34,47 0,52 7,62 2,68 11,73 320,90

Lo18b 42,57 0,14 1,21 3,17 5,09 327,78

Lo19 35,61 0,30 2,36 3,97 8,82 332,90

Lo19b 35,00 0,36 1,87 3,72 6,54 337,80

Lo20 33,76 0,49 6,47 3,01 9,49 345,45

Lo20b 28,22 0,33 8,99 2,27 15,16 349,45

Lo21 34,76 0,28 2,00 2,94 9,99 355,60

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Tabulated raw microprobe data for ankerite CaO

(wt%) MgO (wt%)

MnO (wt%)

FeO (wt%) Total

Height (m) sample

28,538 7,639 4,619 18,077 58,873 127.75 Lo 01

28,665 8,811 4,152 18,162 59,79 127.75 Lo 01

27,994 8,359 4,481 16,795 57,629 127.75 Lo 01

28,856 6,786 2,809 21,214 59,665 127.75 Lo 01

25,334 7,784 4,097 20,625 57,84 127.75 Lo 01

28,346 6,774 3,361 21,083 59,564 127.75 Lo 01

28,273 8,378 4,412 17,343 58,406 127.75 Lo 01

28,33 8,734 3,996 16,677 57,737 127.75 Lo 01

28,954 9,299 3,873 17,573 59,699 127.75 Lo 01

29,198 8,671 0,595 21,274 59,738 249.50 Lo 12

30,12 7,93 0,493 21,205 59,748 249.50 Lo 12

28,132 8,435 0,683 21,736 58,986 249.50 Lo 12

29,734 8,057 0,814 21,674 60,279 249.50 Lo 12

29,169 8,178 0,618 22,703 60,668 249.50 Lo 12

29,498 6,432 1,663 23,312 60,905 320.90 Lo 18

28,413 6,277 1,99 22,145 58,825 320.90 Lo 18

29,298 6,736 1,716 22,887 60,637 320.90 Lo 18

29,162 6,505 1,922 21,958 59,547 320.90 Lo 18

28,945 6,662 1,582 22,501 59,69 320.90 Lo 18

24,836 5,826 1,485 25,184 57,331 320.90 Lo 18

29,958 6,585 1,98 24,659 63,182 320.90 Lo 18

29,568 7,27 1,429 20,559 58,826 345.45 Lo 20

32,028 10,097 1,646 16,473 60,244 345.45 Lo 20

29,643 7,718 1,433 23,006 61,8 345.45 Lo 20

29,719 7,228 1,542 20,822 59,311 345.45 Lo 20

29,388 7,876 1,996 19,693 58,953 345.45 Lo 20

29,391 6,798 1,275 22,733 60,197 345.45 Lo 20

29,423 7,45 1,741 21,34 59,954 345.45 Lo 20

29,143 6,853 1,585 22,309 59,89 345.45 Lo 20

30,967 15,989 1,026 10,525 58,507 145.70 Lo 03

30,278 15,81 1,091 11,197 58,376 145.70 Lo 03

29,251 15,339 1,291 11,547 57,428 145.70 Lo 03

32,787 16,549 1,032 10,863 61,231 145.70 Lo 03

28,743 6,686 1,419 22,138 58,986 145.70 Lo 03

28,78 7,322 1,001 22,081 59,184 145.70 Lo 03

30,019 6,619 0,686 23,49 60,814 191.95 Lo 07

29,235 6,229 0,502 24,418 60,384 191.95 Lo 07

30,309 6,375 0,638 22,525 59,847 191.95 Lo 07

28,124 7,107 0,296 24,976 60,503 191.95 Lo 07

28,776 6,951 0,372 24,809 60,908 191.95 Lo 07

28,115 7,386 0,376 24,392 60,269 191.95 Lo 07

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27,342 7,665 0,84 22,732 58,579 284.70 Lo 15

29,155 10,406 0,841 18,957 59,359 284.70 Lo 15

28,461 10,573 0,465 16,401 55,9 284.70 Lo 15

27,269 8,142 0,461 20,637 56,509 284.70 Lo 15

28,744 8,657 2,687 17,761 57,849 284.70 Lo 15

28,781 9,863 3,216 17,618 59,478 284.70 Lo 15

27,74 10,599 2,662 15,134 56,135 284.70 Lo 15

28,949 6,791 0,766 24,103 60,609 355.60 Lo 21

29,652 7,481 0,635 23,853 61,621 355.60 Lo 21

28,417 6,848 0,732 23,79 59,787 355.60 Lo 21

28,712 7,364 0,581 23,173 59,83 355.60 Lo 21

28,399 7,75 0,733 21,985 58,867 355.60 Lo 21

27,37 6,59 0,791 23,427 58,178 355.60 Lo 21

Tabulated raw data for siderite

CaO (wt%)

MgO (wt%)

MnO (wt%)

FeO (wt%) Total

Height (m) sample

0,417 7,636 7,774 46,508 62,335 127.75 Lo 01

0,813 5,02 3,8 52,547 62,18 127.75 Lo 01

0,928 5,087 4,918 52,43 63,363 127.75 Lo 01

0,692 6,758 6,558 49,552 63,56 127.75 Lo 01

0,715 5,476 5,113 51,861 63,165 127.75 Lo 01

1,341 7,419 7,658 47,419 63,837 127.75 Lo 01

0,554 6,684 6,522 49,772 63,532 127.75 Lo 01

0,364 5,784 1,542 53,814 61,504 249.50 Lo 12

0,889 5,132 1,334 58,582 65,937 249.50 Lo 12

1,262 4,945 1,025 58,005 65,237 249.50 Lo 12

0,508 6,356 0,629 57,331 64,824 249.50 Lo 12

0,276 7,597 0,641 56,016 64,53 249.50 Lo 12

0,249 6,474 0,647 58,059 65,429 249.50 Lo 12

0,267 7,823 1,137 55,451 64,678 249.50 Lo 12

0,995 5,598 2,698 56,332 65,623 345.45 Lo 20

1,08 7,371 3,136 52,924 64,511 345.45 Lo 20

1,157 5,807 2,329 55,422 64,715 345.45 Lo 20

0,766 6,577 2,405 55,223 64,971 345.45 Lo 20

0,824 7,231 1,71 54,857 64,622 145.70 Lo 03

1,097 6,734 1,865 55,469 65,165 145.70 Lo 03

0,649 7,543 2,053 55,757 66,002 145.70 Lo 03

1,089 6,481 2,002 55,671 65,243 145.70 Lo 03

0,311 6,681 1,73 55,736 64,458 145.70 Lo 03

0,398 6,085 1,528 57,181 65,192 145.70 Lo 03

0,407 5,679 0,959 58,332 65,377 191.95 Lo 07

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0,294 4,665 0,65 54,811 60,42 191.95 Lo 07

0,467 5,23 0,976 58,458 65,131 191.95 Lo 07

0,849 5,294 0,916 58,452 65,511 191.95 Lo 07

0,261 5,313 0,882 59,443 65,899 191.95 Lo 07

0,332 5,828 0,593 59,417 66,17 191.95 Lo 07

0,615 4,899 0,895 58,945 65,354 191.95 Lo 07

0,923 6,079 1,226 56,251 64,479 284.70 Lo 15

0,319 5,828 1,346 54,81 62,303 284.70 Lo 15

0,206 8,689 1,411 50,142 60,448 284.70 Lo 15

0,328 5,802 1,262 57,057 64,449 284.70 Lo 15

0,378 6,375 1,62 48,212 56,585 284.70 Lo 15