paleoproterozoic laterites, red beds and ironstones of ... - core

PALEOPROTEROZOIC LATERITES, RED BEDS AND IRONSTONES OF THE PRETORIA GROUP WITH

REFERENCE TO THE HISTORY OF ATMOSPHERIC OXYGEN.

by

HERMAN CHRISTIAAN DORLAND

DISSERTATION

Submitted in the fulfilment of the requirements for the degree

MAGISTER SCIENTAE

in

GEOLOGY

in the

FACULTY OF SCIENCE

at the

RAND AFRIKAANS UNIVERSITY

Supervisor: Prof N.J. Beukes Co-supervisor: Dr. J. Gutzmer

DECEMBER, 1999

DECLARATION

I declare that this research work is my own, conducted under supervision of Prof. N.J. Beukes and Dr. J. Gutzmer. No part of this research has been submitted in the past, or is being submitted, for a degree at another university.

H.C. Dorland

TABLE OF CONTENTS

Acknowledgements i

Abstract

ii

Uittreksel

iii

Chapter 1: Introduction

1.1 Problem statement

1

1.2 Objective

3

1.3 Methods

3

Chapter 2: Geological Setting 2.1 Stratigraphy 5

2.2 Previous Correlation between Transvaal and Griqualand West 6

2.3 New Correlation 6

2.4 Structure and Metamorphism 11

Chapter 3: Oolitic and Pisolitic Ironstone of the Timeball Hill

Formation 3.1 Introduction 13

3.2 Regional Stratigraphic Setting 13

3.3 Distribution of Oolitic Ironstone and Quartzite 15

3.4 Petrography of Oolitic Ironstone 18

3.5 Petrography of Pisolitic Mudclast Conglomerate 25

3.6 Pisolitic Ferricrete 27

3.7 Geochemistry 30

3.8 Depositional Model 32

Chapter 4: The Hekpoort Paleosol

4.1 Introduction 43

4.2 Lithological Description 43

4.3 Petrography 48

4.4 Geochemistry 55

4.5 Discussion 67

4.6 Conclusion 85

Chapter 5: Red Beds of the Dwaal Heuvel Formation

5.1 Stratigraphy 86

5.2 Lateral Variation 93

5.3 Petrography 95

5.4 Major and Trace Element Geochemistry 100

5.5 Discussion 103

5.6 Metamorphism and Diagenesis 116

5.7 Significance of Red Beds for Atmospheric Oxygen 116

Chapter 6: History of Atmospheric Oxygen 118

Appendix I: Analytical Methods

1.1 Fieldwork 125

1.2 Sample Preparation 125

1.3 Microscopy 125

1.4 X-ray Powder Diffraction 126

1.5 Scanning Electron Microscopy (SEM) 126

1.6 Electron Microprobe Analysis 126

1.7 Chemical Analysis 127

Appendix II: Chlorite, Sericite and Pyrophyllite Composition

11.1 Chlorite 128

11.2 Sericite Composition of the Hekpoort Paleosol 131

11.3 Pyrophyllite of the Hekpoort Paleosol 133

11.4 Chlorite in Pisolites of the Dwaal Heuvel Formation 133

Reference List 136

1

ACKNOWLEDGEMENTS

All my praise goes to God my Creator.

There are many people and organizations to acknowledge for contributing in different ways to this thesis.

Prof. N. J. Beukes for his inspirational guidance and patience during this study, and giving me the chance to equip myself for the road ahead.

Dr. J. Gutzmer for his inspirational example, guidance and humour on a daily basis.

Anglo Gold, Goldfields and the Botswana Geological Survey for drill core.

All the members of the Paleoproterozoic Mineralization Research Group for their insets.

Wilna Crous and Hester Roets of the Graphics Department at RAU for their technical support.

My parents, family and friends for their support, encouragement and belief.

The NRF for financial support.

ii

ABSTRACT

The evolution of oxygen in the Earth's atmosphere during the early Precambrian has been a subject of debate for many years. Two fundamental models oppose another. The one by Cloud, Holland and co-workers suggests that the atmosphere was essentially anoxic until about 2.2Ga and then became highly oxygenated due to a sudden rise in oxygen levels. The, other advocated by Dimroth, Kimberley and Ohmoto suggests that the atmosphere was oxygenated as early as 3.5Ga.

The most crucial assumption for the Cloud-Holland model for the evolution of atmospheric oxygen is that the 2.2-2.3Ga Hekpoort paleosol formed under reducing atmospheric conditions. However, regional field, drill core, petrographic and geochemical investigations of the Hekpoort paleosol during this study clearly show that the Hekpoort paleosol in fact represents an oxidised lateritic weathering profile. In addition, the Hekpoort paleosol correlates well to the oxidised saprolites below the Gamagara/Mapedi erosion surface in the Northern Cape Province. The basis for the assumption by Holland and co-workers that a dramatic rise in atmospheric oxygen levels took place at 2.2Ga thus falls away.

During this study extensive red beds, belonging to the Dwaal Heuvel Formation were discovered directly above the Hekpoort paleosol in the Pretoria Group in Botswana and the western Transvaal area. The red beds show two stages of development, firstly fluvial and then deltaic. The red beds are correlated with the Gamagara/Mapedi red beds in Griqualand West.

Apart from this evidence for highly oxygenated conditions immediately above the Hekpoort/Ongeluk lavas, hematitic ferricrete, pisolitic mudclast conglomerate and hematitic oolitic ironstones were also found in the Timeball Hill Formation underlying the Hekpoort lava. Oolitic ironstones are developed over an area of more than 100 000 km2. Several different types of oolites are developed within the oolitic ironstone which contains up to 73wt% Fe203. The ferricrete and hematitic pisolitic mudclast conglomerate contain oncolites. These ferricretes, pisolitic mudclast conglomerate and oolitic ironstones suggest that the atmosphere was already highly oxidising between 2.4 and 2.45Ga, prior to deposition of the Hekpoort lava. Pretoria Group rocks that were deposited in close contact with the atmosphere show no evidence for an anoxic atmosphere. It is suggested that atmospheric oxygen levels may have fluctuated through time but at the same time increased in a steplike manner during deposition of the Transvaal Supergroup. However, at this moment in time we do not have enough information available to develop a quantitative model for the evolution of atmospheric oxygen.

New age data available on the Hekpoort/Ongeluk lava unit indicate that it may be 2.395Ga old i.e. some 200Ma older than thought earlier. Thus, the atmosphere could have been highly oxygenated in very early Paleoproterozoic times. Uraninite, pyrite and siderite present in older Archean sedimentary rocks do, however, argue for more reducing atmospheric conditions at that time. Both the Cloud-Holland and Dimroth-Ohmoto models of atmospheric oxygen development are therefore in need of revision.

111

UITTREKSEL

Die ontwikkeling van atmosferiese suurstof in die aarde se atmosfeer gedurende die vroee Precambrium is al vir 'n geruime tyd die rede vir intense debat. Twee fundamentele modelle opponeer mekaar. Cloud, Holland en medewerkers propageer `n model van 'n essensieel suurstoflose atmosfeer tot en met 2.2Ga, waarna suurstofvlakke dramaties toegeneem het. Dimroth, Kimberley en Ohmoto propageer `n suurstofryke atmosfeer vanaf 3.5Ga.

Die mees kritieke aanname van die Cloud-Holland model is dat die 2.2-2.3Ga Hekpoort paleosol onder 'n reduserende atmosfeer gevorm het. Regionale veld, boorgatkern, petrografiese en geochemiese ondersoeke op die Hekpoort paleosol gedurende die studie wys egter daarop dat die Hekpoort paleosol 'n geoksideerde lateritiese verwerings profiel is. Die basis vir die aanname van Holland en medewerkers dat 'n dramatiese styging in atmosferiese suurstof vlakke plaasgevind het by 2.2Ga val dus weg.

Tydens hierdie studie is wydverspreide rooilae in die Dwaal Heuvel Fonnasie direk bokant die Hekpoort paleosol in die Pretoria Groep, in Botswana en die westelike Transvaal area ontdek. Die rooilae is afgeset tydens twee fases, die eerste fase was fluviaal en die tweede deltas. Die rooilae word gekorreleer met die Gamagara/Mapedi rooilae in Griekwaland-Wes.

Behalwe vir rooilae bo die Hekpoort/Ongeluk lawas, is daar ook hematiet-ryke ferrikreet, pisolietiese modderklast konglomeraat en hematiet-ryke oolitiese ystersteen in die Timeball Hill Formasie gevind, onder die Hekpoort lawa. Oolietiese ystersteen is ontwikkel oor 'n gebied van meer as 100 000 km 2. Verskillende tipes ooliete kom voor in die oolietiese ystersteen wat soveel as 73 gewigs% Fe203 bevat. Die ferrikreet en pisolietiese modderklast konglomeraat bevat onkoliete. Hierdie ferrikreet, pisolietiese modderklast konglomeraat en oolietiese ystersteen dui daarop dat die atmosfeer hoogs oksiderend was in die periode tussen 2.4 en 2.45Ga, voor die afsetting van die Hekpoort lawa. Pretoria Groep gesteentes wat in nabye kontak met die atmosfeer gevorm het, dien geen bewyse vir reduserende atmosferiese toestande nie. Atmosferiese suurstof-vlakke het waarskynlik gefluksueer maar terselfdertyd trapsgewys gestyg tydens die afsetting van die Transvaal Supergroep. Huidiglik is daar nie genoeg inligting beskikbaar om 'n kwantitatiewe model vir die evolusie van atmosferiese suurstof to ontwikkel nie.

Nuwe ouderdomsbepalings dui daarop dat die Hekpoort/Ongeluk lawas ongeveer 2.395Ga oud is, ongeveer 200Ma ouer as wat voorheen gedink is. Die atmosfeer kon dus oksiderend gewees het tydens vroee Paleoproterosaese tye. Uraniniet, piriet en sideriet wat voorkom in ouer ArgeIse sediment'ere gesteentes dui egter op reduserende atmosferiese toestande. Byde die Cloud-Holland en Dimroth-Ohmoto modelle vir die ontwikkeling van atmosferiese suurstof moet dus hersien word.

Ohmoto Model 102

Gamagara red be

Drakenstein/Wolhaarkop oxidised saprolite

10'

- 10° Holland Model

Pyrite-Au-U Hekpoort placer deposits paicosolT-2,

1043 4.6 4.0

2.2 1.9 Present (Ga)

Introduction 1

CHAPTER 1

INTRODUCTION

1.1 Problem Statement

The evolution of oxygen in the Earth's atmosphere during the early Precambrian has been

a subject of debate for many years. Two fundamental models oppose another, the one by

Cloud, Holland and co-workers (Fig. 1.1; Cloud, 1968; Walker, 1977; Holland and Rye,

1997) and the other, originally proposed by Dimroth and Kimberley (1976) and promoted

by Ohmoto (1996 and 1997). The Cloud-Holland model suggests an essentially anoxic

atmosphere, with p02 at about 10 -13% of the present atmospheric level (PAL) prior to 2.2

Ga with a dramatic rise to at least 15% PAL in the period 1.9-2.2 Ga, followed by a

gradual increase to present time (Fig. 1.1). In contrast, the Dimroth-Ohmoto model

suggest an oxic atmosphere for the early Precambrian with minimum p02 at ±50% of

PAL since =4 Ga (Fig. 1.1).

Figure 1.1 Comparison between the two opposing models of the history of atmospheric oxygen (modified

after Ohmoto, 1997).

Introduction 2

In the Cloud-Holland model, the timing of the rise in atmospheric p02 is essentially

based on the comparison between the apparently reduced, Fe 3+-depleted Hekpoort

paleosol at Waterval Onder, Eastern Transvaal (Fig. 1.1; Retallack, 1986) and the Fe 3+-

enriched oxidised Wolhaarkop and Drakenstein oxidised saprolites at the base of the

Gamagara/Mapedi red bed succession in Griqualand West (Fig. 1.1; Wiggering and

Beukes, 1990; Holland and Beukes, 1990). Due to the lack of absolute radiometric age

data, the Cloud-Holland model largely depends on the classic correlation between the

Transvaal and Griqualand West areas (SACS, 1980), which implies that the Hekpoort

paleosol has an age of approximately 2.25 Ga (assuming an age of 2.22Ga for the

Hekpoort lava, Cornell et al., 1996) and that it is distinctly older than the Wolhaarkop and

Drakenstein paleoweathering profiles, thought to be between 1.9 and 2.0 Ga old. It also

depends on stratigraphic models which indicate that the red beds of the Olifantshoek

Group (Beukes and Smit, 1987) are distinctly younger than the Pretoria Group. The latter

was previously thought to be devoid of red beds.

The Dimroth-Ohmoto model challenges all the evidence previously used to prove the

existence of an anoxic atmosphere prior to 2.2 Ga (Ohmoto, 1997). New textural

evidence for the uraninite and pyrite in the Witwatersrand Supergroup, previously

thought to be of detrital origin (Holland, 1984), suggest a diagenetic and/or hydrothermal

origin (Barnicoat et al., 1997). Barnicoat et al. (1997) suggests that oxic groundwater

was responsible for the formation of pre-2.2 Ga uraninite deposits. Ohmoto (1997)

suggest that uraninite and pyrite placers are unusual and cannot be used to prove or

disprove reducing atmospheric conditions. However, recent publications again describe

detrital Archean pyrite, uraninite and siderite grains (Rasmussen and Buick, 1999). This

is strong evidence for an anoxic atmosphere during the Archean (Holland, 1999).

Ohmoto (1996) also re-examined the behavior of Fe compared to Ti in paleosols. He

found that essentially all paleosols, both pre- and post 2.2 Ga in age, have retained some

characteristics of soil formed under oxic conditions, and takes it as evidence for a

constant atmospheric oxygen level as far back as the Mesoarchean (Fig. 1.1).

Furthermore, Ohmoto and co-workers (1993) postulated a new model for the deposition

of Superior-type banded iron-formation in a setting similar to layered iron precipitates in

Introduction 3

the modern Red Sea. This model suggests that only the bottom water of restricted basins

was anoxic in an overall oxic ocean. This model is in sharp contrast to the model by

Cloud (1973) that banded iron-formation precipitated in shallow basins on stable

continental platforms where igneous activity was absent. Precipitation of ferric-

hydroxides occurred when cold Fe2+-rich deep ocean water welled up onto shallow

shelves and reacted with 02 generated locally and seasonally by photosynthetic

organisms. To transport Fe 2÷ from the deep ocean, the model requires an oxygen-

stratified ocean with a globally anoxic deep ocean (Klein and Beukes, 1992). If the Red

Sea-type model is true for the Penge/Asbesheuwels iron-formations, it would permit that

atmospheric oxygen levels were high (Ohmoto, 1997) long before the 2.2 Ga rapid

atmospheric oxygen rise suggested by the Cloud-Holland model.

1.2 Objective

This study was undertaken to try and determine whether the differences between the two

models of atmospheric 0 2 evolution could be resolved from carefully selected units in the

Pretoria Group that were deposited in the period prior to 2.1 Ga, i.e. in the time when the

Cloud-Holland model suggests a dramatic atmospheric 02 rise, and the Dimroth-Ohmoto

model indicates a fully oxygenated atmosphere (Fig. 1.1).

1.3 Methods

In order to reach the outlined objectives detailed studies were undertaken at different

localities in the Transvaal area (Fig. 1.2) of the Hekpoort paleosol, the newly recognized

red beds of the Dwaal Heuvel Formation and the oolitic and pisolitic ironstones of the

Timeball Hill Formation of the Pretoria Group. These units all contain hematite-rich

sedimentary rocks believed to have been deposited in close contact with the oxic or

anoxic atmosphere and which may provide evidence for the ambient atmospheric

composition. Stratigraphic profiles were measured in deep drill cores and, where

necessary, also in outcrop in the northwestern, southwestern, south-central and eastern

part of the Transvaal basin in the Transvaal area (Fig.1.2). Field studies were

Grain Size Formation

CoGr C M F St Sh

Magaliesberg

1000 -

Silverton

0 aspoort

Strubenkop waal Heuvel

Laterite and red beds Hekpoort

Boshoek

Timeball Hill Pisolitic and oolitic ironstone

Rooihoogte/ Duitschland

Siltstone

Unconformity

m

Oolitic ironstone

Group

•=11111 MOM .—. Dolomite Conglomerate

1°4 Diamictite Pisolitic ironstone

A

Lava Quartzite

tt

B ■ Location of measured and sampled

stratigraphic profiles

Borehole 1740, Goldfields Borehole Ethic', Anglo Gold Borehole 13811, Goldfields Borehole BDI6, Goldfields Borehole DPZ-2, Anglo Gold Pretoria Localities Crocodile River Fragment Borehole GD I, Rand Gold Outcrop Profile

Airlie, Oolitic Ironstone Outcrop Waterval Onder Roadcut Borehole Stratal, Botswana Geological Survey

0 100 Km

B-Belfast C-Carolina CAR-Carletonvfile JHB-Johannesburg K-Klerksdorp

+ 260

LY-Lydenburg PO-Potgietersrus POC-Potchefstroom PRE-Pretoria RUS-Rustenburg

W-Wannbad WB-Waterval Boven Z-Zeerust

Introduction 4

complemented by extensive petrographical and geochemical analyses of representative

samples of the different rock types (Appendix I).

Figure 1.2 A) Stratigraphic profile indicating the studied units. Co- Conglomerate. Gr- Gritstone. C- Coarse grained quartzite. M- Medium grained quartzite. F- Fine grained quartzite. St- Siltstone. Sh- Shale. B) Map indicating reference localities of the study and outlines of the outcrop distribution of the Transvaal Supergroup in the Transvaal area (modified after Button, 1973).

Geological Setting 5

CHAPTER 2

GEOLOGICAL SETTING

2.1 Stratigraphy

The late Archean to early Proterozoic sediments provided by the Transvaal Supergroup

(Tankard et al., 1982) makes it an excellent target to test atmospheric and climatic models

developed for this important period of Earth history. The Transvaal succession is up to

12km thick and consists predominantly of sedimentary rocks that are structurally and

erosionally preserved in the Transvaal and Griqualand West areas (Fig. 2.1; Button,

1986). The strata in the two areas were initially deposited as an entity on an extensive

shelf platform that must have covered virtually the entire Kaapvaal craton and perhaps

beyond (Button, 1986; Beukes; 1986). At present, the Transvaal Supergroup is preserved

in an outcrop area totalling about 250 000km 2 (Button, 1986; Beukes, 1986).

23° 26° 29°

0

28°

1. -•

t ist 1 Bramvana .,'"

" !...

Namibia : ....1/ ..'7:'.6110:7D ..14

i s„....4 .;: i .

.,.s.s.... .1 , /

.EOrii=Latr .

h.;„,thA.b.,..-..,-....- - "0-

.*:•••••.....•• • .•:•:::::::::••••• ** •••

'' .....• • .........• . % . :•.:•::: ::::it

. ...::.::.:.:....:.:

,:,...:... --,4.•

:25:11:................

'',1t.:,... Smvoren Fragmmt.1 k;j1

........... ...— itaoibag FragmeM

0

....*:.X.. Marble Hall F.' 0 a. a ..............

::.:: _...........-. ... .®

f :

I s. qt. r I

% \

J C* *......... • ....b.r. fr.m 0

o

.. 4 4.

t I. Transvaal area .: ...

I Undifferentiated post-Olifantshoek Group Strata

i I Namaqua-Natal Granitic and Metamorphic Rocks

Olifantshoek and Kheis Groups

(r• Rooiberg Group —i b )

4,....

' sk Printa Griqualand

.... , tie % ‘s '1) 0

.,

ICimbalay I Pretoria and Postmasburg Groups 1

ats as Chuniespoort and Ghaap Groups e

1

'61 I I Wolkberg Group

c West area Pre-Transvaal Platform Successions

(Pongola, Dominion, Witwatersrand and Ventersdorp) ::• Archean Granitic Suite

100 km Archean Greenstone Belts

Figure 2.1 The distribution of the Transvaal Supergroup on the Kaapvaal craton (modified rom Button and Tyler, 1979).


2.2 Previous Correlation between Transvaal and Griqualand West

The stratigraphy of the Transvaal Supergroup between the Transvaal and Griqualand

West areas are remarkably similar in a broad sense, especially with regards to carbonates

and iron-formation in the lower part of the succession (Figures 2.2 and 2.3).

There are, however, marked differences in siliciclastic sedimentary successions (Figures

2.2 and 2.3). Correlation is further complicated by the presence of regional erosional

unconformities, responsible for the erosion of unquantifiable amounts of stratigraphy. In

the currently accepted stratigraphic column (Beukes and Smit, 1987; SACS, 1980) the

Olifantshoek Group is regarded as unconformably overlying and thus distinctly younger

than the Postmasburg/Pretoria Groups of the Transvaal Supergroup. However, this

stratigraphic column is not based on geochronological evidence, but on the apparent lack

of red beds in the Postmasburg and Pretoria Groups and their obvious abundance in the

Olifantshoek Group (Beukes and Smit, 1987).

2.3 New Correlation

A new correlation for the stratigraphy of the Transvaal Supergroup in the Transvaal and

Griqualand West areas was developed during this study. This correlation is based on the

most recent stratigraphic, geochronological and carbon isotopic data available and

especially on the recognition of red beds in the Dwaal Heuvel Formation of the Pretoria

Group that are very similar to the red beds of the Gamagara/Mapedi Formations in the

Olifantshoek Group (Fig. 2.4).

2.3.1 Correlation of the Gamagara/Mapedi and Dwaal Heuvel Formations and

Underlying Oxidized Paleosol

The Hekpoort paleosol (chapter 4) and the lithologies preserved beneath the Mapedi-

Gamagara erosional surface (Wiggering and Beukes, 1990; Holland and Beukes, 1990;

chapter 4) share one very important characteristic, both have been oxidised.

Siltstone Iron-formation Oolitic ironstone Pisolitic ironstone

Co Gr C M F St Sh

V Lava

77.77 Quartzite Conglomerate

■ 111111=

11110 —MN •1•

1."> 1.`".

Dolomite Diamictite V

1 1 1 1 1 1 I Formation

Bushveld complex

Group

) -

1 -

Vi' — v v

V v V v v

v v v

Dullstroom lava

Mafic to intermediate lava, felsite, pyroclastics, arenite and homfels Dullstroom lava

gi a g•

Quartzite, homfels, carbonate and chert (-0.23) Houtenbek

Quartzite with minor shaly rocks Steenkampsberg -,

(.7.?.."..::::::::• :„:•:: ..: ;:)...:;:.. Argillaceous quartzite, arkose

Homfels/shale Nederhorst -- - - -

- -

.7- Quartzite, feldspathic quartzite, arkose Lakenvlei

Dolomite and chert (-0.63)

Shale Vermont

1-1- _ _

- - -

Quartzite, minor shale Magalsesberg

(' • • •

Chert and dolomite (8.91)

Black shale

Machadadorp lava

Silverton

- - - - - - - - —

- - -

- - - _ - - - - - _ - - - - --=--..-- v v v

pastoeolop - - 7'laacTirshalte shale

he poeiglinaMolglied shale

Amygdaloidal andesitic lava, pyroclastic flows, tuff. Some sediment

Dwaal Heuvel

Hekpoort

v v v v

v v v v v v v v

v v Boshoek

--7c-'

Itacnase wubfgroapginesconglomerate

Rietfonteindam diamictite Black shale Quartzite, siltstone and oolitic ironstone

Black and grey shale

Timeball Hill

-........ -... - . - - -

- - ooihoogte Formation. Lateral equivalent of the Duitschland Formation that consists of diamictite, shale, quartzite, conglomerate and dolomite

(7)

Rooihoogte/ Duitschland

Iron Formation, carbonaceous shale, minor carbonate and breccia. Penge

Chen-free dolomite, primary limestone; carbonaceous shale at base Frisco

Ma

nani Subgroup 1

1 t 1 1

Chert-rich dolomite (-0.42) Eccles 1 I 1 1 1

I Chen-free dolomite Lyttelton

no Chert-rich dolomite Monte Christo mm _tits Dark dolomite - Incorporates carbonaceous shale and quartzite Oak Tree

"1/:1111rormil"derolligolux•To hale and Quartzite

Conglomerate Black Reef

1001

(3) 2.588

Age (Ga) •

(I) 2.065

(2) 2.480

Figure 2.2 General stratigraphy of the Transvaal Supergroup in the Transvaal area (modified from SACS, 1980). Estimated ages are indicated. (1) Age of Bushveld (Walraven and Hattingh, 1993) and Molopo Farms Complex (Reichardt, 1994). (2)Gutzmer and Beukes, 1998. (3) Martin et al., 1998. Carbon isotope values from Buick et al., 1998; Swart, 1999.

Co Gr C M F St Sh I I I 1 I I I

Lithology average ( d"C PDB, in brackets)

Molopo Farms complex

Formation Subgroup Group

I ‘,...4

0

i e.

(2)

Age

(Ga)

(1)

2.065

2.394

2.413

2.465

2 669

::••••:::::•••• Conglomerate and quartzite Neylan

.."....•....•::::::::::.:"..:-...•: Ortho quartzite and subordinate layers of dolomitic limestone (10.45) Lucknow

-I -1 - -

- - -

- Upper Mapedi black shale

Mapedi lava

Marthaspoort quartzite

Lower Mapedi black shale

Hematite-Dabble conelomewte red quartzite and shale

Dolomite, chart, banded jasper and lava (0.19)

Mapedi/ Gamagara

Mooidraai

V V

V

V V

.:-::::: -:-:- .• ....-

-=-=-

Banded red jasper, chert, dolomite, banded iron formation Manganiferous jasper and lava Hotazel

V

V \•

v

V V

Andesitic amygdaloidal pillow lava with occasional bands of red jasper and

agglomerate Ongeluk

j ... ‘..... --- —o 0o

e. .o. ,,, ==-- ....

V. P-A c. .i. Don

_r\ . . D iam ictite Makganyene

1000

Om

....

Iron-formation Rooinekke

Koegas

(-3.94)

- - -

- - -

Ouartz-wacke, shale Nara as

Riebeckitic slate Kwakwas

Iron-formation Doradale

- -

Quartz-wache, shale Pannetiie

Clastic-textured iron-formation Griquatown

Asbesheuwels

Microbanded iron formation Kuruman

Sparry limestone, shale Gamohaan

Campbellrand

(-0.42)

el

i I I Dolomite, limestone Kogelbeen

I I Cherty dolomite Klippan 1

I I Dolomite Papkuil

Cherty dolomite Klipfontein He

Spam/ dolomite Fairfield

I Micritic dolomite Reivilo

I I Dolomite, limestone, shale Monteville

- Shale Lokammona

Schmidslsdrif Dolomite, limestone, shale Boomplaas

. .. ..3.4X.:.34 Quartzite, shale, lava Vryburg

,

Diamictite

V V Lava

-•-•-"7". . NM= g.- --I ir.). :::::.•:.•:. 13W

._._._.._.. Quartzite Conglomerate •MIM Dolomite °L5`

.........-... Siltstone Oolitic ironstone Iron-formation -. --

Figure 2.3 Generalized stratigraphy of the Transvaal Supergroup in the Griqualand West area (modified from SACS, 1980 and Beukes, 1986). Estimated ages are indicated. (1) Molopo Farms Complex (Reichardt, 1994). (2) Romer and Bau, 1998. (3) Gutzmer and Beukes, 1998. (4) Gutzmer and Beukes, 1998. (5) Gutzmer and Beukes, 1998. Carbon isotope values from Buick et al., 1998 and Swart, 1999.


oxidised. The Griqualand West area has been more extensively deformed and uplifted

prior too and/or during erosion, with the Mapedi-Gamagara erosional surface locally

resting on carbonate and iron-formation from the lower part of the Transvaal Supergroup

(Fig 2.4). The Hekpoort paleosol in the Transvaal area (chapter 4) and the oxidised

saprolites preserved beneath the Gamagara-Mapedi Formations in Griqualand West are

both overlain by fluvial red beds. This association of an oxidized weathering profile with

red beds is unique in the Transvaal Supergroup and one of the main reasons for

correlating the Hekpoort paleosol with the Mapedi-Gamagara weathering profiles (Fig.

2.4). It is important to note that red beds have not been previously recognized in the

Pretoria Group (SACS, 1980; Button, 1973).

23.2 Makganyene-Rietfonteindam Diamictite Correlation

More extensive uplift and deformation in the Griqualand West area relative to the

Transvaal area appear to have been the norm during the deposition of the Transvaal

Supergroup. The Makganyene diamictite in Griqualand West is better developed, and

appears to erode much more stratigraphy than the Rietfonteindam diamictite in the

Transvaal area. Siliciclastic successions, including the oolitic and pisolitic ironstones of

the Timeball Hill Formations are preserved beneath the Rietfonteindam diamictite in the

Transvaal area (Fig. 2.4). There appear to be no correlative of Timeball Hill-type strata

in Griqualand West (Fig. 2.4). The Makganyene diamictite in Griqualand West also

appears to cut out the pre-Pretoria Group unconformity present at the base of the

Duitschland/Rooihoogte Formations in the Transvaal area (Fig. 2.4).

2.3.3 Correlation between the Upper Pretoria Group and Lower Olifantshoek

Group

Obvious stratigraphic similarities exist between the lower Olifantshoek Group and

middle-upper Pretoria Group (Figures 2.2, 2.3 and 2.4). These similarities includes a

virtually identical succession of quartzite, shale, carbonate and volcanics in

corresponding stratigraphic positions (Fig. 2.4). Again, the erosional surface on top of


the Lucknow Formation that now marks the top of the Transvaal Supergroup in

Griqualand West, expressed as the Neylan conglomerate (Beukes and Smit, 1987; Van

Niekerk, 1999), is better developed than the top erosional surface of the Pretoria Group in

the Transvaal area, at the base of the Dullstroom lava (Schweitzer, 1998) (Fig. 2.4).

Several cycles of siliciclastic and carbonate rocks preserved below the top unconformity

of the Pretoria Group are apparently absent from the stratigraphy in Griqualand West

(Fig. 2.4). This again indicates more extensive uplift and erosion of Transvaal strata

along the western margin of the Kaapvaal craton.

2.3.4 Correlation Based on Carbonates with Heavy 813C Signatures

The presence of distinct 8 13C excursions in carbonates of the Lucknow and Silverton

Formations (Swart, 1999; Buick et al., 1998) that are located in similar stratigraphic

positions in the Postmasburg and Pretoria Groups further support the new correlation

(Fig. 2.4). These carbonates have a distinct heavy (positive) 8 13C signature. The

Lucknow Formation has an average 8 13C signature PDB of 10.45 (Swart, 1999) and the

Silverton Formation an average 8 13C signature PDB of 8.5 (Buick et al., 1998). The

Silverton/Lucknow positive 8 13C excursion appears to be part of a global trend for

positive 8 13C excursions during the period 2.2 and 2.1 Ga (Karhu and Holland, 1996).

The processes responsible for this heavy 8 13C excursion have been linked to the

development of atmospheric oxygen (Holland, 1978, 1984). The 8 13C signatures of

marine carbonates is determined by a dynamic balance between the rate of relative

oxygen increase (due to the burial of organic carbon) and the rate of oxygen consumption

during weathering and oxidation (Karhu and Holland, 1996). With increased burial of

organic carbon (no reworking), marine carbonates normally with 8 13C signatures of -3 to

+3 become increasingly heavy (above +3) 813C.


2.3.5 Radiometric Age Constraints

The proposed new stratigraphical correlation is backed by recent geochronological work

(Fig. 2.4). A tentative 2.4 Ga age for the Ongeluk-Hekpoort basaltic andesites is

supported by U-Pb SHRIMP zircon ages for tuff beds in the Asbesheuwels (2.465 ±

0.007 Ga, Gutzmer and Beukes, 1998) and Penge Iron Formation (2.480 ± 0.018 Ga,

Gutzmer and Beukes; 1998) beneath the lava successions, and a whole rock Pb-Pb

carbonate age of 2.394 ± 0.026 Ga for the Mooidraai dolomite (Romer and Bau, 1998)

above the lava. The latter age is in good agreement with a poorly constrained SHRIMP

U-Pb zircon age for the Hotazel Formation of 2.413 ± 0.013 Ga (Gutzmer and Beukes,

1998). These ages are regarded as being more accurate than previous age estimates of

2.22 Ga for the Hekpoort-Ongeluk successions that were based on whole rock Pb-Pb and

Rb-Sr dating (Cornell et al., 1996; Armstrong, 1987). The Molopo Farms Complex

(Reichardt, 1994) that intrudes the Olifantshoek Group in Botswana and far northern

Griqualand West are similar in age (2.065Ga) to the Bushveld Complex (Walraven and

Hattingh, 1993) in the Transvaal (Fig. 2.4).

Most importantly, the new correlation suggests that the Wolhaarkop-Hekpoort oxidized

weathering profile may have developed between 2.3 and 2.39 Ga. This has major

implications for the history of atmospheric oxygen as shall be illustrated in this

dissertation

2.4 Structure and Metamorphism

All indications are that very little deformation and only low grade metamorphism

affected rocks studied in this thesis, even close to the Bushveld Complex.

uartzite °S7gR Conglomerate Carbonaceous shale and siltstone

a.'

II Lava

Iron-formation

Diamictite

Griqualand West Area Age (Mn) Group Formation

2065 Molopo Farms complex

(5)2394

2415

, CI

A =

Hartley

Lucknow

Mapedi

Pos t

mas

burg

Mooidraai

Hotazel

Ongeluk

Makganycne

Koegas Subgroup

Asbesheuwel Subgroup

Cambellrand Subgroup

2669 Schmidtsdrif Subgroup

Transvaal Area Formation I GrouplARe (Ma

Bushveld Complex (1) 2065

Dullstroom

Houtenbek

Steenkampsberg

Nederhorst

Lakenvlei

Vermont

Magaliesberg

. E' 2 a.

Silverton

Das oo Stru ens op Dwaal Heuvel

Hekpoort

/ Boskop

Timeball Hill

Duitschland

Penge Iron Fomuttion 2480

Malmani Subgroup

\ Black Reef 2588

Lithologies

Quartzite

Shale

El Dolomite, limestone

Hematite pebble conglomerate, Pisolitic ironstone

Oolitic ironstone

Intrusives

Ofi


Figure 2.4 Suggested new correlation of strata of the Pretoria Group in the Transvaal and the Postmasburg

and lower part of the Olifantshoek Group in Griqualand West. Note the correlation of the Hekpoort

paleosol and Dwaal Heuvel Formation red beds with the red beds of the Gamagara/Mapedi Formations.

Estimated ages are indicated. (1) Reichard, 1994. (2) Gutzmer and Beukes, 1998. (3) Martin et al., 1998.

(4) Reichardt, 1994. (5) Romer and Bau, 1998. (6) Gutzmer and Beukes, 1998. (7) Gutzmer and Beukes,

1998.

Oolitic and Pisolitic Ironstone of the Timeball Hill Formation 13

CHAPTER 3

OOLITIC AND PISOLITIC IRONSTONE OF THE

TIMEBALL HILL FORMATION

3.1 Introduction

The Timeball Hill Formation is a deltaic to shallow-marine succession preserved in the

Transvaal area (Button, 1973). Stratigraphically, the succession is situated in the lower

part of the Pretoria Group and estimated to be between 2.4 and 2.45Ga old (Fig. 2.4).

Three ferruginous rock types are present in quartzite beds of the Timeball Hill Formation

namely 1) oolitic ironstone (definition after Young, 1989), 2) pisolitic mud clast

conglomerate and 3) ferricrete (definition after Bardossy and Aleva, 1990 and Aleva,

1995). All three rock types may provide clues to the oxidation state of the atmosphere

under which they formed (Fig. 2.4).

3.2 Regional Stratigraphic Setting

The Timeball Hill Formation overlies the Rooihoogte Formation (that constitutes the base

of the Pretoria Group) with a sharp contact (Fig. 3.1). The lower part of the Timeball Hill

Formation is composed of a black, finely laminated carbonaceous shale (Fig. 3.1). The

shale becomes less carbonaceous upwards displaying colour variations ranging from

black to dark grey. The shale is highly pyritic in its lower part. Depending on the

location in the Transvaal area, the shale coarsens upwards into either quartzite or oolitic

ironstone (Fig. 3.2). This unit of quartzite/oolitic ironstone is informally known as the

Klapperkop Member of the Timeball Hill Formation in the eastern Transvaal (Schreiber,

1990), and as the Gatsrand Member in the western Transvaal (Visser, 1969). The

quartzitic/oolitic units fine upwards into finely laminated carbonaceous shale. Locally a

diamictite (informally known as the Rietfonteindam diamictite) is developed along the

Basal conglomerate of Boshoek Formation conglomerate Rietfonteindam diamictite

Lithologies 3eneralized profileGrain-size successions Carbon cycles

Oolitic ironstone zone Oolitic ironstone Pisolitic mudclast conglomerate

U Shale Oolitic quartzite

Lithologies Increasing organic carbon

Upper black shale

Flat bedded ortho-quartzite

Pisolitic ferricrete Pisolitic mudclast conglomerate

ma

C_.

Massive oolitic ironstone

Lower black pyritic shale

Top of Rooihoogte/Duitschland Formation

Quartzite Siltstone 44,6 4 .e1747 s•4 Shale-clast diamictite .:11r1.74. Chen breccia Ferricrete

Co Gr C M F St Sh

Unconformiry

V

Conglomerate Oolitic ironstone Pisolitic mudclast conglomerate Shale 00 0 0

o o no 0

Figure 3.1 General profile of the Timeball Hill Formation indicating stratigraphic setting of oolitic ironstone and ferricrete.


top of the Timeball Hill Formation. The diamictite occurs in lens-shaped bodies (Visser,

1969) and is overlain by chert-pebble conglomerate that is the base of the Boshoek

Formation (SACS, 1980). In many places, the diamictite is only recognized by the

occurrence of faceted dropstones in the uppermost shale beds of the Timeball Hill

Formation (Fig. 3.3, localities 3 and 4).

3.3 Distribution of Oolitic Ironstone and Quartzite

The Klapperkop/Gatsrand quartzites and oolitic ironstone are developed at the top of each

of three upward coarsening successions (Fig. 3.2). The ironstone beds associated with

upward coarsening successions are between 2 and 8m thick (Fig. 3.2). Orthoquartzite

caps the succession and erodes into the lower quartzite, shale and oolitic ironstone and

marks a major transgression that concludes ironstone deposition.

On a regional scale the Klapperkop/Gatsrand Member appears to thin from north to south

(Figures 3.2 and 3.3). Ironstone and quartzite isopach maps of the Klapperkop/Gatsrand

Member indicate that the ironstone and quartzite are distributed in two lobe-shaped

sedimentary bodies (Fig. 3.4). One lobe is situated in the eastern and the other in the

western part of the Transvaal area (Fig. 3.4). The western lobe extends further to the

south than the eastern lobe (Fig. 3.4).

Mean paleocurrent directions indicate that the source area for the western lobe was

located to the northwest, and for the eastern lobe to the northeast of the Transvaal area

(Fig. 3.4, Visser, 1969 and Button, 1973). Locally, paleocurrent directions are bimodal

suggesting reworking by marine currents (Button, 1973 and Schreiber, 1990).

The distribution of oolitic ironstone is closely linked to the distribution of quartzite

within the Klapperkop/Gatsrand Members of the Timeball Hill Formation (Fig. 3.4).

Oolitic ironstone is especially prominent in the more distal parts of the depositional lobes

of the Klapperkop/Gatsrand Members. Ironstone is better developed and more abundant

in the eastern depositional lobe (Fig. 3.4). In both northern and southern directions the

Lithologies

=Shale

Siltstone

r7 Quartzite-Fe indicates iron-rich

MI Oolitic ironstone

Pisolitic ferricrete

Pisolitic mudclast conglomerate

Diarnictite

fillilla SO

E

Om

Op ft


oolitic ironstone beds thin, and become less abundant, grading into oolitic quartzite and

quartzite to the north and shale to the south (Fig. 3.4). Oolitic ironstones are sparsely

developed in the westernmost part of the Transvaal area (Klop, 1978; Fig. 3.4). On a

regional scale the Klapperkop/Gatsrand Member appears to thin from north to south

(Figures 3.2 and 3.3). The Timeball Hill oolitic ironstone beds cover an area of more

than 100 000 km 2 (Fig. 3.4; Button, 1973).

Figure 3.2 Fence diagram of the Klapperkop/Gatsrand Member of the Timeball Hill Formation. 1=Drill

core GD1, 2=Drill core DPZ2, 3=Drill core 1740, 4=Combined Pretoria profile, 5=Modified Wagner

(1928) outcrop profile, near Waterval Boven (see figure 3.3 for localities).

100

Delta lobe

Oolitic ironstone

31 °

414 150

.:,.% vill4y at..?

+26.

Vikko,

+24°

100

km =Scarce N:*: Common

Major paleocurrent direction

50 Quartzite thickness contours

Ferricrete

Abundant

Reference points

1-Borehole 1740, Goldfields 2-Borehole Rbkl, Anglo Gold 3-Borehole BB11, Goldfields 4-Borehole BD16, Goldfields 5-Borehole DPZ-2, Anglo Gold 6-Pretoria localities 7-Krokodilrivierfragment,Hartzer,1987 8-Borehole GDI, Rand Gold 9-Klop (1978) 10-Airlie, oolitic ironstone outcrop (Wagner, 1928)

B-Belfast C-Carolina CAR-Carletonville JHB-Johannesburg K-Klerksdorp Ly-Lydenburg PO-Potgietersrus POC-Potchefstroom PRE-Pretoria RUS-Rustenburg W-Warmbad WB-Waterval Boven Z-Zeerust

31°

+ 24°

+ 26°

0

100 ICm

27° + -

Nt

Figure 3.3 Locality map showing the distribution of drill core and outcrop profiles of the Timeball Hill Formation examined in this study. Also shown is the outcrop outline of the Transvaal basin in the Transvaal area (modified after Button, 1973).

Figure 3.4 Quartzite isolith map of the Klapperkop/Gatsrand Members of the Timeball Hill Formation. Illustrated are the development of oolitic ironstone and ferricrete as well as major sediment transport directions (compiled from Visser, 1969; Button, 1973, and Schreiber, 1990).


In the Pretoria area (Fig. 3.4), most notably at Klapperkop and Muckleneuk, 2-5m thick

oolitic ironstone beds with sharp basal contacts are developed (Fig. 3.5). Pisolitic

mudclast conglomerate beds, 1-1.5m thick, rest on erosive basal contacts of the oolitic

ironstone beds (Fig. 3.5). Hematite-rich pisolitic ferricrete that erosively overlies the

oolitic ironstones is also developed in the Pretoria area (Fig. 3.5). This ferricrete is in

turn overlain with a sharp contact by a ripple-marked and crossbedded orthoquartzite

(Fig. 3.5; Visser, 1969).

3.4 Petrography of Oolitic Ironstone

Hematite-oolites of the Timeball Hill Formation are hosted in siliciclastic lithologies

ranging from highly mature orthoquartzites to claystone. They also occur in rocks

composed almost exclusively of hematite. Three types of oolitic ironstone, after

definitions by Richter (1983) and Young (1989), are present as described below.

3.4.1 Type 1 Oolitic Ironstone

The lower oolitic ironstone bed developed at Muckleneuk is representative of type 1

oolitic ironstone. It is characterized by tightly packed oolites with uniform concentrically

laminated cortices and some hematite-coated quartz grains set in a dense, very fine-

grained matrix of dusty hematite and iron-rich chlorite (Fig 3.5 Block D and Fig. 3.6A).

The oolites are between 0.1 and 0.2mm in diameter (Fig. 3.6A). The oolitic ironstone

bed form part of an upward coarsening succession.

Oolite nucleii are predominantly composed of iron-rich chlorite intergrown with hematite

(Fig. 3.6B). Nucleii composed of quartz grains (Fig. 3.6C) are scarce. Cortices of oolites

are composed of fine crystalline hematite laminae (Fig. 3.6B). Most of the oolites in this

ironstone bed are flattened parallel to bedding (Fig. 3.6B). This is thought to represent

evidence for diagenetic compaction. Cortices of neighboring flattened oolites are usually

separated by a chlorite barrier that were indented during diagenetic compaction to


accommodate the oolites that remained spherical (Fig. 3.6B). This suggest that at least

some of the oolites were deposited as hard particles in soft mud.

The outer laminae of composite oolites appear conspicuously fragile, suggesting in-situ

growth of laminae in stationary muddy sediment prior to lithification (Fig.3.6C).

Angular to subrounded, poorly sorted coated quartz grains with diameters ranging

between 0.1 and 0.3mm are abundant throughout the oolitic ironstone bed (Fig. 3.6A).

The quartz grains are coated with hematite laminae very similar to the cortices of

associated oolites (Fig. 3.6B). The thickness of the coating varies from grain to grain,

depending on shape and size of quartz grains acting as nucleii (Fig. 3.6B). The laminae

around the quartz nucleii are irregular in thickness and some may pinch out (Fig. 3.6B).

The hematite laminae coat grains in such a way that irregularities of quartz grains are

smoothed out, with a smooth spherical outer surface as end result.


The lower ironstone bed near Airlie in the eastern Transvaal area (Fig. 3.5 block F)

represents type 2 oolitic ironstone. It is marked by the ditribution of hematite oolites

with variable cortex textures in a quartzite host (Fig. 3.7B and E). Although texturally

different, all the oolites have the same mineralogical composition, consisting

predominantly of hematite with some chlorite (Fig. 3.7B and E). Concentric laminae in

cortices of some of the oolites and composite oolites vary in thickness, and some may

pinch out. The cortices have a porous vaguely concentric appearance (Fig. 3.7E). In

contrast, other oolites in the same ironstone bed (Fig. 3.5 block F) display smooth, well

developed concentrically laminated dense hematite cortices, with laminae of very even

thickness (Fig. 3.7B). Some of the smooth laminae may pinch out providing evidence for

abrasion during transport (Bhattacharyya, 1989; Fig. 3.7B).

Quartz grains and neighboring oolites are indented where they touch due to post-

depositional compaction (Fig. 3.7B; Bhattacharyya, 1989).

Legend for blocks

Oncoidal clast

Pisolite with concentric laminae (Pisovadoid)

Pisolitc with irregular laminae (Pisoncoid)

Hematite indurated mudclast

Oolites and Battened oolites

Hematite-coated, angular and rounded quartz grains Magnetite

Matrix consisting of hematite and chlorite

Legend for profiles

Pisolitic ferricrete

Pisolitic mudclast conglomerate

Quartzite

Oolitic ironstone

Siltstone

Shale

Muckleneuk Grain size cycles Co Gr C M F St Sh A

20 —

Om —

Airlie CoGr C M F St Sh 1111111

0 &al

czit

Oolitic and Pisolitic Ironstone of the Timeball Hill Formation

20

Figure 3.5 Combined profiles of the ICIapperkop member of the Timeball Hill Formation at Pretoria and in

the south-eastern Transvaal near Airlie, indicating characteristic macro- and microscopic features. Insets

A-F illustrate the following: A- The pisolitic ferricrete is composed of a tightly packed poorly sorted

mixture of hematite pisolites, oncoidal clasts, quartz grains and oolites set in a matrix of fine crystalline

hematite and some chlorite. B- Sketch of macroscopic features of the pisolitic mudclast conglomerate

composed of a random mixture of elongated hematite mud clasts, elongated oncoidal clasts, concentric in-

situ pisolites, clasts containing reworked fragmented pisolites, hematitic oolites and quartz grains all set in

a matrix of medium-grained hematite-coated quartz grains cemented by dusty hematite. C- Sketch of

microscopic features of the oolites preserved in an oolitic ironstone clasts incorporated into the pisolitic

mudclast conglomerate. The oolites are almost exclusively composed of hematite. The oolites primary

highly concentric, evenly spaced hematite laminae are well preserved. Secondary magnetite crosscuts

earlier textures. D- Sketch of microscopic features of the oolitic ironstone developed in the basal oolitic

EAT- 13 PEI hi) 1.111- 26 nn Hog- • 923 X-- 20An Photo No..1 • Dolento, tEl

Ei1l 1.i A 'LI IV

1)- nn Hog 91 X flN i lm Photo An...41 • DeLhotor. TETRA •

w. X Dotontor , TENT!!)

61)1 , 16.1111 XI no 211An H Pi10.0 No. :1

lniln H - tthnio 110

®t= 10 on kV Al) 26 11i■ 11,16= 9E4 X notnetnr=. TAMA


ironstone bed at Muckleneuk. The laminae in the cortices of oolites consist of hematite. Matrix between

oolites is composed of a mixture of chlorite and hematite. All oolites are flattened parallel to bedding

illustrating diagenetic compaction. E- Sketch of microscopic features of the upper and middle oolitic

ironstone lenses of the Timeball Hill Formation near Airlie. The oolitic ironstone is strongly

metamorphosed so that most of the hematite is recrystallized or, alternatively, replaced by iron-rich chlorite

and magnetite. Rounded quartz grains are still recognisable in places. F- Sketch of microscopic features of

the lower oolitic ironstone bed of the Timeball Hill Formation near Airlie. The cortices of the oolites

consist predominantly of hematite laminae with a few intercalated chlorite laminae. Some oolites have

vaguely concentric laminae in cortices whilst others display distinct concentric laminae.

Figure 3.6 BSE-SEM-images of the lower oolitic ironstone bed of the Timeball Hill Formation at

Muckleneuk near Pretoria. A - Oolites composed of mixed chlorite and hematite nucleii surrounded by

alternating concentric laminae of hematite and chlorite. Poorly sorted, subangular to rounded hematite-

coated quartz grains are randomly dispersed throughout the oolitic ironstone. The matrix is composed of

densely intergrown hematite and iron-rich chlorite. Octahedral magnetite crystals replace matrix, quartz

grains and oolites, clearly indicating their late metamorphic origin. B- Close up of A. Note the


compactional flattening of oolites parallel to bedding. C- Example of a composite oolite. The outermost

lamina appears conspicuously fragile and is partly replaced by diagenetic chlorite obscuring primary oolitic

textures. Note that the laminae of neighboring oolites and quartz grains are dented in such a way as to

accommodate each other. D- Hematite-coated quartz grain and textureless oolite. Note the omnipresence

of a chlorite barrier separating neighboring oolites and quartz grains from one another.

Quartz grains associated with these quartzite-hosted oolites are typically well rounded

and well sorted and have coatings of hematite and chlorite (Fig. 3.7B). Most of the

quartz grains are between 0.2 and 0.4 mm in diameter, although some grains are smaller

(minimum of 0.1mm diameter) while a few others are bigger (up to 0.7mm diameter).

The bulk of the matrix of the quartzitic oolitic ironstone consists of varying amounts of

finely intergrown hematite and iron-rich chlorite (Fig 3.7B). Overall, transport damage

to the oolites is remarkably small. This suggest that most of the quartzite-hosted hematite

oolites behaved as hard grains during periods of transport and reworking.


Type 3 oolitic ironstone is composed predominantly of hematite with little siliciclastic

material. The oolites in this type of ironstone are composed of remarkably even,

concentric cortices on a microscopic scale (Fig. 3.7C). This type of oolitic ironstone was

only observed as clasts in the pisolitic mudclast conglomerate at Muckleneuk and

Klapperkop near Pretoria (Fig. 3.5 Block C and Fig 3.7C). The laminae are almost

entirely composed of hematite and smooth out any irregularities in the shape of the

original nuclues (Fig. 3.7C). Up to 20 highly concentric hematite-laminae have been

counted in a single oolite, giving it a dense non-porous appearance (Fig. 3.7C).

The matrix of these oolitic ironstone clasts found in the mudclast conglomerate consist of

finely intergrown hematite and chlorite and there are almost no quartz grains present.

Clasts of oolite cortices (Fig. 3.7D) occur in the ironstone. This provides further

evidence for the continuous reworking and repeated transport of these oolites during

deposition.

r --

Figure 3.7

Oolitic and Pisolitic ironstone of the Timeball Hill Formation 24

Text to figure 3.7 Handsample photograph (A) and reflected light photomicrographs (B-F) of oolitic

ironstones of the Timeball Hill Formation. A- Typical massively textured iron-rich oolitic ironstone from

Muckleneuk, Pretoria. B- Association of oolites, rounded quartz grains and chlorite matrix (160x, oil

immersion) in quartzitic ironstone from Airlie in the eastern Transvaal area. C- Oolite with cortex

composed of highly concentric, even hematite laminae. The matrix consists of finely intergrown hematite

and chlorite (160x, oil immersion). Oolite is from an oolitic ironstone clast set in the pisolitic mud clast

conglomerate at Klapperkop, Pretoria. D- Clast composed of part of a oolite cortex (256x, oil immersion)

from an ironstone clast in the pisolitic mudclast conglomerate from Muckleneuk, Pretoria. E- Composite,

irregularly laminated oolite (400x, oil immersion) from quartzitic ironstone near Airlie in the eastern

Transvaal area. F- Example of a hematite oolite replaced by euhedral magnetite (400x, oil immersion)

from Muckleneuk, Pretoria. In all of the microphotographs the brown-red colour is due to the presence of

supergene goethite that replaces the primary hematite and iron-rich chlorite in the zone of modern

weathering. Goethite is absent in drill core samples i.e. core BB11, Potchefstroom.

3.4.4 Diagenesis, Metamorphism and Supergene Alteration

Although primary textures are well preserved, the Timeball Hill Formation's oolitic

ironstones have evidently been affected by diageneses and lower greenschist facies

metamorphism. Iron-rich chlorite originated during diagenesis on the expense of

hematite, quartz and an unknown precursor clay mineral (possibly kaolinite) (Figures

3.6A-D and 3.7B). Powder X-ray diffraction analysis identified the iron-rich chlorite as

chamosite. Compactional flattening is evident in oolites with cortices containing

abundant chamosite (Fig. 3.6B), an observation made in Phanerozoic ironstones

(Bhattacharyya, 1989). Hematite-rich oolites subjected to compaction during diagenesis

are usually little affected and display little or no deformation of their concentric shape

(Bhattacharyya, 1989). Varying compaction suggests that at least some of the chamosite

is primary.

Large magnetite octahedra formed at the expence of hematite during metamorphism

(Figures 3.6A and 3.7F). With increasing grade of metamorphic recrystallization, the

sedimentary textures of the oolites are obscured by coarse crystalline magnetite and/or

recrystallization of hematite to a textureless mass (Fig. 3.5 block E). Rounded quartz


grains, on the other hand, are often still recognisable even at higher metamorpic grades

(Fig. 3.5 block E).

Modern supergene oxidation results in the martitisation of magnetite and leads to goethite

staining along grain boundaries and joint planes in the oolitic ironstone.

3.5 Petrography of Pisolitic Mudclast Conglomerate

In the Pretoria area, and especially at Muckleneuk and Klapperkop, pisolitic mudclast

conglomerates are well developed. The pisolitic mudclast conglomerates consist of a

random mixture of elongated hematite-indurated mudclasts, hematite-rich oncoidal (in a

textural sense; Brand and Veizer, 1983) clasts (Fig. 3.8A and B), concentric apparently

in-situ pisolites (Fig. 3.8B and C), clasts containing reworked hematite-rich pisolites (Fig.

3.8A) and hematite-rich oolites set in a matrix of medium-grained hematite-coated quartz

grains cemented by fine crystalline hematite (Figures 3.5 block B and 3.8 B and E). The

mudclasts itself consist predominantly of fine crystalline hematite and some chlorite (Fig.

3.8E). The clasts are typically elongated, and up to 50 mm long (Fig. 3.8A and B).

Elongated mudclasts which contain oncoids have lengths of between 30 and 100 mm

(Fig. 3.8A and B) and are randomly dispersed throughout the conglomerate. These

oncoidal clasts incorporate quartz grains, smaller oncoidal clasts, pisolites and oolites

into their structure (Fig. 3.8E). The internal structure of the oncoidal clasts consists

almost entirely of fine crystalline hematite (Fig. 3.8D and E).

Both reworked and in-situ pisolites occur in the pisolitic mudclast conglomerate. The in-

situ pisolites have diameters of up to 10 mm and are randomly dispersed (Fig. 3.8B).

They have nuclei consisting of fine crystalline hematite and chamosite (Fig. 3.8C) and

cortices consisting of even, micron-thick, concentric dense hematite and chamosite

laminae (Fig. 3.8C).

.7,

Figure 3.8


Text to figure 3.8 Photographs and secondary-SEM-images of the pisolitic mudclast conglomerate form

Klapperkop, Pretoria. A- Photograph of the pisolitic mudclast conglomerate consisting of a mixture of

hematite indurated mudclasts, irregular laminated pisolites, and oncoidal clasts. The clasts are set in a

medium-grained quartzite matrix cemented by hematite. Length of handsample is 12.5cm. B- Photograph

illustrating in-situ pisolites. Length of handsample is 5.5cm. C- BSE SEM-image of concentrically

laminated pisolite. D- Secondary SEM-image of oncolite with structure and cortex composed of hematite.

Note the quartz grains trapped by the structure of the oncolite. E- Secondary SEM-image displaying a

cluster of oncolites. Note the reworked pisolite with a core of quartz grains cemented by fine crystalline

hematite. F- Secondary SEM-image of elongated mudclast. Note the matrix consisting of quartz grains

cemented by hematite.

Reworked pisolites also occur as clasts (Fig. 3.8A). They are irregularly laminated and

up to 5mm in diameter (Fig. 3.8E). The pisolitic clasts are indurated by fine crystalline

hematite. The nucleii of reworked pisolites consist of fine crystalline hematite and quartz

grains (Fig. 3.8E) with cortices composed of irregular, vaguely concentric hematite and

chamosite laminae (Fig. 3.8E). The laminae enclose siliciclastic grains (mainly quartz,

Fig. 3.8E). Some of the reworked pisolites display partly eroded cortices (Fig. 3.8A).

The matrix of the pisolitic mudclasts conglomerate consists of medium-grained, well-

sorted, sub-rounded to angular quartz grains and hematite ooids, cemented by massive,

fine crystalline hematite (Fig. 3.8E). In slightly weathered samples goethite replaces

hematite and obscures primary sedimentary textures.

3.6 Pisolitic Ferricrete

Despite being developed only locally in the Pretoria area (Fig. 3.4; Visser, 1969), the

ferricrete of the Timeball Hill Formation is of utmost importance to this study as it

provides clear evidence for the accumulation of hematite-rich sediments in subaerial

continental environments. The pisolitic ferricrete is a massive, poorly sorted mixture of

tightly packed irregular laminated hematite pisolites mixed with hematitic oncoidal

(textural sense) clasts, hematite-coated quartz grains and a few oolites set in a fine

crystalline hematite matrix (Fig. 3.9 A and B).


The pisolites in the ferricrete have diameters ranging between 2 and 15 mm (Fig. 3.9A

and B). Almost every pisolite in the ferricrete is marked by irregular oncoidal (in a

textural sense) semi-concentric lamination (Fig. 3.9A and D). However, concentric

laminae also constitute part of the cortices of the pisolites (Fig. 3.9D). All the laminae of

the pisolites are composed of hematite and chamosite (Fig. 3.9C and D) and entrap quartz

grains (Fig. 3.9E). Nucleii of the pisolites consist of fine crystalline hematite and

chamosite (Fig. 3.9A-E).

Elongated oncoidal clasts that are up to 30 mm long are randomly dispersed throughout

the pisolitic ferricrete (Fig. 3.9B). The laminae of these clasts consist of fine crystalline

hematite and chamosite and incorporate not only quartz grains, but also smaller pisolites

(Fig. 3.9B).

The coarser grained components of the pisolitic ferricrete (pisolites, oncoidal clasts,

hematite coated quartz grains and oolites) are indurated by a matrix consisting of fine

crystalline hematite (Fig. 3.9F).

It appears as if diagenesis and metamorphism had little impact on the ferricrete. X-ray

powder diffraction and petrography indicate the presence of chamosite replacing quartz

and hematite. This suggest a diagenetic origin for most of the chamosite. In near surface

samples extensive goethite staining, and recrystallization of fine crystalline hematite into

structureless massive hematite by modern weathering, sometimes severely obscures

sedimentary textures.

Text to figure 3.9 A - Photograph of pisolitic ferricrete. Length of handsample is 11.5cm. B - Photograph

of pisolitic ferricrete illustrating oncoidal clasts. Note the round oncoid in the middle of the handsample.

Length of handsample is 8.5cm. C- Secondary SEM-image of hematitic pisolites with cortices consisting

of hematite and chamosite. D- Secondary SEM-image of in -situ pisolite consisting of hematite and

chamosite. E- Secondary SEM-image of core of pisolite consisting of smaller pisolites and quartz grains

indurated by fine crystalline hematite. F- Secondary SEM-image of matrix of pisolitic ferricrete consisting

of quartz grains indurated by hematite.

Figure 3.9


3.7 Geochemistry

Only three samples of the oolitic ironstone were analysed (Table 3.1), but this is thought

to be sufficient to characterise some of the general geochemical characteristics of the

oolitic ironstones of the Timeball Hill Formation and to compare them with their better

known Phanerozoic counterparts described by Harder (1989). All in all, Timeball Hill

ironstones are very similar in chemical composition to ore-grade ironstones of the

Phanerozoic described by Maynard (1983) (Fig. 3.10).

Table 3.1 Major (oxide weight%) and selected trace element (ppm)

composition of the oolitic ironstones of the Timeball Hill Formation.

Sample OTVL/14 TBH/MCN2 TBH/MCN3 Average Hematitic Si02 24.5 12.8 26.5 21.26 5.4 (A) TiO2 0.26 0.32 0.13 0.24 A1203 6.1 5.9 4.8 5.6 3.3 (A)

Fe2O3T 64.4 75.4 62.9 67.56 Fe203 51.29 73.32 61.47 62.02 67.8 (A) FeO 11.8 1.87 1.29 4.98 10 (A) Mg0 0.8 0.2 0.2 0.4 CaO 0.01 0.04 0.03 0.03 Na20 0 0 0 0 K20 0 0.05 0.01 0.02 P205 0.15 0.15 0.18 0.16 2.3 (A) L.O.I. 2.39 3.28 4.80 3.49 Total 98.61 98.14 99.55 98.76

Zn 138 162 155 151 Cu 119 88 134 113 120 (B) Ni 49 53 69 57 V 355 344 273 324 Cr 132 295 166 197 Sc 18 19 17 18 Co 61 69 123 84 Mn 650 276 212 379

OTVL/14 from upper oolitic ironstone bed near Airlie. TBH/MCN2 and

TBH/MCN3 from lower oolitic ironstone bed at Muckleneuk, Pretoria. A)

Average values for goethitic and hematitic oolites from Maynard (1983) and B)

Harder (1989). L.O.I.-loss on ignition. Analysis performed by B&B

Laboratories by XRF using standard procedures as outlined in Appendix I.

The Si02 content in the oolitic ironstones is directly linked to the abundance of

siliciclastic components, i.e. detrital quartz and original clay minerals (now chlorite)

(Table 3.1). The average bulk Si0 2 content of the Timeball Hill oolitic ironstones (21.2


wt%) is much higher than the average Si02 concentration of hematitic (5.4 wt%) and

goethitic (9.9 wt%) oolites occurring in ore grade Phanerozoic ironstone (Maynard,

1983). The Timeball Hill ironstones is also somewhat enriched in A1203 (4.1-6.1wt%)

relative to Phanerozoic ironstones containing on average 3.3wt% A1203 (Maynard, 1983).

However, most of the Si02 and A1203 in the Timeball Hill irontone are restricted to the

matrix.

Fe203 is the dominant constituent of the Timeball Hill oolitic ironstone (51-73 wt%)

(Table 3.1). It is similar to the composition of oolites occurring in Phanerozoic

ironstone-deposits in Newfoundland (up to 67.8 wt%; Maynardt, 1983). Most of the

ferric iron is present as hematite, with small amounts contained in metamorphic

magnetite and supergene goethite. Significant amounts of FeO can be attributed to

diagenetic and metamorphic reduction. In both the Pretoria and eastern-Transvaal areas,

chlorite incorporates Fe 2+ into its structure. The oolitic ironstone of the eastern Transvaal

carries a higher concentration of FeO, an observation that is readily explained by a

greater abundance of magnetite associated with later metamorphic recrystallization of

hematite. The average FeO concentration for hematitic oolites occurring in Phanerozoic

ironstone-ore is 10 wt% (Maynard, 1983). Where well developed, the oolitic ironstone of

the Timeball Hill Formation contains more than 60 wt% Fe. This is almost double the

iron concentration of 38wt% Fe of average ore-grade oolitic Phanerozoic ironstones in

Europe (Walther and Zitzman, 1977).

TiO2 concentrations range between 0.13 and 0.32 wt% (Table 3.1) in Timeball Hill

ironstones, similar to those found in oolites of Phanerozoic ironstones (Maynard, 1983).

The TiO2 may be present either as Ti0 2-bearing heavy minerals of detrital origin, or,

alternatively, it may be substituted into chlorite and Fe-oxides. Concentrations of alkali

and earth alkali elements are extremely low, indicating the compositional maturity of the

sediment. The average CaO concentration is 0.03 wt%, indicating that the hematite in

the oolitic ironstone did not form by carbonate replacement. P205 concentrations range

between 0.15 and 0.18 wt%, much lower than the average (2.3 wt% P205) for oolites in

Phanerozoic ore-grade ironstone (Maynardt, 1983).

A120, Si02

Fe203+MgO

0 Bulk chemistry of Timeball Hill oolitic ironstone

Hematitic and goethitic oolites

Chamositic oolites

Kaolinitic oolites


Trace element concentrations are surprisingly uniform in all three samples, with values

comparable to those of ore-grade hematitic Phanerozoic ironstones (Harder, 1989). Mn

(212-650 ppm) is enriched in all the samples but is especially high in the sample from the

eastern Transvaal area compared to average Mn concentrations (80-200 ppm; Table 3.1)

in Phanerozoic ore-grade hematitic oolitic ironstones (Harder, 1989). Figure 3.10

displays how the bulk chemistry of Timeball Hill oolitic ironstone compares to the

chemistry of oolites in Phanerozoic ironstone.

Figure 3.10 Bulk chemistry of the Timeball Hill oolitic ironstones compared to chemistry of oolites in

Phanerozoic oolitic ironstones. Diagram modified from Maynard (1983).

3.8 Depositional Model

3.8.1 Comparison with Phanerozoic Ironstones

The oolitic ironstones of the Timeball Hill Formation are remarkably similar to

Phanerozoic hematitic and goethitic oolitic ironstones in terms of stratigraphy, texture

and chemistry (Maynard (1983) and Young and Taylor (1989)). However, they have a

known surface exposure of more than 100 000 km 2 , at least two orders of magnitude


larger than any of their known Phanerozoic counterparts described in Young and Taylor

(1989). The oolitic ironstones of the Timeball Hill Formation have an ore reserve of 6

billion tons at 40-55% Fe (Schweigart, 1963) and in excess of an estimated 40 billion

tons total resource. All of Europe's Paleozoic oolitic ironstone ore (including every type

of ironstone, not only hematite/goethite oolitic ironstones) amounts to 13 billion tons at

38% Fe, and the total resource amounts to about 36.5 billion tons. Europe's most

important oolitic iron ores were deposited during the Ordovician and resources amounts

to about 6 billion tons at 30% Fe (Walther and Zitzman, 1977).

A depositional model, proposed by Harder (1989), based on stratigraphy, mineralogy and

geochemistry proposed for Phanerozoic oolitic ironstones appears to be at least partly

applicable to the Timeball Hill Formation (Fig. 3.11). This model suggests that lateritic

weathering is the source of iron for oolitic ironstones, and that iron is transported as Fe-

rich colloids and detritus via rivers to the site of deposition. Here, iron-rich particles are

deposited together with organic matter in isolated transitional basins (Fig. 3.11). The

accumulated organic matter in this isolated anoxic basin forces the Eh down to reducing

conditions and redisolution of iron as Fe 2+ takes place. Depending on pH, either

chamosite or siderite will form in the sediment during redisolution. However, most of the

remobilized iron is transported from this transitional basin to a more alkaline agitated

oxygen-rich marine environment where it precipitates as Fe 3+ forming the laminae of

oolites (Harder, 1989; Fig. 3.11).

3.8.2 Depositional Environment of the Timeball Hill Ironstones

Based on the dominance of upward coarsening shale-quartzite depositional cycles,

. largely unimodel paleocurrent directions, and the distinct thickness distribution of

quartzites and oolitic ironstones of the Timeball Hill Formation's oolitic ironstone,

Button (1973) proposed that the ironstones were deposited in a deltaic setting (Figures

3.1 and 3.12). The vast amount of iron held within the Timeball Hill Formation's oolitic

eie


Source of Iron

Transport via River

Site of Deposition

in solution: sedimentation of Flocculation and accumulation of iron-rich 10 ppm SiO, sand, silt and clay colloids together with siliceous and car-

detritus bonaceous organic matter in suspension: Fe-rich colloids Organic productivity and detritus High Low

Brackish Marine

aims

Figure 3.11 Source, transport and diagenesis of iron in Phanerozoic oolitic ironstones (Harder, 1989).

ironstones suggests not only that weathering in the source area was intense but also that

the transport of iron from the source area was very efficient.

3.8.2.1 Source of Iron

Pisolitic ferricrete accumulated on inter-channel and inter-lobel areas during the

deposition of the oolitic ironstones of the Timeball Hill Formation.(Figures 3.4 and 3.12)

provides direct evidence that lateritic weathering conditions prevailed during the time of

oolitic ironstone deposition (McFarlane, 1976). Weathering conditions within the delta

areas was probably not much different from those experienced outside of the delta lobes,

although there is no geological record of this continental area. High CIA (see below)

values for the oolitic ironstones provide further evidence, although indirect, that the iron

of the Timeball Hill Formation's oolitic ironstone was derived from an extensive lateritic

source area where aggressive chemical weathering took place (Fig. 3.12). The CIA

values for the oolitic ironstones are uniformly high, ranging between 97.9 and 99.7%.

The chemical index of alteration (CIA) is calculated as follow:


CIA = [(A1203)/(A1203 + CaO + Na20 + K20)]*100 (Al, Ca, Na and K in molucular

weight %) (Nesbitt and Young, 1984). The CIA is used as a measure of the intensity and

duration of chemical weathering. The CIA is ultimately controlled by the mineralogy of

the sedimentary rocks, a factor that is not only controlled by the conditions of chemical

weathering in the source area, but also by the processes of transport and deposition

(Schaefer, 1998).

The CaO* + Na20 - K20 - A1203 plot (CaO* represents CaO in silicate phases only) can

also be used to graphically estimate the extent of chemical weathering in the source area

of the oolitic ironstone (Nesbitt and Young, 1982 and 1984). Leaching of calcium,

succeeded by the removal of K20 during advanced weathering, moves the bulk

composition of weathering residue towards the A1203 apex. The position of all three

analysed ironstone samples virtually on the A1203 apex again indirectly indicates that the

source area for the oolitic ironstones of the Timeball Hill Formation was exposed to

intense chemical weathering (Fig. 3.13). The aggressive chemical weathering needed to

release such an amount of iron needed to form the Timeball Hill oolitic ironstones could

possibly also be related to the occurrence of higher CO2 values in the atmosphere during

that Archean and Palaeoproterozoic (Rye et al., 1995).

Figure 3.13 Ca0* + Na 20 - K20 - A1203 plot indicating the maturity of the oolitic ironstones of the

Timeball Hill Formation.


Although mainly used to determine the intensity of weathering in the source area, the

CIA may also serve as a good indicator of the maturity of sediments (Young and Nesbitt,

1998). High CIA values such as those calculated for the oolitic ironstones suggest that

the oolitic ironstones are very mature sedimentary rocks (Fig. 3.13).

3.8.2.2 Transport and Precipitation of Iron

Fluvial channels that eroded into the pisolitic ferricrete situated on the delta lobe and near

shore oolitic ironstones during periods of lowest sea level stand (Fig. 3.13) probably also

eroded into lateritic weathering profiles further inland. These channels were filled by

erosional relics of lateritic ferricrete and oolitic ironstone in an upwards fining manner

with a pisolitic mudclast conglomerate forming the base of channel-fills that fine upwards

into quartzites containing reworked (Fig. 3.14). The channel fills are cemented by fine

crystalline hematite. Hematite probably precipitated during dry seasons when Fe 2+-rich

groundwater migrated into rivers (Fig. 3.13). Fe 2+ was oxidised to Fe3+ when it came into

contact with atmospheric oxygen. The pisolitic mudclast conglomerates represent the

earliest continental red beds (Turner, 1980) in the Transvaal Supergroup.

Hematite in the oolitic ironstones precipitated in a different setting to hematite in the

fluvial channelfills (Fig. 3.14). During the wet season, deltas became flooded. The

flooded deltas formed shallow basins that became increasingly acidic and anoxic with

deposition of organic matter. Fe 3+ was reduced to Fe 2+ and remobilized in this acidic,

anoxic environment. Iron precipitated on oolites when acidic Fe 2+-rich water came into

contact with alkaline oxygenated shallow marine water (Figures 3.12, 3.14 and 3.15;

Harder, 1989).

Wet season

Fe' *ferricrete

Lower, iron depleted lateritic weathering profile.

Waterlogged conditions. Almost no penetration of oxygen into weathering profile and leaching of pallid zone and laterite by groundwater. Influx of ferruginous sediment into rivers from eroded lateritic weathering profiles. Accumulation of sediment by pisolites. Transport of Fe' =rich particles to anoxic flooded deltas and mobilization of iron to Fe' Reprecipitation of iron as hematite in oxygenated alkaline shallow ocean to form oolitic ironstone.

Dry season

• •

Fe' 'ferricrete

Lower, iron depleted lateritic weathering profile

Sediment dry out. Influx of Fe' with groundwater into rivers. Accumulation of hematite laminae on pisolites and coated grains during oxygenation of sediment. Development of mudcracks during very thy conditions. Some 0, penetration of laterite with lowering of water table followed by the precipitation of hematite. Very little iron moblized to precipitate as Fe' in shallow oceans.

Groundwater flow


Figure 3.14 Model for transport and precipitation of iron in the Timeball Hill depositional system.

3.8.2 Origin of Coated Grains

3.8.3.1 Oolites

The identification of several different textural types of oolites confirms that the Timeball

Hill oolitic ironstones were deposited in a dynamic and variable depositional

environment, similar to those associated with a delta-front (Fig. 3.12). The Timeball Hill

Formation shows many mineralogical and textural similarities to subclass A2 oolites in

the classification of Kearsley (1989). The A2 subclass oolites are described as being

comprised of smooth rounded cortices composed of concentric goethite and kaolinite

laminae with a detrital core deposited in a low C org environment (Kearsley, 1989). The

development of texturally different types of oolitic ironstone appears to be related to

different energy environments. Some of the Timeball Hill Formation's hematite oolites

(type 3-see petrography) have cortices consisting of remarkably even, highly concentric,

micron-thick laminae. Some carbonate oolites forming in the Bahamas today have the

same texture as the above mentioned oolites and are known to form in a shallow marine,


agitated environment where oolites remain in constant motion. Siliciclastic input into

these environments is nominal and carbonate laminae precipitate rapidly (Hine, 1983).

This type of hematite oolites may thus be inferred to have been deposited in

approximately the same type of shallow marine, agitated environment, with only minor

siliciclastic input. The influx of siliciclastics (mud, silt and sand) into the depositional

environment of the Timeball Hill oolitic ironstones is negatively correlated with the

precipitation of the main chemical component, iron (Fig. 3.15A). This suggests that

during the time of deposition of the oolitic ironstones, the amount of siliciclastic input

fluctuated locally and probably regionally.

In contrast, hematite oolites characterized by irregular lamination (type 2 ironstones) and

associated with quartz grains were probably deposited in a depositional environment with

significant siliciclastic input. The quartz grains are well rounded and well sorted

suggesting relatively high energy conditions. The type of cortex texture displayed by

these oolites has also been observed in younger biogenic carbonates deposited in low

energy depositional environments. These carbonate oolites are known as microoncoids

(Peryt, 1983b and Young, 1989). The association of rounded quartz grains with hematite

oolites in the Timeball Hill Formation suggest that these oolites were deposited in a

medium to high energy environment.

Type 1 hematite oolites are associated with mud sized siliciclastics. Their primary

textures are partly obscured by diagenetic chamosite, but it appears as if these oolites

formed in a deeper clay-rich, lower energy environment compared to the other types of

oolites (Bhattacharyya, 1989).

3.8.4.2 Pisolites and Oncolites

Pisolitic mudclast conglomerate are closely associated with the oolitic ironstones of the

Timeball Hill Formation on interlobel areas. They contain pisolites and oncoidal lasts

that incorporate siliciclastic grains into their structure. Irregular laminated carbonate

pisolites that trap siliciclastic grains have been described from modern and several


Phanerozoic successions and are thought to be of biogenic origin (Nickel, 1983). The

textural evidence suggests that pisolites (genetic term-pisoncoids) and oncoidal clasts of

80

.70

60

Eh

40

1.0

0.8

0.6

0.4

0.2

-0.2

-0.4

-0.6

Fe'

Amorphous

B

Fe(OH), (+Illite)

A

a

.8

I

Q TBHMCN2

increase in siliciclastic component

I

O 0Tvl I 4

0 TBEIMCN3

Fez*

possible Eh-pH conditions for Timeball FBI! Formation's oolitic ironstones

• Charn'osite Glauconite

(+Mita)

chamosite 10 20 30

Si0,+A120,

pH 2 4 6 8 10 12

Figure 3.15 A - Binary diagram indicating the silisiclastic input (Si02+A1203) vs. the chemical input

(Fe203) into the Timeball Hill oolitic ironstone depositional system. B- Eh-pH relationships between

chamosite and glauconite, suggesting that chamosite (Fe 2+) should favor lower pH and iron-oxyhydroxides

higher pH conditions. Drawn for seawater activities of K=10 -2 •2 and A1=10-6'8 ; Si=10-2-7. From Maynard

(1983).

the Timeball Hill Formations ferricrete might be of biogenic origin, and that they

developed within a soil profile that experienced fluctuating groundwater levels (Fig. 3.14;

Nickel, 1983; Gutzmer and Beukes, 1998).

The basal conglomerate contains pisolites with alternating evenly spaced concentric

hematite and chomosite laminae. Younger carbonate pisolites with the same concentric,

even texture are known to form in a vadose environment (Folk and Chafetz, 1983), and

have the genetic term pisovadoids connected to them, based on their shape, size and

environment of deposition (Folk and Chafetz, 1983). Hematite laminae on pisovadoids


may have precipitated when the channel fills dried out, and Fe 2+ migrated with

groundwater into fluvial channels (Fig. 3.14). Fe 2+ precipitated to Fe 3+ when it came into

contact with atmosperic oxygen that easily diffused into the porous sediment (Fig. 3.14).

Chamosite laminae may have developed during the wet season under anoxic conditions.

3.8.5 Evidence for the Primary Origin of Hematite

Several arguments may serve to support the primary origin of the hematite in the oolitic

ironstones. Hematite is the dominant constituent of the oolites and preserves accretionary

textures in every detail. It constitutes the nuclei and cortex of oolites as well as the

coating of coated quartz grains and occurs as cement in the surrounding matrix.

Replacement of hematite by either chlorite or magnetite effectively obscures these

primary textures. The large number of hematite laminae forming the cortices of the

oolites further suggests that the depositional environment remained stable over an

extended period of time. The primary nature of hematite in the oolitic ironstones of the

Timeball Hill Formation suggests that hematite precipitated from normal seawater (Fig.

3.15B).

3.8.6 Diagenesis and Metamorphism

Effects of diagenesis as well as metamorphism only partly obscured the primary minerals

and textures in the oolitic ironstones of the Timeball Hill Formation. Diagenetic

reduction is indicated by the presence of chamosite in the oolitic ironstone. Magnetite, in

contrast, may be attributed to incipient metamorphism. Higher grades of metamorphism

in the east of the Transvaal area are possibly related to the intrusion of the Bushveld

Complex.


3.8.7 Conclusion

The oolitic ironstones of the Timeball Hill Formation developed under virtually identical

conditions to those of Phanerozoic ironstones. The primary nature of hematite in the

pisolitic and oolitic ironstones of the Timeball Hill Formation suggest the presence of an

oxidising atmosphere at around 2.45Ga. Ferricrete and pisolitic mudclast conglomerates

are evidence for the development of the first continental red beds (Turner, 1980) known

in the Transvaal Supergroup. The fact that iron was remobilized under oxygenated

atmospheric conditions suggest that organic carbon was present in terrestrial

environments, as suggested earlier by Gutzmer and Beukes (1998).

Hekpoort Paleosol 43

CHAPTER 4

HEKPOORT PALEOSOL

4.1 Introduction

Correlation of the Hekpoort paleosol (Button, 1979; Holland, 1984; Retallack, 1986;

Rye and Holland, 1998) in the Transvaal area with oxidised saprolites beneath the

Gamagara unconformity (Wiggering and Beukes, 1990; Holland and Beukes, 1990) in

the Griqualand West area (Fig. 2.4) suggests that across virtually the entire Kaapvaal

craton, a surface area of more than 800 000 km 2 was exposed to an extended period of

erosion, chemical weathering and oxidation between 2.4-2.2 Ga. In the Transvaal

area, this erosional surface rests only on the Hekpoort basalt, and a very characteristic

weathering profile (known as the Hekpoort paleosol) developed throughout this area

(Fig. 2.1). The paleosol is overlain with a marked erosional unconformity by red beds

and quartzites of the Dwaal Heuvel Formation (Fig. 4.1).

4.2 Lithological Description

4.2.1 Type Profile from Stratal in Botswana

The most complete section of the Hekpoort paleosol recognized in this study was

intersected by a deep diamond drill core (Stratal) in Botswana, in the northwestern

part of the Transvaal outcrop area (Fig. 1.2 and Fig. 4.1). In this drill core, the

Hekpoort paleosol comprises of five distinctive lithological zones which are

remarkably similar in color, texture, mineralogy and chemistry to lateritic weathering

profiles as described by Schellmann (1982), Bardossy and Aleva (1990) and Thomas

(1994) in the modern tropics. The zones include, from bottom to top, the saprolite,

pallid zone, mottled zone, laterite and reworked laterite (or ferricrete) (Bardossy and

Aleva, 1990).


Chloritized Hekpoort lava underlies the Hekpoort paleosol (Fig. 4.1 and Fig. 4.2E).

In Strata 1, the Hekpoort lava is composed of massive as well as amygdaloidal lava.

No pyroclastic flows have been recognized in the Strata 1 section, although they are

common in the Potchefstroom area (drill core Rhkl). Hydrothermal alteration have

chloritized the Hekpoort basalt extensively (Oberholzer, 1995). The chloritized

Hekpoort lava grades upward along a weathering front (Bardossy and Aleva, 1990)

into a visibly altered zone, the saprolitic zone of the Hekpoort paleosol. It is difficult

in core to determine exactly where the saprolite grades into the fresh basalt due to the

fact that the lower saprolite is well indurated, and displays similar texture and color as

the chloritized parent basalt. The top of the saprolite (Fig 4.1 and Fig. 4.2D) is

marked by the uppermost occurrence of recognizable amygdaloidal textures,

characteristic of the parent basalt. Some of the amygdales are filled with impure iron-

oxides. This preservation of amygdaloidal textures is used to distinguish the upper

saprolite from the lower pallid zone. The pallid zone is separated from the saprolite

by a second weathering front (Bardossy and Aleva, 1990). The pallid zone can be

subdivided into a lower green and an upper yellow sub-zone. Abundant dark green

chloritic spots set in a light green matrix composed of a mixture of sericite and

chlorite are characteristic of the green sub-zone. The matrix of the green pallid zone

grades from light green down to dark green at its base, where it grades into the

saprolite. The yellow pallid zone is marked by its white, grey and yellow coloration

with small (1-2 mm wide, 5-10 mm long) green chlorite lenticles (Fig. 4.2C).

A sharp gradational contact exists between the hematite deficient (yellow pallid zone)

and hematite bearing (yellow and red) mottled zone. The mottled zone appears

massive in texture and is very fine grained (Fig. 4.2B). However, it is characterized

by a mosaic of colors ranging from red to greyish-white grading into each other.

These color changes define iron-rich (red) and iron depleted (yellow) mottles. Iron

depleted mottles form lenses and bands around red hematite bearing mottles. Lens-

like mottles are up to 10 centimeters in diameter, and rapid gradations are observed

over distances of a few centimeters from grey-white to red. The mottles are always

horizontally elongated, parallel to bedding. Red iron-rich mottles get smaller and less

abundant downwards where the mottled zone grades into a finely laminated

ferruginous zone, that grades towards its base into a thin massive hematite-indurated

section constituting the base of the mottled zone. Upwards, the mottled zone grades


into the red hematite-indurated lateritic zone that has not been affected by reworking

or erosion. The lateritic zone is overlain with sharp contact by elastic textured

ferricrete (Fig. 4.2A). The ferricrete consists of lateritic clasts and clasts derived

from the pallid zone set in a hematite-rich matrix with some hematite cement.

101 —

Stratal

Dwaal Huivel red beds 102 — Gamagara-Dwaal Huivel erosional surface

Ferrierete (reworked laterite) Laterite

103 —

Mottled zone 104 —

Finely laminated zone grading into massive ferrigunous zone at base of mottled zone

105 —

106 — Yellow sericitic pallid zone

107 —

108 — Green chloritic pallid zone. No amygdales

109 —

110 —

Saprolite containing amygdales 111 —

112 —

113 —

114 — Saprolite grades downward into chloritized lava

Figure 4.1 Profile of the Hekpoort paleosol as preserved in borehole Stratal, Botswana.

Text to figure 4.2 Hand specimen photographs of the Hekpoort lateritic profile. A-Ferricrete

(borehole Rhkl). Up to top of photograph; B- Mottled zone (Stratal). Up to left of photograph; C-

Sericite-rich yellow pallid sub-zone. Hammer head points to top (from Waterval Onder, Eastern

Transvaal); D- Bottom of saprolite. Amygdaloidal structure still visible at base of core sample (Rhkl).

Up to right of photograph; E- Chloritized but not weathered Hekpoort lava (borehole Rhkl). Up to left

of photograph.

Figure 4.2

0 m-

Stratal Shallow marine orthoquartzite -

I I Quartzite I

Pisolitic quartzite 5)- .:. ::...... ..- ..: • ••• e.-1.■ •■ •• •••

CI .‘,,,,.. ---,..-r* v_.,

Nre.. .., O., ...,._ -.‘0,

DPZ-2

W Bring

Ferricrete ,T 77'

Laterite -to

Mottled zone Pallid zone Saprolite

Hekpoort lava

Waterval Onder

H 1

uoge

unod

Lam

m p

uma

Regional unconformity

I

Rhk I v. sa

Pretoria CD


4.2.2 Lateral Variation of the Hekpoort Paleosol

Although the Hekpoort basalt must have been exposed over large surface areas along

the Dwaal Huivel erosional surface, the Hekpoort weathering profile is not equally

well preserved everywhere below the Dwaal Heuvel erosional surface (Figures 4.3

and 4.4). It appears as if progressively less of the Hekpoort paleoweathering profile

(as observed in drill core Stratal, Botswana) remains preserved below the erosional

surface at the base of the Dwaal Heuvel Formation from west to east across the

Transvaal outcrop area (Fig. 4.3). In fact, the ferricrete, laterite and mottled zones of

the weathering profile have only been identified in the western Transvaal area (Fig.

4.3).

Figure 4.3 Lateral variance in development and preservation of the Hekpoort lateritic profile across the

Transvaal area.

Previously it was believed that the road cut at Waterval Onder represented an almost

complete section of the Hekpoort weathering profile (Button, 1979; Holland, 1984;

27° 29°

+ 26°

Reference points

1-Borehole Rhkl, Anglo Gold 2-Borehole DPZ-2, Anglo Gold 3-Pretoria localities 4-Waterval Onder roadcut 5-Borehole Strata 1, Botswana Geological Survey

100 Km

31 ° + 24°


Rye and Holland, 1998). However, it was established in this study that the section at

Waterval Onder represents only the lowermost part of the characteristic Hekpoort

lateritic profile preserved in Botswana and elsewhere in the western Transvaal area

(Fig. 4.1 and Fig. 4.3). Erosion is believed to be responsible for the removal of the

top zones of the weathering at Waterval Onder and other localities in the central and

eastern Transvaal area.

B-Belfast C-Carolina CAR-Carletonville JHB-Johannesburg K-Klerksdorp Ly-Lydenburg PO-Potgietersrus POC-Potchefstroom PRE-Pretoria RUS-Rustenburg W-Warmbad WB-Waterval Boven Z-Zeerust

Figure 4.4 Locality map indicating the distribution of boreholes and outcrop profiles of the Hekpoort

paleosol examined in this study. Also shown is the outline of the Transvaal sub-basin (modified from

Button, 1973).

4.3 Petrography

Following the macroscopic subdivision, the Hekpoort paleosol is petrographically

described in its five distinctive zones from bottom to top, starting with the Hekpoort

basalt.


The Hekpoort basalt has been described by Button (1973) and Oberholzer (1995).

According to Button (1973), the Hekpoort lava could be best described as an andesitic

basalt. Andesitic basalts are composed mainly of plagioclase, pyroxene, hornblende

and very little olivine (Levin, 1990). Under the microscope, very little of the original

plagioclase and pyroxene are preserved even in the least altered samples of the

Hekpoort lava. The original minerals are extensively chloritized (Appendix

indicating that the lava underwent intense early hydrothermal alteration, as also

suggested by Oberholzer (1995). The distinctive greenish tint of the "fresh" Hekpoort

lava in core samples (Rhkl) confirms this suggestion. However, the extent of

hydrothermal alteration and indications that this alteration predates the formation of

the paleosol suggest that sea floor alteration (spilitization) may have been responsible

for chloritization.

Under the microscope, the saprolite shares characteristics of both the green pallid

zone and the chloritized but unweathered lava (Fig. 4.6E). It is characterized by the

gradual disappearance of amygdaloidal textures due to chemical leaching. The

mineralogy of the lower part of the saprolite is dominated by quartz and replacive

chlorite (Table 4.1A and B; Appendix II). Towards its top, pyrophyllite, kaolinite and

sericite are found in minor amounts (Table 4.1A).

The green pallid sub-zone is totally devoid of amygdaloidal structures. The texture of

the pallid zone fines increasingly towards its top. In its lower part, the green pallid

sub-zone consists mostly of replacive iron-rich chlorite (Fig. 4.7A; Appendix II) and

quartz. Towards its top increasing but variable amounts of fine-crystalline sericite

(Fig. 4.7B) and pyrophyllite (Fig. 4.7C) create a finer texture than that of the lower

green pallid sub-zone (Table 4.1A and C). Minor amounts of kaolinite are present in

the green pallid zone (Table 4.1A). Ti-oxides are preserved throughout the yellow

and green pallid sub-zones.

The yellow pallid sub-zone consists essentially of a dense scaffold of 5-151.rm large

sericite platelets with interstitial pyrophyllite (Table 4.1A and B). The sericite

platelets have no preferred orientation and obviously replace the pre-existing

pyrophyllite (Fig. 4.6B and C). Small amounts of kaolinite have been detected by X-

ray diffraction throughout the paleosol (Table 4.1A and B) and diaspore is sometimes


present (Table 4.1B). Rarely, apatite is present as large euhedral grains that are

replaced by both sericite and pyrophyllite. Chlorite is present in small but varying

amounts in the yellow pallid zone (Table 4.1 A-C; Appendix II).

The mottled zone is marked by rapid variation in the concentration of fine crystalline

hematite (Table 4.1A and B) clustered to form hematite-rich mottles of varying size

(Figures 4.6A and 4.5B-E). The hematite is set in a matrix of extremely fine

crystalline sericite and pyrophyllite, finer than that in the yellow pallid zone. Powder

X-ray diffraction indicated the presence of minor amounts of kaolinite (Table 4.1A

and B), diaspore (Table 4.1B) and chlorite (Appendix II; Table 4.1A and B).

Although hematite concentrations are highly variable, it is the presence of this mineral

that clearly distinguish this zone from the hematite-depleted pallid zone (Table 4.1A

and B). The bottom of the mottled zone is characterized by rapid vertical variations in

the concentration of hematite, resulting in a finely laminated appearance (Fig. 4.5E).

Similar to the pallid zone, euhedral sericite plates form a dense scaffold, replacing the

finer grained interstitial pyrophyllite (Fig. 4.5C).

The laterite, composed of hematite, pyrophyllite, sericite, chlorite and small amounts

of kaolinite (Table 4.1A and B), is characterized by the extremely fine crystalline and

massive intergrowth of all its constituents (Fig. 4.5A). Sericite replaces pyrophyllite

and chlorite replaces all the other minerals (Appendix II). No clastic textures are

observed within the laterite. However, in the overlying reworked laterite or ferricrete,

minute clay clasts of pyrophyllite are coated by thin rims of fine crystalline hematite.

Sericite overgrows pyrophyllite in the clasts. Multiple hematite coatings are often

visible (Fig. 4.5A). The clastic texture of the ferricrete distinguishes it clearly from

the laterite immediately underneath. Both the ferricrete and laterite appear indurated

by fine crystalline hematite.

011-,-15.08 kV 00- 05 on 20mm H Photo No.24

Moe, 284 X DoLoolmr. 110000

EHT-20.110 kV 288n -

WO., /5' Photo 1o. ,24

HO)- 288 X Dormotor= 1+11,0

00 - 25 um 01n X ',how NO . -31 Dorm:Imp= TEMA

\let

EMF=I5.MM 00

D uPo H

Hekpoort Paleosol

51

Figure 4.5 BSE-SEM images of the Hekpoort lateritic profile from samples of drillcore Stratal,

Botswana A- Ferricrete showing pyrophyllite clasts coated by hematite and set in a matrix of chlorite

and sericite. B- Large subhedral apatite grain in the mottled zone. C- Example of randomly dispersed

hematite clusters set in a matrix of sericite and pyrophyllite in the mottled zone. D- Fine crystalline

hematite clusters set in a fine crystalline matrix of sericite and pyrophyllite in the mottled zone. Note

that the sericite blades are euhedral, replacing interstitial pyrophyllite. E-Vertical variations in

hematite concentration near the bottom of the mottled zone, giving rise to the finely laminated

appearance of this zone.

Photo No 211

Uu 21 nn Hog= 11.15 K X . 1101.0c Lop , 501110

1 1 0010 No ..32

• A it 4 UI) 25 nn • Mg , 125 0

Dol.oc tor- TETRA

11 % - nn flu e,- 102 .X

1,0000 NO. Dol.00 tor - 117111,1

1.111* 15.10) 11101 ■ 0

1:1)0- 721) 51 1.0 map. H

1i i o H

fa: WO= 25 00 • Hag-- 2.31 K X . •

now Ho. , 21 Dot On torz.11:1170

SIT. 15.00 00 3itn

Hekpoort Paleosol

52

Figure 4.6 BSE-SEM images of the Hekpoort lateritic profile from samples in drill core Strata 1,

Botswana. A- Sharp-gradational transition between the mottled zone (right) and yellow (left) pallid

zone. Note the concentration of hematite at the bottom of the mottled zone. Large grains of Ti-oxide

are visible at the top of the pallid zone. B- Sericitic yellow pallid sub-zone composed of a massive,

dense scaffold of sericite blades with interstitial pyrophyllite. Large, euhedral apatite crystals occur

dispersed throughout the pallid zone. C- Close up of sericitic yellow pallid sub-zone displaying the

dense intergrowth of pyrophyllite and sericite. D- Example of dense recrystallized assemblage of

chlorite, sericite and pyrophyllite that form the bulk of the green chloritic pallid sub-zone. E- Example

of chloritized Hekpoort lava showing alteration to quartz, calcite and chlorite.


A

B 0 5 10 15

cPs

400?

cA•

cc.

20 Ensen (kW}

C 10 15 20

Energy (keV)

Figure 4.7 Examples of EDS spectra of minerals found abundantly in the Hekpoort lateritic

weathering profile. A-Fe-rich chlorite. B- Sericite. C-Pyrophyllite


Table 4.1A Mineralogy of the Hekpoort lateritic paleosol as observed in samples from drill core

Stratal, Botswana. Corresponding sample depths are shown in Table 4.2.

Sample Sample depth Zone Qtz Chl Cal Pyr Kao Ser Hem

(m)

10 102.5 Ferricrete ++ +++ + +++

9 102.6 Laterite ++ +++ + +++

6Bbo, 6A, 7, 103.1-104.56 Mottled +(1) + +++ + +++ ++(2)

8

4, 5, 6Bon 104.57-106.1 Pallid-yellow +(3) +++ +++

3 107.33 Pallid-green +++ +++ + + +

2 109.1 Saprolite +++ +++ + + ++

1 112.9 Chloritized +++ +++ ++

lava

(1)-Quartz is only present in sample 6Bbo, at the base of the mottled zone. (2)-Hematite concentration

varies throughout the mottled zone from + to +++. (3)-Chlorite contents vary from + to ++ in the

sericitic pallid zone.

Estimated amounts: +++(dominant: >30%), ++(major: 10-30%), +(minor: <10%). Abbreviations:

Qtz- quartz; Chl- chlorite; Cal- calcite; Pyr- pyrophyllite; Kao- kaolinite; Dia- diaspore; Ser- sericite;

Hem- hematite.

Table 4.1B Mineralogy of the Hekpoort lateritic profile as observed in samples from drillcore Rhkl,

Potchefstroom.

Sample Zone Sample

depth (m)

Qtz Chl Cal Pyr Kao Dia Ser Hem

P,Q Ferricrete 773.97- ++ +++ +++

774.1

0 Laterite 774.35 ++ +++ +++ +++

H, I, J, K, L, M, Mottled 774.8- (1) +(2) +++ +(3) +(4) +++ +(5)

N 776.75

D, E, F, G Pallid-

yellow

777-778.2 +(6) +++ +(7) +++

C Pallid-

green(8)

778.2 +++ +++ +

B Saprolite 779 +++ +++ +

A Chloritized

lava

871 +++ +++ +

(1)-A small amount of quartz is present in samples H and J at the bottom of the mottled zone. (2)-

Samples J and I near the bottom of the mottled zone contain significant amounts of chlorite. (3)-

Kaolinite is present only in sample J. (4)- Significant amounts of diaspore are present in the upper part


of the mottled zone. (5)-Hematite contents increase at the top of the mottled zone to ++ values. (6)-At

the bottom of the yellow pallid zone, chlorite is present in ++ amounts. (7)-Diaspore is only present at

the top of the yellow pallid zone. (8) The green pallid zone is weaker developed here than in Stratal.

Abbreviations and symbols as in Table 4.1A.

Table 4.1C Mineralogy of the Hekpoort lateritic paleosol at Waterval Onder.

Sample Zone Qtz Chl Cal Ser

OTVL 1 1 Pallid-yellow +++

OTVL5 Pallid-green ++ + ++

The top of the remnant weathering profile shows no

hematite enrichment, which suggests that the regional

unconformity cut into the pallid zone. Abbreviations

and symbols as in Table 4.1A.

4.4 Geochemistry

For this study samples from drill core Strata 1 were submitted for quantitative

chemical analysis by XRF to B&B Laboratories, Johannesburg (Appendix I). Rare

earth elements were analysed by M. Bau, Germany (Appendix I). The research group

of Prof. H. Ohmoto (Penn State University, USA) provided unpublished geochemical

data for the Hekpoort paleosol and Dwaal Huivel red beds for samples from drill core

Rhkl near Potchefstroom.

4.4.1 Major and Trace Element Geochemistry

Si02 behaves in a similar fashion in both the Strata 1 and Rhkl paleosol profiles (Fig.

4.8A and B; Table 4.2A and B). Increasing upwards depletion of Si02 is evident from

the unweathered but chloritized Hekpoort basalt to the ferricrete in a somewhat

irregular fashion. The ferricrete and laterite are depleted in Si0 2 (Fig. 4.8A and B).

Si02 shows strong negative correlation with Fe203 (Table 4.3A and B).

Except for the laterite and ferricrete, TiO2 is enriched throughout the weathering

profile in Stratal. The base of the mottled zone, however, is marked by strong

depletion of Ti02. TiO2 is positively correlated with A1203 (Fig. 4.8; Tables 4.2A and


4.3A). In contrast, TiO 2 values appear fairly constant from the unweathered but

chloritized lava upwards to the top of the pallid zone in Rhkl. In this core, the

mottled zone is enriched in Ti02. However, in the middle and near the top of the

mottled zone, TiO2 appears to be depleted, as it is in the laterite and ferricrete (Fig.

4.8B). TiO2 is positively correlated with A1203 in Rhkl (Tables 4.2B and 4.3B).

Positive correlation between A1203 and TiO2 in both Stratal and Rhkl suggests that

both elements behaved immobile during weathering.

FeO (mainly a constituent of chlorite) is enriched in the saprolite and basal part of the

green pallid zone but strongly depleted from the yellow sericitic pallid zone up to the

mottled zone (Fig. 4.8A and B). The mottled zone is marked by varying enrichment

and depletion of FeO and Fe203. This variable behavior of iron is also reflected in the

change of the Fe2O3/FeO ratios in the mottled zone. This variation in Fe2O3/FeO

ratios is more pronounced in Rhkl than in Stratal (Table 4.2A and B). The ferricrete

and laterite zones are strongly enriched in iron as Fe203 (in hematite) and to a lesser

extent in FeO (in chlorite; Fig. 4.8A and B).

Except for MgO enrichment in the saprolite (as a component of chlorite), CaO and

MgO are strongly depleted throughout Strata 1 and display a correlation coefficient of

0.97 with each other (Fig. 4.8A, Tables 4.2A and 4.3A). This is a clear indication of

efficient chemical weathering. In Rhkl, CaO and MgO concentrations follow the

same pattern as in Stratal, except that their concentrations are highly variable

throughout the mottled zone (Fig. 4.8B and Table 2B). CaO and MgO correlates

strongly in Rhkl (Table 4.3B).

Na20 and K20 behave similar throughout Stratal, as expressed by a correlation

coefficient of 0.93 (Table 4.3A). Na20 and K20 are strongly depleted in the saprolite

zone. Na20 remains somewhat depleted throughout the lateritic profile while K20 is

strongly enriched, showing evidence of secondary sericitization (Fig. 4.8A and Table

4.2A). The laterite and ferricrete are again strongly depleted in Na20 and K20. K20

has a correlation coefficient of 0.88 with A1203 (Table 4.3A). In Rhkl, Na 20 is

strongly depleted in the pallid zone. Except for strong enrichment near the base of the

mottled zone, Na20 concentrations are very minor from the yellow pallid sub-zone

upwards into the mottled zone, laterite and ferricrete (Fig. 4.8B and Table 4.2B). K20


displays a highly variable pattern, but little enrichment occurs throughout the

weathering profile (Fig. 4.8B). Na2O, K2O and A1203 do not correlate in Rhkl (Table

4.3B), a result very surprising given the fact that the mineralogy of Stratal and Rhkl

are essentially identical. Corestone like weathering may be responsible for this effect.

In Strata 1, P2O5 is slightly enriched at the base of the mottled zone and laterite but

does not display any significant correlation with any of the other major elements (Fig.

4.8A). In Rhkl P2O5 displays a similar distribution pattern, but enrichment is more

pronounced (Fig 4.8B).

In Strata 1, A1203 displays marked upward enrichment from the saprolite into the

chloritic pallid zone, and a further drastic increase into the sericitic pallid zone. A1203

concentrations are back to fresh rock values in the laterite, due to the predominance of

Fe2O3 (Fig. 4.8A; Table 4.2A). In Rhkl, A1 203 displays the same pattern of

enrichment upwards from the base of the pallid zone to the top of the mottled zone.

A1203 is drastically depleted in the laterite and ferricrete. Surprisingly, A1203 appears

drastically depleted in the middle of the mottled zone (Fig. 4.8B).

Within the paleosol, the major elements appear to form subgroups based on their

distribution and correlation. A1203 and TiO2 behave immobile and become relatively

enriched with increasing weathering intensity (Maynard, 1992). Fe2O3 is enriched in

the top part of the profile and FeO is enriched at the bottom of the profile. CaO,

MgO, Na2O and K2O are increasingly depleted with increase in weathering intensity

(Maynard, 1992). However, K2O concentrations have been increased by later

sericitization. The irregular behavior of P2O5 in the weathering profile is difficult to

explain, but may be related to the original concentration in the parent basalt.

In Stratal, Y (0.94) and Mo (0.96) display strong positive correlation with Fe 2O3

(Table 4.3A). They are enriched in the top and bottom parts of the lateritic profile and

strongly depeleted in the central section (Fig.4.9A). These elements are therefore

expected to be immobile in an oxidizing environment. U/Th ratios are always smaller

than one, except in the ferricrete, where reworking has occurred. Co (0.74), Mn

(0.63) and Ni (0.87) display strong positive correlation with FeO (Table 4.3A). Mn

and FeO are both constituents of chlorite. Th (0.88), Hf (0.94), Ba (0.77), Zr (0.81),


Rb (0.84) and Ga (0.94) display strong positive correlation with A1203 (Table 4.3A)

and are enriched in the upper chloritic and sericitic pallid zone (Fig. 4.9A) indicating

that these elements behaved immobile during weathering or were enriched during

later sericitization. Th (0.84), Hf (0.86), Ba(0.82), Rb(0.96) and Ga(0.73) displays

strong positive correlation with K2O and are enriched in the saprolite, pallid and

mottled zones (Table 4.3A; Fig. 4.9A), indicating that these elements became

enriched during sericitization.

None of the trace elements display any significant positive or negative correlation

with Fe2O3 or FeO in drill core Rhkl (Table 4.3B). Sc(0.77), Hf(0.7) and Ga(0.88)

display strong positive correlation with A1203, indicating immobility during

weathering or later enrichment during sericitization. Rb(0.74) displays the expected

positive correlation with K 2O (Table 4.3B). P2O5 does not display any significant

correlation with trace elements in drill core Rhkl (Table 4.3B).

4.4.2 Rare Earth Element Geochemistry

In Stratal, La, Ce, Pr, Nd, Sm, Eu and Gd show overall enrichment from the saprolite

into the green pallid zone, suggestive of residual enrichment, perhaps as REE-bearing

apatite. Strong enrichment occurs in the laterite (Fig. 4.10A). Relative enrichment is

also observed at the base of the mottled zone, matched by relative depletion at the top

of the mottled zone. The enrichment at the base of the mottled zone is progressively

less well developed towards the heavier rare earth elements (Fig. 4.10A).

Intermediate to heavy REE, namely Tb, Dy, Ho, Er, Tm, Yb and Lu display slight

enrichment along the top of the saprolite zone with enrichment becoming

progressively more prominent towards Lu. Depletion of REE in the chloritic pallid

zone, and also in the sericitic pallid zone with slight enrichment at the base of the

mottled zone, is followed by depletion at the top of the mottled zone and strong

enrichment in the laterite (Fig. 4.10A). The heavy REE Lu (0.89), Yb (0.91), Tm

(0.92), Er (0.94), Ho (0.95), Dy (0.95), Tb (0.95), Gd (0.92) and Eu (0.73) display

very strong positive correlation with Fe2O3 (Table 4.3A).

Table 4.2A Geochemistry of the Hekpoort lateritic paleosol and Hekpoort lava Major elements in oxide weight%. Trace and rare earth elements in ppm.

Str1/10 StrU9 Str1/8/1 Str1/8 Str1/5x Str1/7 Str1/6Bbo StrU6on Str1/6A Str1/5 Str1/4 Str1/3x StrU3 Stria: Str1/1 128.12 Depth 102.5 102.6 103.1 103.2 104 104.53 104.56 104.57 104.6 105.26 106.1 106.23 107.33 109.1 112.9 128.12

SiO2 18.4 23.5 44.6 43.9 35.5 38.4 36.9 44.6 44.6 46.9 55.3 54.4 54.7 37.3 47.3 54.8

TiO2 0.56 0.63 1.41 1.58 1.52 1.5 0.15 1.75 1.58 1.79 1.25 1.33 1.24 1.21 0.65 0.85

A1203 15 18.5 36.1 37.5 27.9 30.4 30.2 38.4 37.6 37.7 30.3 29.9 29.2 26.8 15.6 18

Fe203 44.35 39.22 2.04 1.1 18.93 15.21 11.88 1.07 0.72 0.55 1.03 1.19 1.21 1.81 1.79 0.36

FeO 13.1 9.34 0.5 1.44 4.2 1.7 7.04 0.43 0.29 0.14 1.4 3.2 2.87 17.9 14.5 8.1

Fe2O3T 58.9 49.6 2.6 2.7 23.6 17.1 19.7 1.55 1.04 0.71 2.59 4.75 4.4 21.7 17.9 9.36

MgO 1 0.4 0.1 0.2 0.2 0.2 0.3 0.1 0.1 0.1 0.2 0.3 0.4 1.3 9.5 4.4

CaO 0.15 0.31 0.04 0.09 0.05 0.16 0.15 0.13 0.18 0.12 0.09 0.09 0.19 0.15 1.06 1.36

Na2O 0 0.2 1 0.9 0.6 0.7 0.7 1 0.9 0.8 0.4 0.4 0.3 0.3 0 5.6

K2O 0.56 1.31 7.19 7.64 3.69 4.04 3.9 5.5 5.06 4.48 2.85 2.54 2.86 2.32 0.07 1.57

P2O5 0.07 0.18 0.07 0.1 0.02 0.11 0.09 0.16 0.16 0.09 0.05 0.05 0.03 0.03 0.07 0.11

L0.1. 3.93 3.99 5.68 5.27 5.64 6.44 5.75 6.72 6.96 6.93 6 5.9 5.82 7.13 6.86 2.94

Total 98.57 98.62 98.79 99.88 98.72 99.05 97.84 99.91 98.18 99.62 99.03 99.66 99.14 98.24 99.01 98.99

Fe2O3/FeO 3.39 4.2 4.08 0.76 4.51 8.95 1.69 2.49 2.48 3.93 0.74 0.37 0.42 0.1 0.12 0.04

Fe•fri 91.24 72.21 1.67 0.8 14.55 11.82 7.9 0.95 0.53 0.36 0.96 1.05 1.14 1.73 3.23 0.49 Feziri 29.94 19.11 0.46 1.18 3.58 1.47 5.21 0.42 0.23 0.1 1.45 3.11 3.01 19.05 28.9 12.33

FeT/Ti 121.18 91.31 2.14 1.98 18.13 13.29 13.12 1.38 0.76 0.47 2.43 4.15 4.16 20.79 32.1 12.82

Laterite 0.31 0.41 1.17 1.14 0.76 0.84 0.88 1.13 1.16 1.23 1.77 1.75 1.8 1.3 2.72 2

Sc 28 37 45 43 96 64 81 93 58 56 56 80 25 40

V 284 391 187 184 606 770 251 277 343 379 275 375 166 151

Cr 258 161 244 202 366 415 264 283 608 715 483 793 412 642

Mn 400 217 0 54 0 0 132 91 0 0 94 300 1939 1300 Co 96 14 24 5 24 30 26 8 14 29 26 65 130 63 Ni 161 124 5 44 89 58 80 123 57 107 107 577 243 109 Cu 207 101 0 30 0 0 42 25 0 0 28 0 58 9 Zn 46 37 0 10 7 6 12 9 6 8 23 47 272 50

Ga 17 29 37 36 26 18

Ge 0 0 0 1 0 0

Rb 8 65.8 325 385 90 228 240 217 128 110 132 58 9.8 42

Sr 32 152 81 81.2 53 198 197 153 46 38 53.8 30 8.7 109

Y 494 25.3 64.2 49 31.9 90.8 18.5

Zr 26 136 220 285 117 295 314 297 221 218 202 109 110 134

Mo 13 1 4 1 1 0

Cs 4.31 5.58 5.9 5.01 4.35 8.31 5.62

Ba 0 201 570 1021 522 865 884 746 157 268 273 152 25.6 929

Hf 4.37 7.87 8.21 8.64 8.18 5.68 2.83

Pb 61 49.3 32 15.5 55 16.7 20.1 12.3 25 29 5.29 27 9.24 24

Th 17 9.6 25 18.8 16 18.9 22.1 21.2 12 18 13 14 7.79 14

U 20 6.69 1 3.62 8 8.76 5.66 6.68 0 0 6.75 2 2.87 I

U/Th 1.18 0.7 .0.04 0.19 0.5 0.46 0.26 0.32 0.52 0.14 0.37 0.07

La 114 48.8 52.4 119 60.9 78.1 72.1 6.95

Ce 195 105 117 260 143 134 137 14.8

Pr 25 13.4 20.2 36.1 20.9 16.2 18.8 1.8

Nd 116 50.6 101 150 85.8 56.8 72 6.65

Sm 35.3 9.52 29.4 33 18.6 8.93 12.6 1.69

Eu 9.91 2 6.76 7.76 4.3 2.17 3.17 0.478

Gd 68.7 7.49 22.4 26.2 13.8 9.23 15 2.36

lb 16.9 1.12 3 3.171 1.52 1.61 3.03 0.43

Dy 102 6.37 17.1 14.6 7.56 12 21.6 2.92

Ho 18.3 1.13 2.89 2.34 1.36 2.93 4.7 0.623

Er 46.4 2.96 6.74 5.5 3.66 9.6 14.5 1.94

Tm 5.92 0.408 0.845 0.738 0.517 1.54 2.2 0.304

Yb 33.3 2.721 4.68 4.36 3.24 10.1 14.5 2.01

Lu 4.66 0.39 0.628 0.638 0.466 1.63 2.21 0.32

Samples taken from drill core Stratal, Botswana. Ferricrete 102.2-102.5m. Laterite 102.5-103.1m. Mottled zone 103.1-104.56m Pallid zone 104.56-106.1. Saprolite 106.1-128m.

9 O

.75 N V

N

N

8

6

O

2 0 0.

N

8 2

N

'81

2

0 .9

y

Y

I

O

= O

8

000, 0 ,f ■ NW00,7■■ NcOm,WMM OMOWQNOVN.1.0N.4. ■■ MNOOMNVm ■ ..,.0-.,,,p ,,,,"000m0 mNm 464-666a6--cia8davi6a NN,,,WW ■

M ■ ■ N -64"acicid - d6a6 ON 6 6 -

q .000 qdd99 0000 q.": 0!qqqR -, -: 0.6 ,4 ,n,-,-6.00;40, ,000006,m006 ,00,0,0 0,, -0 , 1-i 0m6m-0.00.6

a 6 6aa66aa-6a66-ma ocoa66.-6.6aaommaq.acc000aaaaavaa-Nr-m ‘06coaa-66aaavaa vidia-adda6--66g66a6a M ■ NMN0W ■ ,.. NooN0,6

■ darA 666 - 6.666 g406 ad 66666

0, N0.1. 0.1. N0W ■■■ WN NON-, N0 .1. MWO, MM0,VCI,NNOs0.4. 00.4.0MMV00.4.NwIN.4.M Mchoomv,o0M0w)Nmm ■ mt ci 4 - - 6 .4 a 6 -; a 8 oa c.66.6.666 - dac.id daaaadadad6666

- 6- 4-

nt CO 00 0 ••-• .0/

NNNOM,NNVI0V1 ■ MN00000 00.0M ■ N00, 0M0, ■■ NN ■ 000VINMON0N.VM ■ W,M MWINwl,MNOVN ■ M ■ M vida 66, m-.6a666-,4 viacco„,6 oa'6d66 - 6666 664aada6a66666 -

CONNNO ■ NMW0.e-0WNNV0 ,* ON , ONV1 , ■ 000 ■ 0MOM ■ 00 ■ Chas? ■ WM ■ 0 ■ .1WM ChWIMN00, 00WW 464 - acivia-cidagda6da

6 6 vici6a666"dacid - 64 -- aciadadeici6d

MoostMV0NNWOO ■ Omt. MNWNM NmooMNV0NWI ■ OV ■ .0., ■ 000MN0000,N ■ WO ,10d wIr■ N .4. NW0VINNNM ■ M 666 ciadaa 66. R64 - 6a

6 m6a.5a.oca-" 6 ,6666-6. 666 a-aaadadad6666

N Os -•

666a6c!ccam-m-a6 -6-6 6voama--6666a6aooa6oca6ac.am6aa awma66aaawaaaa 6d a.-; -da6d6666 6 6 a --. m - -

a - a ,,,- d Os 0 ■ N ■ vo as '•+,,6i - N,4 ,i,,aad-, - 6.666 aaaacida6a66666 6 cca - a

000, VM ■■ N ■■■■ ONMN ■ 0N MMN ■ MMOMN000NMCAVIMOS000 ■ N ■ N ■■ OWON,V M.d. CA,N ■ MW00W0NN

-aM666.66666 0'6666- , A" am ."6 a66666 N -- 6. cn a 6 6vi.-a 4a6 6aadaM4.-;a-46a6

a v. Cn N ■ 01 n 01 ■

■ M ■ .10 ,4:0 ■ N ■■ NNM NN0NO.V. 0 ■ W00,400, ,10 ■ 00WWINWM ■ N ■ WON,.d. N,Wstoovioo,o0VNO.Mat

-c566dvia a 66. aacc6=66. N 6- 6 6 6saani6 a6aaa666 4a6 vi6a-a 6666 dad 66- 6 et c0 N

M ■ 0 ■ MNN0N ■ NNNO.M ■ VVIM ON , V. NVINNOSVIOWNNMOM0000 ,-!NWWNMN000■ M M ■ 0, 000NWNONN.Q.4. V. a-46666aa6-.66R66ada moa m6a6-,4 a6a

■ ■.6.4 o6a666 -. a...;6 6a aa6vidad6cscid

M co m

mspao6aoa-ma--aooaco6 ON-aovo66 ,qoadamda0006ard-a66a6a a-6 66 666-666 0,,N0 ■ M N_,MV ■ d

■ ■ N ciN0.600 06 , 6a666-a-4646 a a " a - -a

0N0W0NON ■■ 0 ■ 01 ,M VINN0WN.10,d,VN ■ 7 ■ 0000MNMNet0MON ■ NNV N0.41. 0, ■ 0,0,V. VNV. V.VM vi aci-.6 66666 w cid-;-;- a a aa_4aaaa,n6Ra,6,,na a.6cia 66,- a6a6-66 - 66a6 6-4 s°46--.6-a666 aM N^- -

WN0 ,70.4:0N ■ M0 ■ MN NON-* 0 , -. VDNOWN000MM0N000 ■ .cte4M00c0M0,0,0,NM C9WWW0!.** ,13WINWWW1W

f46.■ .-. ddd-6 a d 6-. 6 " v 66*. a66-6. daa466-;a6"da a -a -,666a6d ada6 a 6 -a6%0 6m-.6 a-. 6 6w 6-

a a -

a66d6,p6macc.”.mm6aa6a oaa4.666maawawoa6-0006aawamaaoaam a-.-ammaam66m6m iAdc,1.66.-.64a 666gd666a

-. -. a CO NO 01 00 0, oo ■ . m " os - 66A6000v 66d aaac.i6adad 666 6 -a

0N ■ MOMON ■■ Wo ■ ,N MOM- U.IN0WNOWOMNNCANN0o10 0MM MOWO,Chd MONNMM ■ oNcs,p, ■ 0v0 O-a a 6 66666a 6 66. m Y"4ga *, '-6 64-.c66- 64a6.6a..66 .'adad -:666a6.-;6-a6 6 6- N •-•

ONst ■ N0N0M ■ NO, ■ 00 MOMN MN0 ■ CXM ■ N0 .. a6a4 4 66666 6 Ni

S en 00 -" S.'6"""'t6 aa64ad

a6,66. 4a66666 .'6"66 viaa4a666 ^ -;ada 6 am

Ni Ni 1 6aamomo6-6a66- acia6 aawoamo6-a6maa6a...o.-.66.6waamaN66com 6aoaa66.7,acoacca6

a 666666a Ni 6 ,-; a E N-at 6

2, NN Ni -.. -- gig-g,tz:ci da a M a - 6 6 °` 6 6 vid F....: it.; <, tz::2: -4. ,.0: - 6 6 a 6 0 ■

1 N N ..7. .....,4

[-. 0, 0, 0, mv. 0.4. ■■ 0 ■ 00M,MW ■ V ■ WIWNV. M0W ■ W0r0W0000 ■ VV. Nnn.e. 0 ■ MNTI: 0,-.-:me.:0,01- ,nm.0.01.0m -6a66d 6666d a--.-6.6 t- -. (-- cz , " " "." 6 •-• a 4 a a a a .. 6-"o ^ 6666 oa6a6...co ao6o o

di nt en .

,42-."... MO,NW ■

N N MN M

=

.O., ...'"': .4"-.0.-.0 "T".,40 ■ 00 000^ .1 ■■ SNNO ■ N,M ■ NNNW300 ■ ChM0, ■ NdWNWN0NM VetNnMW00,MN0ONVN 4 .-.N.A ,A 6666daa a m.-; M

,,,,,, a '- '6 -. 6a6a6 dviaa6a-4d 6 , ,GO ,,A, •,,,,,,.4 •4,:,,,i,:;_,6,46 Ch wiN V Mas ■■ ' N N0 M

:2 Ni _

K mNmv,moopm...... ■ ce0,NNNN V ■ WM.0. 00,MMNWIN00N0000 ,41,0, ■ MNNNNNMN 0NN ■ NO,-..IN, ■■ W ■

A M a ei d a ci ci 6 6 a am a O d6 •-; sit" ,/,... ."4• 6 66.6a66 ,, ,- _, " 1T+ -a- a486-466^6646 -. 6 -; 6 6 ci aci6d- 6 6 ci -.

9 Ni Ni

":., (".. .D.2;NO ,ONM ■ 000 ,4:M ■ NN OMV. ONNO0MNO,OVINN00 ■ NN0VONntMM ■ MVN VO.N.,NooMNInknoo ■ cr, ■. 40.ig.4-4 666 aa-6 6- 6---co "°, 6 -., 6 - A"d daaap, - 666"daad aad66aoadd6ci 6 0.a 04 h

mv,, M0m0m-VVIOnm ,00 ,0V N-.0, 0.0000V0 00nW,m,m ,ri ■ Mm.,1 ,0 T N vloOVCnnNop ■ -elal -66a a 6666 aa a a6 accm -4a a.

ci ,n , 6

- q -

a oi vi

2 w ..aal 666 -"-a66 a46-occoo caoo6o

A Noo , w0a00.1-NONNV VOV ■ VNVNChOm ■ oor...0 ,0 ,0nmo2.1000N ,OvVo■ wooOmV ,n04 Nn-VVr.V.INViaOMMM

Cisaoi CO aciciddaa a a 6- a6a -.66m"-,6d,,,;-,6

.... ...,..-.A ay:mad' aad 66 a6d6dacs-cid tN Os N N 01 N •-• CO

NNNNM ■ OVNNN00.10.,10 ■■ N ■ MO, N,MNN ■ MO,V. M. WMMOONM ■ MMNN .O. N0NNV. MCn ■ VM, ■ MNVMNMN N N N Vi •-■ N Ci CS Ci en r-: 4 e4 (: vi -

co .4....‘r-,4 4-6006,606,6 N ■ V' N

(-4,6444d 6"66Md d6 4666-6-.66.6 N 6 6

8 -Rd 6 cidd 6 g6.0 , ,0 ,tS,10.70 M OCh.0.0 0 0 CI 0 00

■■

-*VV10400010700 -.ON T.9O`OT d OS el CO 0 si0 o■ 6 cc co 6 m 0■ 000, 01 s0 N 00 ■ NNNN 0200 al " ‘o N ■ 4(.4 (.4 66666 d't" "d 7taM''''6 64 - 6.6.6 aAa-ad'daad 6-.ada66666 NN N T -•

N

oo no, .poso, ,4 o ggi vp,so r...a.,Nmv.,wm g o,o ,i , ,,,,,,I ,..s . .4d,:ig g dcidddag NC nnt, aa66 viad6c44c6-666- ^"60o-. man° ..0 0,40.-.0.-.0

0' 01 N 01 •0 ,t n

NO

.' 0 "W".".., ' vez,.7.12 2 ,1 01 ,n ,Delonel s - 1 ,t r ,g7,,g .0.--Nm06, 00.°0 .0-0.0 -,., ,., m m-- -4i,o ,o, --m-00,00.0,0 0 pa=aogo- Roo cAfAceipla.6a vom --,- -.

.. a - ;.'

A

▪

.06 ,.:JG d 6 cida ct: Ci ••••

0.,NMZ4,0 ,00M

•

0 "TON:51NN ■ W ■ W

6(5 1A2 (4 9 ,93 0,68 9 2 4 '0PactcuL 2uzx07.4mlitc8X,1 01,:;>ddidAcgd...taae3g74g3Fe.iigA 1642Aj3g&XOU3

.9

t.,

ra

44

U

00 0 -• Cn

cid

Os a. d 0

00

1••• 00 ".

0. O 0 CA Ci ciddd

••e rn ‘o coos cidcidd

000 ono N 00 00 00 d d o o o o

o h a

c

S dn h d

dc da

•

4 d V VI

csO 0

dcdcd

M CV en ,1 h a cel c,„

cr . 0 0 0 0 0 0 0

N 00 Os en eV 0. ON VI -• c0 " " (.4 d "1 ,1 ,1 .1 .1 °9 cl ddd 0 0 0 0 0 oo

h 00 00 cl• •••• Os en 00 rn el en 0 0. ct h dddci d d d 0 ci o o

CO 00 00 00 CO Os CO SO SO Ces Vi h VI h h h -• h n 00 Ces 00d00000dd ° 6

'A A • A A A 9 A' ; '4 4 4 A g s, 000000 dci ciddedd

■:,' Pl c4 4 N Ag a AfgrA r.i.VA .2 ci;cf. Si q ci;c0 ci; S;c3c3dddci q N sO CO h 00 -• .0 VI - On 0. On h h

e P•N d °1 .1 0.? ,,: .c.. e-:"',. -t. ,,-. 6_, dd o o o o o o o o o o od ea

.--.a a ac ,oa enenna, r- ,a.nena.-.r- an; •tnenen • e^ "cida ,1 01 "q 999 90 o o o o 9

.A / 449 ,q&' .2 9A43A a rair.8 °' 000 1404 daidddadd f a

1.6 0 6 1. " APIta; a A 'a.1 8744 1:18 ..°,1 0 40 a 00 °004 10 0 0000

' .5;%?; 4 4 `t P " A 49.64 &PS c t3 cid qi da °° dddciciddcid

1-• as as •-• •-■ O. 00 es sa as 47 a, sO d•-• ••••• O. Os 0 Os Cn

a VI n a n VI a 00

ddc cd d dc cdc c d q 7 0: •

'" .7, ""I 4 ra- 3`21 66 ddddd 000 000 o o o odd; ci

4 '444:4P.A.ISPI-. 'A gnss g s'ci,q.ci'c'ci;c5 0000 0 0909009090

9 9 9 9 rn S 4c, P4 4 !-5. °3;g1 g Pi EX00040004d cidoaciad cid c.

U N al CV al al al .0.• -• 8 6' 2 6' " 4 " "" " SO a• cr en -• h

cce O.

4004 00ddda 0 00 0060°5dd

r- b so h a 01 02 eV r- ‘o a, r b 0- 00 •d• •-•• •••• •-•• vl V0 VI V) '1 en 0 eV •1• •I• al al ON en ON 00 -• Os h VI • Os h 9. 9. 0 d a d ci ci ci ci ci ci a d ci d ° d a

N° en'" 4° v.,- - C' 7i .41 A. Fj a! 4g 41 44 59 4n 59 59 95 qgfi q cii aaaaaaaaa 9

.--t g 00 N 1::: P. P 'A ,1 ',?, .94 ',Q a- S t7 N 'A .9,1 :2 6 2 O. ....1 el g N g 04. ddd ddcido a cid ddddcidaciddd d00

enav, ,lanar.a. os 00 00 se, VI v) Os sll oo 00 •-• on In - s.0 .-. es 00 on ces N en ON Crs Os Os Os Os Ces Os ca a a a VI sO or al Os el al ct O. n N- Cl e • CO 0 a d d c c c c 3 00 0 0 00000d0g

100 a

" " ` ' ' - " ' " '4 '4 4' 4 4 P I '-- "" r rz -,1 g.' ,c,' w. ^ 6 ' " 7 2 .‘ `. 8 ' 44999999999994990999909.009R

ch m " w ' ';' 74 'S : -1 S 73 16- 2, r;-, ,--,. g 4 8 4' A 9 4" Ft% PI LI g: A ..1 8 Ci c'ef' ci l' I ' 9 ' 9q4; 9i5i9c0c0q999999999 0 99°

2 2 8 8 O 8 8 ' t 9 4 : ', r,,- P 4 9 g; 8 %I 4 8 P 8 P r '.'-.' 8 g .1 IP g Ill 'en' A 00000 0004440400000000 00 0 00 0 0 00 00

"`""""" P- iz: $ 3 " G ea ` la: " 8 " ' 1 ' Z'r r,1 „- A `P N N g 2 - " ' 9

ep

ci. q c.; 4; 9 c1. 4319 qc; Ogg c011q qd9 gq9 coq 9a qq a cs rz tz ■-°- g P. 1 2 rz ;*, G S 2 -. 'A ?I.! 4 '6' ',2 4 ,,g: A g."' '43 PI 4 A r.' '2 P 1;.: r 4' 8 4.

0 ci ci d ci d ci a ciciddedd dd '90099990409090

`e : el se, '41 A c', '4 ,s9 6" ' 8 a ' 9 " 8 O- N" 8 ° 6' I 2 2 9 / '4' .ai 2 : i 9 A' .:1 g A adcidddciciddddddcidaqd 009099999099990

'l lelLci scl 17;i.«. r... ra A S 3 9 . . 8= ,a4 A g g 9 9 P1 R. 8 ..1 .8 g s 8 A 2 ,t:i 9 0 0 0000000000000009 00 90 0 00 0000 d

4 4 4 ;7. P- , 9 :2 A 8 $ ^ A ' 1 " ° 3 8 zi. 1:3 '8 S 7; PI :ft'. !°.. :2 P. ' a A A '4 ? / ' q A' ! l - g

N 0 •-•• Cf' q 4' q q g f; c; Sid da ci ci 40090900000 9 49999 9 40 cid d ° 4 d

4 4 4' 4 'A A 74 2 F'.. PI '1 '41 4 P " 8 " ''' " " O`•

ci; g0q c 00 desadadd`c2 0 "ce2 ; ";1°.. "2a 2 ;";:i 4 '''4 r4;' ,; tcl .1 5 =''' 4 6 8 6 8 6 6 S N : / 7, 2 :2 N 2 r,':,' N " -. " " ''' " " N " a Vi 99999999990999999999909C;;;4 0,g 4 4; CO' 00 N Fi 'Z 24 .1. 4 "4

A P P,"' PI g.' P A 1, gl ';:'; R P S :z 49, 74 g. 4 EI 4 i. 4' '41I 4 N Ig g: . -1 PI C7 g:Ae3P1 n n A A E7, cl. gg 'zI c ci5 qq4; ' 00000000 d 0000000 d 00 d a ddd 0000 a

sel sO 00 Cn ON 0 0 -• 0. -• en -• en al VI CO -• Os Os en y) Cn SO ON •-•• ON Os Of VI el al --• en ..-, .o . co ,0 e, n 1••• CO 00 00 co a a co 00 w, a -.. a o al el VI sO h en en sue a .., ,,,, .-. el n - e, oo -. a el al VI -• -• VI VI N -* ci d ci ci ci ddd a ciddcid ci ci d d c. d00°00ddd0 o

11

a

PIPP,A.P: P,' 3 9 gl .7. F,',' g , 1,3 9 9 8 :II '6:' S 8 ' 9 4 P. 9 ea ; g ,' A 2 9 " t 2 3 :".3 A 9 .13 P.' t." -,- s3 ril cidaddci d . . . . . . . . .o

d 00009aa g d g 00 ' d 99 9 99 ° 9 ° ddd 6000.0d 0

'Fos g. F ;' g P O. 'I: ri g2 a ,,,-; 'T.!. 'A' V 7 T...'' S PI 72. 'A' A 1, A F-.." 7.: PI g ' 7. "S 'A g 8 1 9 A 9 'A 8 t" a : PI aciciddciciodciddddciddddddcidddd . dadd . 9 9. oda 99o9 ° d

""''''''' 4 4 4 9 8 1=1 g ; 4 '4 g 8 2.2 ',1 g 1::: 8 06 .7, ,`"4 3 '4 ' 1 S' f., - Z' 8 ' ^ '4' 8 ' g'S ; r - S g. ' P ;7, 9999 cis qc00 °°°°° °q°q°°c1;6 1°°° ° °qc'qq. q 99 4cii°°° 9 41qg 0 0 Os O. h en Os on •-• en O. •••• 00 os4 1.-•• Os VI 00 h VI 00 vi In. 00 71 ••••_ sr? hhhhh -• en 0 en 0 - CO -• h 0 en h sO 0 VI ON h -• ct 0 sun el sa sc• .0 Z51' .s5 ‘P,1% 9 9499999900 0 0090090900 d 0 d 000040 d dd 0 9 4 9 g°

co rn se• 00 0 Os h 4 •••• Cn el h en en Os en VI .0 4 v.. 8 en 00 0 -• ct CO el s -4. h h • 00 oo co oo • h • en ct •-•• sO el 00 en -• -• 0 el Os 0. 0 al 0 st 0. 0 O CO rt 99 9999 9 9 9 9 9 9 0 9 00 9 9 000 9 9 ° 9 0 9 9 9 00 99900

A

E U

9 a 0 6 6 6 4 u1 g33 A 2,t63 g R. X" al 6 t4 JA 6 6 > 6 (3 11. 2- (1.' if it if

Q

a 00

olg

N t-. 00 O. 0 O. cicid

ch‘or-.o. 0 oo ON at ol addo

Va M ,41 CO Qs 00 Os Os Cr, 01 dddoc;

M OS ••• h 00 0' co CO ON 01 01 01 cacidodd

ggo ro, cicicidado

N 00 00 CT VI C.. h CO 00 00 d Os cidcidd 0 d

-• 00 V) •••• h 00 CO CONO 00ON \ 00 a, 600000dd.

ID 10 N e0 N N 6 M M 10. M ID 1.0 OS c; 0 ci 0 d o 0 ci ci

d M 00 00 M d 0'

N N sea c; 0 0 Ca c Ca 0 cid

ON t tO M Os M II. VI ON h M cicicicicidoddddo

M WI tel N m 00 ID 11. V1 CO hM M 6 IMM CO 00 00 Os dddci o d ci o ci ci ci

01 CO N N 00 el M dmdm -• el el N en en mdvo. 00600000dd d d c;

9 8 8 8 8 6 8 O ° A- .

•

9 0 1 ° 666 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

O

O

U

00

0

0:

O

0

U9

O

O

O

10.2

0

O

O

0

O. h+ - 6 Noe gtb N 1.0

000CCC3 CalaCa00: 0"" g;ig iC;

r

•

d. 4' 8 4 g P, 4 h 54 4 4' ° A ci d ci o ci O

ci ci

0 N

▪

:‘? g A A' °

o

N 9 9 9 9 00000000 000 00

8 8 G S 9. 9. 8 PI A '5 8 ° " 9 qcicidciddcidd ° dqci

to <7. A A P, t A A 444. 88A •10 688 9 0 ,600 ,0 6° 0000.6 ,60d ° ciciciq

S S - .7 7-1" A 8 V ,g 0 9 1 99 0009 9 000004 cs l a O CO Os 01. ON hM d et V1 m CO N VI 0 01 h 1.0

O

vw, ,r, SO No. ,1 .1 VI •O •1 d ',vv., CV CI •-•

0dcicid0dcidcicicicic; dcicicicaq

' 88018A "''''''-' 888 ° A88 9 ,8 9, 8 000090005 00010 66 666 0 6 00

CO 00 M 1/1 CO •-• 2.! 00 0. •-■ d 00 Cy..I 0 4 .2, 8 .„N ,s r,-; A CI N CV el el CV CO M et el el d et to Ca 0 Ca Ca 0 ci cia do cid ci 0 d

,

•

g 2, 9 A9.=8880886PFgg dM

0 ° 006,60000.65;40 6 00 6 0 6 00 M h N CO 43 el en on n o 09 01 M te) 01 00 N -■ el CV dmele, N t•I N dd t4h .11 dd on o M n n-4 on a

ci (idea cido ciod ci 0 0 0 C:5 Ca 0 -.-

[71 g fro c g ho r./1 3 g (0% A ° 8 .= " 09. r, ci ci cid de; cacicio0 odOcid°60dOci

<4 o. - co - v., ■0 el <4 ,c.. - 00 0, el q m 0 CI ID CO 1p m •- ■ CI d ,1,1 CO •-• l41

C3 ; ; ; ; ; ; q ; ; 9 '; ; 44 c;; ,1. 9 `4',; ,v q`c-g cci, `4g . .„ ., c, - N ..., g N. .., ._. -,. - ... 1 ON 0 *t --. '0' t, VI NO 00 00 '0. en •0 co r-

, r". a ci <1 <-: <I .1 t"! 0 <''' '-' N M et el d It d CV el CO d 6 d. d . v., reli dciddcicidddcici 0 oo ddqd 900090 q

'41 .9; gg 4 '4' '4 4 8' 8 4 V 8 8 04 = d ° 'A 8 A 8 ,°..' 4 °.;,' Z54 PI 8' F.' 3 <7. g Uciciaddcidciciodciddaci ci d 0 4 0 4 d d d q ci ci 0 0

8 " " PI P. 8 9 " °` A. Pi " ''' "*. .8 ° 8 4 ' P. ' A " 8 g A 8 g 8 A 8' A

>dc; ci d a d a cid cidd ° d ° a ciddadciciciciqciddcid

'F'. 8 N 8 8 8 4 A. 4" A N^ 9 8 8 O, ° ° 8 8 4 4' 8 8 A 8 °$ 8 8 g= , '8 "

.,:i,:icicicicacici0cidcidciqq dciqdq a ciq 0 000440

n •-• 07. M M M se) d el el CO M d 6 0,

NO

F.,14 a, ts. so s orl 0 6 el el CV el •- ■

cis 0000000900000 6 6 6 0000 60 0 6 0 6666 0

O 3 8 O ° G 8 g 8 8 8 8 P Q 6 6 S 8 P. 74 8 8 " 66666 0 6 0000000 9 01 6 0 6 00 6 0 ° 10 69 10 6 0

O

t.- A. Af4 A g: A' 4 4' 8, '4' 17 ° g g 8 P. '9 9, A 8 8 701 8 g 4

o ci ci o d ci ci c; ci ci 0 ci ci o o d d 0 d 0 ci g O ci q 0 ci ci ci d

439 000 9 0001009 960 0

d

O999 9999900999

09000

-. 01 CV 00 s ,0 on on CV CO 00 01 h CO CO VI 0 10 00 CI h 00 M CI C4 M 0, co .... so so v. v os v, N st .0 st 41 en • N N 6 el I* cr. el d M M d 0 e. 9 0 0 .1 .... d -1 ,1 ,1 -:'' t..-!<1 <la! <1 a!q<1 a. dociddci 00°0000dd o.... 909 00900900099 o

N M M el e, ON h V1 ON o, e, O. e, a, N --. o '0 •- ■ N 8 on -4 00 CO N 00 '0 d 00 SO VI m 00 .0. 0 0 N N g 6 ci N d el el el ei C. el d el d N M ID 0 0 Ca ci ci ci ci ci d dcici

v., en on ov a d4-,-. Ch h .--. O. h Os VI el CO el Ca00000 ocicidloddcidqdci

. , , 25 gi z..,t. 4 q .74 g g ;. 4 4 4 '4 '8 $ '4 ° 8 A 4 8 d :2, r-2 s g g.,* g 0.:...' A .3,- 00 P. 8 9 8 g 8 g Z 0 o 0 o 60000dt:3dd 00 0 q 9 0 0 qdciqdciaqci 9 0

' si '", :3 ', 7; A ,;'-; PI A 8 8 g A A 8 8 8 ° '4' g,1 8 8 'RI 8 P P '4 N r, r ri s g g..,' -- P- g F. 9 G G Ndoci d ci dcido c:3 ci ci ci ci o q ci ciOci a ci.ci d cild cidc; cisid ci a ci

<3 s' 3: rd. : g g g '09, G ,°'; 07. G 8' S S Fl ;',1 ° g °C 3 •Jil 8 8 8 8 P. 8 9 8 8 CA g 8 " 8 g fg P 8' n g-,' 8 >' cicia do 000 6060,061 060060600 cid q cidd . <acid cidd

S 7" 81 ' " S " " S 8 8 g g 0'''' ° '8 8 8 'A N O"" 8 9 8 9 S 8 9 8 9. PI 8 A S 9 9' 9

O. el CV v h (4 el el el el 0 01. Ch h•-• N O. 5, 2t 0.2 _<;., r <4 t;- 4 4 .4 at el el el dt101.0 V1 n1 VI 6 el CV CO 00 00 N Cl el •-• 0. 66 .6 __,6661 9 9555 C4 1000000100000 00 00 6 00 6 0 0 00 9 9

M d h 00 •-• WI el h Os Ch V) •-. 'I el z VI 0 CI VI tO CI 00 N es at s :„.,, s ch ,.-,. - a F., 4 CAI g 6 -,.. s 2 6 s ,,, 2 pl... N el N CO d. d M sO 01 11. e. e. el e. '-' q ,i a a 9 a! c$ • I : <1 . . . . . . . . . . a a a a ,i, a a a 000090000000900 ° 90 9 ' 9°9999°9999°

N PI 8 8 '9! 9 9 8 n' 2-9 PI 08. A A 4 A' ° PI 8 8 8 8 8 P P, .,'; 'A' 8 ° g 6 8 c's Pi ::.." 8' A 9 '8 A 8 9 9 7 g, ci ci ca ci a ci ddciddc; a ci q 00006006906 a ci 4 9 a a d a ci ci 0 ci cii ci ci

' '''' ''' ' " 7 " 8 9 9‘.. 'A A 8 A A 9 0 A '8 8 8 PI' 8 '8 '4 9 .' g '9 9 8 'A' 4 8 Pi = A 2' n - gi 2 - PI A 8 8 o cicicidcidcidd cid cio ci ci q q ci d q ci ci q ci ci . d 0 0 ci c; ci q ci 0 q ci ci a ci q<0 a ci

r< . ti 4 4 8 8 A -•1 4 4 g 8 8 '4' 8 O ° P.' 3 '...-1 8 G 8 9 9 Q^ 8 9. (7.1 i41 g g 2 8 r, ..7. g N^ A' 4 S '8 8 8 9 ci d ci ci o ci ci a ci d ci a d 0 4 9190 6c5 900 6 0004'6 0 °d 6 0 6 000 a 6 90°°

4 n ° S 8. 4 8 8 4 , .4.• 8 8 '8 ' 8 G 8 8 '8 P 8 8' 8 e.. .-. r... 4 4 ..n ,o. o 4 ,.1 ,-+. ..-■ o .--.. Ci; Ci; 0999 09900 9 9 9 6 00 6 0 6 0 9 g 0 6 00000 6 0000000 66666

8 8 '1 - " " " - 8 '8 8 g g S N ° 8 ^ - S P'. 9 A S 8 9 9 A 0-.' " o r,!:-..E,-- m = 8' 8 9 -: g 8 '4 g 8 8 PI 00 9 000 4" qq 606 qc; ci f' qcf' cf“f' ci; g1 ci; 90 9 000 9 00000 9 q a 0 66

r- : 9 " 8 8 A P. 8 PI .9 A b^ 8 8 ° 4' 8 N' A 8 8 "" tP, 8 8 ' " t; ; 7:, 8 e," - 8 '4' 8 '''' 8 8' ' c.' " (4 r., i4 PI " sq A !,.... 000 9 000000000000 00°00°000 6 0 9 6 00000 6 0000 9 00 6666°66

8. .. -1-': M. 12. 7 7,1 A G 8 ,7; °-'-', "2 ?- ‘4 .<7.1 o7. 1. r--. ° V Pi P. '9 8' P-. '8 Pi 8 8° 8 1 '05.' 3 G. ":' 8 8 4 8 "' 1 " " - ^ ''' "' 6" ^ ''' 9999990000000000 00 6 00 6 000 6 00 6 00 9 00 6 0g 0;; a g."at dga0"; rCni

a' ^" Pi n F1: g ,, F1 - " gi, A' $ 0 8 A ?., 1 A. 8l 4 '"? i Zi ' n r< 1 T 3 4 S e G 8 e, .7, g P. 8 ' g 2 - 8, 8 8 8 8 8 " 1 8 9999999999 00 9 000 00°00° 9 0 9 °00°00000 6 000 9 0 9 0°° 6 ° ° ° °66

8 ' '''' " 8 °A P. A " P0. " 8 4° '4 8 Pi 0 8 '8 '0'.; P. "1 :8 g N 8 t4" . g A Pi N1 P ,' n 4 A 8' N^ 4 PI - 8 A -: g 8 9 O ° P. A 4' 00 9 00 00 9 0 9 0000.0 00 6 g 96 0 6 0 6 00 6 000 666 000. 00 966° 00 6 0 666

6 8 8 8 8 8 0 ° 8 g os, 0^ 9 08 08 ° 4' N O ^ 8 4 '00, PI 8 '4' 'A' 8 G 8 8 A p^ 8 .8 '8 8 O ° 2' g s •,-.1 g N ,T, I^ 8 S Po' °A 000000 0 0 000000 000 9 0 6 000 6 100000 0 6 0000 96 ' 6 0 6 0 6666666

^ 9 9 as P. r. " '9 8 4 4 4 8 A N ° 8 6 <7- . 9 i I I^ A :f.' N° (‘.'i rr.-; 8 S 8 O % G ^ 8 A 8 0^ * N n 0 9 2 9 9 S - 8 A A 011000000000000 0 00 0 0000 60 0 9 666 00 6 9 6 6 0 6 0 6 6 10 6 001 9 0 6 0

S ■Zit.1 A 8 A r n g g?, O? ,:,., g 2, 2 e g 4 8 9 8 8 $ .9 A ?...' 8 P' '4 ; g 1 O ^ 8 8 PI 8 8 8 r 9 V n. ... g 4 p n e, O. .3• % ,73 ,se, 0060000 ,60000 0 0 000006009009 600066666660006090010001 = t.12 C.Lt N^^ ON 9N. ,00_, 7 7 - 00 S „.,to soet ov. Lit 2 cn, - 9. , 7, 7 7 7

- 9' M" M" da" o" o71 ci ci d ci ci a .6 c

▪

i cid 0 cisiod o cyo 9 o 090000990

vre " " N 8 O^ S 8 8 '8 G g 00 ° P. g :I. 8 A A A . A84SG 08888 ss 8 08 $ P S 8 4 A so 0. 0 ci ci ci ci ci ci d ci c q' 6000610600060 106000060006011° 00000

6. 0 0 02 .6 . 6 q: o z c1 2 at Z A Z-* 6 .1 0, 1t .4 A.° 03 > g .A g X a r.5 CI' if R. P

Figu

re 4

.8A

Prof

iles

of m

ajor

ele

men

t con

cent

ratio

ns a

cros

s th

e H

ekpo

ort p

aleo

sol,

Stra

tal.

50

40

0: 30

rz., 20

10

0

105 110 115 120 125 130

100 105 110 115 120 125 130

0.2

c$ 0.15

0.1

0.05

0

100 105 110 115 120 125 130

20

15 O LL■

10

5

0

100 105 110 115 120 125 130

0

100 105 110 115 120 125 130

2

1.5

0.5

0 100 105 110 115 120 125 130

60 50

„, 40 ° 30 Crl 20

10 0

100 105 110 115 120 125 130

100 105 110 115 120 125 130

6 0

z 4

2

0

100 105 110 115 120 125 130

1.5

O

0.5

0

100 105 110 115 120 125 130

l0

0 8

6 4

2

0

100 105 110 115 120 125 130

50 -

40 -

0 30 .7d 20 -

10 -

0,, 60 4)" a) "al 4" 40 3 .19

u 20 ,Z • pp O 0

80

0.30 0.25 0.20 0.15

80

60

0,40 U

20

0

0.10 0.05 0.00

770 780 790 800 810 820 770 780 790 800 810 820

15

O a) 4.4 . 5

10

0 770 780 790 800 810 820

3.0 2.5 2.0 1.5 1.0 0.5 0.0

770 780 790 800 810 820

80

60

40

20

0

2.5

2.0

1.5 crr

1.0

0.5

0.0 770 780 790 800 810 820 770 780 790 800 810 820

0

0

0 0

a)

a)

cn 0

cn

0

a-,

U (.) O U

a>

0 E 0 0 a)

•

tg u.

Cel °C?

an

50 -

40

0: 30 -

.71d 20 -

10-

0

770

10

8

O 6

C.) 4

2

0 780 790 800 810 820 770 780 790 800 810 820

3.0 - 2.5 2.0 -

O 1.5 - 1.0 0.5 -0.0

770 780 790 800 810 820

O

770 780 790 800 810 820

60 50 -

O 40 - *(7) 30 -

20 - 10 -

• • •• O

0

790 800 810 820

a)

a) .10 3 a)

vt: * 0

0.25

0.20

0.15

0.10

0.05

0.00

770 780 AtItt 00_

„I:INN 0

4.4

790 800 810 820

ell "ci

c7)

Figu

re 4

.9A

Dis

trib

utio

n of

trac

e el

emen

ts in

the

Hek

poor

t pal

eoso

l, St

rata

l.

• •

100 105 110 115 120 125 130 100 105 110 115 120 125 130

A

15

10

2500

2000

1500

1000

500

0

•

♦ • • 100 105 110 115 120 125 130 100 105 110 115 120 125 130

ISO -

100 -

50 -

'AL 0

100 110 120 130

100 105 110 115 120 125 130 110 115 120 125 130 100 105

400

200

500

300

100

0 •

100 105 110 115 120 125 130

10 -

8 - •

6 - • It • 4 - ♦ •

2 -

0

100 105 110 115 120 125 130

120 125 130

600 - 500 - • 400 300 - 200 - 100 -

0 • •

100 105 110 115 120 125 130 115

30 - 25 - 20 -

g 15 - 10 - 5 - 0

40

30

20

10 4.

•

0

300 250 200

4= 150 100 50 0

100 115 105 110 120 130 125

•

10

a 6 4 •

•

2

0

100 105 110 115 120 125 130

250

200

c_.4 150 -

100 -

50 -

0

100 105 110 115 120 125 130

800

600

400

200

0

1200 -1000 -800 -600 -400 200

0

100 105 110 115 120 125 130

• 110 105 115 120 125 130

1000 -

800 -

V 600 -400 -

200 -

0

100

400

300 -

200 -

100 -

0 100 105 110 115 120 125 130

•

250 -

200 -

150 -

100 -

50 -

0

120 100 - 80 -

e_.> un 60 -

40 - 20 - 0

25

20

15 10

5

0

100

1000 -

800

600 400

200

0

00 105 110

a4

200

150

100

SO

820 770 780 790 BOO 810

790 800 810 820 780

Fil

10

6

780 790 800 810 820

4 790 BOO 810 820

770 780 790 800 810 820

Ir N

400

300

MOO

100

0 770 780 790 800 810 820

k ig

ure

4.9

1$

llis

tnbu

tion

of

trac

e e

lem

ents

in t

he H

ekp

oort la

va a

nd

pal

eoso

l, R

hk

l

760

790

800

810

820

780

790

800

810

820

770

780

790

800

810

820

780

790

800

810 B20

780 790

800

810 820

770

780

790

800

810

820

770

780

790

800

810

820

770

780

790

BOO

810

820

770

780

790

800

810

820

Up

1000

4 600

200

800

0 770

0 770

60 50

CO ...0 30 20 1 00 0

10 8

0 770

0 770

1200 1000

g Boo t.,,j 600

400 200

0 7-70

15

10

0

12 10

6

2 0

770

0

200

250

: Og SO

7 150 100

13 P4

250 200

50 0

770 780 790 800 810 820

✓

770 780 790 800 810 820

1

12

2.5 2.0 1 .0 1.0 0.5 0.0

770

2 0

800 -

ctt 600 -

0:1 .00 -

200 -

0 770 780 790

10

V1 6

2 41\ O

30 25 zo 13 C/) 10

CI)

z 4

600

200

800

0

770 780 790 800 810 820

1000 C)

1500

2000

SOO

0

810 820

810 B20

810 820

770 7B0 790 800

770 780 BOO

770 780 790 800

O

1200 1000 800 600 400 200

0

820 BOO 1370

770 780 790 BOO B 1 0 820

25 20 15 10

0 770 780 790 800 810 820

01

BOO 810 Br

MIL ic.b I

/eft

4): • ^O CS 4 ii. 4 2 I . C ) CO N N 6.-- CI) 03 .-1 .,-. CV/ZPIZ at = >

ci.. Cl.).= CZ *Cif LI 4.4 rtCC Cil

0 ay tr)

app 0

E ...., al C4 C.)

770

1000 BOO

400 200

600

0

100

C150

30

0 770 780 790 BOO 810 B20


In Rhkl, in contrast, all the REE display concentrations similar to the parent rock in

the pallid zone from where they are sharply enriched in a somewhat irregular fashion

to the middle of the mottled zone. The upper mottled zone is depleted in La, Ce, Pr

and Nd while the laterite and ferricrete are enriched in these elements (Fig. 4.10B).

None of the REE elements display any significant correlation with any of the major

elements (Table 4.3B).

PAAS normalized rare earth element signatures for the lateritic profile as graphically

displayed in Figure 4.10A and B. In the saprolite and green pallid zone of Stratal, the

light REE are depleted relative to the heavy REE (Fig. 4.11A). In the yellow pallid

zone this trend inverts and the light REE are enriched relative to the heavy REE (Fig.

4.11A). A positive cerium anomaly typically observed in modern laterite profiles

(Braun et al., 1990) is absent from the Hekpoort paleosol. It might reflect the primary

scarcity of cerium in the parent lava. In Rhkl, the saprolite and green pallid zones

displays relatively flat patterns (Fig. 4.11B). The mottled zone is depleted in light

REE and enriched in heavy REE The laterite and ferricrete are marked by a flat

pattern with a small positive Ce anomaly (Fig. 4.11B), which might indicate Ce

enrichment during oxidation.

The Hekpoort lava is marked by a negative Eu anomaly (Fig. 4.11B). This may be

due to hydrothermal alteration, that mobilized Eu from plagioclase as it were altered

to chlorite. The paleosol displays a positive Eu anomaly (Fig. 4.11A and B) that may

be related to seritization.

4.5 Discussion

4.5.1 Conditions of Weathering During Development of the Hekpoort Paleosol

Laterites are products of intense subaerial chemical weathering and consist

predominantly of the mineral assemblages goethite, hematite, aluminium hydroxides,

kaolinite and quartz. The SiO2: (A1203+Fe203) ratio of a laterite (Table 4.2A and B

"laterite") must be lower then that of the parent rock (Schellmann, 1982; Bardossy

and Aleva, 1990).

102 104 106 108 110 112 114 102 104 106 108 110 112 114

102 104 106 108 110 112 114 102 104 106 108 110 112 114

20

15

10

5

0

102 104 106 108 110 112 114 102 104 106 108 110 112 114

120 - 100 - 80 -

0 60 - 40 - 20 - 0

300 - 250 - 200 -

0 o 150 -

100 - 50 -

0

110 112 114 102 104 106 108 110 112 114

200 -

150 -'O Z 100 -

50 -

0

102 108 106 104

50 -

40

W 30

20

10

0

20

15 0

10

5

0

40 -

30 - 4: 20 -

10-

0

102 104 106 108 110 112 114 102 104 106 108 110 112 114

102 104 106 108 110 112 114

I I 4.)

' M 4.)

4, = 0 N

= c:.

1 4.) a 0 N

cc: 0...

I G)

.174 76)i...

va

I I

4.) I-. 4.) as

0) :2 4 1

>, t 4

80

6 -0

0

C.7 40

20

0

102 104 106 108 110 112 114

8

6 E

E-4 4

2

0

102 104 106 108 110 112 114

102 104 106 108 110 112 114

I

I I I 4)

= 4.) 0.) CI

N 0

4.) N N 0 t) 44 1-. 41

-0 C:). fl

=

•—) 4.) ... -a' V]

3) 0 0-4 "E -›-.' . N

4 . g -c.2 1

Figu

re 4

.10A

Dis

trib

utio

n o

f rar

e ea

rth

elem

ents

in

the

Hek

poor

t pal

eoso

l, St

rata

l.

r ig

ure

4.1

Uli D

istri

butio

n of

rare

ear

th e

lem

ents

in th

e H

ekpo

ort l

ater

itic

pale

oso l

, Rhk

l.

•—J 0.5

25

20

15

10

5

0

"Cl

1.5

1.0

W

8

6 -

4 -

2 -

770 780 790 800 810 820

5 4

2 -0

1 0

770 780 790 800 810 820

0.0

770 780 790 800 810 820

8-

6

4

2

0

770 780 790 800 810 820

300 250 200 150

0 100 50 0

770 780 790 800 810 820

15

10

A 5

0 770 780 790 800 810 820

20

15

10

5

0

770 780 790 800 810 820 780 790 800 810 820

1.2 -1.0 -0.8 - 0.6 -0.4 -0.2 -0.0

770

200

150

cti 100

50

0

820

3.0 2.5 2.0 1.5 1.0 0.5 0.0

770 780 790 800 810 820 770 780 790 80% 810

100

80

60

40

20

0

770 780 790 800 810 820 0

770 780 790 800 810 820

25

20

15

Q. 10

5

0

770 780 790 800 810 820 780 790 800 810 820

3.0 2.5 2.0 1.5 1.0 0.5 0.0

770

La Ce Pr Nd

Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

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

svvd

oidu

ms

0.1

30

10

1

30

10

0.1

Pallid zone Saprolite Green Yellow Mottled zone Laterite

I I II I I I 0 Str1/1 ❑ Strl/2 111 Str1/3 X Str1/5 A Str1/6A Str1/7 CI Str1/8 n Str1/9

Figure 4.11A Rare earth element signatures of the Hekpoort paleosol,drill core Stratal .

Laterite FerrIcrete Mottled zone

La Ce Pr Nd

Sm Eu Gd lb Dy Ho Er Tm Yb Lu

La Ce Pr Nd

Mottled zone

778.95 e 779.1 E 779.3 0 779.5 C 779.8 Q 780 i 0 7803

Saprollte Pallid zone

•780.6 0 781 1 1 7781.3

Chbrttized lava Saprolite

I> 783 0'783.6 1 1 0 786 <782.1 ill 782.6 ❑ 795.4 M 799.4 +802.6 x 812.6 • 1091 1

1 I 1 0 777.5 ❑ 777.6 U 777.7 +777.8 X 777.9 A 778 •778.01 A 778.3 1:1778.6 In 778.9

5

1


La Ce Pr Nd

Sm Eu Gd Tb Ho Er Tm Yb Lu

Dy

La Ce Pr Nd


100

10

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

5

1

0.1

0.04

5

1

0.1

0.04

5

1

0.1

0.04

0.1

0.04

5

1

0.1

0.04

7111tIllik

js '111■1116*- -

1 1 1 1 1 1 1 I I I I

Figure 4.11B PAAS normalized rare earth element signatures of the Hekpoort paleosol and lava in drill core Rhkl.


The Hekpoort paleosol, although weakly metamorphosed and affected by K-

metasomatism (Button, 1979), displays all the features of a typical lateritic

weathering profile in the modern tropics and subtropics (Schellmann, 1982; Bardossy

and Aleva, 1990; Table 4.1A and B and Table 4.2A and B).

In the Hekpoort lateritic profile, lithologies of different zones of the paleosol consist

predominantly of fine crystalline sericite, pyrophyllite, hematite, iron-rich chlorite and

subordinate amounts of diaspore and kaolinite (Table 4.1A and B, Appendix II). Due

to diagenisis, low grade metamorphism and sericitization, this mineral assemblage

obviously developed from a suitable precursor. It is problematic to determine

precursor minerals in the metamorphosed rock containing a completely recrystallized

mineral assemblage. It is therefore necessary to revert to the bulk chemical

composition of the weathering profile to determine possible precursor minerals.

Except for the addition of K20 and possibly Na20, Ba and Rb during later

sericitization, it appears likely that the bulk chemistry of the Hekpoort weathering

profile remained relatively unchanged during diagenesis and metamorphism (Holland,

1984).

A1203 is virtually immobile during lateritic chemical weathering processes and

therefore Si02/A1203 ratios usually decrease progressively with increasing degree of

chemical weathering (Retallack, 1990; Bardossy and Aleva, 1990 Thomas, 1994 and

Table 4.2A and B). An increase in Fe20 3 concentration (vs. FeO) relative to an

immobile element reflects increasing oxidation (Ohmoto, 1996). If Fe203 and Fetotat

increased sympathetically, absolute introduction of Fe into the profile is indicated.

Trends displayed in Fe203-A1203-Si02 ternary diagrams may therefore be of great

help to recognize precursor minerals from the recrystallized assemblage present in the

Hekpoort weathering profile (Fig. 4.12).

The relationship between A1203, Si02 and Fe203 in weathering profiles reflect the

interaction between weathering and oxidation. In the Hekpoort weathering profile for

Strata 1 and Rhkl, A1203 increases relative to Si02 from the parent basalt upwards

into the pallid zone until the molar A1203:Si0 2 ratio is exactly 1:1, the A1203:Si02

ratio for kaolinite (Fig. 4.12; Miicke et al., 1999). Kaolinite is associated with an

Hekpoort Paleosol

73

Figure 4.12 Fe203-A1203-Si02 (in mole %) ternary diagrams for the Hekpoort weathering profile from

different localities. A) Stratal, Botswana. B) Rhkl, Potchefstroom. C) Waterval Onder.profile

(Button, 1979; Holland, 1984). D) Drakenstein, Griqualand West (Wiggering and Beukes, 1990).

optimum state of chemical weathering and is the most common clay mineral in soils

affected by efficient chemical weathering i.e. laterites and laterite-type soils

(Bardossy and Aleva, 1990). When this 1:1 A1203:Si02 ratio is reached only Fe203

(hematite) is added in the mottled zone, laterite and ferricrete, indicating

progressively increasing oxygenation towards the top of the weathering profile.

The samples from these zones plot on a line towards the Fe203 apex, normal to the 1:1

A1203:Si02 ratio (Fig. 4.12A and B). It is significant to note that Fe203 is only


present along the top of the lateritic profile. This is in consensus with models

proposed for modern laterites with iron enrichment at the top of the profile and the

presence of a Fe-poor pallid zone (Fig. 4.19; MacFarlane, 1976). No crosscutting

hematite veins or any other textural evidence have been observed within the studied

Hekpoort weathering profiles that would suggest epigenetic enrichment of iron. This

is also supported by the fact that at the Pretoria locality a highly ferruginised pisolithic

quartzite directly overlies the pallid zone of the Hekpoort paleosol (Fig. 4.3). Despite

this, the pallid zone is essentially free of Fe2O3, indicating that virtually no Fe2O3

seepage took place from the overlying red beds into the paleosol. The Waterval

Onder profile displays a significant upwards decrease in its Si02:A1203 ratio

(Fig.4.12C). However, the profile does not indicate any significant increase in Fe2O3.

A slight increase of the Fe203/Ti ratio is observed at the very top and bottom of the

Waterval Onder profile. The fact that iron enrichment is all but absent from the

weathering profile at Waterval Onder is a strong argument that only the saprolite and

part of the pallid zone and maybe the very base of the mottled zone remained

preserved beneath the Dwaal Heuvel erosional surface in the east of the Transvaal

area (Fig. 4.3).

The calculated CIA values (Young and Nesbitt, 1998) for the Hekpoort paleosol,

along the Dwaal Heuvel erosional surface, are listed in Table 4.4 and illustrated in

Figure 4.13. CIA values for Stratal (Fig. 4.13A) and Rhkl (Fig. 4.13B) are high,

ranging between 70-90%, in spite of strong sericitization (Table 4.4). Such high CIA

values suggest efficient chemical weathering, as expressed by the strong leaching of

alkali and earth alkali elements (Nesbitt and Young, 1982). CIA values for the

Waterval Onder profile (Fig 4.13C) are less than those calculated for Stratal and

Rhkl (Table 4.4). However, K2O values are higher for the Waterval Onder profile

(Holland, 1984), indicating more severe sericitization (Appendix II).

The Ca0*+Na20-K20-A1203 ternary plot (CaO* represents CaO in silicate phases

only), can also be used to evaluate the extent of weathering of the weathering profiles

preserved beneath the Gamagara-Dwaal Huevel paleo-erosional surface (Nesbitt and

Young, 1982, 1984). For profiles Stratal (Fig. 4.14A) and Rhkl (Fig. 4.14B),

leaching of CaO, usually preceeded by the removal of K2O (Nesbitt and Young,


1982) during advanced weathering, moves the bulk composition of the weathering

residue towards the A1203 apex.

Table 4.4 CIA values for A- Strata 1; B-Rhkl; C- Waterval Onder

(Button, 1979; Holland, 1984); D- Drakenstein (Wiggering and Beukes,

1990).

A

Sample CIA Sample CIA Sample CIA Sample CIA

Str1/10 93 777.5 91 13 71 720.73 64

Str1/9 87 777.6 90 12 74 721.29 65

Str1/8B 74 777.7 90 11 69 723.34 60

Strl/8 74 777.8 97 10 70 724.23 60

Str1/5x 81 777.9 90 9 94 724.55 63

son 80 778 90 8 69 725.48 53

Str1/6Bbo 81 778.1 94 7 69 726.7 47

Str1/6Bon 79 778.3 88 6 69 729.76 46

Strl/6A 80 778.6 94 5 68

Strl/5 82 778.9 99 4 68

Str1/4 85 778.95 97 3 69

Str1/3x 86 779.1 96 2 69

Str1/3 85 779.3 97 1 69

Str1/2x 86 779.5 95

Strl/1 91 779.8 98

Str128 56 780 53

780.5 93

780.6 97

781 94

781.3 48

782.1 98

782.6 99

783 51

783.6 54

786 58

795.4 54

799.4 52

802.6 51

812.6 52

1091 49

For what appears to be the more complete weathering sections, the Hekpoort lateritic

profile plots in the advanced weathering zone (Fig. 4.14A and B).

The Waterval Onder profile (4.14C) shows less intense weathering than the profiles at

other localities in the Transvaal area. However, this is due to higher K20

concentrations (Button, 1979; Holland, 1984; Appendix related to sericitization.

O

100

110

120

130

778 -

779 -

782 -

783 -

784 -

785 -

786 -

B N.) 4 ON 00 8 N.) 4 a

0 s 000 ..

0 0 0 0 0 0 0 0 0

I I I I I 777 ,. t ,

C ■—• t.) La ;a I./. 0 \ -4 00 \ 0

0 0 0 0 0 0 0 0 0 0 0

2

4

6

8

10

12

14

D 0 0

7200

o—• 0

t.) 0

444 4. 0 0

721

722

723

724

725

726

727

728

729

730

731

A

787

VI CA ....1 0 0 0

Decreasing depth(weathering path)

Figure 4.13 Vertical variation of CIA values for A- Stratal; B- Rhkl; C- Waterval Onder (Button, 1979; Holland, 1984); D- Drakenstein (Wiggering and Beukes, 1990). Y-axis of A, B and D represent depth. Y-axis of C represent sample number.

Figure 4.14 (Ca0*+Na20)-1(.20-A1,03 plot used to decipher the weathering trend of selected weathering profiles preserved beneath the Gamagara-Dwaal Heuvel erosional surface. A- Stratal; B- Rhkl ; C- Waterval Onder (Button, 1979 and Holland, 1984); D- Drakenstein (Wiggering and Beukes, 1990).

A

B

C

D A O.

O

FeT/Ti

Fe/Ti

Fe/Ti

O o 777

778

779

780

781

782

783

784

785

786

787

100

105

110

115

120

125

130

—N N ta C Ln

0 0 0 0

Fe2+/Ti • FeT/Ti Fe3+/Ti

—N N W W A A 0 ,-A0v.0t.AOLA0v. 0 IQ A ON Oa

720

721

722

723

724

725

726

727

728

729

730

731

Hekpoort Paleosol

77

Figure 4.15 Ti normalized Fe2+, Fel* and Fe(totai) profiles for profiles preserved beneath the Gamagara-

Dwaal Huivel erosional surface. A) Stratal, Botswana; B) Rhkl, Potchefstroom; C) Waterval Onder

(Button, 1979, Holland, 1984); D)Drakenstein (Wiggering and Beukes, 1990). Y-axis of A, B and D

indicate depth. Y-axis of C indicate sample number (Holland, 1984).

Ohmoto (1996) suggested that mineralogy, together with the atomic ratios of Fe 2÷/Ti,

Fe3+/Ti and Fe(totaDai of a weathering profile can be used to distinguish, if leaching or

retention of iron took place, i.e. to determine if the paleosol developed under

oxidizing or reducing conditions. Hematite contains iron as Fe 3+, and requires

oxidising conditions for its precipitation (Garrels and Christ, 1965). Hematite is a

major mineral constituent of the mottled zone, laterite and ferricrete of the Hekpoort

paleosol (Table 4.1A and B). Fe/Ti plots for the weathering/oxidation profiles in the

Griqualand West and Transvaal areas are displayed in Figure 4.15.

For Stratal, the Fe 2+/Ti profile is marked by strong variation in Fe 2+ concentrations,

but the general trend for Fe 2+ is upward depletion (Fig. 4.15A). Fe 2+ is not expected

in an oxidised lateritic weathering environment, although the presence of chlorite has

also been noted in younger paleosols (Bardossy and Aleva, 1990). The Fe 3+/Ti profile

is similarly variable and marked by slight enrichment of Fe 3+ in the mottled zone and

strong enrichment in the laterite and ferricrete. The Fe(totaVri profile combining Fe 2+

and Fe3+ concentrations, is marked by enrichment towards top and bottom and

efficient depletion in the pallid zone (Fig. 4.15A).


The Rhkl profile displays depleted Fe 2+/Ti ratio throughout the paleosol (Fig. 4.15B).

The Fe3+/Ti ratio, in contrast (Fig. 4.115B), is lowered in the pallid zone only. The

top of the mottled zone, laterite and ferricrete are strongly enriched in Fe 3+. There is

an increase in the Fe 3+/Ti ratio in the saprolite that may indicate oxidation of the

whole weathering profile before chloritization (Appendix II). The FeT/Ti profile

parallels that of Fe3+/Ti.

At Waterval Onder, the Fe/Ti plots display patterns broadly similar to those seen in

the saprolite and pallid zones in Stratal and Rhkl (Fig. 4.15C). Iron is enriched as

Fe2+ in the saprolitic zone, while it is almost completely depleted in the pallid zone.

Enrichment of Fe 3+ towards the top of the profile is not evident, indicating again that

the mottled zone, laterite and ferricrete are eroded away. However, the base (sample

13) and very top (sample 1) of the profile display higher Fe 3+/Ti ratios than Fe 2+/Ti

ratios suggesting that the whole profile was oxidised before chloritization (Appendix

II) and that the very base of the mottled zone is still preserved (Fig. 4.15C).

The Drakenstein Fe/Ti profiles display enrichment as Fe 2+ at its base, depletion of

iron in its upper central part and enrichment of iron as Fe 3+ in its upper part (Fig.

4.15D). The normalised Fe/Ti plots obtained for the Hekpoort lateritic profile,

retaining iron as Fe3+ towards its top are typical for oxidized type paleosols as defined

by Ohmoto (1996).

Fe3+/Ti ratios plotted against Fe 2+/Ti ratios display patterns that are not described by

Ohmoto (1996) (Fig. 4.16A-D). This type of binary plot describes the nature of iron

in lateritic weathering profiles in the western Transvaal area well (Fig. 4.16A and B).

In the different zones of the weathering profile, iron is either almost totally leached

(pallid zone) enriched as Fe2+ (saprolite) or Fe 3+ (mottled zone, laterite, ferricrete). It

is important to note is that Fe 3+ is enriched at the top and Fe 2+ at the base of the

lateritic profile indicating a shift from oxidising to reducing conditions. At Waterval

Onder, samples within the saprolite display both Fe 2+ and Fe3+ enrichment (Fig.

4.16C). Samples within the pallid zone are virtually depleted in iron (Fig. 4.16C).

Chlorite crosscuts hematite, pyrophyllite and sericite in the top zones of the Hekpoort

weathering profile (Fig. 4.5A).

B 25 Saprolite+chloritized lava 20

t 15 10 5 0

80 100 0 50 100 150 200 250 300

Fe/Ti

0 20 40 60

Fe" /Ti

C D

12 10 0 2 4 6 8

Fe" /Ti

15 5 10

Fe" /Ti

30 25 20 15 10 5 0

6

4 ti)

Mottled zone?

Chloritic pallid zone and saprolite

A 40 Ferricrete

Saprolite+chloritized lava

OFLaterite Depleted mottled zone and pallid zone

Mottled zone 0

Laterite

epleted mottled zone and pallid zoneFerricrete Laterite and mottled zone

Hekpoort Paleosol

79

Figure 4.16 Ti normalised Fe247Fe3+ plot. A- Stratal; B- Rhkl; C- Waterval Onder (Button, 19;

Holland, 1984); D- Drakenstein (Wiggering and Beukes, 1990).

However, there is also evidence for hematite crosscutting iron-rich chlorite and

sericite (Figure 4.5B and D; Appendix II). It is therefore suggested hematite was the

primary iron-rich mineral in the weathering profile. Iron-rich chlorite (Appendix II)

replaced hematite during a period of metamorphism later than sericitization. Iron that

was mobilized during this period of late metamorphism later recrystallized as

hematite. It is clear that since weathering of the Hekpoort paleosol started, iron was

always present in the top part of the weathering profile. Thus, weathering must have

taken place under oxidising conditions to retain iron (Ohmoto, 1996). It is therefore

suggested that at least the iron in the top zones of the paleosol (and possibly the lower

ones) was present as Fe 3+ before chloritization during late metamorphism (Appendix


Sericite forms up to 10i.tm large euhedral platelets with interstices filled by anhedral

pyrophyllite, diaspore, chlorite and hematite (Fig. 4.6B). This textural evidence

indicates that sericite is replacing the metamorphic pyrophyllite (Fig. 4.6B) and that

■

0

2. a 0

Sillimanite 4

40-." Andalusite

al

1

az = 0

4' -, 7' Kyanite 0

0.

300 400 T(°C)

500


sericitization post-dated early metamorphism (MacFarlane and Holland, 1991).

Therefore, assuming that kaolinite is the precursor mineral to pyrophyllite and

diaspore, as indicated by bulk chemistry and powder x-ray diffraction, the following

dehydration reaction characteristic for early peak-metamorphism of the paleosol can

be written:

Kaolinite —> Pyrophyllite + Diaspore + Water

2Al2[(OH)4Si2O5] ---> Al2[(011)2Si4010] + 2A100H + 2H20

This reaction suggests lower greenschist metamorphism at 300-350°C, corresponding

to lithological pressure between 2 and 3kbar (Fig. 4.17).

Figure 4.17 Phase diagram showing stable equilibria in the system A1203-Si02-H20. Hatched area

outlines peak metamorphic conditions suggested for the Hekpoort paleosol (modified after Anovitz et

al., 1991).


Adding K20 to diaspore and pyrophyllite, the following reaction can be written for

the sericitization process:

Diaspore + Pyrophyllite + K20 —> Sericite + Water

6A100H + 3Al2[(OH)2Si4O 10]+ 2K20 —> 4ICA12(OH)2(A1Si3010) + 2H20

The negative Eu anomaly displayed throughout the Hekpoort lava suggests that it was

intensely hydrothermally altered, mobilizing Eu from plagioclase during chloritization

predating weathering (Appendix II). Chlorite also appears to have formed during

development and diagenesis of the weathering profile (Appendix II). Chlorite is

known to form as a primary mineral in lateritic weathering profiles (Bardossy and

Aleva, 1990). However, some of the chlorite overgrows sericite and is of late

metamorphic origin (Appendix II).

4.5.3 Correlation with Paleoweathering Profiles in Griqualand West and

Environmental Significance

In Griqualand West, different lithologies of the Transvaal Supergroup are

successively intersected by the erosional surface below the Gamagara/Mapedi

Formations that may be correlated with the Dwaal Heuvel erosional surface on a

stratigraphic basis (chapter 2). These highly oxidized alteration profiles have been

described in detail (Fig. 4.18; Wiggering and Beukes, 1990; Holland and Beukes,

1990). The Gamagara/Mapedi erosional surface does not intersect lithologies higher

up in the Transvaal Supergroup than the Mooidraai dolomite of the Voelwater

Subgroup (Figure 2).

The weathering and alteration profiles preserved beneath the Gamagara-Dwaal

Heuvel erosional surface display significant mineralogical and chemical variation in

the Griqualand West area, as they are developed on different host rocks (Wiggering

and Beukes, 1990; Holland and Beukes, 1990). The same weathering profile in the

Transvaal area was developed only on the Hekpoort lava, and mineralogical and

chemical changes reflected by the paleosol remain fairly uniform (Fig. 4.18).

.-‘ o ,— — .;.I.,' E

. _ to

. I ..... o :.:--

o g OA E o = G. o U -o o. c 0 c 2

...+4 c — .....

0 , 3 a co) -o. -0,

c:4 2P A

I IIU . Mil

..CI 'CI

X

..... 'll': 0

.'..ti

0

c -o

: o-

/

.....

•

.0 0 U T.) .0 a

l''.1,0 11'1,1'.111: 'Ir I I ■ ' ' .. . ' • -' ' ' • t?..

.-"'",;" g

E

-a 0.. .60 c

o — ,..... 0 0 i

li ...

u .,', L li i'..g1:,, 'cl. /,',

04 X -A • •

40 ,-, 0 -0 0

..AlldIIIIIIIMIIIIIIIkllb..... IIIM14:4'. iiiiiiii.

o

' i'l,vit.-!,;.: .; :t.,-;-.,.; . ''.i. : .•:.,. ',, -,....,.f.,L,,, '' . 1.1.1 I. .= .N

0 0

t3 ,.. 0“. ..

.. -0 0 ; -0 0 -0 .E 0.

I 1 X 0

a E 0 u•• cl 0

a ,

— k Yil

Fi4 i'l °


Fe203-A1203-Si0 2 ternary plots for weathering profiles below the Gamagara-Dwaal

Heuvel erosional surface developed on the Ongeluk lava in the Griqualand West area

indicate that these profiles are less strongly weathered, and that oxidation played a

more dominant role than in the weathering profiles below the Dwaal Heuvel

unconformity (Fig. 4.12D). The increase in A1203 is less pronounced than in the

Transvaal area profiles, while a shift towards the Fe2O3 apex is significant, even for

slightly weathered samples near the base of the weathering profiles (Fig. 4.12D). The

Drakenstein profile displays negative correlation between Fe 2+ and Fe3+ (Fig. 4.16D)

illustrating that iron behaved relatively immobile within the saprolite, with Fe 2+ being

progressively more oxidized to Fe 3+ towards the top of the erosional surface (Fig.

4.16D). CIA values for the Drakenstein alteration profile (developed on the Ongeluk

lava) are low, ranging between 40-65% (Table 4.4 and Fig. 4.13D), and possibly

pronounced by high K 2O values due to seritization, are significantly less than the CIA

values for Hekpoort weathering profiles in the Transvaal area.

Lateritic weathering profiles did not develop below the Gamagara erosional surface in

Griqualand West area, because uplift was to fast causing the saprolite to erode before

a pallid zone and laterite could develop (Fig. 4.19). Oxygen penetrated deep into the

weathering profile and Fe2+ was oxidized in—situ to Fe3+ in this well drained

weathering profile (Fig. 4.19). Some residual enrichment of Fe 3+ took place at the top

of the weathering profile (Fig. 4.19). The Drakenstein profile could be best described

as a well drained oxidised saprolite (Fig. 4.19).

4.5.4 Model for the Development of the Hekpoort Weathering Profile

The Hekpoort lateritic weathering profile developed on a stable, flat, low lying area to

allow deep, intense chemical weathering and avoid mechanical erosion under warm

and humid climatic conditions (MacFarlane, 1976 and Bardossy and Aleva, 1990)

(Fig. 4.19). During development of the Gamagara-Dwaal Heuvel erosional surface in

the Transvaal area, mobile elements were leached out of the weathering profile by

meteoric and ground water, while hematite and kaolinite accumulated in the residual

weathering profile (Fig. 4.19) (Mcfarlane, 1976). Fe/Ti profiles suggest that the

whole weathering profile was oxidised early on. However, the whole weathering

profile soon became waterlogged and anoxic during the wet season, with Fe 3+ only

V V V Drakenste in Wolhaarkop Strata I

RiversGriqualand West Oxidising atmosphere

' Downward and lateral erosion

Uplift in western part of Kaapvaal craton Stable interior of craton

Waterval Ondcr V

Transvaal

O.

Microbial activity

Oxidised saprolite (more than 100m thick)

Ongeluk lava

Well drained saprolite in Griqualand West. Oxygen penetrates up to 100m into saprolite, oxidising Fe" to Fe' in place. Fe' enrichment at top of profile.

Groundwater flow

Hekpoort paleosol: Wet season

Lateritic weathering profile

Hekpoort lava

Duitschland, Timeball Hill and Boshoek Formations

Penge iron formation

Oxidised saprolite

Ongeluk lava

Makganyene diamictite

Asbesheuwels iron-formation

Waterlogged conditions. Poor groundwater drainage. No percolation of 0, beyond laterite. Organic carbon from microbial mats mobilize iron under oxygenated atmospheric conditions. Leaching of iron from laterite and mottled zone

Top of groundwater table

Microbial vegetation Upper soil Fe laterite

Mottled zone

Kaolinitic pallid zone

Saprolite Weathering front Chloritized- Helcpoort lava

Hekpoort

Lowering of groundwater table during dry season. 0, penetration and oxidation of Fe' to Fe' in mottled zone

Groundwater flow --1

paleosol: Dry season

Microbial vegetation Upper soil Fe' laterite Mottled zone Top of groundwater table. Maximum depth of 0, penetration (bottom of mottled zone) Kaolinitic pallid zone

Saprolite Weathering front Chloritized- Hekpoortlava

Rain + *

1111 rf— •

4111

•

II

0

- r

Fb

Fe"

Groundwater flow —1

Griqualand West oxidised saprolites Rain +

Figure 4.19 Schematic diagrams to illustrate the development of the Gamagara-Dwaal Heuvel erosional surface.


precipitating during the dry season when the water table migrated downwards and

oxygen penetrated into the mottled zone (Fig. 4.19).

Leaching of iron under oxidised atmospheric conditions also give indirect evidence

for the accumulation of organic matter in a terrestrial environment (Gutzmer and

Beukes, 1998) (Fig. 4.19).

4.6 Conclusion

Weathering profiles that usually have a very low preservation potential are widely

preserved below the Gamagara-Dwaal Heuvel erosional surface over an area of more

than 800 000km2. Oxidation of the Gamagara-Dwaal Heuvel erosional surface was

evidently important in both areas. In the Transvaal area it concentrated hematite

along the top of lateritic weathering profiles developed on the Hekpoort lavas. It is

ironic that at the type locality of the paleosol at Waterval Onder, where it has been

used as evidence to establish the lower limit of oxygen accumulation in the

atmosphere (Holland, 1984; Rye and Holland, 1998), only the iron-depleted pallid

zone has been preserved from erosion prior to the deposition of the overlying Dwaal

Heuvel Formation. Evidence that the atmosphere was already oxidizing during

Hekpoort paleosol times will have a major impact on the established oxygen evolution

model (chapter 6). Paleo-oxidation along the Gamagara—Dwaal Heuvel erosional

surface played an important role in the upgrading of banded iron and manganese

formations to high grade world class iron and manganese deposits in Griqualand West

(Van Schalkwyk and Beukes, 1986; Gutzmer, 1996).

27° 29°

+ 26°

Reference points

1-Borehole 1740, Goldfields 2-Borehole Rhkl, Anglo Gold 3-Borehole BB11, Goldfields 4-Borehole DPZ-2, Anglo Gold 5-Pretoria localities 6-Waterval Onder roadcut. 7-Borehole Strata 1, Botswana Geological Survey

31 ° + 24°

100 Km

Red Beds of the Dwaal Heuvel Formation 86

CHAPTER 5

RED BEDS OF THE DWAAL HEUVEL FORMATION

5.1 Stratigraphy

The Dwaal Heuvel Formation was studied in several deep drill core intersections and

also in outcrop (Fig. 5.1).

B-Belfast C-Carolina CAR-Carletonville JHB-Johannesburg K-Klerksdorp LY-Lydenburg PO-Potgietersrus

POC-Potchefstroom PRE-Pretoria RUS-Rustenburg W-Warmbad WB-Waterval Boven Z-Zeerust

Figure 5.1 Locality map showing the distribution of borehole and outcrop profiles of the Dwaal

Heuvel Formation examined in this study. Also shown is the outlines of the Transvaal sub-basin

(modified from Button, 1973).

A major finding was that the Dwaal Heuvel Formation in the western part of the

Transvaal area represents a very well-developed succession of fluvial red beds. One

of the best intersections through the red beds come from diamond drill core Stratal in


Botswana (Fig. 5.2). Here, as mentioned in the previous chapter, the Dwaal Heuvel

Formation rests with a sharp erosional contact on the Hekpoort paleosol. The Dwaal

Heuvel Formation is composed of red quartzite and shale arranged in both upward

fining and upwards coarsening sedimentary successions. The quartzites fine upwards

into red siltstones and red mudstones. Red ferruginous quartzites in the succession

typically fine upward from a coarse grained scoured base along which mud clast

conglomerates may be concentrated (Figures 5.2 and 5.9A and B). Upward

coarsening shale-quartzite successions are capped by transgression surfaces along

which coarse-grained ferruginous quartzite with mudclasts may be developed (Figures

5.2 and 5.9B). These mudclast bearing quartzites may in turn fine upward into

mudstone that display upward changes in colour from black carbonaceous to red

hematitic. Some of the mudstone beds display mudcracks (Figures 5.2 and 5.9D)

indicating exposure to the atmosphere. The top of the Dwaal Heuvel Formation is

marked by a major transgression, and the deposition of the carbonaceous shale of the

Strubenskop Formation.

In addition to Stratal, the Dwaal Heuvel Formation was also studied in several drill

core intersections in the Potchefstroom area. In borehole 1740 (Fig. 5.1), red

mudstones directly overly the ferricrete capping of the Hekpoort paleosol (Fig. 5.3).

This mudstone changes upwards in color from black to red, indicating shallowing

water levels and decreasing organic carbon concentrations. It grades into red siltstone

and an oolitic quartzite, in a typical upwards coarsening deltaic succession. Renewed

transgression is marked by another mudstone succession grading in color from black

to red and coarsening upwards into intercalated siltstone and red fine-grained

quartzite with some intercalated mudstone beds. The mudstone layers are

mudcracked, indicating direct exposure to the atmosphere (Fig. 5.9C). A third

upwards coarsening sedimentary succession follows, grading from green mudstone

into a fine grained oolitic quartzite. The top of the Dwaal Huevel Formation is

marked by the deposition of the black shales of the Strubenskop Formation,

suggesting a major transgression (Fig. 5.3).

Channel Red quartzite with scattered red mud clasts

Siltstone ....••••••••+:•9••••.:•:••••••-••:

0 Ripple marks Oolitic quartzite

Co Gr C M F St Sh Sample(m) Contacts and grain size cycles Lithologies Depositional environment

Large red basal mud clasts Wavy and lenticular laminated red mudstone and siltstone

Red oolitic flat bedded quartzite with scattered mud clasts

Basal red mud clasts Red quartzite, siltstone and mudstone. Mudcracked Wavy and lenticular bedding Trough and flat bedded red oolitic quartzite

Red quartzite

Channel

Crevasse splay

Channel Ml+" •

Delta

— --

Str 1 /15green 90.25

Strl/l5rooi — _

<— 91.9 Str1/14

Strl/12 —V--- IC— 99.8

•

101.27 StrI/11

Red mudstone with irregular green mudstone bands and lenses

Coarse red quartzite with mud clasts

Red quartzite with scattered red mud clasts Channel

Ferricrete of the Hekpoort paleosol

Ferricrete Quartzite with mudclasts Mudstone with green lenses and bands

Green shale grades into black carbonaceous shale

Green mudstone. Base of Strubenskop Formation .4-- Transgression Green siltstone. Top of Dwaal Heuvel Formation

Green mudstone partings Flatbedded red quartzite with ripple cross lamination towards top

10 —

Om —

Red mudstone with irregular bleached green bands

Red quartzite Red mudstone and siltstone

Red, flat bedded quartzite

4—Transgressional surface

Channel

Floodplain

Transgressional surface

/ \

Figure 5.2 Profile of the Dwaal Heuvel Formation from drill core Stratal, Botswana (see figure 5.1 for locality). Sample positions are indicated.

Ferricrete Mudstone Siltstone with mudcracks Quartzite with coated grains

Co Gr C M F St Sh Sample Contacs + grain size cycle Lithologics Depositional environment 1111111

Grey/green mudstone

Green siltstone. Base of Strubenskop Formation Red quartzite. Top of Dwaal Heuvel Formation

Green/grey mudstone Red quartzite with coated grains Red siltstone and mud Green mud

Red quartzite with red mudstone partings Mud cracks in every mudstone bed

Red mudstone Red and green mudstone Green mudstone

Black/grey mudstone

Coated grains

Red siltstone

Red mudstone with green clasts

Black/grey mudstone

Light green mudstone

Red mudstone with green bands and clasts Ferruginous quartzite Ferricrete of time Hekpoort paleosol

Transgression

Crevasse splay

Quartzite with ripple marks

I f

329 5

337.7

7 338.7

\ /

355m

Flood plain

Transgression

♦ Transgressional surface

Slow deepening

Transgression


Figure 5.3 Profile of the Dwaal Heuvel Formation from drill core 1740 near Potchefstroom (see figure 5.1 for location). Sample positions are indicated.

Co Gr C M F St Sh Depositional environment Samples(m)1 Contacs + grain size cycles Lithologies

Transgressional surface Lateritic weathering NRed mudstone . .

cTruidier osfilivotoertwaeguoristone clasts

Clasts Siltstone Quartzite Ferricrete Mudstone 410

Carbonaceous shale of the Strubenskop Formation

Ferrigunous quartzite with mud clasts. Top of Dwaal Heuvel Transgressional surface Formation Green and red siltstones and mudstones Ferruginous quartzite with red mudclasts

Green and red siltstones and mudstones

Ferruginous quartzite

Green and red siltstones and mudstones Transgression

Ferruginous quartzite with mud clasts Transgressional surface

Green and red siltstones and mudstone

Green siltstone

<-764.6 4-765.3

4-766.4

<-768.1

4-770

<-775.5 <-776 4t 3367: 5 '37731 1

—10

—Om


In another core intersection near Potchefstroom (drill core Rhkl; Fig. 5.1),

ferruginous quartzite with red mudstone clasts is developed directly on top of

ferricrete of the Hekpoort paleosol (Fig. 5.4). A red mudstone is developed on top of

the ferruginous quartzite and grades up into a siltstone that changes color upwards

from green to red (Fig. 5.4). The red siltstone is eroded into by a red clay-pebble

conglomerate. It is in turn overlain by siltstone that changes upwards in colour from

green to red before it grades into a fine quartzite (Fig. 5.4). The fine quartzite sharply

grades into green siltstone that become red in colour upwards in the core. This

mudstone is overlain by two beds of red mud-clast conglomerate, both with sharp

erosional base (Fig. 5.4). The upper one is overlain by red mudstones and black

carbonaceous shale of the Strubenskop Formation (Fig. 5.4).

Figure 5.4 Profile of the Dwaal Heuvel Formation near Potchefstroom from drill core Rhkl. Sample

positions are indicated.

In borehole BB11, near Carletonville (Fig. 5.1) red siltstone forms the base of the

Dwaal Heuvel Formation that rests with sharp contact on Hekpoort paleosol (Fig.

Slow decpenig

and shallowing

of lake

Green bands in red finely laminated mudstone

Transgression

Red finely laminated mudstone

Red siltstone Femcrete of the Hekpoort paleosol

Ferricrete Siltstone Mudstone

ConGr c M F St Sh Contacs and grain size cycles Lithology Depositional environment

710m

Finely laminated black shale

Grey finely laminated shale. Base of Strubenskop shale Top of Dwaal Heuvel Formation Red mudstone Green finely laminated mudstone

Green and red finely laminated mudstone

10 —

Om —

/ \


5.5). The red siltstone grades upwards into a mudstone that changes colour upwards

from red to green. The top of the Dwaal Heuvel Formation is marked by another

color change from green to black, the black mudstone marking the base of

Strubenskop Formation (Fig. 5.5).

Figure 5.5 Profile of the Dwaal Heuvel Formation from drill core BB11, near Carletonville (see figure

5.1 for locality).

Gr Co C M F St Sh Contacts and grain size cycles Lithology 1111111

Mudstone t t Ripple marks Pisolitic quartzite Quartzite with mudclasts

Finely laminated carbonaceous shale. Base of Strubenskop Formation

Red quartzite with ripple marks

Basal mudclasts Mudstone Red ripple marked quartzite with basal mudclasts

Pisolithic quartzite zone of Hekpoort paleosol.

Pallid zone of Hekpoort paleosol

Pallid zone of Hekpoort paleosol

Depositional environment

Transgression

Channel

Trangressional surface

Channel

Channel



At Pretoria (Strubenskop and Daspoort tunnel), in the central part of the Transvaal

area, the Dwaal Heuvel Formation is only five meters thick (Fig. 5.6). The base of the

formation is formed by a red medium to coarse grained, moderately sorted upward

fining ferruginous quartzite containing ripple marks and mudclasts. It grades upwards

into mudstone (Fig. 5.6). A second scour-based, fining upward quartzite overlies the

lower one. It is in turn overlain by black shales of the Strubenskop Formation (Fig.

5.6). A very prominent feature of the Dwaal Heuvel quartzite at this locality is the

presence of pisolites (2-15mm in diameter) in the lower quartzite (Figures 5.6 and

5.9E; Wagner (1928)).

Figure 5.6 Combined profile of the Dwaal Heuvel Formation near Strubenskop in Pretoria.

Much further to the east, at Waterval Onder (Fig. 5.1), the Dwaal Heuvel Formation

consists of shallow marine quartzites (Button, 1975). These quartzites are very well

sorted, contain gritty and pebbly intercalations and are marked by symmetrical and

assymmetrical ripple marks and cross bedding. Red discolouration or hematite coated

grains are conspicuously absent (Button, 1975). A coarse grained to pebbly quartzite

erodes into the sericitic zone of the Hekpoort paleosol and marks the base of the

Dwaal Heuvel Formation. This basal quartzite fines upward into a siltstone and

further into a mudstone. Pebbly quartzite erodes into the mudstone. The pebbly


quartzite is overlain by mudstone that coarsens up into coarse grained quartzite (Fig.

5.7; Button, 1975) .

5.2 Lateral Variation

From the profiles described above, a fence diagram could be constructed illustrating

lateral variations in the Dwaal Heuvel Formation. In the west of the Transvaal area,

the red beds of the Dwaal Heuvel Formation are marked by both fining and

coarsening upwards sedimentary successions (Fig. 5.7). To the southeast,

sedimentary cycles are mainly upwards coarsening (Fig. 5.7). Red and green

mudstone are preserved in drill core BB11 (Fig. 5.11). A pisolitic quartzite mark the

base of the Dwaal Heuvel Formation in the Pretoria area. Thinly developed upward

fining sedimentary successions dominate the Dwaal Heuvel Formation in the central

Transvaal area (Figures 5.7 and 5.8). In the east of the Transvaal area, the Dwaal

Heuvel Formation is marked by orthoquartzite (Figures 5.7 and 5.8).

On a whole, the thickness of the Dwaal Heuvel Formation in the Transvaal area varies

from less than 5 metres in the south to more than 100 metres in the north (Fig. 5.8).

From east to west, the thickness of the Dwaal Heuvel Formation remain relatively

even (Fig. 5.8). In most sections, the Dwaal Heuvel Formation displays a lower

upwards fining sequence and an upper upward coarsening sequence (Fig. 5.8).

Quartz pebbles

Green siltstone

Mudclasts

Red mudstone

Pisolitic quartzite

Carbonaceous siltstone

Green mudstone

Mudcracks and ripple marks Ferruginous quartzite and coated grains

Ferruginous quartzite

Carbonaceous mudstone

Quartzite

Red siltstone SE-Tvl

West East

Hekpoort paleosol

CoO CMF StSh

-1

cy

J 0 0 0

Strata].

*

Lithologies

Figure 5.7 Fence diagram of Dwaal Heuvel Formation. Localities are indicated on figure 5.1.

2• l•

■ Reference points 25

1-Borehole 1740, Goldfields 2-Borehole Rhkl, Anglo Gold 3-Borehole BB11, Goldfields 0 100

ICm 4-Borehole BD16, Goldfields 5-Borehole DPZ-2, Anglo Gold 6-Pretoria localities 7-Waterval Onder roadcut. 8-Borehole Stratal, Botswana Geological Survey

27° 29°

N

50

31° + 24°


Figure 5.8 Isopach map of the Dwaal Heuvel Formation (Modified from Button, 1973 and 1975).

Thicknesses in north-western part of Transvaal area from Klop (1978) and in the central part of the

Transvaal area from Hartzer (1987).

5.3 Petrography

The mineralogy of the red beds of the Dwaal Heuvel Formation in the western and

central parts of the Transvaal area was characterized using samples from three drill

core intersections, namely Stratal, Rhkl and 1740, and outcrop sample's from

Strubenskop, Pretoria (Tables 5.1A to D).

Table 5.1A Mineralogy of the sedimentary rocks of the Dwaal Heuvel Formation

from drill core Stratal in Botswana.

Depth(m) Sample Quartz Chlorite Pyrophyllite Berthierine Hematite

68.5 Str1/21 +++(1) ++(2) ++(3) +++(4)

73.5 Str1/20 +++(1) +++(3) +++(4)

74 Strl/19 +++(1) ++(3) +++(4)



76.4 Strl/18 +++(1) ++(3) +++(4)

79.05 Strl/17 +++(1) +(2) +(3) + +++(4)

81.9 Strl/16 +++(1) +++(3) +++(4)

90.25 Str1/15 ++(1) +(2) +++(3) +++(4)

91.9 Strl/14 +++(1) +(2) +++(3) +++(4)

95.3 Strl/13 +(1) +(2) +++(3) +++(4)

99.8 Strl/12 +(1) +(2) +++(3) +++(4)

101.27 Str1/11 +++(1) +(2) +++(3) +++(4)

(1)- Detrital quartz grains. (2)- Chlorite concentrated in lamina of green mudstone.

(3)- Concentrated in mudstone lamina and clasts. (4)- Concentrated in matrix and in

coatings of quartz grains and mud granules. Estimated amounts: +++(Dominant:

>30%), ++(Major: 10-30%), +(minor: <10%).

Table 5.1B Mineralogy of the sedimentary rocks of the Dwaal Heuvel Formation in drill core Rhkl

near Potchefstroom.

Depth(m) Sample Quartz Chlorite Pyrophyllite Berthierine Hematite Diaspore

762.18 W(1) + + +++(2) +++(3)

765.57 V(4) + +++(5) +++(2)

772.2 U(4) + +++(5) +++(2)

773,54 T(1) + +++(2) + +++(3) +

773.76 S(1) + +++(2) +++(3)

774 R(6) +++(7) + +++(3)

(1)- Red mudstone. (2)- Pyrophyllite is concentrated in mud lamina mud clasts. (3)- Hematite is

concentrated in matrix and in hematite-coated quartz grains. (4)- Green mudstone. (5)- Chlorite is

concentrated in laminae of green mudstone. (6)- Ferruginous quartzite. (7)- Quartz present as detrital

grains.

Table 5.1C Mineralogy of the sedimentary rocks of the Dwaal Heuvel Formation

from drill core 1740 near Potchefstroom.


329.5 4(1) +++(2) +++(3) + +++(4)

337.7 6(5) + + +++(3) +++(4)

338.7 7(6) +++(7) +

355.8 8(1) +++(2) + +++(3) +++(4)

(1)- Red, poorly sorted, sub-rounded quartzite. (2)- Quartz present as detrital grains.

(3)- Pyrophyllite concentrated in mud laminae and mud clasts. (4)- Hematite

concentrated in matrix and coatings around detrital grains. (5)- Red mudstone. (6)-

Green mudstone. (7)- Chlorite concentrated in green mudstone laminae.


Table 5.1D Mineralogy of the pisolitic quartzite of the

Dwaal Heuvel Formation at Strubenskop, Pretoria.

Sample

Zone Quartz Chlorite Hematite

Piso 1,2,3,4 Pisolitic quartzite +++ ++ +++

The Dwaal Heuvel Formation is represented by three main rock types namely

ferruginous quartzite, mudstone and pisolitic quartzite. The petrography of each of

these are now to be described in more detail.

5.3.1 Quartzite

The ferruginous quartzites of the Dwaal Heuvel Formation are predominantly

composed of quartz, hematite, pyrophyllite and chlorite (Tables 5.1A-D). Small

amounts of berthierine and diaspore are sometimes present (Table 5.1A). Size,

angularity and sorting of quartz grains vary significantly. Quartz grains in poorly

sorted fluvial quartzite are fine to coarse grained, angular to sub-rounded and poor-

medium sorted (Fig. 5.10A). Quartz grains of all different shapes and sizes are

hematite coated (Fig. 5.10B and C). These coatings thicken and thin in such a way as

to smooth out irregularities on the quartz grains, approaching a smooth spherical

shape (Fig. 5.10E). The hematite coatings consist of several, less than a micron thick,

dusty hematite (crystal size 2-7gm) coatings (Fig. 5.10D). The matrix consists of fine

crystalline hematite and chlorite (Fig. 5.10B). Mudclasts present in the quartzites

(Fig. 5.10A) consists mainly of finely intergrown pyrophyllite and hematite.

5.3.2 Mudstone

Red and green mudstone occurs in layers, irregular bands and clasts. Red mudstone

consists predominantly of pyrophyllite, chlorite and hematite (Tables 5.1A to C).

Pyrophyllite is the most abundant mineral in green mudstone (Tables 5.1A to C and

Fig. 5.10F) together with chlorite (Tables 5.1A and B).


5.3.3 Pisolitic quartzite

Pisolites are composed of a nucleus surrounded by a cortex composed of

concentrically laminated hematite. Depending on the composition of the nucleii, two

different types of pisolites are recognized in the Dwaal Heuvel Formation. The first

type contains nucleii composed of fine crystalline hematite with subordinate chlorite

(Fig. 5.11A). The second type contains complex nucleii composed of a mixture of

hematite-coated grains and fine crystalline hematite and chlorite (Fig. 5.11C and D).

The shape of hematite laminae in cortices of pisolites varies from a virtually

concentric evenly spaced variety, forming a none porous cortex (Fig. 5.11A) to

irregular, thickening and thinning, sometimes even pinching out laminae, which trap

siliciclastic grains giving the cortex a very porous appearance (Fig. 5.11C).

Laminae sometimes grow around two smaller pisolites to form a double-cored pisolite

(Fig. 5.11B). The grains trapped in the cortices of the pisolites are exactly similar in

composition to grains occurring in the surrounding parent rock (Fig. 5.11D).

Similar to the other red quartzites of the Dwaal Heuvel Formation, the ferruginous

quartzite hosting the hematite pisolites, consists of angular to sub-rounded, poor to

medium-sorted quartz grains with diameters ranging between 0.1 and 0.5mm, set in a

matrix of chlorite and fine crystalline hematite (Fig. 5.11D to F). Most of the quartz

grains are coated by fine crystalline hematite. However, no true hematite oolites have

been noted.


Fibrous green chlorite occurs throughout the siliciclastics of the Dwaal Heuvel

Formation (Figures 5.10B and 511F, Appendix II), replacing quartz, hematite and

pyrophyllite (Fig 5.10B). The replacive chlorite is inferred to be of late metamorphic

origin. Some of the pisolites are compacted (Fig. 5.11B).

Figure 5.9


Text to figure 5.9 Photographs of handspecimen of the Dwaal Heuvel Formation red quartzite. All

samples from Stratal. Up towards top of photographs. A- Ferruginous quartzite. Sample from

Stratal, Botswana. B- Ferruginous quartzite with mudstone clasts. Sample Stratal, Botswana. C-

Ferruginous quartzite with red mud drapings. Some leached spots are visible in the hematitic mudstone

drapings. Some grey mudstone clasts with red rims are visible. These clasts appear to have been

leached of ferric iron. D- Mudcracks in green mudstone are filled with red quartzite. The top layer of

mudstone appears to be leached, possibly indicating that it contained more carbonaceous material than

the bottom layer. Clasts of green to red mudstone with dark red rims are present in the red quartzite

below the mudcracks. E- Pisolitic quartzite, four times enlargement.

5.4 Major and Trace Element Geochemistry

For this study, samples from borehole Strata 1 and Strubenskop were submitted for

quantitative chemical analysis by B&B Laboratories, Johannesburg. REE analyses

were supplied by M. Bau. The research group of Prof. H. Ohmoto (Penn State

University, USA) provided unpublished geochemical data for samples from borehole

RHK1. The concentrations of the major, trace and rare earth elements of the Dwaal

Heuvel Formation and the basal part of the Stubenskop Formation are listed in Tables

5.2 A and B.

Ferruginous and pisolitic quartzites of the Dwaal Heuvel Formation contain on

average 39,15wt% Si0 2 ,0.58wt% Ti02, 11.01wt% A1203, 36.84wt% Fe203, 5.67wt%

FeO, 0.69wt% MgO, 1.01wt% CaO, 0.05wt% Na20, 0.92wt% K20, 0.6wt% P205.

Red mudstone and red shale of the Dwaal Heuvel Formation contain on average

52.56wt% Si02, 1.lwt% Ti02, 22.94wt% A1203, 10.25wt% Fe203, 2.09wt% FeO,

0.66wt% MgO, 0.31wt% CaO, 0.22wt% Na20, 2.26wt% K20, 0.15wt% P 205.

Green mudstone and siltstone of the Dwaal Heuvel Formation contain on average

55.53wt% Si02, 1.3wt% Ti02, 24.4wt% A1203, 2.68wt% Fe203, 5.38wt% FeO,

0.69wt% MgO, 0.27wt% CaO, 0.2wt% Na20, 1.61wt% K20, 0.15wt% P205.

Throughout the Dwaal Heuvel Formation coarser-grained material contain more

Fe203 than finer grained material indicating that oxygen could penetrate the more

porous sediments better to precipitate hematite.

RD= nn Hue, 2:i J .

Photo no..15 Doloclor- TETRA cur-ItTOO kV :Wpm H

•

-C ER1=2,11.110 R1 WH= ?..! ■ m ■ "" 42? 701. 0 H Photo No.-111 • Detect°, TETRA

DeteutOM. TETRA

1142=uiAi kV Min . H •

ED , Ti mn

444,00 Nn.-14

11044= DO I. OGLO, TETRA

4 . . 4,44

214pn H.... l'Antn . DoLontor , ;TETRA:;•

Red Beds of the Dwaal Heuvel Formation

101

Figure 5.10 BSE-SEM-images of the Dwaal Heuvel Formation. A- Angular to sub-rounded, poorly

sorted fine grained quartzite. The matrix consists of fine grained clay minerals and dusty hematite. B-

Hematite-coated quartz grains with a matrix of diagenetic chlorite and fine crystaline hematite. C-

Hematite coated quartz grains and clay clasts in a matrix of fine chlorite and hematite. The coatings

consist of both massive and crystalline hematite. D- Clay clasts in a fine matrix of chlorite and

hematite. E- Hematite-coated quartz grain clast. The coatings consists of massive and dusty hematite,

and pyrophyllite. Galena is developed in the massive hematite. F- Chloritic siltstone (west) grading

into chloritic mudstone (east).

.., tiog.%vplieV...-=',;;;:--4-0;-

At. LI I r=1.,.!111. 411Wly

IF-FAW7e,ir6VA:

Red Beds of the Dwaal Heuvel Formation

102

Figure 5.11 BSE-SEM-images of the pisolitic quartzite. A- Hematitic pisolite with a dense hematite

nucleus and laminae of fine crystalline hematite. The inner laminae of the cortex appear concentric and

dense with little sediment trapping. However, outer laminae trap an appreciable amount of sediment.

B-Composite hematitic pisolite with laminae trapping pisolites and sediment. C- Hematitic pisolites

with different nuclei. The nucleus of the pisolite on the right appears to be a clay clast. It has dense

closely spaced, evenly thick elliptical hematite lamina. The pisolite on the left has a nucleus consisting

of trapped sediment and somewhat irregular hematite laminae. D- Pisolite with nucleus consisting of


trapped matrix sediment and oddly shaped outermost laminae trapping an appreciable amount of clastic

sediment. E- Matrix consisting of siliciclastic sediment. Note the angular quartz grains next to the

clay clasts. Hematite is coating both quartz grains and clay granules. F- Matrix consisting of rounded

quartz grains coated by massive as well as fine crystalline hematite. Note chlorite replacing the quartz

and hematite of the coated grains.

Total iron concentrations of the green siltstones and mudstones (mostly as chlorite)

are lower than those of red siltstones and mudstones (mostly as hematite) indicating

that iron was more easily oxidised than reduced. The Fe 2O3/FeO ratio of red

mudstones and siltstones is at least twice that of the Fe2O3/FeO ratios of green

mudstones and green siltstones (Tables 5.2A and B).

K2O/Na2O ratios range between 5 and 19.3 indicating that K2O was introduced to the

rocks, probably during sericitization (Tables 5.2A and B).

U concentrations range between 3.36 and 11.05 ppm in the Dwaal Huivel Formation

(Tables 5.2A and B).

5.5 Discussion

5.5.1 Provenance and Degree of Weathering in the Source Area

Thickness estimates and paleocurrent directions indicate that source areas for the

Dwaal Heuvel Formation was located to the north and east of the Transvaal area

(Button, 1973; Fig. 5.8). However, exact location and composition of the source

terrain remain speculative.

Condie and Wronkiewicz (1990) suggest that Cr/Th ratios correlates with Co/Th

ratios in pelites across the Kaapvaal Craton and appear to monitor changes in source

composition. Siltstones and mudstones of the Strubenskop and Dwaal Heuvel

Formations display similar trends to that displayed for the Pretoria Group by Condie

and Wronkiewicz (1990). They also display similar trends to that of the most

weathered zones of the Hekpoort paleosol (Fig. 5.12). It appears as if the Cr/Th and

Sc/Th ratios become less with increased weathering of the Hekpoort lava.

Table 5.2A Geochemistry of the Dwaal Heuvel Formation. Major element concentrations in oxide weight%. Trace and rare earth element concentrations in ppm.

Rock type P RQ RM RQ GM RM RQ GS GS

Sample Pisol Str1/11 Strl/12 Str1/14 Str1/1Sgreen Str1/15rnoi Str1/17 Strl/22 Str123 Depth (m) 101.27 99.8 91.9 90.25 90.3 79.05 68.5 64.5

Sl02 42.8 60.1 56.1 53.6 62.9 53.9 Na 50.2 54.5

TiO2 0.67 1.15 1.37 0.35 1.55 1.26 Na 1.59 0.92

A1203 7.5 20.6 22.5 8.9 22.1 25.2 Na 33.8 23.8

Fe203 39.2 5.71 5.39 25.51 1.61 5.25 Na 1.14 0.83

FeO 2.16 4.31 3.45 5.03 3.74 3.02 Na 0.65 7.76

Fe202T 41.6 10.5 9.22 31.1 5.77 8.61 Na 1.86 9.45

MgO 0.4 0.5 0.7 0.4 0.5 0.6 Na 0.2 1

CaO 0.22 0.26 0.19 0.37 0.32 0.24 Na 0.74 0.27

Na2O Bdl 0.2 0.4 0 0.2 0.4 Na 0.5 0.2

K2O Bdl 1.21 3.07 0.88 2.21 3.55 Na 4.99 2.92

P202 • 0.53 0.13 0.09 0.2 0.15 0.11 Na 0.29 0.09

L0.1. 4.8 4.25 4.45 2.46 4.25 4.7 Na 5.3 6.03

Total 98.52 98.9 98.09 98.26 99.95 98.57 99.47 99.18

CIA 95 90 84 85 88 84 82 86

K20/Na20 6.05 7.68 11.05 8.88 9.98 14.6

SiO2/A1203 5.71 2.92 2.49 6.02 2.85 2.14 1.49 2.29

Fe203/FeO 18.15 1.32 1.56 5.07 0.43 1.74 1.75 0.11

Cu 78 42 193 326 51 36 Na 20 148

Zn 66 21 33 30 25 20 Na 12 49

Ni 85 97 103 63 68 60 Na 18 74

Mn 488 263 116 194 308 79 Na 11 424

Cr 234 212 172 195 210 201 Na 210 144

Mo 2 2 3 MI Bdl 6 Na 1 Bdl

V 194 141 204 244 99 174 Na 221 124

Co 34 38 32 46 30 18 Na 11 35

Ga 19 24 29 18 30 31 Na 36 29

Ge 1 1 Bdl Bd1 1 Bdl Na Bdl 1

Sc 18 43 31 18 22 31 Na 62 25

Rb Na 32.9 160 38.2 179 107 39.6 227 138

Sr Na 42.7 107 39.1 140 87 53.8 112 95.8

Y Na 20.4 36.1 18.1 40.5 45 15.7 50.4 24.3

Zr Na 123 318 100 363 567 87.7 293 232

Cs Na 3.73 12.6 2.83 13 7.24 3.72 8.14 11.3

Ba Na 125 410 104 487 289 193 1036 324

Hf Na 3.14 9.19 2.77 9.74 15.1 2.43 7.85 6.24

Pb Na 16.3 11.7 23.5 13.6 18.6 33.7 11.4 10.1

Th Na 9.48 15.7 16.6 25.4 21 11.8 22.7 18.6

U Na 3.36 7.03 6.96 7.53 7.34 7.91 12.3 6.29

U/Th 0.35 0.45 0.42 0.30 0.35 0.67 0.54 0.34

Th/Sc 0.22 0.51 0.92 1.15 0.68 0.37 0.74

La/Sc 0.47 1.64 0.79 3.35 1.36 1.05 2

La/Ib 2.12 3.24 0.86 2.90 2.01 1.19 2.86 2.69

Cr/Zr 1.72 0.54 1.95 0.58 0.35 0.00 0.72 0.62

La Na 20.1 50.8 14.2 73.6 42.3 14 64.9 50

Ce Na 36.4 97 30.1 139 86 27.3 130 96.2

Pr Na 4.47 11.4 4.25 15.3 9.8 3.65 15.4 10.8

Nd Na 15.8 39.9 15.7 51.9 35 13.3 55.1 36.1

Sm Na 5.03 7.58 3.8 9.55 6.82 3.56 11.2 6.58

Eu Na 1.45 1.87 0.93 2.15 1.57 1.01 2.62 1.39

Gd Na 5.7 7.91 4.13 8.58 7.06 4.17 10.39 5.97

Tb Na 0.7 1.2 0.69 1.21 1.12 0.65 1.66 0.85

Dy Na 3.92 6.85 4.1 7.16 7.2 3.64 10.1 4.65

Ho Na 0.74 1.36 0.77 1.45 1.53 0.73 2.02 0.88

Er Na 2.14 3.88 2.28 4.21 4.67 2.1 5.88 2.6

Tm Na 0.31 0.57 0.36 0.63 0.73 0.32 0.88 0.41

Yb Na 2.07 3.69 2.31 4.18 4.79 2.16 5.88 2.62

Lu Na 0.31 0.57 0.35 0.65 0.78 0.32 0.85 0.42

Sample Piso from Strubenskop, Pretoria The remaining samples are from drill core Stratal in Botswana. Piso to Str1/17 from Dwaal Heuvel Formation. Str122 and Str123 from Strubenskop Formation. P-Pisolitic quartzite. RQ-Ferruginous quartzite. RM-Red mudstone. GM-Green mudstone. GS-Green Shale. BS-Black shale. Na-Not analysed. Bdl-Below detection limit. See figure 5.2 for sample positions.

Table 5.2B Geochemistry of the Dwaal Heuvel Formation. Major element concentrations in oxide weight%. Trace and rare earth elements in ppm.

Rock type FQ FQ FS FS RM RM RM RM RM GM GM GM GM RM RM Sample 762.1 762.3 762.75 763.2 764.6 765.3 766.4 768.1 770 775.5 776 776.5 777.1 777.11 777.3

SiO2 28.36 31.85 41.57 44.86 53.29 54.17 53.8 53.53 59.07 55.85 54.54 52.72 51.62 54.69 53.23

TiO2 0.61 0.69 0.87 0.98 0.98 1 1.02 1.07 0.93 1.07 1.12 1.21 1.55 1.34 1.28

A1203 13.69 13.95 18.54 21.18 20.97 21.71 22.04 22 18.99 21.32 21.9 29.26 27.4 29.22 30.04

Fe203 37.22 45.42 29.74 15.92 8.37 10.13 7.09 11.49 8.63 5.13 3.79 1.75 1.1 7.41 3.31

FeO 9.81 Bd1 Bd1 5.85 4 Bd1 2.59 Bd1 0.33 6.74 6.85 4.94 4.64 Bd1 3.74

Fe2O3T 48.12 45.42 29.74 22.42 12.81 10.13 9.97 11.49 9 12.62 11.4 7.24 6.26 7.41 7.47

MnO 0.04 0.05 0.02 0.03 0.03 0.03 0.06 0.06 0.03 0.07 0.04 0.03 0.02 0.01 0.02

MgO 0.98 0.97 0.48 0.7 0.84 0.82 0.84 0.71 0.86 1.11 0.99 0.51 0.28 0.38 0.37

CaO 2.1 1.36 0.9 0.52 0.35 0.23 0.23 0.24 0.21 0.44 0.26 0.15 0.16 0.19 0.14

Na2O 0.08 0.07 0.15 0.15 0.15 0.16 0.19 0.15 0.1 0.16 0.09 0.31 0.26 0.26 0.29

K2O 0.92 0.96 1.51 2.22 2.36 2.51 2.83 2.05 1.93 1.12 0.71 2.05 1.94 1.4 1.47

P202 1.1 0.82 0.52 0.25 0.19 0.12 0.04 0.02 0.1 0.25 0.17 0.07 0.12 0.11 0.08

L0.1. - - - 5.59 - 5.59 5.74

Total • 96 96.14 94.3 93.31 91.97 90.88 91.02 91.32 91.22 94.01 91.22 99.14 89.61 100.6 100.13

CIA 77 81 84 83 83 84 82 86 85 90 94 89 89 92 92

K20/Na20 11.5 13.71 10.07 14.80 15.73 15.69 14.89 13.67 19.3 7 7.89 6.61 7.46 5.38 5.07

Si02/A1203 2.07 2.28 2.24 2.12 2.54 2.50 2.44 2.43 3.11 2.62 2.49 1.80 1.88 1.87 1.77

Fe2O3/FeO 3.79 - 2.72 2.09 - 2.74 - 26.15 0.76 0.55 0.35 0.24 - 0.89

Ba 202.62 224.8 337.06 477.6 479.39 545.05 543.96 409.94 392.81 201.16 136.16 636 353.12 568.7 440

Rb 41.26 43.18 69.49 103.92 113.34 116.93 132.32 94.62 90.09 53.35 34.82 110 77.67 89.52 81

Sr 104.1 85.62 90 99.82 92.28 95.71 93.66 88.15 76.05 101.46 97.86 88.6 135.86 163.15 115

Y 87.2 54.5 43.8 41.1 37.1 32.5 32.4 45.6 27.8 37.5 29.4 27 32.8 32.4 32

Zr 125.48 156.91 181.14 216.44 201.97 213.58 204.42 343.73 270.6 274.26 249.77 211 227.48 221.53 214

Nb 5.17 6.65 10.66 14.31 14.16 15.01 14.57 15.12 13.38 14.57 14.44 13 17.58 12.16 13

Th 24.03 23.47 26.09 26.82 21.27 22.8 22.07 20.54 17.97 20.57 21.23 14.7 18.76 17.71 16

Pb 46 41 50 25 14 12 13 24 9 8 10 19 28 18 32

Ga 28 29 30 34 31 34 33 32 28 31 33 32 30 39 33

Zn 66 63 30 54 158 71 99 90 110 131 141 79 68 51 66

Cu 8 Bd1 16 21 93 72 197 Bdl 61 33 49 116 126 283 130

Ni 89 103 104 101 190 96 94 106 67 249 167 103 88 103 95

V 482 413 352 291 155 155 159 125 107 145 163 322 293 320 320

Cr 348 298 316 251 174 176 186 217 124 165 154 414 429 316 333

Hf 3.11 4.3 4.93 5.92 5.63 5.84 5.72 8.98 7.11 7.32 6.59 6.2 7.4 5.98 6.2

Cs 5.29 5.8 6.89 9.03 10.2 9.53 11.12 9.59 9.31 6.87 5.68 7.3 7.32 8.79 7.3

Sc 45 42 39 40 24 25 27 23 19 23 26 55 53 58 56

Ta 0.54 0.73 1.15 1.39 1.528 1.60 1.50 1.48 1.38 1.35 1.42 1.29 1.48 1.41 1.34

Co 32.3 32.6 23.4 27.7 35.3 34.4 27.7 21 35.2 59.2 61.2 16 9.5 10.6 9.8

Be 4 4 4 4 3 3 4 3 3 2 2 BdI 3 4 Bd1

B 0.72 0.97 1.06 0.58 0.4 0.42 0.63 0.39 0.35 0.17 0.15 0.1 0.95 0.36 0.3

U 11.05 10.66 8.52 7.4 6.33 6.31 5.89 5.82 4.71 5.9 5.22 5.95 6.63 8.26 7.08

W 13.68 24.17 15.5 10.18 25.52 22.98 20.73 25.03 83.99 48.2 27.6 1.3 3.02 7.33 1.1

Sn Bd1 BdI Bdl Bdl Bdl Bd1 Bdl Bdl Bd1 Bdl 1341 2.8 5.6 Bd1 6.9

Mo 2.85 3.32 6 2.35 1.26 1.95 2.66 0.43 1.73 0.71 0.95 2.1 1.97 0.42 0.6

Au 0.7 0.42 0.57 1.84 1.1 0.96 1.02 0.91 0.77 0.47 0.33 0.59 0.57 0.81 0.46

Ge 3.6 3.2 2.7 2.8 2.6 2.5 2.5 2.8 2.5 3.2 3 2 3.3 3.3 3

As 12 14 13 8 8 Bdl 10 Bdl 11 8 Bdl Bd1 11 6 8

Ag 13fil Bdl Bdl WI Bd1 BdI Bdl Bdl BdI Bdl Bdl Bd1 Bdl Bdl BdI

Sb 16.06 15.21 12.8 7.37 3.34 2.51 2.8 3.49 1.67 1.29 1.42 1.99 3.6 2.83 3.2

Ir Bdl Bd1 0.1 Bd1 Bd1 Bdl Bdl Bdl Bd1 Bdl Bdl Bdl BdI BdI Bdl

U/11 0.46 0.45 0.33 0.28 0.3 0.28 0.27 0.28 0.26 0.29 0.25 0.40 0.35 0.47 0.44

Th/Sc 0.53 0.56 0.67 0.67 0.89 0.91 0.82 0.89 0.95 0.89 0.82 0.27 0.35 0.31 0.29

La/Sc 0.74 0.80 1.23 1.52 2.46 2.60 2.31 2.69 2.61 2.89 2.56 0.45 0.86 1.29 0.57

La/Th 1.38 1.43 1.83 2.27 2.78 2.85 2.83 3.01 2.76 3.23 3.13 1.68 2.44 4.24 2.01

Cr/Zr 2.77 1.90 1.74 1.16 0.86 0.82 0.91 0.63 0.46 0.60 0.62 1.96 1.89 1.43 1.56

La 33.13 33.57 47.82 60.83 59.02 65.07 62.43 61.94 49.62 66.52 66.47 24.7 45.76 75.1 32.1

Cc 76.55 75.98 103.71 126.76 121.64 135.02 128.54 132.27 103.07 138.42 135.39 54.6 86.16 163.24 70

Pr 7.934 7.69 9.44 10.73 10.29 11.16 10.59 11.18 8.84 11.9 10.86 5.63 8.02 13.39 7.12

Nd 36.9 34.34 40.59 42.89 41.28 43.49 42.33 45.23 34.35 49.03 42.12 21 34.4 54.59 26.4

Sm 8.61 7.84 8.03 8.06 7.63 7.55 7.24 7.78 5.92 7.92 6.79 4.23 6.52 9.33 5.25

Eu 2.40 2.08 1.96 1.73 1.63 1.69 1.63 1.80 1.37 1.73 1.55 1.02 1.5 2.32 1.217

Gd 11.03 9.34 8.49 8.48 8.15 7.91 7.39 8.43 6.2 8.3 7.24 3.83 5.79 9.59 4.83

Tb 2.36 1.58 1.27 1.14 1.03 0.97 0.95 1.1 0.78 1.11 0.9 0.75 0.91 1.2 0.89

Dy 17.35 10.98 8.12 7.5 6.34 5.67 5.42 6.92 4.73 6.33 5.19 4.66 6.09 6.39 5.69

Ho 3.58 2.32 1.64 1.46 1.22 1.06 1.01 1.4 0.91 1.23 1.02 0.96 1.21 1.17 1.17

Er 11.02 7.09 4.74 4.57 3.56 3.58 3.23 4.37 2.79 3.93 3.04 2.83 3.87 3.76 3.58 Tm 1.6 1.03 0.73 0.67 0.62 0.47 0.46 0.65 0.4 0.52 0.43 0.47 0.55 0.53 0.57 Yb 10.74 7.25 4.86 4.79 3.33 3.39 3.17 4.07 2.86 3.62 3.02 2.92 3.74 3.61 3.66 Lu 1.69 1.12 0.82 0.76 0.59 0.56 0.56 0.67 0.47 0.58 0.51 0.13 0.6 0.58 0.536

Data courtesy of H. Olunoto. FQ-Ferruginous quartzite. FS-Ferruginous siltstone. RM-Red mudstone. GM-Green mudstone. See figure 5.4 for sample positions.

r1/2)c

S 1/1 A

Str1/4

Strip

s Strl/

Dwaal Heuvel and Strubenskop Form

0 10

20 Sc/Th

1 2 3 Sc/Th 4

Str1/7 ►

Str1/9

tr1/5

5x

10 • Str1/

/22 Strongly weathered ock

0 0 1 2 3

Sc/Th 4 5 6

40 30

30

.c 20

20

(.)

10 10

0 0

Strl/

Str1/15gr r n

60

50

40

.c 30

20 1/10

Strl

Slightly weathered rock

C

■


Transport within a fluvial environment may decrease the Cr/Th and Sc/Th ratios even

further.

Figure 5.12 Cr/Th ratio of siltstone and mudstone of the Strubenskop Formation, Dwaal Heuvel

Formation and Hekpoort lava and paleosol as a function the Sc/Th ratio (after Condie and

Wronkiewicz, 1990). A- Cr/Th to Sc/Th plot of Hekpoort lava, Hekpoort paleosol, Dwaal' Heuvel

Formation and Strubenskop Formation (only Str1/22 and Str1/23). Data from drill core Stratal. B-

Cr/Th to Sc/Th plot of Hekpoort lava and paleosol. Data from drill core Rhkl. C- Cr/Th to Sc/Th plot

of Dwaal Heuvel formation. Data from drill core Rhkl.

Triangular plots of Th-Hf-Co and La-Th-Sc (Fig. 5.13) are similar to those of the

Strubenskop Formation as displayed by Jahn and Condie (1995). Again, the Dwaal

Heuvel and Stubenskop Formations display similar trends to that of the weathered

zones of the Hekpoort paleosol (Fig. 5.13).

Figure 5.13 Composition of mudstones and siltstones of the Dwaal Heuvel and Strubenskop Formations on Th-Hf-Co and La-Th-Sc diagrams compared to the compositions of the Hekpoort paleosol and lava. A and B from drill core Stratal. B-F from drill core Rhkl. See figure 5.14 for symbols.


This illustrate that the composition of siliciclastic rocks is not only dependent on the

type of tectonic setting (source rock; Condie and Wronkiewicz (1990)) but also to a

large extent on the type of weathering the source rocks have been exposed too.

During intense lateritic weathering even the least mobile trace elements may be

mobilized. These type of plots may therefore not be a reliable way to determine the

tectonic setting of the source rocks.

The sedimentary rocks of the Dwaal Heuvel and Strubenskop Formations display

similar rare earth elements distributions as those given by Condie and Wronkiewicz

(1990) and Jahn and Condie (1995) for the Silverton, Strubenskop and Timeball Hill

Formations (Fig. 5.14A and B).

Heavy rare earth elements are enriched relative to light rare earth elements (Fig.

5.14A and B), similar to that of the weathered zones of the Hekpoort paleosol (Fig.

5.14A and B). The chondrite normalised REE patterns display negative Eu anomalies

for the Dwaal Heuvel Formation, Strubenskop Formation and Hekpoort lava (Fig.

5.14A and B). The paleosol displays a negative Eu anomaly in Stratal (Fig. 5.14A)

and both positive and negative Eu anomalies in Rhkl (Fig. 5.14A and B).

Condie and Wronkiewicz (1990) suggest a granitic source for the Pretoria Group

pelites that might have been associated with continental rifting. However, it is

apparent that the Dwaal Heuvel and Strubenskop rocks display trends similar to the

Hekpoort paleosol and lava. This suggest that at least some of the detritus for the

Dwaal Heuvel and Strubenskop Formations was sourced by the Hekpoort lava.

The degree of weathering in the source area can be estimated from the chemical index

of alteration (CIA) (Nesbitt and Young, 1982). CIA values for the Dwaal Heuvel and

Strubenskop Formations range between 80% and 90% (Tables 5.2A and B),

indicating a source areas that experienced efficient chemical weathering. On a

CaO*4-Na20-K2O-A1203 plot (CaO* represent CaO in silicate phases only) the

sedimentary rocks of the Dwaal Heuvel and Strubenskop Formations are distributed

within the field of advanced chemical weathering (Fig. 5.15).

400

u 100

10 La Ce Pr Nd

LTI Strl/9

X StrI/5

Sm Eu Gd Tb Dy Ho Er TmYb Lu

1:11 Str1/8 11111• Str1/7 A StrI/6A

• Strl/3 ❑ Strl/2 0 StrI/1

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

Str1/23 4 Str1/22 ❑ StrI/170 Str/15rooi

Str1/15green 0 Str1/14 e Strl/12 StrI/11

600

u100

0 .c

10 8

400

100

0 .0

10

5



Sm Eu Gd Tb Dy Ho Er TmYb Lu

0 762.1 ❑ 762.3 • 762.75 + 763.2 X 765.3


0 776 e 776.5 13 777 . 1 61 777.11 ID 777.3

A 764.6 • 766.4 A 768.1 [I 770 In 775.5

0 777.5 ❑ 777.6 • 777.7 + 777.8 X 777.9

300

10

C 0

100

0

200

100

10

6

Figure 5.14A Chondrite-normalised REE distribution of Dwaal Heuvel, Strubenskop, Hekpoort lava and Hekpoort paleosol. A- Hekpoort paleosol, Stratal. B- Dwaal Heuvel and Stubenskop Formations (Str1/22 and Strl/23), Stratal. C-E Dwaal Heuvel Formation, Rhkl. F- Paleosol, Rhkl.

al 778.95 e 779.1 I] 779.3 la 779.5 C 779.8 • 778.01 A 778 A 778.3 CI 778.6 121 778.9

780 0 780.5 • 780.6 0 781 V 781.3 <I 782.1 1 782.6 r> 783 11110 783.6 0 786

100

6.)

0 0 0

10

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

200

0 0 .c

10

❑ 795.4 ■ 799.4 + 802.6 X 812.6 A 1091

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

I 100 0

.0

50

3 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er TmYb Lu

10 8 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb to

800

100

4.)

0 0 0

Figure 5.14B Chondrite normalised REE distribution of the Hekpoort lava and paleosol, Rhk 1 .


Figure 5.15 Ca0*+Na20-A1203+K20 ternary plot for the Dwaal Heuvel and Strubenskop Formations.

CaO* represent CaO in silicate phases only (Nesbitt and Young; 1982).

5.5.2 Depositional Model and Primary Development of Red Beds

Studies of the Hekpoort paleosol suggest that the Dwaal Heuvel erosional surface was

exposed to a period of extensive lateritic weathering prior to deposition of the Dwaal

Heuvel Formation (Fig. 4.19). During development of the Dwaal Heuvel erosional

surface, some rivers eroded into the weathering profile after uplift of the Kaapvaal

craton (Fig. 5.16). In the central Transvaal area, these river channels were filled with

highly ferruginous sands, with at least some of the sediment evidently derived from

the Hekpoort paleosol and lava. Iron-concentrations were further increased when Fe 2+

migrated with groundwater from the lateritic profiles into the rivers when the water

level of the rivers dropped during dry seasons (Fig. 5.16). Fe 3+ precipitked as

hematite laminae on coated quartz grains when exposed to atmosphere resulting in

extremely Fe 3trich red bed sediments. Pisolitic ironstone also developed in this type

of setting (Fig. 5.16). Pisolites preserved in pisolitic quartzite near Pretoria reveals up

to 6 different periods of siliciclastic sediment trapping (Fig. 5.11A and B).

Progressively larger amounts of clastic sedimentary matrix material are incorporated

by the lamina towards the outside of the pisolite cortex (Fig. 5.11A and B). Hematite

laminae precipitated during dryer periods, when Fe 2+-rich groundwater migrated


towards rivers (Fig. 5.16). Grain trapping took place during wetter periods (Fig.

5.16). This suggests that the sediment became waterlogged for longer periods

towards the end of pisolite forming conditions (Fig. 5.11A and B). Irregularly shaped

outer laminae of the pisolites mark the end of pisolite development (Fig. 5.11A).

These irregularly shaped laminae may indicate some biogenic influence in the

precipitation of hematite under waterlogged conditions (Dahanayake, 1983).

Uplift in the western part of the Transvaal area was not sufficient for rivers to erode

the upper ferricrete and laterite of the Hekpoort paleosol (Fig. 4.19). This area

appears to have been relatively low lying with mostly waterlogged conditions during

the development of the Hekpoort paleosol (Fig. 4.19).

In the eastern Transvaal area the top zones of the Hekpoort paleosol were eroded

away by rivers prior to the deposition of the Dwaal Heuvel Formation. Paleocurrent

directions by Button (1973) and Schreider (1990) suggest that a marine front (Button,

1973) then transgressed from the southeast, reworking fluvial sediments into a

shallow marine orthoquartzite (Fig. 5.16).

Overall, it appears as if the Transvaal area was slightly warped during the deposition

of the first stage of the Dwaal Heuvel Formation (Fig. 5.16). The central- and

westernmost areas were anticlinal and the mid western- and easternmost areas were

synclinal areas.

A second stage of sedimentation of the Dwaal Heuvel Formation is broadly marked

by a upward coarsening succession of shale to quartzite (Fig. 5.7; Button, 1973). In

the east, the upward coarsening succession is marked by black shale coarsening up

into orthoquartzite, thought to represent a prograding delta (Button, 1973) in a

shallow marine setting (Fig. 5.16).

In the mid western part of the Transvaal area, successions of green mudstone

coarsening upward into ferruginous quartzite also indicate a prograding deltaic

environment (Fig.5.16). Towards the northwest (Stratal), fluvial sediments become

increasingly important (Figures 5.7 and 5.16). Some of the quartzites in the western

part of the Transvaal area contain abundant mud cracks, coated grains and are


extremely Fe 3+-rich (Tables 5.2A and B). This suggests regular dry periods with

Fe2+-rich groundwater migrating into the fluvio-deltaic environment.

In the central part of the Transvaal area, near Pretoria, this second phase of

sedimentation in the Dwaal Heuvel Formation is thinly developed (Figures 5.7 and

5.16).

An isopach map of the second stage of Dwaal Heuvel sedimentation indicates that in

the easterly most part of the Transvaal area, the succession thickens into a south

westerly direction (Fig. 5.16), indicating a source area towards the north east. In the

mid west (Stratal and Potchefstroom), the succession thickens extend in a south

easterly direction (Fig. 5.16), suggesting a source area towards the north west. This

indicates a subtle shift of source from a northerly area during the first stage of Dwaal

Heuvel deposition to a northwesterly area during the second stage of Dwaal Heuvel

deposition.

Hematite is a major constituent of the red beds. It constitutes the nucleii and cortices

of in-situ pisolites (Fig. 5.11A and B). Hematite preserves every textural detail of

these pisolites (Fig. 5.11A). Replacement of hematite by chlorite obscures the

primary textural details (Fig. 5.11F). Hematite-coated grains are abundant in the

ferruginous quartzites of the Dwaal Heuvel Formation. Again hematite preserves

textural details of coatings (Fig. 5.10B). This indicate that hematite was precipitated

as a primary phase and not during a secondary process (Turner, 1980).

5.5.2 Correlation of Dwaal Heuvel Formation and Gamagara/Mapedi

Formations

The Dwaal Heuvel Formation in the Transvaal area and the well described red beds of

the Gamagara/Mapedi Formation in Griqualand West (Beukes and Smit, 1987;

Holland and Beukes, 1990; Wiggering and Beukes, 1990; Gutzmer and Beukes, 1998)

display many similarities. Transvaal strata, exposed by the Dwaal Heuvel/Gamagara

erosional surface (Fig. 4.19; Holland and Beukes, 1990), are overlain by fluvial

hematite pebble conglomerate marking the base of the Mapedi/Gamagara red beds

Sta 1- Fluvial Phase

Oxidised sepiolite Ongeluk lava


Asbesheuwels iron formation

Lateritic weathering profile

Hekpoort lava

Duitschland, Tuneball Hill and Boshoek Formations Paige iron formation

Wet season

Medd OW mos 011ortlbe■ ww.... 0.1212.11fteg. leos

Waterlogged conditions. Almost no penetration of oxygen into weathering profile and leaching of pallid zone and laterite by groundwater. Influx of ferruginous sediment into rivers from eroded lateritic weathering profile.

•■)100. Groundwater flow

paealid moan

16.1sor. 14.11deb pal as

Cdatelse1WILIsals Ceaerltls■ ogsdba Okelslad /Wow Iowa

Sediment dry out. Influx of Fe with groundwater into rivers. Accumulation of hematite laminae on pisolites and coated grains during oxygenation of sediment. Development of mudcracks during very dry conditions. Some 0, penetration of laterite with lowering of water table followed by the precipitation of hematite.

i7.-Yealle aft ad Melo Medd so raid. parlaa•

Isopach map indicating depositional environments during first stage of the Dwaal Heuvel Formation

Dry season

111115:4%7"14 -UN"0410

Oxidised sepiolite Ongeluk lava


Asbesheuwels iron formation

Stage 2- Deltaic Phase

Deltaic phase of Dwaal Heuvel Formation Fluvial phase of Dwaal Heuvel Formation

Hekpoort lava

Deutschland, Tuneball Hill and Boshoek Formations

Penge iron formation

Trangression cause flooding of erosional surface and fluvial red beds. Shallow marine sediment are deposited in the eastem part of the Transvaal area In the western part of the Transvaal area and in the Griqualand west area, deltaic and fluvial red beds are deposited.

Isopach map indicating depositional environments during second stage of the Dwaal Heuvel Formation

Dapoddoul anima.

(=j edveiw delleomarbe combo

1=1 Wade to Oaks deslo anksamase vah mw sof bah

INsp wax etahroal erld scsedbm∎ imeea•Ocie einslaptd

Griqualand West Oxidising atmosphere Transvaal alaWillOPMIESINIVIIGEIMIEWLSMUVRillIV-VM11.1•PAWSEVN.411COMIRMSM71

IiIirWirKW.33111106W9NUMWEV41,06W

V *V

Sudsidence of the Kaapvaal craton

Figure 5.16 Schematic diagram to illustrate the development of the Dwaal Heuvel Formation.

Wolhaarkop

Upwards coarsening sequence

TransL essionalsur a pwar ming

sequence

Simondium South

t Section west of thru

Rooinekke

Ongeluk lava Kuruman BIF Koegas Subgroup Campbellrand Subgroup

Ijf Section east of thrust 4— Mapedi lava

1.-Sishen

Lucknow/Upper Mapedi _ 1—

Marthaspoort/Lower Mapedi

Gamagara red beds

Valwater

Formation Hartley

CoGr C M F SI Sh 1111111

14--

I00

Om -I

Venn dolomite

Volop

Neylan • — •—

Lucknow

••••-,

Upper Mapedi shale

Lithologies

Quartz pebble conglomerate

Quartzite and arkose

Carbonaceous siltstone

Lava

Red Beds

Mapedi lava

0 0 0 o o,_po o BIF-rich conglomerate

Limestone/Dolomite am-rma-r•Im-0

Agglomerate

Carbonaceous shale

Marthaspoort

Red quartzite

Red shale Lower Mapedi shale

Red siltstone

r Hematite pebble conglomerate

Formations North Gamagara red beds 4lotazel/Beaumont

100

Om

Figure 5.17 Fence diagram of the Gamagara/Mapedi Formations in the Griqualand West area along the Western edge of the Kaapvaal craton (modified from Beukes and Smit, 1987). Also indicated are a proposed new stratigraphic subdivision of the Transvaal Supergroup and Olifantshoek Supergroup (after Van Niekerk, 1999). Note the difference in the scale of the profiles.


(Fig. 5.17; Beukes and Smit, 1987). This conglomerate (Gutzmer and Beukes, 1998)

fines upwards into shale (Figure 5.17). Lateritic weathering profiles containing

hematite pisolites are well developed in the lower conglomerate, quartzite and shale

units of the Gamagara/Mapedi red beds (Gutzmer and Beukes, 1998). Iron

enrichment related to lateritic weathering constitutes part of the ore reserves at Sishen

iron-ore mine.

An iron pebble conglomerate eroding into the shales of the lower upwards fining

succession marks a transgressional surface (Fig. 5.17). The shales of the

Gamagara/Mapedi red bed succession deposited during the transgression are marked

by red to green and green to red cycles (Fig. 5.17; Holland and Beukes, 1990). The

shale coarsens up into a ferruginous quartzite (Fig. 5.17).

Gamagara/Mapedi red beds correlate well with the Dwaal Heuvel Formation. The

sedimentary and volcanic successions of the Mapedi Formation above the red beds

(Fig. 5.17), correlates excellently with sedimentary and volcanic successions above

the Dwaal Heuvel Formation in the Transvaal area (Fig. 2.4).

5.6 Metamorphism and Diagenesis

The Dwaal Heuvel mudstones and Strubenskop shales displays K2O/Na2O ratios

greater than 6, indicating that alkali metasomatism may have affected them (Tables

5.2A and B). K20 enrichment occurs only in the finer grained rocks. The presence of

replacive iron-rich chlorite (Appendix II) in the red beds suggests late metamorphic

reduction (Figures 5.10B and 5.11F). The Dwaal Heuvel Formation displays no

evidence for metamorphism higher than lowest greenschist facies.

5.7 Significance of Red Beds for Atmospheric Oxygen

Correlating of the Gamagara/Mapedi red beds and the Dwaal Huivel red beds suggest

an areal extent of red bed formation of more than 500 000km 2 (Fig. 5.18). The

occurrence of primary fluvial red beds containing mudcracks over such an extensive


area is conclusive evidence for a highly oxygenated atmosphere during the deposition

of the Dwaal Heuvel Formation.

23° 26° 29°

I I I

o

---------------- --M7

— 25°

— 28°

■ - - Lycle

Of 0

Zeerust Rustenbur: 0

i Pretori '• )1t>

— Jo annes ur .

t

• bur!

P mo km

1 Sishe 0•:.-.

0

Plostmasbur

7D 48

• Kimber ey Olifantshoek Group Minimum areal extent of Gamagara/

Z Dwaal Heuvel red beds

:73, p nri rip 1 and West area .ii

Pretoria and Postmasburg Groups PFiesic

g .

c .tc c• ' -Y Chuniespoort and Ghaap Groups

1

Figure 5.18 Minimum arial extent of the Gamagara/Mapedi and Dwaal Heuvel red beds.

History of Atmospheric Oxygen 118

CHAPTER 6

HISTORY OF ATMOSPHERIC OXYGEN

With regards to the history of atmospheric oxygen the laterites, red beds and

ironstones of the Pretoria Group must be seen in context of immediately adjacent rock

units and the stratigraphic evolution of the Transvaal succession as a whole. The

succession provides us with only a few terrestrial exposure surfaces (erosional

surfaces) from which some constraints of the redox state of the paleo-atmosphere can

be estimated. The loss of unknown amounts of stratigraphy below the erosional

surfaces further complicates deciphering of the history of oxygenation of the

Paleoproterozoic atmosphere. However, indirect evidence about the oxidation

potential of the atmosphere are also provided by certain chemical, siliciclastic and

volcanic units in the succession. From this, the history of atmospheric oxygen can be

summarized as follows:

Several stromatolitic carbonate units distributed throughout the Transvaal Supergroup

(Fig. 6.1) provide evidence for the primary production of oxygen through

photosynthesis, and therefore a potential increase in the oxidation potential of the

paleo-atmoshere. These carbonate units were deposited on a stable shelf in shallow,

warm and clear water as evident from their size, composition and structure (Button,

1973; Beukes, 1986; Fig 6.1). The oxygen that was produced by photosynthesis could

eventually have led to the precipitation of the Penge/Asbesheuwels-Koegas banded

iron-formations (Figures 6.1 and 6.2). Cold reduced deep ocean water with Fe 2+ in


solution welled up onto a shallow shelf and mixed with oxygenated water and iron-

oxides precipitated (Klein and Beukes, 1992). Precipitation of iron-oxides may

therefore have acted as efficient sinks to keep atmospheric oxygen at low levels

(Holland, 1984; Fig.6.2). However, the Rooinekke iron-formation at the top of the

Koegas Subgroup contain manganiferous beds (Visser, 1954; Van Wyk, 1980)

indicating that the oxidation potential, and therefore the 02 content of the atmosphere

increased (Figures 6.1 and 6.2). Oxygen (a non-greenhouse gas) increase and CO2 (a

greenhouse gas) burial (marked by the occurrence of carbonates with heavy 8 13C

signatures in the Duitschland Formation, Buick et al., et al., 1998; Swart, 1999) may

have led to climatic cooling and an ice age represented by diamictite in the

Duitschland Formation (Fig. 6.1). After this ice age, the ocean transgressed, and the

black shales of the Timeball Hill Formation were deposited. This transgression was

initiated by subsidence of the Kaapvaal craton.

The oldest known oolitic ironstone and pisolitic ferricrete developed in the Timeball

Hill Formation (Fig. 6.1) after deposition of a thick shale succession. These rocks

probably developed during a period of extensive lateritic weathering under highly

oxygenated atmospheric conditions (Fig. 6.2). Therefore, the paleo-atmosphere

probably contained significant amounts of oxygen before the precipitation of the

Hotazel banded iron-formation and giant manganese deposits (Gutzmer, 1996) (Fig.

6.1 and Fig. 6.2).

The Makganyne diamictite in the Griqualand West occurs in a stratigraphic position

below the Ongeluk lava and is correlated with the Rietfonteindam diamictite directly

above the Timeball Hill Formation's pisolitic ferricrete (Fig. 6.1). Paleomagnetic


evidence suggest a low latitude setting (11°±5°) for this glaciation (Evans et al.,

1997). The Makganyene/Rietfonteindam ice age may have developed, similar to the

Duitschland ice age, due to the "build up" of the non-greenhouse gas oxygen

(indicated by the ferricrete and oolitic ironstone of the Timeball Hill Formation) and

the loss of greenhouse gas through carbon burial (as indicated by the thick succession

of Timeball Hill black shale) (Figures 6.1 and 6.2). More and more sun energy was

reflected away from the surface of the earth by growing ice caps, the so called ice-

albedo effect, resulting in increasingly lowered temperatures, eventually leading to a

snowball earth (Fig. 6.1; Evans et al., 1997 and Beukes and Klein, 1992). The

glaciation probably terminated the existence of most of the photosynthesizing oxygen

producers, resulting in a period of stable oxidation potential and 02 content of the

atmosphere (Fig. 6.2; Hoffman et al., 1998 and Christie et al., 1999). The oceans

were separated from the atmosphere by a layer of ice and they became oxygen

depleted (Klein and Beukes, 1992). This cycle of ever decreasing temperature was

only broken by the emission of large amounts of greenhouse gasses by volcanoes

(Ongeluk/Hekpoort Formations) increasing the temperature (Fig. 6.1) but leading to a

decrease in the oxidation potential and oxygen concentrations of the atmosphere (Fig.

6.2). As the temperatures increased, ice caps melted, ocean circulation re-

commenced, 02 mixed into the shallow oceans that led to the precipitation of iron and

manganese oxides that form the Hotazel Formation (Tsikos et al., 1999; Kirschvink et

al., 1999; Fig. 6.1) resulting in a decrease of atmospheric oxidation potential (Fig.

6.1). Ice melt induced a bloom of oxygen producing photosynthesizers (Mooidraai

dolomite), increasing the oxidation potential and 02 concentration of the atmosphere

(Fig. 6.1). The transition of iron-formation to carbonate succession represent a sea

level fall (Fig. 6.1; Klein and Beukes, 1992).


The next clue in the stratigraphic column about the redox state of the paleo

atmosphere is situated above the Mooidraai dolomite, and is expressed as the oxidized

lateritic weathering profiles below the Gamagara/Mapedi-Dwaal Heuvel erosion

surface (chapter 4, Figures 6.1 and 6.2). Modern day lateritic weathering profiles

occur exclusively in tropical-subtropical regions (Fig. 6.1, Bardossy and Aleva,

1990). Atmospheric oxygen levels of at least 15% PAL (with pCO2 at 100x PAL)

have been estimated for the oxidation profiles in the Griqualand West area (Holland

and Beukes, 1990). The lateritic and oxidized profiles are directly overlain by the

Gamagara-Dwaal Heuvel red beds (Fig. 6.1), providing good evidence for oxygenated

atmospheric conditions (Holland and Beukes, 1990; Figures 6.1 and 6.2). Above the

red beds, thick successions of carbonaceous shale are developed, suggesting not only

extraction of CO2 from the atmosphere through burial of organic carbon, but also a

resulting increase in the oxidation potential of the atmosphere (Fig. 6.1). This system

of organic carbon burial may have terminated in the deposition of carbonates with

very positive 8 13C values at the of the Magaliesberg and Lucknow quartzites, possibly

indicating a third ice age during the deposition of the Transvaal Supergroup (Fig. 6.1).

During this study it became clear that the Hekpoort paleosol is a product of lateritic

weathering under oxidising atmospheric conditions. This resulted in the accumulation

of Fe3+ as hematite in its top section, a fact that was previously unknown. The

atmosphere must therefore have become highly oxygenated before 2.2Ga (Fig. 1.1).

The Cloud-Holland model of atmospheric oxygen (Cloud (1968) and Holland (1999)

thus needs at least to be corrected and the timing of oxygen rise in the atmosphere re-

assessed.

Waterberg red beds Terrestrial evidence

2.1 2 1.9

I I I I 1

2.6 2.5 2.4 2.3 2.2 Ga

• Marine evidence

Timeball Hill

Magaliesberg/Luclmow positive D"C carbonates Lateritic weathering

and red beds ferrierete

Ongeluk/Hekpoort lavas — ----- _ — — Soativephic hiann — —

Carbon-rich shales X--- Hotazel Mn and Mooidraai carbonates

Rooinekke tvtii « r . r Makganyene diamictite

Duitchland diarnictite and high D"C carbonates

••• Penge/Asbeshuiwels iron-formations

Oxi

datio

n bo

unda

ries o

f r-I

- and

Mn '

'

Mn


Figure 6.2 Qualitative model for the evolution of atmospheric oxygen based on the oxidation of iron

and manganese during the deposition of the Transvaal Supergroup

The first indirect evidence for a highly oxygenated atmosphere are the highly positive

813C values for carbonates in the Duitschland Formation. The first direct evidence for

a highly oxygenated atmosphere, are ferricrete, pisolitic mudclast conglomerate and

oolitic ironstone in the Timeball Hill Formation.

However, several lines of evidence, especially from shallow marine uraninite and

pyrite grains, suggests that the atmosphere was anoxic in the Archean (Holland,

1999). Therefore, the Dimroth-Ohmoto model for oxygen evolution (Ohmoto, 1997)

is also partially flawed (Fig. 1.1).


Evidence against existing models of atmospheric oxygen development thus

necessitates development of a revised model in which the suggestion is that

development of oxygen in the atmosphere took place in a cyclical or fluctuating

manner. However, it is still impossible to give real oxygen levels for any model.

Conclusive age determinations have to be awaited for further quantification. On a

whole, too little is known to establish an exact oxygen evolution model at this point in

time.

125

APPENDIX I

ANALYTICAL METHODS

I.1 Fieldwork

Fieldwork was conducted at various localities in the west, central and eastern outcrop

area of the Tranvaal Supergroup. Outcrops were mapped in detail. Because fresh

material is a necessity in this type of investigation, detailed studies were conducted on

fresh drill core material.

1.2 Sample Preparation

Preparation of sample material for chemical and X-ray powder diffraction analysis was

done in the sample preparation facility at the Department of Geology, RAU. Drill core

samples were cut by diamond saw and then washed and dried. The least weathered

pieces were sawed out of outcrop samples, washed and dried. The dried samples were

crushed in a Siebtechnik swing mill, using a Tungsten-Carbide steel set and rings. The

set and rings were cleaned after each sample by wiping with clean paper towel, washing

with water and acetone and then dried using compressed air. Glass was milled and the

whole washing and drying process repeated in between two samples in order to ensure

minimal contamination between samples. Sample powder was stored in clean glass viles.

1.3 Microscopy

Reflected and transmitted light microscopy work was performed at the Department of

Geology, RAU using Leitz Orthoplan and Leica DMLP research microscopes.

126

1.4 X-ray Powder Diffraction

Measurements were done at the Centralized Analytical Facility at RAU using a Phillips

PW 1710 diffractometer. Routine measurements were carried out with the following

diffractometer settings:

Tube anode material Cobalt

Generator tension 40kV

Generator current 30mA

Wavelength Kai 1.78896A

Wavelength Ka2 1.79258A

Intensity ratio IKaiilica2 2

Divergence diaphragm 1°

Detector diaphragm 0.1mm

Angle range 5-70°28

Step size 0.02°20

Scan rate 2s per step

Scan type Step

1.5 Scanning Electron Microscopy (SEM)

SEM studies were conducted on polished thin sections, first at the Council of

Geosciences, Pretoria, with a Cambridge Stereoscan 440 SEM and later at the Centralized

Analytical Facility at RAU with a JEOL JSM-5600 SEM.

1.6 Electron Microprobe Analysis

Microprobe analysis was performed on a Cameca CAMEBAX 335 electron microprobe

equipped with a Link EXL EDS detector at the Centralized Analytical Facility, RAU.

127

Routine spot analysis was performed at an anode tension of 15kV and a beam current of

15mA, with a time constant of 80s per spot analysis.

1.7 Chemical Analysis

Chemical analysis were performed on powered sample material by B&B Laboratories,

Johannesburg, using standard X-ray fluorecence methods. Fused beads were analysed for

major elements and pressed powder pellets for Na and trace elements. Fe 2+

concentrations were determined titrimetrically. REE analysis were conducted by Michael

Bau at the Geoforschuangszentrum in Potsdam, Germany using ICPMS (inductively

coupled plasma mass spectrometry). H. Ohmoto provided unpublished chemical analysis

for drill core Rhkl.

128

APPENDIX II

CHLORITE, SERICITE AND PYROPHYLLITE

COMPOSITION

II.1 Chlorite

Hekpoort Lava

Chlorite developed in the Hekpoort lava replaces plagioclase, amphibole and pyroxene.

The chlorite replacing pyroxene and amphibole (Fig. ILA) occurs in well defined crystals

shapes, in contrast to chlorite replacing replacing plagioclase (Fig. 11.1B). CaO

concentrations determined in chlorite vary significantly (between 1.7 and 8wt%).

Chlorite replacing plagioclase appears to have higher CaO concentrations than chlorite

replacing pyroxene and amphibole. However, CaO enrichment could also be a secondary

effect related to calcite veining in the lavas, as chlorite usually contain very little CaO

(Deer et al., 1965, 1976) (Fig. IL 1F). The FeO content of the lava chlorite varies between

23.9 and 27.6wt% (Table II.1). The chemical composition of chlorite from the Hekpoort

lava is similar to that of diabantite (Hawes, 1875; Bannister and Whittard, 1945).

11.1.2 Hekpoort Paleosol

11.1.2.1 Saprolite

Chlorite developed in the saprolite appears to replace sericite and pyrophyllite (Fig. TLC).

The chlorite contains on average 29.6wt% FeO (Table 11.1). The CaO content is less than

0.2wt% (Table It 1). The chlorite from the saprolite is enriched in FeO relative to those

from the lava. The chemical composition of the saprolite chlorite is similar to that of

ripidolite (Ross, 1935; Tilley, 1938).

129

11.1.2.2 Chloritic pallid zone

The chlorite of the chloritic pallid zone is similar in composition to daphnite (Orcel,

1927). It contains on average 22.7wt% Si02, 0.2wt% Na20, 22.8wt% A1203,

2.2wt%MgO, 42.4wt%FeO, 0.1wt% MnO and 0.1wt% K20 (Table III). There is a sharp

increase in FeO concentrations in chlorite from the green pallid zone compared to that

from saprolite.

11.1.2.3 Sericitic pallid zone

Chlorite appears to replace sericite in the sericitic zone at Waterval Onder (Fig. MD and

E). Because of extreme intergrowth of chlorite and sericite, Si02 and K20 contamination

are apparent (Table 11.1). However, the analyses could give some indication of the

chemistry of the chlorites.

Table Hi Quantitative scanning electron microprobe data for chlorite from the Hekpoort lava and

Hekpoort paleosol.

Na20 MgO A1203 SiO2 K20 TiO2 CaO MnO FeO P2Og Total Lava Bdl 16 13.9 29.5 bdl na 2.7 0.6 27.6 na 90.4 Rhkl Bdl 17.1 12.7 31.5 bdl na 3.1 bdl 25.1 na 89.6

Bdl 17.2 13.9 29.3 bdl na 1.7 bdl 26 na 88.1 Bdl 17.8 14.4 29.5 bdl na 1.8 bdl 26.6 na 90.1 Bdl 15.2 9.5 35.7 bdl na 6.4 bdl 24.9 na 91.7 Bdl 14.9 9.9 34.1 bdl na 8 bdl 23.9 na 91

Average 16.4 12.4 31.6 4.0 25.7 90.2

Saprolitic zone Str1/1 Bdl 14.5 21.8 25.6 bdl na 0.1 bdl 29 na 91

Bdl 14.5 22.2 25.5 bdl na 0.1 bdl 29.2 na 91.5 Bdl 13.8 22 23.9 bdl na bdl bdl 30.1 na 89.8 Bdl 14.6 22 25.5 bdl na bdl bdl 29.7 na 91.8 Bdl 14.7 21.9 25.1 bdl na 0.1 bdl 29.9 na 91.7

Average 14.4 22.0 25.1 29.6 91.2

Sericitic pallid zone Waterval Onder 0.3 9.8 1.1 45.1 0.4 na 13.5 bdl 29.8 na 99.6

0.3 5.5 28 30.4 2 na 0.2 bdl 40 na 106.3 Bdl 2.9 23.9 23.7 bdl na bdl bdl 46.8 na 97.7

Mottled zone Strl/6B Bdl 1.7 25 18 bdl 2.6 bdl bdl 45.9 na 93.1

Bdl 1.7 24.9 18 bdl 2.4 bdl bdl 46.3 na 93.3

130

Na20 MgO A1203 Si02 K20 TiO2 CaO MnO FeO P205 Total Bdl 2 24.7 18.8 0.2 1.6 bdl bdl 46.5 bdl 93.8 Bdl 2 26.3 18.2 bdl 0.5 bdl bdl 46.9 bdl 93.9 Bdl 1.7 26.1 18.5 bdl 1.2 bdl bdl 46.4 bdl 93.7

Strl/8 Bdl 1.5 27.7 19.6 bdl bdl bdl bdl 43.8 bdl 92.6 Bdl 1.8 28.5 20.8 0.3 bdl bdl bdl 43.1 bdl 94.4

Average 1.8 26.2 18.8 0.1 1.2 45.6 93.5

Ferricrete Str1/10 Bdl 3.1 23.3 23.3 bdl bdl bdl bdl 43.2 bdl 92.8

Bdl 3.4 23.9 23.2 bdl bdl bdl bdl 44.4 bdl 94.8 Bdl 3.2 24.2 23.2 bdl bdl bdl bdl 44.6 bdl 95.1 Bdl 3.3 24.7 23.4 bdl bdl bdl bdl 44.4 bdl 95.8

Bdl 3.3 24 23.3 bdl bdl bdl bdl 44.5 bdl 95.1

Average 3.3 24.0 23.3 44.2 94.7

FeO concentrations increase with decreasing MgO concentrations (Table 11.1). High Fe-

chlorites compare well to daphnite (Orcel, 1927) while low Fe-chlorites compare well to

ripidolite (Ross, 1935 and Tilley, 1938).

Table 11.2 Microprobe data for chlorite of the Hekpoort paleosol..

Green pallid zone Na20 Si02 A1203 MgO FeO MnO TiO2 K20 CaO Total

Str1/3 0.2 21.8 22.2 1.2 48.1 0.2 bdl 0.2 bdl 93.9 0.2 20.9 21.6 1.6 43.1 0.1 bdl 0.2 bdl 87.9 0.2 24.6 23.7 2.8 36.0 0.0 bdl bdl bdl 87.4 0.2 22.9 23.5 2.2 45.1 0.1 bdl bdl bdl 93.9 0.2 23.3 23.0 3.3 39.6 0.2 bdl bdl bdl 89.6

Average 0.2 22.7 22.8 2.2 42.4 0.1 0.1 90.5

Mottled zone 0.7 26.5 23.3 0.2 37.2 0.1 3.5 1.1 0.2 92.8 Strl/6 0.3 22.1 26.2 1.6 38.7 0.1 2.0 1.0 bdl 91.8

0.3 19.5 26.9 1.5 41.5 0.1 0.7 0.0 bdl 90.6 0.1 26.7 26.2 1.7 36.6 0.1 0.4 1.7 bdl 93.5

Average 0.4 23.7 25.6 1.2 38.5 0.1 1.7 1.0 92.2

Strl/8 20.3 27.4 1.7 40.0 0.1 0.1 bdl bdl 89.6 Ferricrete

Strl/9 0.5 17.1 17.5 1.0 56.0 0.2 1.6 0.6 0.1 94.8

11.1.2.4 Mottled zone

The presence of small amounts of K20 in the chlorite analyses of the mottled zone

indicate some sericite contamination. FeO concentrations of chlorite in the mottled zone

varies. Chlorite with average FeO concentration of 38.5wt% (Table 11.2) compare well to

131

ripidolite (Tsermak, 1891) while chlorite containing 45.6wt% FeO (Table 11.1), compare

well to daphnite (Orcel, 1927)

11.1.2.5 Ferricrete

Chlorite in the ferricrete contains on average 44.2wt% FeO, 3.3wt% MgO, 24wt%A1203

and 23.3wt% Si02 (Table 11.1). Chlorite analyses were sometimes contaminated with

Fe203 from hematite (Table 11.2). The chlorite appears to replace sericite (Fig. 4.5A).

The composition of chlorite from the ferricrete is similar to that of daphnite (Orcel,

1927).

Overall, FeO concentrations increase in chlorite upwards in the paleosol. Replacement of

sericite by chlorite indicates that the chlorite formed during a late metamorphic event.

This late chlorite may have totally replaced earlier chlorite in the saprolite.

11.2 Sericite Composition of the Hekpoort Paleosol

Sericite in the different zones of the Hekpoort paleosol display a relatively constant

chemical composition laterally and vertically (Tables 11.3 and 11.4). At Waterval Onder

sericite displays slightly higher K20 values than that of drill core Stratal (Tables 11.3 and

11.4).

TableII.3 Quantitative scanning electron microprobe data for sericite from the Hekpoort

paleosol.

Samples Na20 MgO A1203 Si02 CaO FeO K20 TiO2 Total

Sericitic pallid zone 0.7 0.2 34.7 42.5 bdl 1.7 10.3 bdl 90

0.4 0.2 34.1 43 0.1 0.8 9.9 bdl 88.5

0.5 0.2 34.9 43.1 0.1 1.5 9.8 bdl 90.1

0.6 0.1 34 41.8 bdl 1.1 10.2 bdl 87.8

0.5 0.2 34.5 43 0.2 1.6 9.9 bdl 89.9

0.6 0.3 32.7 43.8 0.6 1.8 8.6 bdl 88.4

Average 0.6 0.2 34.15 42.9 0.2 1.4 9.8 89.1

Na2O MgO A1203 SiO2 CaO FeO K2O TiO2 Total

Strl/6B 1.2 bdl 34.9 42.7 bdl 1.2 7.6 bdl 87.5

0.6 bdl 34.1 42.4 bdl 1.1 7.8 bdl 86

0.6 bdl 33.6 41.5 bdl 1.2 8.5 bdl 85.4

0.8 bdl 33.3 41.1 bdl 1 7.8 3.2 87.2

0.7 bdl 33.8 48.4 bdl 1.2 5.4 bdl 89.5

1.3 bdl 35 46.4 bdl 1.3 7.3 bdl 91.2

Average 0.9 34.1 43.8 1.2 7.4 87.8

Mottled zone

Str1/8 1 bdl 35.4 44.7 bdl 1.4 6.3 bdl 88.8

1.3 bdl 37.4 41.5 bdl 1.1 4.6 bdl 86

0.7 bdl 35.2 43.4 bdl 1 9.1 bdl 89.4

0.8 bdl 35.2 43.2 bdl 1.5 8.3 1 90

0.9 bdl 35.1 41.9 bdl 2.1 7.8 1.7 89.5

Average 0.9 35.7 42.9 1.4 7.2 0.5 88.7

Table 11.4 Microprobe data for sericite from the Hekpoort paleosol

Na2O SiO2 A1203 MgO FeO MnO TiO2 K2O Total Strl/3 0.5 46.2 33.9 0.3 4.8 bdl 2.2 6.8 94.7

Strl/6 1.0 45.0 38.2 0.5 1.0 bdl 0.1 5.2 90.9 1.1 42.0 28.6 0.2 17.6 bdl 1.2 4.0 94.7 0.9 44.3 37.9 0.1 0.9 bdl 1.3 5.8 91.1 0.8 43.0 39.5 0.1 1.5 bdl 0.1 5.0 90.0 0.8 44.1 39.4 0.1 1.2 bdl 1.0 3.7 90.3 0.6 45.4 40.0 0.1 1.2 bdl 0.7 3.0 90.8 1.1 47.5 37.3 bdl 0.9 bdl bdl 5.3 92.1 1.3 43.2 38.0 0.0 2.1 0.1 3.6 4.2 92.5 0.7 44.6 38.4 0.1 1.5 bdl 0.1 5.4 90.8 1.2 45.9 36.4 bdl 1.1 bdl bdl 7.8 92.5 0.5 44.6 38.7 0.8 2.2 0.1 bdl 3.1 89.9

Average 0.9 44.5 37.5 0.2 2.8 0.7 4.8 91.4

Strl/8 2.3 49.6 37.5 bdl 0.6 bdl bdl 5.5 95.6 0.8 47.3 36.6 bdl 0.8 bdl 0.9 7.5 93.8 0.8 49.6 37.9 bdl 0.7 bdl 0.1 3.9 93.1 1.7 46.0 39.6 0.1 0.7 0.2 0.1 4.0 92.4 1.8 47.2 36.8 0.0 0.7 0.1 0.1 6.4 93.1

Average 1.5 47.9 37.7 0.0 0.7 0.1 0.2 5.5 93.6

Strl/9 1.0 44.8 32.7 bdl 1.8 bdl bdl 6.6 86.9 0.9 47.7 35.4 bdl 1.0 bdl bdl 7.9 92.8 1.5 44.0 33.6 0.4 10.8 bdl 0.1 4.7 95.0

Average 1.1 45.5 33.9 0.1 4.5 6.4 91.6

132

133

Sericite from the Hekpoort paleosol displays similar chemical composition to that of

sericite and muscovite described by Deer et al. (1965).

11.3 Pyrophyllite of the Hekpoort paleosol.

The chemical composition of pyrophyllite from the Hekpoort paleosol (Table 11.5), is

similar to that of typical pyrophyllite described by Hendricks (1938) and Bosazza (1941).

Table 11.5 Microprobe data for pyrophyllite from the Hekpoort paleosol.

Na20 Si02 A1203 MgO FeO MnO TiO2 K20 CaO Total St 1/3 0.2

0.1 67.3 65.1

29.8 28.3

0.03 0.02

0.6 0.6

bdl 0.02

0.1 0.1

0.6 0.3

0.02 bdl

98.7 94.5

11.4 Chlorite in Pisolites of the Dwaal Heuvel Formation

Chlorite partly replacing pisolites in pisolites from the Dwaal Heuvel Formation are

similar in composition to daphnite described by Orcel (1927). Some of the chlorites are

extremely iron-rich (Tables 11.6 and 11.7).

Table 11.6 Quantitative SEM data for chlorite from pisolites in

the Dwaal Heuvel Formation, Strubenskop, Pretoria.

MgO A1203 Si02 FeO P203 Total Pisolites 2.9 22.9 21 50.8 bdl 97.6

2.5 22.4 23.1 44.7 0.4 93.2 Average 2.7 22.7 22.1 47.8 0.2 95.4

Table 11.7 Microprobe data for chlorite in the pisolites from the Dwaal Heuvel

Formation, Strubenskop , Pretoria.

Na20 Si02 A1203 MgO FeO MnO TiO2 K20 CaO Total 0.5 20.4 19.6 1.8 43.9 1.8 0.0 0.2 0.3 88.5 0.1 21.3 22.4 1.6 44.9 0.0 1.3 bdl 0.1 91.7 0.0 25.1 22.4 2.0 40.7 0.2 0.1 bd1 0.1 90.6 0.3 24.9 21.5 1.9 41.8 0.1 bdl 0.2 0.1 90.7 0.1 22.3 21.1 3.3 38.9 0.2 0.1 0.3 0.1 86.5 0.3 21.6 21.5 2.0 44.1 0.2 bdl 0.1 0.0 89.8 bdl 21.4 23.5 3.1 41.7 bdl bdl bdl 0.2 89.9 0.1 25.8 21.4 0.9 36.3 0.2 bdl 0.2 0.2 85.0 0.4 25.5 21.5 1.2 35.7 0.2 bdl 0.2 0.2 84.8 0.2 21.5 22.2 1.5 43.9 0.2 bdl 0.0 0.1 89.5

134

Na20 Si02 A1203 MgO FeO MnO TiO2 K20 CaO Total 0.2 21.4 22.6 2.6 39.9 bdl 0.1 0.1 0.1 87.0 0.0 28.2 19.8 1.3 37.3 0.2 0.1 0.1 0.1 87.0 0.3 22.8 21.6 1.5 41.8 0.1 bdl 0.1 0.0 88.2 0.3 28.8 24.1 1.1 32.9 0.2 bdl 0.2 0.2 87.9 0.2 13.2 12.1 0.7 51.2 0.3 bdl 0.2 0.3 78.2 0.0 25.8 23.7 3.5 34.9 0.1 bdl bdl 0.1 88.1 0.2 22.0 23.6 4.1 38.2 0.1 bdl 0.1 0.1 88.4 0.1 27.3 25.0 3.0 35.2 0.2 bdl 0.1 bdl 90.9 bdl 24.4 22.9 3.6 38.5 0.1 bdl 0.1 bdl 89.6 bdl 23.0 24.5 2.2 41.5 0.2 bdl 0.1 0.1 91.7

Average 0.2 24.6 23 2.3 42.3 0.2 0.1 0.1 0.1 92.8

Text to figure II.1 Secondary SEM-images of Hekpoort paleosol. A- Chlorite (on left) replacing

amphibole (on right) in the Hekpoort lava. Note the chlorite's relatively sharp crystal edges. (From drill

core Rhkl, 1145m). B - Chlorite (in centre) replacing plagioclase. (From drill core Rhkl, 1145m). C-

Chlorite (grey) replacing sericite and pyrophyllite (black) in the saprolite of the hekpoort paleosol. (From

drill core Stratal). D- Chloritization of sericite in the sericitic pallid zone, Waterval Onder. E- Chlorite

(white) replacing sericite and pyrophyllite in the sericitic zone of the Hekpoort paleosol. (From drill core

Stratal). F- BSE-SEM-image of chlorite and calcite in the saprolite.

Figure HI

136

REFERENCE LIST

Aleva, G.J.J. (1995). Laterites. Intern. Soil Ref. Ctr., Wageningen, 159 pp.

Anovitz, L.M., Perkins, D., and Essene, E.J. (1991). Metastability in near surface rocks

of minerals in the system A1203-Si02-H20. Clays Clay Min., 39, 225-233.

Armstrong, R.A. (1987). Geochronological studies on Archean and Proterozoic

Formations of the foreland of the Namaqua Front and possible correlates on the

Kaapvaal craton. Unpubl. PhD. thesis, University of the Witwatersrand, Johannesburg,

274 pp.

Bannister, F.A. and Whittard, W.F. (1945). A magnesian chamosite from the Wenlock

limestone of Wickwar, Gloucestershire. MM. Mag., 27, 99.

Bardossy, G. and Aleva G.J.J. (1990). Lateritic Bauxites. Elsevier, Amsterdam, 624 pp.

Barnicoat, A.C., Henderson, I.H.C., Knipe, R.J., Yardley, B.W.D., Napier, R.W., Fox,

N.P.C., Kenyon, A.K., Muntingh, D.J., Strydom, D., Winkler, K.S., Lawrence, S.R., and

Cornford, C. (1997). Hydrothermal gold mineralization in the Witwatersrand basin.

Nature, 386, 820-824.

Beukes. N.J. (1986) The Transvaal Sequence in Griqualand West. In Annheausser, C.R.

and Maske, S. (eds.), Mineral deposits of Southern Africa, I. Geol. Soc. S. Afr.,

Johannesburg, 1020 pp.

Beukes, N.J. and Smit, C.A. (1987). New evidence for thrust faulting in Griqualand

West, South Africa: Implications for stratigraphy and age of red beds. Trans. Geol. Soc.

S. Afr., 90, 378-394.

137

Bhattacharyya, D.P. (1989). Concentrated and lean oolites: example from the Nubia

Formation at Aswan, Egypt, and significance of the oolite types in ironstone genesis. In

Young, T.P. and Taylor, W.E.G. (eds) Phanerozoic ironstones, Geological Society

Special Publication, London, No 46, 251pp.

Bosazza, V.L. (1941). Further notes on the absorption of malachite green by some clay

minerals. Amer. MM., 26, p396.

Brand, U. and Veizer, J. (1983). Origin of coated grains: Trace element constraints. In

Peryt (ed.), Coated grains, Springer-Verlag, Berlin, 655pp.

Buick, I.S., Uken, R., Gibson, P.L. and Wallmach, T. (1998). High-8 13C

Paleoproterozoic carbonates from the Transvaal Supergroup, South Africa. Geology,

26(10), 875-878.

Button, A. (1973). A regional study of the stratigraphy and development of the

Transvaal Basin in the eastern and northeastern Transvaal. Unpubl. PhD. thesis,

University of the Witwatersrand, Johannesburg, 352 pp.

Button, A. (1975). A paleocurrent study of the Dwaal Heuvel Formation, Transvaal

Supergroup. Trans. Geol. Soc. S. Afr., 78, 173-183.

Button, A. (1979). Early Proterozoic weathering profile on the 2200 M.Y. old Hekpoort

Basalt, Pretoria Group, South Africa: Preliminary results. Inform. Circ. Econ. Geol. Res.

Unit, No. 133, University of the Witwatersrand, 133 pp.

Button, A. (1986). The Transvaal sub-basin of the Transvaal Sequence. In Annheausser,

C.R. and Maske, S. (eds.), Mineral deposits of Southern Africa, I. Geol. Soc. S. Afr.,

Johannesburg, 1020 pp.

138

Button, A. and Tyler, N. (1979). Precambrian palaeoweathering and erosion syrfaces in

South Africa: Review of their character and significance. Inform. Circ. Econ. Geol. Res.

Unit. No. 135, 1-37.

Christie, B.N., Sohl, L.E., Kennedy, M.J., Hoffman, P.F. and Schrag, D.P. (1999).

Considering a Neoproterozoic snowball Earth; discussion and reply. Science, 284, 5417.

Cloud, P. (1968). Atmosphere and hydrospheric evolution of the primitive earth.

Science, 160, 729-736.

Cloud, P. (1973). Paleoecological significance of the banded iron-formation. Economic

Geology, 68, 1135-1143.

Condie K.C. and Wronkiewicz, D.J (1990). The Cr/Th ratio in Precambrian pelites from

the Kaapvaal Craton as an index of craton evolution. Earth and Planetary Science

Letters, 97, 256-267.

Cornell, D.H., Schutte, S.S., and Eglington, B.L. (1996). The Ongeluk basaltic andesite

formation in Griqualand West, South Africa: submarine alteration in a 2222Ma

Proterozoic sea. Precambrian Research, 79, 101-123.

Dahanayake, K. (1983). Depositional environments of some Upper Jurassic Oncoids. In

Peryt, T.M. (ed). Coated Grains. Springer-Verlag, Berlin, 655pp.

Deer, W.A., Howie, R.A. and Zussman, J. (1965). Rock-forming minerals: Vol. 3, Sheet

silicates. Longmans, London, 270pp.

Deer, W.A., Howie, R.A. and Zussman, J. (1976). An introduction to the rock forming

minerals. Longman, London, 528pp.

139

Dimroth, E. and Kimberley, M.M. (1976). Precambrian atmospheric oxygen: evidence in

the sedimentary distributions of carbon, sulfur, uranium and iron. Canadian Journal of

Earth Sciences, 13(9), 1161-1185.

Evans, D.A., Beukes, N.J. and Kirschvink J.L. (1997). Low latitude glaciation in the

Palaeoproterozoic era. Nature, 386, 262-266.

Folk, R.L. and Chafetz, H.S. (1983). Pisoliths (Pisoids) in Quaternary Travertines of

Tivoli, Italy. In Peryt, T.M. (ed). Coated Grains. Springer-Verlag, Berlin, 655pp.

Garrels, R.M. and Christ, C.L. (1965). Solutions, minerals and equilibria. Harper and

Row, New York, 450pp.

Gutzmer, J. (1996). Genesis and alteration of the Kalahari and Postmasburg manganese

deposits, Griqualand West, South Africa. Unpubl. PhD. thesis, Rand Afrikaans

University, Johannesburg, 266pp.

Gutzmer, J. and Beukes, N.J. (1998). High grade manganese ores in the Kalahari

manganse field: Characterisation and dating of ore forming events. Unpubl. report,

Rands Afrikaans University, Johannesburg, 221pp.

Gutzmer, J. and N.J., Beukes. (1998). Earliest laterites and possible evidence for

terrestrial vegetation in the Early Proterozoic. Geology, 26(3), 263-266.

Harder, H. (1989). Mineral genesis in ironstones: a model based upon laboratory

experiments and petrographic observations. In Young, T.P. and Taylor, W.E.G. (eds),

Phanerozoic ironstones, Geological Society Special Publication No. 46, 251pp.

Hartzer, F.J. (1987). Die geologie van die Krokodilrivierfragment, Transvaal. Unpubl.

MSc. thesis, Rand Afrikaans University, Johannesburg, 201 pp.

140

Hawes, G.W. (1875). On diabantite, a chlorite occurring in the trap of the Connecticut

Valley. Amer. Journ. Sci., 9, 454.

Hendricks, S.B. (1938). On the crystal structure of talc and pyrophyllite. Zeit., 99, 264.

Hoffman, P.F., Kaufman, A.J., Halverson, G.P. and Schrag, D.P. (1998). A

Neoproterozoic snowball earth. Science, 281, 1342-1346.

Holland, H.D. (1978). The Chemistry of the Atmosphere and Oceans. John Wiley and

Sons, New York, 351pp.

Holland, H.D. (1984). The Chemical Evolution of the Atmosphere and Oceans. Princeton

Univ. Press, Princeton, 582 pp.

Holland, H.D. (1999). When did the Earths atmosphere become oxic? A reply. The

Geochemical News, 100, 20-22.

Holland, H.D. and Beukes, N.J. (1990). A palaeoweathering profile from Griqualand

West, South Africa: Evidence for a dramatic rise in atmosperic oxygen between 2.2 and

1.9 bybp. Am. J. Sci. 290-A, 1-34.

Holland, H.D. and Rye, R. (1997). Evidence in pre-2.2 Ga paleosols for the early

evolution of atmospheric oxygen and terrestrial biota. Comment: Geology, 25, 857-858.

Jahn, B. and Condie, K.C. (1995). Evolution of the Kaapvaal craton as viewed from

geochemical and Sm-Nd isotopic analyses of intracratonic pelites. Geochimica et

Cosmochimica Acta, 56(11), 2239-2258.

141

Karhu, J. and Holland, H.D. (1996). Carbon isotopes and the rise of atmospheric oxygen.

Geology, 24, 867-870.

Kearsley, A.T. (1989). Iron-rich ooids, their mineralogy and microfabrics: clues to their

origin and evolution. In Young, T.P. and Taylor, W.E.G. (eds), Phanerozoic ironstones,

Geological Society Special Publication No. 46, London, 251pp.

Kirschvink, J.L., Gaidos, E.J., Bertani, L.E., Beukes, N.J., Gutzmer, J., Maepa, L.N. and

Steinberger, R.E. (1999). Paleoproterozoic Snowball Earth: Extreme Climatic and

Geochemical Global Change and its Biological Consequenses. Proceedings of the

National Academy of Sciences of the United States of America, in press.

Klein, C. and Beukes, N.J. (1992). Time distribution, Stratigraphy, and Sedimentologic

Setting, and Geochemistry of Precambrian Iron-Formations. In Schopf, J.W. and Klein,

C. (eds), The Proterozoic biosphere. Cambridge Univ. Press, Cambridge, 1348pp.

Klop, A.A.C. (1978). The metamorphosed sediments of the Pretoria Group and the

associated rocks northwest of Zeerust, western Transvaal. Unpubl. MSc. thesis,

University of Pretoria, Pretoria.

Levin, H.L. (1990). Contemporary physical geology. Saunders College Publishing,

Philadelphia. 623pp.

MacFarlane, A.W. and Holland, H.D. (1991). The timing of alkali metasomatism in

paleosols. Canadian Mineralogist, 29, 1043-1050.

Martin, D.M., Clendenin, C.W., Krapez, B and McNaughton, N.J. (1998). Tectonic and

geochronological constraints on late Archean and Paleoproterozoic stratigraphic

con-lation within and between the Kaapvaal and Pilbara cratons. Journal of the

Geological Society of London, No 155, Part 2, Geological Society of London, London,

311-322.

142

Maynard, J.B. (1983). Geochemistry of sedimentary ore deposits, Springer-Verlag, New

York, 305 pp.

McFarlane, M.J. (1976). Laterite and Landscape. Academic Press, London, 151 pp.

Medvedeff, D.A. and Wilkinson, B.H. (1983). Cortical fabrics in calcite and aragonite

ooids. In Peryt, T.M. Coated Grains, Springer-Verlag, Berlin, 655pp.

Mucke, A., Badejoko, T.A. and Akande, S.O. (1999). Petrographic-microchemical

studies on the origin of the Agbaja Phanerozoic Ironstone Formation, Nupe basin,

Nigeria: a product of a ferruginized ooidal kaolin precursor not identical to Minette-type.

Mineralium Deposita, 34, 284-296.

Nesbitt, H.W. and Young, G.M. (1982). Early Proterozoic climates and plate motions

inferred from major elements of lutites. Nature, 299, 715-717.

Nesbitt, H.W. and Young, G.M. (1984). Prediction of some weathering trends of the

plutonic and volcanic rocks based on thermodynamic and kinetic considerations.

Geochimica Cosmochimica Acta, 48, 1523-1534.

Nickel, E. (1983). Environmental significance of freshwater oncoids, Eocene Guarga

Formation, Southern Pyrenees, Spain. In Peryt, T.M. (ed). Coated Grains, Springer-

Verlag, Berlin, 655pp.

Oberholzer, J.D. (1995). Die geologie van die piroklastiese gesteentes in die Hekpoort

Formasie, Transvaal Opeenvolging. Unpubl. MSc. thesis, University of Pretoria, 116 pp.

Ohmoto, H. (1993). Abst. With Programs GSA Annual Meeting, Boston, MA A89.

143

Ohmoto, H. (1996). Evidence in pre-2.2Ga paleosol for the early evolution of

atmospheric oxygen and terrestrial biota. Geology, 24, 1135-1138.

Ohmoto, H. (1997). When did the Earth's atmosphere became oxic. The Geochemical

News, 93, 12-27.

Orcel, J. (1927). Recherches sur la composition chimique des chlorites. Bull. Soc.

Franc. Min., 50, p75.

Peryt, T.M. (1983). Classification of coated grains. In Peryt, T.M. (ed). Coated Grains.

Springer-Verlag, Berlin, 3-6.

Peryt, T.M. (ed) (1983). Coated Grains. Springer-Verlag, Berlin, pp 655.

Rasmussen, B. and Buick, R. (1999). Redox state of the Archean atmosphere: Evidence

from detrital heavy minerals in ca. 3250-2750 Ma sandstones from the Pilbara Craton,

Australia. Geology, 27(2), 115-118.

Reichardt, F.J. (1994). The Molopo Farms Complex, Botswana: history, stratigraphy,

petrography, petrochemistry and Ni-Cu-PGE mineralization. Exploration Mining

Geology, 3, 264-284.

Retallack, G. (1986). Reappraisal of the 2200 Ma-old paleosol near Waterval Onder,

South Africa. Precambrian Research, 32, 195-232.

Retallack, G.J. (1990). Soils of the past. Unwin Hyman, Boston, 520 pp.

Richter, D.K. (1983). Calcerous ooids: A synopsis. In Peryt, T.M. (ed). Coated Grains.

Springer-Verlag, Berlin, 71-90

144

Romer, R.L. and Bau, M. (1998). 2.4Ga secondary lead age for the Mooidraai Dolomite:

Implications for the early evolution of the atmosphere. Chinese Science Bulletin, 43,

109.

Ross, C.S. (1935). Origin of the copper deposits of the Ducktown type in the southern

Appalachian region. U.S. Geol. Surv., Prof Paper, 179, p. 66.

Rye, R., Kuo, P.H., and Holland H.D. (1995). Atmospheric carbon dioxide

concentrations before 2.2 billion years ago. Nature, 378, 603-605.

Rye, R. and Holland, H.D. (1998). Paleosols and the evolution of atmospheric oxygen: A

critical review. American Journal of Science, 298, 621-672.

Schaefer, M. (1998). Stratiform Manganese Deposits in Mid Proterozoic Red Bed

Successions of South Africa. Unpubl. M.Sc thesis, TUFreiberg, Germany, Johannesburg.

Schellmann, W. (1982). Eine neue Laterit definition, Geologishes Jahrbuch, D. 58, 32-

47.

Schreiber, U.M. (1990). A paleoenvironmental study of the Pretoria Group in the

eastern Transvaal. Unpubl. PhD. thesis, University of Pretoria, 308 pp.

Schweigart, H. (1965). Iron ores of South Africa. Economic Geology, 60, 269pp.

Schweitzer, J. (1998). The Dullstroom Basalt Formation and the Rooiberg Group:

volcanic rocks associated with the Bushveld Complex. Unpubl. P.hD. thesis, University

of Pretoria, Pretoria.

South African Committee For Stratigraphy (SACS), (1980). Stratigraphy of South

Africa. Part 1 (Comp. L.E. Kent). Lithostratigraphy of the Republic of South Africa,

145

South West Africa/Namibia, and the Republics of Bophuthatswana, Transkei and Venda.

Handbook Geol. Surv. S. Afr., 8, 690 pp.

Swart, Q.D. (1999) Carbonate rocks of the Paleoproterozoic Pretoria and Postmasburg

Groups, Transvaal Supergroup. Unpubl MSc.thesis, Rand Afrikaans University.

Tankard, A.J., Jackson, M.P.A., Eriksson, K.A., Hobday, D.K., Hunter, D.R. and Minter,

W.E.L. (1982). Crustal evolution of southern Africa. Springer-Verlag, New York, 523

pp.

Thomas, M.F. (1994). Geomorphology in the tropics. A study of weathering and

denudation in low latitudes. Wiley, New York, pp19-113.

Tilley, C.E. (1938). The status of hornblende in low-grade metamorphic zones of green

schists. Geol. Mag., 75, 497.

Tschermak, G. (1891). Die Chloritgruppe. Sitzber. Akad. Wiss. Wien, 100, p29.

Tucker, E.M. (1991). Sedimentary petrology: an introduction to the origin of

sedimentary rocks. Blackwell Scientific Publications, Oxford, 260pp

Tsikos, H., Moore, J.M. and Harris, C. (1998). Stable isotope evidence of the origin,

diagenesis and alteration of the Palaeoproterozoic Hotazel Fe-Mn Formation, Kalahari

Manganese Field. South African Journal of Earth Sciences, 28.

Turner, P. (1980). Continental Red Beds. Elsevier, Amsterdam, 562 pp.

Van Niekerk, H.S. (1998). The Paleoproterozoic Olifantshoek-Kheis Orogen and its

Mineral Potential. Unpubl. progress report, Rand Afrikaans University, Johannesburg 1-

26.

146

Van Schalkwyk, J. and Beukes, N.J. (1986). The Sishen iron ore deposit, Griqualand

West. In: Annhaeusser, C.R. and Maske, S.S. (eds.). Mineral deposits of Southern

Africa. Geol. Soc. S. Afr., Johannesburg, 931-956.

Van Wyk, J.P. (1980). Die geologie van die gebied Rooinekke-Matsap-Wolhaarkop in

Noord-Kaapland met spesiale verwysing na die Koegas-Subgroep, Transvaal-

Supergroep. Unpubl. Msc. thesis, Rand Afrikaans University, 159 pp.

Visser, B.J.L. (1954). Deposits of manganese ore on Rooinekke and neighboring farms,

District Hay. Transactions of the Geological Society of South Africa, 57, 61-75.

Visser, J.N.J. (1969). 'n Sedimentologiese studie van die Serie Pretoria in Transvaal.

Unpubl. PhD. thesis, University of the Orange Free-State, 263 pp.

Wagner, P.A. (1928). The iron deposits of the Union of South Africa. Geol. Surv. S.

Afr., Pretoria, 120pp.

Walker, J.C.G. (1977) Evolution of the atmosphere. Macmillan, New York, 318pp.

Walraven, F. and Hattingh, E. (1993). Geochronology of the Nebo Granite, Bushveld

Complex. S. Afr. J. Geol., 96, 31-41.

Walraven, F., Armstrong, R.A. and Kruger, F.J. (1990). A chronological framework for

the middle to late Precambrian stratigraphy of the Transvaal, South Africa:

Tectonophysics, 171, 23-48.

Walther, H.W. and Zitzman, A. (eds) (1977). The iron deposits of Europe and adjacent

areas. Bundsant. Geowiss. Rohstoffe. Hanover, Federal Republic of Germany.

147

Wiggering, H. and Beukes, N.J. (1990). Petrography and geochemistry of a 2100-2200

m.y. old hematitic paleoalteration profile on the Ongeluk basalt of the Transvaal

Supergroup, Griqualand West, South Africa. Precambrian Research, 46, 241-258.

Young, G.M. and Nesbitt, H.W. (1998). Processes controlling the distribution of Ti and

Al in weathering profiles, siliciclastic sediments and sedimentary rocks. Journal of

Sedimentary Research, 68, 448-455.

Young, T.P. (1989). Phanerozoic ironstones: an introduction and review. In Young,

T.P. and Taylor, W.E.G. (eds). Phanerozoic ironstones. Geological Society Special

Publication, No. 46, London, pp ix-xxv.

Young, T.P. and Taylor, W.E.G. (1989). Phanerozoic ironstones. Geological Society

Special Publication No. 46: London, 251pp.

paleoproterozoic laterites, red beds and ironstones of ... - core

Documents