The abundances and distribution of some trace elements in the selected South African Shales

229 Pages·2016·3.73 MB·English

THE ABUNTIANCES A.lifI) TIISTRIBUTION OF SOME
TRACE ELEME~TS IN SOME SELECTED
SOUTH AFRICAN SHALES

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P.K. HOFMEYR
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Thesis submitted in f'ulrilment
or the requirements ror the
degree or Doctor Philosophy
in the Faculty or Science,
-~--JJnl i.v ersity or Cape Tmm •
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or Geochemistry,
or Cape Town.
October, 1971.
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The copyright of this thesis vests in the author. No
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quotation from it or information derived from it is to be

published without full acknowledgeement of the source.
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The thesis is to be used for private study or non-
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commercial research purposes only.

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Published by the Universit y of Cape Town (UCT) in terms
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of the non-exclusive license granted to UCT by the author.
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(i)
1 SUMMARY
The concentrations of fifteen trace
elements Be, Ga, Cu, Zn, Co, Ni, V, Cr, Sc, Y, Yb, Zr, No,
Th and Pb, have been determined in. 315 Southern African
argillaceous rocks which range in age from the early
Precambrian Fig Tree Series to the Triassic Beaufort Series
of the Karroo System. Less than two micron clay
fractions have been separated from thirty-three of these
sediments, and they have been analysed for the same trace
elements. The determinations have been carried out by
X-ray fluorescence and emission spectrographic analysis.
In addition, forty-four selected samples have been
analysed for U and Th by gamma-ray spectroscopy. The
results obtained for several international rock standards,
which were analysed simultaneously,.emphasize the high
quality of the data of this work.
A literature survey pertaining to
the formation of clay minerals has been carried out and
from this it is·concluded that the general consensus of
.opinion is that clay minerals are principally formed in
the weathering environment and not in the environment o·f
deposition. Consequently, the bulk of the trace element
content of clay minerals is incorporated in the weathering
environment and trace element data is most useful in pro
viding information concerning the composition of the source
rocks and possibly the prevailing climatic conditions
during weathering. This is shown particularly for the
Fig Tree Series sediments which are considered to have been
d·erived from a source area containing ultrabasic rocks,
basic rocks and granites in the proportion 2:3:2 respectively.
Chemical weathering is thought to have occurred in a
reducing, oxygen-less, primordial atmosphere.
(ii)
The trace element data and inter
element ratios are used to delineate the amount of varia
tion in the trace element content of the sedimentary
sequences studied. Correlation coefficients are used to
determine groups of associated elements and an attempt is
made, where possible, to correlate the trace element
content with mineralogy. Carbonaceous shales f'rom the
Northern Ecca Facies of the Karroo System have been studied
in most detail. It is shown that, in general, their
trace element content is very variable but several elements
show an association with the organic content. The deve
lopment of authigenic iron minerals has not usually
played a controlling role in the distribution of trace
elements in these carbonaceous shales. Trace element con
centrations were not found to vary systematically with
height in the succession, but decreases in concentrations
were noted along a NW-SE trend lirie f'rom near the edge of
the Northern Ecca basin of sedimentation to the centre of
the Natal Trough. Possible reasons for this trend are
given. The trace element abundances of the shales dredged
f'rom the Agulhas Bank are similar to those of the Bokkeveld
Series and Malmesbury Formation shales and they cannot be
used to positively adentify to which stratigraphic sequence
these shales belong.
The abundances and distribution of the
trace elements in the sediments of this work have been dis
cussed individually. A strong coherence has been observed
between Ga and Al and a mineralogical control Of ·the Ga/Al
ratio has been noted. Chlorite-rich shales tend to have
the lowest and kaolinite-rich shales the highest Ga/Al ~atios.
V and Cr are often correlated with Ti and Al in the non-car-
(iii)
bonaceous shales, and with Ti and the organic content in
the carbonaceous shales. The development of a V and Cr
bearing authigenic Ti mineral, possibly anatase, is
postulated. This phase is associated with the clay
fraction in non-carbonaceous shales and is adsorbed on
the coaly plant material of carbonaceous shales. The con
centrations of U and Th in most shales are found to be
significantly higher than those for similar shales from
other parts of the world and the possibility of Southern
Africa being a high Th and U province is suggested.
Trace element variations with
geological age have been studied in some detail. It was
found that Ni and Cr decreased and Zr increased slightly
with decrease in the age of the sediment. Yb and Be
increased until.Malmesbury times and then remained
relatively constant. Any systematic variation in the
concentrations of the remaining elements was masked by·
the normal spread of concentrations.
The possibility of using trace elements
as indicators of environment of deposition is discussed
comprehensively. · The trace element concentrations were
examined in shales of undoubted marine origin, the Bokke
veld Series, and the undoubted fresh-water origin, the
Witteberg Series and parts of the Ecca Series. These
shales were also selected because they all contain
dominant illite. It was found that V and Cr were enriched
in marine and Cu in fresh-water shales. Two axis and
triangular plots were constructed of these element/Al
against each other and it is shown that a good separation
(iv)
into marine and f'resh-water 11fields II is obtained. It
is suggested that these elements are potentially usef'ul
in paleo-environmental studies but that fUrther research
should be directed towards gaining· more information on
the actual sorption behaviour of these elements in the
sedimentary environment.
A few suggestions for fUrther research
on South African shales are given.
CONTENTS
VOLUME I
PAGE
1. SUMMARY (i)
2. INTRODUCTION 1
3. ANALYTICAL PROCEDURES 4
3.1 Sample Preparation 4
3.2 Clay Mineral Separation 6
3.3 X-ray Diffraction 10
3.4 Emission Spectrography 12
3.4.1 Introduction 12
3.4.2 Method of Analysis 14
3.4.3 Quality of IBta 17
3.5 X-ray Fluorescence Spectrometry 23
3.5.1 Introduction 23
3.5.2 Zinc, Copper and Ntckel 25
3.5.3 Zirconium, Niobium, Yttrium,
Thorium and Lead 30
3.5.4 Chromium 34
3.5.5 Gallium 35
3.6 Gamma Spectroscopy - Thorium,
Uranium 36
4. NOTES ON THE ORIGIN AND DISTRIBUTION
OF TRACE ELEMENTS IN SHALES 37
5. THE TRACE ELEMENT GEOCHEMISTRY OF
SOUTH AFRICAN SHALES 5 0
. 5.1 Introduction 50
5.2 The Swaziland System 55
5.3 The Kheis System 67
5.4 The Witwatersrand System 68
5 •. 5 The Sinclair Group 70
5.6 The Damara System 72
5.7 The Malmesbury Formation 73
5.8 The Cango Formation 75
CONTENTS (cont.)
PAGE
~.9 ·The Nama System 76
5.9.1 Kuibis Series 76
5.9.2 Schwarzkalk Series 78
5.9.3 Schwarzrand Series 78
5.9.4 Fish River Series 79
~.10 The Cape System 79
5.10.1 Bokkeveld Series 79
5.10.2 Witteberg Series 85
_5;11 The Karroo System 88
5.11.1 Dwyka Series 88
5.11.2 Northern Ecca Facies 91
s.11.2.1 Tntroduction 91
5.11.2,2 Borehole Bh 134 and
A Series Boreholes,
Eastern Transvaal 93
5.11.2.3 GB Series Boreholes,
Northern Transvaal 100
5.11,2.4 Darmhauser Boreholes,
Central Natal 103
5.11.2.5 Somkele Borehole,
Northern Zululand 104
5,11.2.6 Boreholes from the Vier
fontein and Bothaville
Districts, Western
Transvaal 108
5.11.2.7 Lower Ecca Shales 109
5.11.3 Central Ecca Facies 110
5.11.4 Southern Ecca Facies 111
5.11.5 Western Ecca Facies 112
5.11.6 Beaufort Series 114
5.12 Shales dredged from the Agulhas Bank 115
6. SOME OBSERVATIONS ON THE DISTRIBUTION OF
TRACE ELEMJ<~NTS IN SOUTH AFRICAN SHALES 118
6.1 Introduction 118
6.2 Beryllium 118
6.3 Gallium 123
6.4 Copper, Zinc, Lead 128
6.5 Cobalt, Nickel 137
6.6 Vanadium, Chromium 144
6.7 Uranium, Thorium 150
CONTENTS (cont. )
PAGE
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6.8 Zirconium 160
6.9 Niobium 164
6.10 Scandium, Yttrium, Ytterbium 168
7. SOME OBSERVATIONS ON THE VARIATION
OF TRACE ELEMENT CONTENT WITH
GEOLOGICAL AGE 177
8. THE USE OF TR.ACE :F~LEMENTS AS
INDICATORS OF ENVIRONMENT OF
DEPOSITION 183
9. SUGGESTIONS FOR FUTURE RESEARCH ON
SOUTH AFRICAN SHALES 196
10. ACKNOWLEDGEMENTS 198
11. REFERENCES 200
VOLUME II
APPENDIX I S.AM:PLE DESCRIPTIONS AND
LOCALITIES
APPENDIX 2 ELEMENT CONCENTRATIONS AND
SOME INTER-ELEMENT RATIOS
APPENDIX 3 TABLES AS REFERRED TO IN
VOLUME I
APPENDIX 4 FIGURES AS REFERRED TO IN
VOLUME I
APPENDIX 5 COMPUTER PROGRAM FOR ZINC,
COPPER AND NIC~L DETERMI
NATIONS
.l. •
2. INTRODUCTION
A glance at a geological map of southern
Africa will immediately reveal that much of it is cover
ed by sedimentary rocks. These vary from the schistose
and dolomitic sequences of the Damara System in the north
west of the area and the sandstone thicknesses of the
Cape System in the south and south-east of the area to the
tremendously thick and extensive shale horizons of the
Karroo System in the central part of the area and the con
glomerates and dolomites of the Transvaal System in the
north of the area. Now in general, the extent of
geological knowledge of any area is usually dependent upon
its economic significance, and the detailed knowledge
of any formation is normally related to its economic
mineralisation. Consequently the various members of
the South African stratigraphic column have not been
equally studied geologically. Although structural and
mineralogical studies of coarse-grained sedimentary
rocks and structural studies of fine-grained sedimentary
rocks have been undertaken, virtually no mineralogical
and geochemical data have been accumulated for fine
grained sedimentary rocks in this country.
Consequently this work was undertaken with
the object of collecting information on the mineralogy
and major and trace element·geochemistry of shales
occurring in southern Africa.· Particular attention is
devoted to the Karroo System, as it blankets most of the
Republic of South Africa, and within it the Ecca Series,
as this series contains the bulk of the shales found in
southern Africa: • However, most of the more important
shale-containing representatives of the South African
stratigraphic column are included in this study.
The sequences analysed range in age from