fique scienti Actualité Revised Earth history

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Actualité scientifique
N° 388
November 2011
Potential treasures lie
concealed under the West
African savannah in some
of the Earth’s poorest
countries. Extensive gold
fields occur, over hundreds
of kilometres, from
Senegal to Niger. The
rapid rise in precious
metal prices over the past
five years has prompted
hugely intensified mineral
exploration. Yet new veins
must be found if this
African gold rush is to
continue.
A new discovery,
published in Nature
Geoscience, is now
shaking up our
understanding of the Earth
and the prospects for
exploration. The research
team4, led by an IRD
geologist, used innovatory
modelling software to take
a fresh look at the origins
of plate tectonics in the
light of the geothermal
history of the gold
deposits in the West
African gold fields. These
investigations will lead to
better ways of locating the
emplacement of veins and
their depth. This result is a
fundamental one for the
science which furthermore
offers promising
applications for West
African countries.
Actualidad cientifica
Revised Earth history
favours the search for African gold
© IRD / F. Colin
Scientific news
Gold extraction is the primary mining industry in West Africa (here in Essakane gold mining camp in Burkina Faso).
The Earth’s history is 4.5 billion years long and is
marked by several great episodes of intense
magmatic activity. Such geological upheavals are
associated with the formation of immense granite
provinces and metalliferous deposits –including
gold-bearing ones– like those observed in West
Africa. Such crises arrived in succession and
greatly modified the composition and structure of
the lithosphere1 and also its resistance to tectonic
stresses. A fundamental change in our planet’s
working systems and processes then came about
with the onset of modern-style plate tectonics2. In
other words, similar to the system we know today:
“cold” subduction zones3 develop at the boundaries between oceanic plates and continental collision zones give rise to mountain chains.
A fresh look at Earth’s history
Scientists considered up to now that the onset of
this modern-style plate tectonics occurred just
900 millions years Before Present (B.P.). However,
a recent international study 4 , led by Jérôme
Ganne, geologist at the IRD and published in the
journal Nature Geoscience, has fundamentally
upturned Earth history as we knew it. Ancient
rocks dating back more than 2 billion years, to
the Palaeoproterozoic era, were discovered in
West Africa that were produced by “cold” subduction. This find pushes back the chronological
reference point more than one billion years. A
real upheaval for our knowledge and understanding of plate tectonics.
Greenstones
the key to transition history
For these investigations, the research team
collected samples in the belts of greenstones5
stretching over several hundred kilometres, from
eastern Senegal to western Niger. The sedimentary and volcanic rocks contained in the belts
underwent metamorphic transformations6 during
the time of onset of the new tectonic regime. The
For more information
team pinpointed the temperature and pressure
conditions which prevailed at the time of the metamorphic rock formation. They focused on foliated
minerals, micas and chlorites –often associated
with the presence of gold– which crystallize at low
temperature and are difficult to analyse.
Innovatory method
The geologists applied a technologically unique
method, based on a novel metamorphic model
software. They ran metamorphic models established for their samples and found evidence that
these minerals were formed at high pressure,
along a “cold” geothermal gradient representing
less than 15°C per kilometre. That is a feature
characteristic of modern-style subduction zones.
The exact age of these micas and chlorites is now
known. They are in fact over 2 billion years old. The
geodynamic systems of the present already
existed at the time, the Palaeoproterozoic era.
The African gold rush continues
These investigations considerably improve
understanding of the geothermal and mechanical
evolution of the lithosphere during the course of
the Earth’s history. They also provide information
crucial for guiding future mineral exploration in
West Africa’s mineral fields, particularly for deter-
mining the depth of gold veins that remain
unworked. The prospects for the continent are
immense, where underground half the world’s
gold reserves are harboured.
Precious metal prices have been rising steeply
since 2007, and gold mining companies’ prospecting efforts have multiplied. More than 30 countries
currently produce gold, mainly in West and Central
Africa. A total of over 600 tonnes of gold are
extracted each year, representing around onequar ter of annual world production. Among
producers South Africa is leader. Ghana follows,
after all historically known as the Gold Coast, with
more than 75 tonnes per year, then Mali with an
average of 50, Guinea producing 10 to 20 tonnes,
followed by Senegal, Burkina Faso, Niger and the
Ivory Coast.
Contacts
Jérôme Ganne,
researcher at the IRD
Tel: +221 77 802 29 22
[email protected]
UMR 234 Géosciences environnement
Toulouse - GET (IRD / Université Paul
Sabatier - Toulouse 3/ CNRS)
Address
Observatoire Midi Pyrénées
Université Toulouse
14 Av. E. Belin
31400 Toulouse
References
Ganne Jérôme, De Andrade V.,
Weinberg R., Dubacq B., Vidal O.,
Kagambega N., Naba S., Baratoux Lenka,
Jessell M., Allibon J. Modern-style
plate subduction preserved in the
Palaeoproterozoic West African Craton.
Nature Geoscience, 2011.
DOI: 10.1038/NGEO1321.
Given that the world’s largest known gold deposits
–Abitibi in Canada and Yilgarn in Australia– lie in
ancient lands dating back 2.5 billion years, the
research team’s discovery, a major one both for the
fundamental science and its applications, opens
up vast prospects for the West African gold rush .
Key words
Plate tectonics, gold, mines, West Africa
Copy editor – Gaëlle Courcoux - DIC, IRD
Translation – Nicholas FLAY
1. Literally “stone sphere”, the lithosphere is the Earth’s rigid outermost shell, made up of the crust and the surface part of the
mantle.
2. A theory elaborated in 1968, the dominant one now for 40 years.
3. Subduction is the process whereby one tectonic plate slides under another.
4. T his work was conducted jointly with research scientists from the University of Ouagadougou in Burkina Faso, from
Brookhaven National Laboratory in New York, Monash University in Melbourne, Australia, University of Toulouse III, University
Joseph Fourier in Grenoble, the University of Cambridge in the United Kingdom and the University of Lausanne in Switzerland.
5. All the rocks with green as dominant colour and of diverse origins.
6. In other words, mineralogical, and sometimes chemical, changes in the rock induced by the effect of very high pressures and
and/or temperatures
Coordination
Gaëlle Courcoux
Information and Culture
Department
Tel: +33 (0)4 91 99 94 90
Fax: +33 (0)4 91 99 92 28
[email protected]
Frome Senegal to Niger, immense gold fields stretch under the vast arid lands. The. Research team’s discovery (centre) will bring improvements to mineral
exploration (right, in Essakan mining camp in Burkina Faso).
© IRD / F. Colin
© IRD / J. Ganne
© IRD / L. Baratoux
Indigo, IRD Photo Library
Daina Rechner
Tel: +33 (0)4 91 99 94 81
[email protected]
You can find IRD photographs on this topic,
copyright-free for media reproduction at
www.indigo.ird.fr
44 boulevard de Dunkerque,
CS 90009
13572 Marseille Cedex 02
France
© IRD/DIC, november 2011 Design and graphics: L. CORSINI
Media Contact
Cristelle DUOS
Tel: +33 (0)4 91 99 94 87
[email protected]
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