388 - Revised Earth history favours the search for African gold
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 team ( 4), 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.
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 lithosphere( 1) 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 tectonics ( 2). In other words, similar to the system we know today: “cold” subduction zones ( 3) 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 greenstones( 5) stretching over several hundred kilometres, from eastern Senegal to western Niger. The sedimentary and volcanic rocks contained in the belts underwent metamorphic transformations( 6) during the time of onset of the new tectonic regime. The 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 determining 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 one-quarter 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.
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 .
(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) This 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.
Rédaction DIC - Gaëlle Courcoux
Translation - Nicholes Flay
