Precious nuggets

How can we achieve the cleanest possible ecological transition? Thanks to geologists. Three researchers from the Géosciences Montpellier laboratory* are working to gain a better understanding of the processes involved in the formation of rare metals used in so-called "green" technologies, in order to relocate and facilitate their exploitation.

Lithium, tungsten, germanium, gallium, indium... These metals all have one thing in common: they are indispensable to new technologies. Tungsten? Precious in certain aeronautical alloys. Lithium? Essential to automotive and computer technologies. As for the rare metals gallium, germanium and indium, they are necessary for the manufacture of solar panels, making them essential ingredients in the development of renewable energies.

Bénédicte Cenki-Tok explains: "To reduce our carbon emissions, renewable energies are the preferred choice, and are sometimes considered to be clean energies. But the only clean energy is the one we don't use," adds the geologist. And with good reason: the supply chain for these famous rare metals used in green technologies can hardly be described as "clean".

Clean energy

At present, the global rare metals market is dominated by China," explains Alexandre Cugerone, who wrote his thesis on the subject. Europe is almost totally dependent on Asia, the Americas and Africa. The import of metal resources for our 21st century technologies, some with a strong 'green' or 'renewable' connotation , from faraway countries with lax or non-existent environmental rules on exploitation, is particularly paradoxical", deplores the researcher.

This is where geologists come in. " These metals are essential for developing these technologies, so what we need to do is try to exploit them in a thoughtful and intelligent way," explains Émilien Oliot. And to make the most of them, we first need to understand how they are formed and concentrated in nature. " We've long known about the processes that give rise to metamorphic rocks and mountain ranges, and the temperature and pressure conditions inherent in their formation. This knowledge, which may seem outdated, is in fact precisely what enables us to understand the processes involved in the formation and concentration of critical metals used in modern technologies", adds Bénédicte Cenki-Tok.

Mining heaps

The geologists' aim: to inspire new methods of exploration and eco-responsible recovery of certain waste products from past mining operations. For these precious metals are not only to be found at the end of the world, but sometimes right under our noses, or almost. " For some rare metals, we know which minerals are the basic carriers, so we know where to look for them," explains Emilien Oliot. Where to find them? In mining spoil heaps, for example, artificial hills built by accumulating the by-products of mining operations.

These critical metals are found either in minute quantities, disseminated in base ores such as zinc and copper, or sometimes in hyper-concentrated minerals, smaller than a tenth of a millimeter. To better understand the significance of these heaps, the researchers use a culinary analogy. "Consider a single cake, with chocolate powder evenly distributed in the cake batter, and chocolate chips. In which form is the chocolate easiest for gourmands to retrieve once the cake has been made? Chips, of course. The principle is the same in our study: it's easier to extract critical metals concentrated in small minerals - our chocolate chips - rather than scattered in the base mineral - the chocolate powder in the cake mass."

Chocolate chips

Researchers have shown, for example, that the deformation of zinc sulfide ore, contemporaneous with the formation of mountain ranges, favors the re-concentration of germanium in hyper-concentrated minerals, "our famous chocolate nuggets, found in the heart of the Pyrenees". For geologists, this makes it all the more interesting to look for mining sites where deformation by natural geological processes has acted as a "natural concentrator" of rare metals.

"Many mining sites were once exploited solely for their base metals, and the spoil heaps from this past exploitation could be valorized, particularly in the Pyrenees, the Massif Central, but also in the Alps, or in the Scandinavian mountains of northern Europe. They could be potential rare metal resources," emphasize the geologists.

Redistributing resources

With another major advantage: these metals could be easier to extract. "When a rare metal, such as germanium, is disseminated in the ore, extraction is complex and requires cumbersome processes. On the other hand, if these rare metals are concentrated - like chocolate chips - in small minerals, their separation could be simplified", explains Alexandre Cugerone.

A promising way out of European countries' almost total dependence on rare metals. The social and environmental impacts of this kind of mining are worrying, and we can't simply relocate the pollution generated by it," warn the geologists. Above all, we need to work inclusively with producer countries and integrate all stakeholders, in order to better redistribute resources." Let everyone have their share... of the cake.

* Geosciences Montpellier (UM - CNRS - University of the West Indies)
Redistribution of germanium during dynamic recrystallization of sphalerite. Alexandre Cugerone, Bénédicte Cenki-Tok, Emilien Oliot, Manuel Muñoz, Fabrice Barou, Vincent Motto-Ros, Elisabeth Le Goff, Geology (2020) 48 (3): 236-241