Stage 15 | Rodez – Carcassonne / Hard rock with element Lithium

Today’s stage is mostly flat, but the riders are crossing a region that was once a big mountain belt. The peloton will start in Rodez that is half built on crystalline rocks (mainly granite, old magma chambers, the ones that exist below explosive volcanoes). The other half is built on sedimentary rocks that were deposited in the Permian (299-252 million years ago) on top of the old Massif Central. From there, the Tour will find its way through the sediments of the Aquitaine and Carcassonne Basins that were deposited in shallow seas and rivers during the last 200 million years. But between these two basins, the riders first have to cross a narrow zone known as “la Montagne Noire”, where we see the southernmost edge of the Massif Central. And this “Black Mountain'' hides a resource that is in high demand these days.

Recharge your batteries in the Black Mountain

The Massif Central, and the Black Mountain, corresponds to a large area of rocks that were once so hot that they partly started to melt between 320 and 300 million years ago, during the ‘Variscan’ mountain building stage that formed the supercontinent Pangea. The Black Mountain rocks were then buried to a depth to ~20 km where they suffered temperatures of ~700°C, and under those conditions, certain minerals in the rock started to melt. Out of such partially molten rocks (that we call migmatite, see stage 17) comes granitic magma that is full of the elements that like it best to be molten (so-called ‘incompatible elements’ ). When that magma cools and solidifies they often form rocks with enormous crystals (sometimes tens of cm across) that we call pegmatites. And those pegmatites, which are present in Montagne Noire, are enriched in incompatible elements that can form ores.

Hard rock Lithium

Some pegmatites in the Montagne Noire contain mineral deposits that are rich in elements that we need for economic purposes that they are ores. That is the case with the Brassac pegmatite. It contains quartz, feldspar, muscovite, and tourmaline that are common minerals of pegmatite. More special is that this pegmatite also contains lepidolite and elbaite. These two last minerals do not only have uncommon names that most of us have never heard of before, they host a chemical element that is in high demand nowadays: Lithium. When a pegmatite is rich in Lithium, it is often also rich in other chemical elements such as Niobium, Tantalum, Cesium, or Tin. These elements belong to the list of critical substances defined by the European Union commission. Also other granites and pegmatites in the Massif Central are Lithium ores, such as the Beauvoir Granite and the Monts d’Ambazac pegmatites.

Lithium and the green technologies

Findings of ore deposits often give mixed feelings. On the one hand, Lithium is critical for industry, where it is used to strengthen ceramics or glasses, but it is also an important constituent of batteries. The green energy revolution has thus increased the demand for lithium, and many other elements that are needed in electric devices, solar panels, or windmills. On the other hand, winning these minerals requires mining, which is straining local environments. The debate between the cost and demand will likely only rise in the future. The riders will only climb Montagne Noir today, but perhaps they can recharge their batteries on the top for the final sprint to Carcassonne!

I am a metamorphic petrologist and a field geologist. I have training in numerical modeling of geodynamics processes. I love to be outside hammering rocks before preparing them for EPMA analysis. For a few years I have now specialized in the behavior of metal elements during the melting of rocks. Check the Geo-TdF-team-2022.

Alexis Plunder