Geological map of the Rhine Graben.
© BRGM - B. Sanjuan
To support the energy transition, demand for lithium for use in electric vehicle batteries is increasing very sharply and is expected to accelerate further. Rubidium is another strategic metal, although less well known. Part of a niche market, it is used in advanced technologies such as optics (for example in certain photovoltaic cells), electronics and atomic clocks.
How can Europe meet this growing demand for strategic metals, while reducing its dependence on China? Certain stakeholders are already considering reopening mines in Europe, but – even in this case – the first step still involves assessing the resources in our subsurface.
And what if there was already a potential solution right under our feet when it comes to lithium and rubidium? More specifically in the Rhine Graben, between France and Germany, in the deep waters already being used to generate geothermal heat and electricity.
A recent study quantified the region’s lithium and rubidium potential, which looks very promising. This is all the more true given that these elements are found in the hot, highly saline underground waters that are already being exploited for deep geothermal energy production. However, a number of technical and environmental challenges will need to be overcome before this potential can be exploited.
How do these metals end up in geothermal waters?
Unlike conventional mineral deposits, the lithium and rubidium here do not need to be extracted from rock but from hot water, which interacts chemically with the rocks in their geothermal reservoirs.
In the case of the Rhine Graben, the deep geothermal brines (highly saline waters) have been formed from a mixture of ancient seawater – which covered the region in the past and has since largely evaporated – and rainwater.
However, we needed to understand why and how these metallic elements are found in the brines. Previously, there had been no chemical or isotopic analyses carried out on the lithium contained in the rocks and minerals of deep boreholes in the Rhine Graben. We therefore performed these analyses and coupled them with those of the geothermal brines. This enabled us to better understand how the brines become enriched with lithium.
The data appear to confirm that the weathering of micas, which are transformed into illite (a clay mineral commonly observed in hydrothermal weathering processes), is, in all likelihood, the main mechanism responsible for releasing lithium and rubidium in solution and controlling their concentrations in the geothermal brines.
Rubidium (left) and lithium (right) concentrations in geothermal brines in the Rhine Graben.
© BRGM - B. Sanjuan
Quantifying the region’s potential
These very hot (with temperatures exceeding 150°C) and salty geothermal brines, which circulate at a depth of more than 2,500 metres in the Rhine Graben, are therefore rich in lithium and rubidium. On average, they contain around 174 milligrams per litre (mg/l) of lithium and 25 mg/l of rubidium. These concentrations are among the highest in the world, making this area a prime candidate for the extraction of these elements, coupled with deep geothermal energy production.
Lithium reserves in the Rhine Graben are currently estimated at between 1 million and 16 million tonnes, with an average estimate of 6.2 million tonnes. By way of comparison, global production in 2023 stood at 230,000 tonnes of lithium and could reach around 800,000 tonnes of lithium per year by 2031. The Rhine Graben could therefore represent a major strategic resource for Europe.
As regards rubidium, these resources are estimated at between 150,000 and 2.3 million tonnes, with an average estimate of 900,000 tonnes. That is a significant figure, since global production is currently estimated at just under 8 tonnes per year, underlining the potential economic benefits of exploiting the region’s resources.
Several industrial initiatives in France and Germany are already looking to exploit this potential for lithium production. In 2022, pilot projects in Alsace produced the first few kilogrammes of lithium using geothermal water from the Rittershoffen power station.
Within the next decade, other projects aim to achieve industrial production of several thousand tonnes a year. The aim is clear: to turn the Alsace region into a European hub for "low-carbon" lithium, produced locally and coupled with renewable energy production.
