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The formation of Rare Earth ore deposits facilitated by hydrothermal complexation with carbonates
Modelling ore-forming processes for the Rare Earth Elements (REE) requires understanding their behaviour in geological melts, hydrothermal fluids and minerals. In-situ X-ray absorption spectroscopy measurements reveal that REE-carbonate complexes can promote high REE concentrations in geological fluids, a new mechanism to concentrate these critical metals in the Earth s crust.
The switch from fossil to non-carbon energy sources relies on efficient and sustainable access to a number of critical metals, including the Rare Earth Elements (REE). REE are a group of 15 metals (La to Lu) with closely related chemical properties; Sc and Y are often added as they share similarities in their chemical behaviour. REE have
become critical to a variety of industrial and technological applications and are notably present in multiple components of smartphones (magnets, electronics, audio and video screens), wind turbines and electric cars (permanent magnets, rechargeable batteries, alloys).
The name Rare Earths is misleading as REE are relatively widespread in Earth s crust, with average La and Yb concentrations reaching close to 50 and 5g/ton of rock, compared to gold and platinum, which are only present at <1g/ton. Some of the world s largest REE deposits are associated with igneous rocks called carbonatites, which are geological oddities with fewer than 500 known locations in the world, of which only five are currently mined (Bayan Obo, Maoniuping and Dalucao in China; Mountain Pass in the USA; Mt Weld in Australia). How magmatic and hydrothermal processes combine to form economic REE deposits in these particular environments is still a matter of debate.
Field studies suggest that aqueous fluids were present during the crystallisation of high-grade ores in and around carbonatite intrusions. These fluids were hot (200-500°C), solute-rich (few to tens of wt% of dissolved Cl , F , SO42 and CO32 ) and vary in pH from alkaline (pH > 8) to near-neutral and acidic (3 < pH < 5) conditions. Previous experimental studies and thermodynamic models established that the REE form increasingly stable complexes with Cl or SO42 with increasing temperature under acidic conditions . However, a long-standing issue with REE transport by acidic fluids has been that addition of fluorine, a ubiquitous element in these systems, rapidly leads to the precipitation of insoluble fluoride phases and thus precludes further transport and concentration of the REE.
In comparison, little was known about the potential of alkaline (pH > 10) fluids to mobilise and concentrate the REE at typical ore-forming temperatures (T > 100°C). The aim of this study was to investigate REE solubility and speciation in NaOH and Na2CO3-bearing high-temperature fluids. In-situ X-ray absorption spectroscopy (XAS) measurements were conducted in a high-temperature autoclave at the BM30 FAME beamline. This unique setup, developed at the Institut Néel in Grenoble, allows for data acquisition on dilute fluids to 500°C and 1500 bars , and offers unmatched opportunities to study the solubility and complexation of metals under the typical pressure and temperature conditions of ore formation in the Earth s crust.
It was found that La, Gd and Yb aqueous concentrations may reach from 100s to 1000s g/ton in Na2CO3-bearing fluids, significantly higher than observed in NaOH solutions (<1-50g/ton) and approaching the high concentrations found in very acidic Cl-rich fluids (>1000s g/ton) (Figure 8) . Interestingly, La solubility was found to increase with temperature, whereas it and other REE have retrograde
Fig. 8: Evolution of La, Gd and Yb aqueous concentration with increasing temperature and different fluid composition, as derived from the absorption of the experimental solutions.