Carbonate complexation enhances hydrothermal transport of rare earth elements in alkaline fluids

Abstract

Rare earth elements (REE), essential metals for the transition to a zero-emission economy, are mostly extracted from REE-fluorcarbonate minerals in deposits associated with carbonatitic and/or peralkaline magmatism. While the role of high-temperature fluids (100 < T < 500 °C) in the development of economic concentrations of REE is well-established, the mechanisms of element transport, ore precipitation, and light (L)REE/heavy (H)REE fractionation remain a matter of debate. Here, we provide direct evidence from in-situ X-ray Absorption Spectroscopy (XAS) that the formation of hydroxyl-carbonate complexes in alkaline fluids enhances hydrothermal mobilization of LREE at T ≥ 400 °C and HREE at T ≤ 200 °C, even in the presence of fluorine. These results not only reveal that the modes of REE transport in alkaline fluids differ fundamentally from those in acidic fluids, but further underline that alkaline fluids may be key to the mineralization of hydrothermal REE-fluorcarbonates by promoting the simultaneous transport of (L)REE, fluoride and carbonate, especially in carbonatitic systems.

Fig. 1. Examples of fluorescence spectra used to evaluate La (A), Sm (B), and Yb (C) hydrothermal concentrations.

Enhanced solubility of REE in the hydrothermal fluids is evidenced by an increase of the fluorescence absorption edge jump, which is shown as eH on (C). La data were collected at 40 MPa, other REE data were collected at 80 MPa.

Fig. 2. Effect of temperature on the concentrations of La(A), Gd (B), and Yb (C) in Na₂CO₃ ± NaF solutions.

Solubility trends for La and Gd are here represented as the absorption edge height (eH) value from fluorescence spectra. This value is correlated to the concentration of the REE in solution, but may be affected by changes in density and absorption of the different solutions with increasing P–T (see “Methods” section). Therefore, it is only an indication of solubility behavior and cannot be used to directly calculate the concentration of La and Gd in the solution. For Yb, absolute concentrations could be calculated from the transmitted spectra, and are reported as ppm Yb in solution.

Fig. 3. EXAFS spectrum (A) and corresponding Fourier transform (B) for Yb in Na₂CO₃ ± NaF solutions at 200 °C and 80 MPa.

The spectrum is compared to that of Yb oxide crystalline compound and Yb dissolved in Cl-rich acidic solution. The fits are reported as a dashed line over the experimental spectra. The two vertical dashed lines on (A) underline differences in the shape and position of the EXAFS oscillations between alkaline and acidic solutions. Bold arrows in (B) point to the features arising from different scattering paths to oxygen (Yb–O) and carbon (Yb–C) first neighbor atoms or oxygen and ytterbium atoms in the second shell (Yb–O/Yb).

Fig. 4. Proposed structures for the Gd (hydroxyl-)carbonate complexes.

Structures were obtained by static quantum mechanical calculations using the Amsterdam Density Functional (ADF) program: A [REE₃(CO₃)₂(OH)₄(H₂O)₁₂]⁺, B [REE₃(CO₃)₃(H₂O)₁₂]³⁺. The atoms are gadolinium (purple), oxygen (red), carbon (brown), and hydrogen (white).