Hydrothermal controls on the genesis of REE deposits: Insights from an in situ XAS study of Yb solubility and speciation in high temperature fluids (T<400°C)

High pressure-high temperature experiments are used to investigate the effect of temperature, pH and fluid composition on the transport and deposition of REE and provide constraints on the role of hydrothermal fluids in the formation of rare metal deposits. In situ X-ray absorption (XAS) measurements were conducted at 50MPa to determine simultaneously the solubility of Yb compounds (Yb₂O₃, Yb₂Si₂O₇ and YbPO₄) and Yb speciation from 200 to 350-400°C in fluids involving Cl, F and P ligands. Yb concentrations were monitored down to tens of ppm from the height of the absorption edge while XANES and EXAFS analyses were used to refine the aqueous speciation of Yb in the high temperature fluids. The XAS analysis suggests that Yb solubility is retrograde from 200°C and that 10 to 100ppm Yb can be dissolved in acidic fluids (pH<2) at T<400°C, most probably as Yb[(H₂O)_7,8]³⁺ species. Increasing pH and adding phosphorus trigger the precipitation of Yb compounds from 200 to 400°C. Hence buffering of acid orthomagmatic fluids and leaching of P from host rocks could be major mechanisms in the development of (H)REE enrichments in magmatic-hydrothermal systems.