Hydrothermal mineral replacement of bastnäsite by rhabdophane and monazite: effects of temperature on mineralogy, REE immobilisation, and fractionation

• Published in: Contributions to mineralogy and petrology Vol. 179; no. 11; p. 101
• Main Authors: Knorsch, Manuel, Bamforth, Tobias G., Xia, Fang, Deditius, Artur P., Pearce, Mark A., Schoneveld, Louise, Roberts, Malcolm P.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.11.2024; Springer Nature B.V
• Subjects: Bastnasite; Clean technology; Earth and Environmental Science; Earth Sciences; Electrons; Experiments; Fluids; Fractionation; Geology; Hydrothermal alteration; Immobilization; Mineral deposits; Mineral Resources; Mineralogy; Minerals; Monazite; Original Paper; Petrology; Phosphates; Rare earth elements; Renewable energy technologies; Temperature; Temperature dependence; Temperature effects; Yttrium; Bastnäsite; Rhabdophane; Mineral replacement; REE immobilisation; REE fractionation; Monazite
• DOI: 10.1007/s00410-024-02183-4
• ISSN: 0010-7999

The rare-earth elements (REEs, La–Lu, Y) are essential for the development of renewable technologies. Bastnäsite (REECO 3 F) is a common REE ore mineral that is often subject to hydrothermal alteration at all crustal levels. Mechanisms of hydrothermal bastnäsite alteration therefore govern the evolution of REE deposits, though these mechanisms remain poorly understood. This experimental work investigates the hydrothermal replacement of bastnäsite by rhabdophane (REEPO 4 ∙xH 2 O, x = 0–1) and monazite (REEPO 4 ) in phosphatic fluids. Two temperature-dependent alteration pathways were identified; both follow the coupled dissolution-reprecipitation (CDR) mechanism. At 90 °C, bastnäsite was replaced by highly-porous metastable rhabdophane which was then replaced by monazite, forming an inner layer of rhabdophane and an outer layer of monazite. At 220 °C, bastnäsite was replaced directly by monazite. Although replacement initiated more quickly at 220 °C, greater overall replacement occurred at 90 °C (~ 61 wt.% after 500 h, compared to ~ 13 wt.% at 220 °C) due to surface passivation by monazite at 220 °C. Geochemical analyses showed REE fractionation during bastnäsite alteration. At 90 °C, rhabdophane was enriched in heavy REEs (Eu–Lu, Y), likely due to the evolving fluid chemistry, while at 220 °C secondary monazite was enriched in Sm and Ho compared to bastnäsite. These results indicate that: 1) the hydrothermal alteration of bastnäsite by rhabdophane and monazite in ore deposits leads to REE immobilisation, with little net loss of REEs to solution; 2) rhabdophane is metastable relative to monazite at 90 °C, and; 3) variable temperatures can cause different mineral textures and REE fractionation trends during hydrothermal alteration and mineral replacement.