Hydrothermal Ree Transport In Carbonatites

Rare earth elements (REE) have been classified as ‘critical metals’ by a number of international organisations due to China controlling the current market, the ever growing demand, low recycling rates, and poor substitutability of these elements. Currently, carbonatite deposits are the main source of Light REE (LREE) due to their high REE grade and easy to process minerals. Interestingly, few carbonatite deposits worldwide also show economic Heavy REE (HREE) enrichment. Often, REE economic concentrations are hosted by minerals precipitated during hydrothermal alteration, rather than the early magmatic REE minerals. However the role of hydrothermal activity in enhancing ore grades of REE carbonatite deposits is poorly understood.
This study considers two contrasting deposits, both hydrothermally reworked; the Okorusu carbonatite complex, Namibia, considered to be a ‘common’ carbonatite deposit, based on its LREE enriched pattern, and the Dashigou mine part of the Huanglongpu carbonatite deposit, China, which is one of the few carbonatite deposits showing a HREE enrichment, leading to an unusual flat REE pattern. The work achieved during this PhD has allowed the recognition of fundamental characteristics about hydrothermal REE reworking and beneficiation processes in carbonatite. The REE are remobilised by hydrothermal fluids from the REE-bearing magmatic minerals, which can be transported over sufficiently large distance to create an enrichment of the more altered carbonatites. This transport has been found effective at a temperature below 100 °C. The final REE pattern and content of a carbonatite strongly depends on the magmatic REE source and the nature of the hydrothermal fluid. The unusual HREE enrichment of the Huanglongpu carbonatites is attributed to a HREE enrichment at the final magmatic stage of the carbonatite and subsequent alteration by sulfate-rich hydrothermal fluids. Finally, relatively small volumes of hydrothermal fluids, such that the magmatic oxygen isotope signature is not greatly modified, are sufficient to rework magmatic REE from gangue minerals to form secondary REE mineralisation.