EXPERIMENTAL DERIVATION OF THE REE PARTITIONING BETWEEN FLUID-CALCITE AS A TRACER OF HYDROTHERMAL METAL TRANSPORT AND MINERALIZATION PROCESSES

Carbonatite and alkaline igneous systems are typically associated with intraplate crustal settings and can contain anomalously high concentrations of rare earth elements (REE), in some cases sufficient enough to form potential ore deposits [1]. The REE are highly valued commodities imperative in the manufacturing of both high technology and green industries. The cause of metal enrichment in intraplate settings has been postulated to be related to a metasomatic ground preparation of the crust by H₂O-CO₂-bearing fluids [2]. Our current ability to quantify and understand the processes involved in the hydrothermal mobilization/mineralization of the REE in the crust is very limited [3]. A fundamental understanding of the partitioning of REE between fluid-minerals could potentially be used as a tracer of crustal fluid-rock interactions in these systems. To better elucidate the role of these hydrothermal processes in the mobilization of REE, we performed a series of laboratory experiments.

Calcite synthesis experiments were performed by mixing aqueous NaHCO₃- and CaCl₂-bearing fluids in a stirred titanium Parr reactor at 200°C and water vapor saturation pressure. Calcite crystals were produced over a period of two weeks and studied at the end of the experimental duration using SEM. The aqueous solutions were sampled in situ and later analyzed using ion chromatography. With increasing reaction time, the Ca²⁺ and HCO₃^- activities decreased in the experiments as the aqueous solution approached saturation with calcite and steady-state conditions. Future experiments will be doped with REE chloride-bearing aqueous solutions to study the partitioning behavior of the REE between the calcite and the fluid. These results will allow us to develop a partitioning model that will be compared to the composition of natural hydrothermal calcite veins found in intraplate igneous systems. The broad applicability of this research is expected to impact and advance REE mineral exploration strategies while also providing fundamental partitioning data for the study of crustal metasomatism.

[1] Smith M.P., Henderson P., Peishan Z. (1999), Contrib. Mineral. Petrol. 134, 294-310. [2] Martin R., De Vito C. (2014) Can. Mineral. 52, 165-181. [3] Gysi A.P., Williams-Jones A.E. (2013), Geochim. Cosmochim. Acta 122, 324-352.