Rare Earth Elements in Mineral Deposits: Speciation in Hydrothermal Fluids and Partitioning in Calcite

Abstract

is properly cited.Studying the speciation and mineral-fluid partitioning of the rare earth elements (REE) allows us to delineate the key processes responsible for the formation of economic REE mineral deposits in natural systems. Hydrothermal REE-bearing calcite is typically hosted in carbonatites and alkaline rocks, such as the giant Bayan Obo REE deposit in China and potential REE deposits such as Bear Lodge, WY. The compositions of these hydrothermal veins yield valuable information regarding pressure ( ), temperature ( ), salinity, and other physicochemical conditions under which the REE can be fractionated and concentrated in crustal fluids. This study presents numerical simulation results of the speciation of REE in aqueous NaCl-H 2 O-CO 2 -bearing hydrothermal fluids and a new partitioning model between calcite and fluids at different --conditions. Results show that, in a high CO 2 and low salinity system, bicarbonate/carbonate are the main transporting ligands for the REE, but predominance shifts to chloride complexes in systems with high CO 2 and high salinity. Hydroxyl REE complexes may be important for the solubility and transport of the REE in alkaline fluids. These numerical predictions allow us to make quantitative interpretations of hydrothermal processes in REE mineral deposits, particularly in carbonatites, and show where future experimental work will be essential in improving our modeling capabilities for these ore-forming processes.

Fractionation of Rare Earth Elements in Greisen and Hydrothermal Veins Related to A-Type Magmatism

“…The strong indications that the Cl-rich greisen and vein fluids most likely had a circumneutral pH mean that they could have contributed to keep LREE preferentially in solution as Clcomplexes. The predominance of chloride complexation typically increases to higher pH with increasing temperature [5], which suggests that the most substantial REE-chloride aqueous transportation occurred close to the granite, as corroborated by high salinities in the high-temperature fluid inclusions [12].…”

“…The low REE concentrations in the studied hydrothermal system and the naturally low hydroxide contents in acidic fluids suggest that hydroxide was not a significant transporting agent at early greisen stages where the combination of low pH and temperatures above 350°C [11,12] disables complex stability. Hydroxyl REE complexation was likely also insignificant during vein formation since the hydroxyl species is only considered to impact REE mobility below 200°C [12].…”

“…Carbonate ligands have been shown to form strong complexes with MREE and HREE at both high (600-800°C; [66]) and low (25°C; [67]) PT conditions, whereas data for REE partitioning of carbonate complexation is absent in the moderate hydrothermal regime [5]. Modelling of REE speciation and fluid/calcite partitioning in H 2 O-NaCl-CO 2 solutions in temperatures ranging between 100 and 400°C predicts bicarbonate or carbonate complexation at low salinity but suppresses carbonic species in favor of chloride complexes at high salinity [12]. In our vein system, with fluids rich in NaCl and with temperatures up to 430°C, calcite precipitation was impeded despite probable calcite saturation [12] and high CO 2 pressure until fluid temperature and pressure successively decreased enough to enable H 2 CO 3 dissociation into the aqueous species [68].…”

“…Modelling of REE speciation and fluid/calcite partitioning in H 2 O-NaCl-CO 2 solutions in temperatures ranging between 100 and 400°C predicts bicarbonate or carbonate complexation at low salinity but suppresses carbonic species in favor of chloride complexes at high salinity [12]. In our vein system, with fluids rich in NaCl and with temperatures up to 430°C, calcite precipitation was impeded despite probable calcite saturation [12] and high CO 2 pressure until fluid temperature and pressure successively decreased enough to enable H 2 CO 3 dissociation into the aqueous species [68]. This feature is consistent with the low abundance of calcite in greisen and also implies that carbonate complexation had a negligible influence on REE fractionation at the high-temperature hydrothermal stages.…”

“…The preferred complexation of LREE by chloride and fluoride and HREE by hydroxide and carbonate ligands experimentally established at low temperatures (generally below 100°C or even at ambient conditions) is not as straightforward at higher temperatures, where pH and redox conditions and coexisting species in the solution have stronger impacts on REE selectivity [5]. Experimental data of REE-carbonate and -hydroxide complexation at elevated temperatures is particularly scarce, although recent simulations predict abundant transportation of REE carbonates in solutions with high CO 2 and low salinity [12]. Fluidrock and fluid-fluid interactions add further complexity to REE fractionation in natural systems, yet the applicability of experimentally determined partitioning has been implemented in geochemical, physical, and thermal evolution models of magmatic-hydrothermal systems at Strange Lake granitic pluton, Canada [3,13] and in carbonatites such as those at Bayan Obo, China [14] and Songwe Hill, Malawi [15].…”