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REE Mineralogy and Resources Chapter 279 161 For example, at pH 3 and t 1⁄4 400°C dissolved Nd, 200 ppm in concentration, mainly occurs as NdCl2+ (Fig. 17A). At the pH 5, however, the Nd solubility decreases to less than 1 ppm, because HF dissociation makes more fluoride ions available, resulting in precipitation of fluocerite-(Nd) (NdF3). Decrease of temperature also promotes HF dissociation, reducing NdF3 solubility (Fig. 17B). Thus despite the high stability of REE fluoride complexes, fluo- rine acts a binding ligand that promotes precipitation of REE minerals such as fluocerite-(Ce) and more importantly bastnäsite-(Ce). Phosphate complex is also not able to become the major REE transporter because of the very low solubility of monazite-(Ce). Therefore, chloride species are expected to transport the REEs in most hydrothermal systems. The deposition of the REEs as bastnäsite-(Ce) is shown by the reaction: REECl2+ +HF+HCO31⁄4REECO3FðbastnäsiteÞ+2H+ +Cl Increase of pH and/or HCO3 activity or decrease in Cl activity will lead to deposition of bastnäsite. At Bayan Obo, the host for the REE mineralization is dolomite. Reaction of the ore fluid with dolomite provides a very effective mechanism for ore deposition because of the reaction: 2H+ +CaMgðCO3Þ2 1⁄4Ca2+ +Mg2+ +2HCO3, which leads to sharp increases in both pH and HCO3 activity. This explains the intimate association of bastnäsite-(Ce) with fluorite in the deposit. Like REE fluorides, fluorite is extremely insoluble, and thus any free fluorine will deposit as fluorite when calcium becomes available (Williams-Jones et al., 2012). 2.6 Transportation and Deposition of REEs in Submarine Hydrothermal System 2.6.1 Concentrations and Compositions of REEs in Submarine Hydrothermal Systems Submarine hydrothermal fluids at mid-oceanic ridges have been recognized as an important source of several elements in the oceans. Studies hydrothermal fluids from the Pacific vent sites have shown that they are enriched in REEs relative to seawater concentrations by as much as 10–100 times with signifi- cant positive Eu anomalies, several thousand times higher than seawater (Michard, 1989; Fig. 18). Hydrothermal plumes from the venting sites rise and mix with ambient seawater, until neutral buoyancy is achieved after which the plume disperses laterally. Studies of REEs in seafloor vent fluids have focused mostly on basalt- hosted hydrothermal systems along mid-oceanic ridges where fluids have remarkably uniform chondrite-normalized REE patterns characterized by LREE enrichment and large-positive Eu anomalies (Klinkhammer et al., 1994; Michard, 1989; Michard et al., 1983). Mitra et al. (1994) reported that the endmember hydrothermal fluids from sulfide-rich black smokers arePDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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