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156 Handbook on the Physics and Chemistry of Rare Earths (bastnäsite) (Xu et al., 2010). Balance between carbonate (calcite) and phosphate (apatite) results in the concentration of heavy or light REEs in the residual melt. Xu et al. (2010) discussed this enrichment process in the Miaoya deposit in cen- tral China. Their results show that due to extremely low partitioning of LREEs to calcite, even though apatite and monazite precipitated early in the evolution, REEs, especially LREEs, were enriched to an economical grade of 1.72 wt.% REO in the fractionated carbonatites, and were finally incorporated in REE fluorocarbonates such as bastnäsite-(Ce) and synchysite-(Ce). The abundance of REEs of peraluminous melts is low and consequently the abundances of REEs in peraluminous granites and pegmatites are low, but their solubilities greatly increase with increasing peralkalinity of the melts. This leads to high concentration of high-field strength elements (HFSE) in peralka- line granites by extreme fractionation of peralkaline magma. Such examples are observed at Khaldzan Buregtey deposit in Mongolia (Kovalenko et al., 1995) and Nechalacho (Thor Lake) deposit in Canada (Sheard et al., 2012), although subsequent hydrothermal processes played a major role for the con- centration of REE in these deposits (Salvi and Williams-Jones, 2005). 2.5 Partitioning to Fluids For many years, REEs were considered to be immobile in hydrothermal fluids. This idea is supported by very low concentration of REEs in low temperature hot spring water (van Middlesworth and Wood, 1998). However, it has become apparent that REEs can be concentrated to ore grades by hydro- thermal processes (Migdisov and Williams-Jones, 2014; Samson and Wood, 2005; Williams-Jones et al., 2012). Recent studies of REE mineralization associated with carbonatites and peralkaline igneous complexes have revealed that the REE mineralization in these igneous complexes is exclusively related to hydrothermal activities that followed magmatic processes (eg, Migdisov and Williams-Jones, 2014; Sheard et al., 2012; Wall et al., 2008; Williams- Jones et al., 2012). The only apparent exception is the Mountain Pass deposit in the United States, which is assumed to have formed by fractional crystalli- zation of carbonatite magma without significant hydrothermal alteration (Castor, 2008a). This section reviews the recent studies on the complexation of REEs, and REE partitioning between fluids and melts. 2.5.1 Complexation of REEs Except under very acidic conditions, simple hydrated trivalent ions are not capa- ble of transporting significant quantities of the REEs. Rather, the formation of strong aqueous complexes is required. Because aqueous REE3+ ions are “hard” in the sense of Pearson (1963), they form stronger complexes with “hard” ligands such as F, SO42, CO32, PO43, and OH than with “soft” ligands (eg, HS, CN, Br, and I). The “borderline” ligand Cl forms comparatively weakPDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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