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216 Handbook on the Physics and Chemistry of Rare Earths Zudong, Zhaibeiding, and Datian granites (mostly muscovite-bearing) are characterized by enrichment in HREEs (LaN/YbN1⁄40.11–1.5; Fig. 40G), Y/Ho ratios ranging widely from 25 to 47 (Fig. 40H), and occurrences of hydrothermal minerals (Bao and Zhao, 2008; Huang et al., 1989; Wu et al., 1990). In contrast, less fractionated biotite granites tend to show lower Y/Ho ratios 15–31 (Fig. 40H), and they are significantly enriched in LREEs (LaN/YbN 1⁄4 1.4–70; Fig. 40G). 4.2.3 REE-Bearing Minerals and Their Susceptibilities to Weathering In the process of granite weathering elements, including REEs, can be released from the primary magmatic and hydrothermal REE-bearing minerals, and sec- ondary REE-bearing minerals which are formed by alteration and replacement to the primary minerals at ambient temperatures. REE fluorocarbonates dissolve in acidic soil water near the surface, however, other REE-bearing minerals such as phosphates, oxides, and silicates are likely to dissolve more slowly (Sanematsu and Watanabe, 2016). REE fluorocarbonates in igneous rocks occur along grain boundaries, as veinlets and in cavities, and it has been suggested that they were formed by F- and CO2-rich fluids in deuteric alteration at the last stage of fractional crys- tallization (eg, Bea, 1996; Berger et al., 2008; Huang et al., 1989; Ishihara et al., 2008; Maruejol et al., 1990; Sanematsu et al., 2013). Occurrences of these minerals may be indicated by high Y/Ho ratios as previously explained. In the Longnan deposits, the muscovite granite, which is the main ore body of the HREE-rich Zudong intrusion (Fig. 37), contains abundant F-bearing minerals such as fluorite, synchysite-(Y), and topaz, whereas the biotite gran- ite of the Zudong intrusion only contains some F-bearing minerals such as biotite and apatite (Huang et al., 1989). Wu et al. (1992) studied HREE-rich granites (not all are related to REE deposits) regionally in the Nanling Range and demonstrated relatively high F contents of 340–6700 ppm with a median value of 1400 ppm. The REE fluorocarbonate minerals generally dissolve in acidic soil water during weathering of parent granites (Huang et al., 1989). Allanite and titanite are considered to be the important REE sources (par- ticularly LREE and HREE sources, respectively) for ion-adsorption ores as well as REE fluorocarbonates (Sanematsu and Watanabe, 2016; Sanematsu et al., 2015, 2016). Allanite and titanite, which are common REE- and REE-bearing minerals in monazite-free granite, contribute significantly to whole-rock REE contents (Bea, 1996; Gromet and Silver, 1983), and they can be precursor minerals of REE fluorocarbonates by hydrothermal alter- ation. Partition coefficients for REEs between allanite and melt indicate that allanite predominantly incorporates LREEs rather than HREEs (eg, Giere and Sorensen, 2004), and that titanite is moderately rich in HREEs in contrast to allanite (eg, Prowatke and Klemme, 2005). Modal and EMPA analyses ofPDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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