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REE Mineralogy and Resources Chapter 279 227 thick weathering profiles are likely to be developed in areas with moderate rainfall not to accelerate a transport of weathered materials but to promote chemical weathering. Southern China is located in the areas of temperate cli- mate and the annual temperature is a range of 18–21°C and annual precipitation is a range of 1500–2000 mm on average (Wang et al., 2013). Climates control rainfall and temperature, and they are consistent with denudation rates, which are basically equivalent to annual discharge of suspended sediments from vari- ous drainage basins (Einsele, 2000; Milliman and Meade, 1983). 4.2.8 Conclusions The parent granites for the REE deposits consist mainly of biotite granite and muscovite granite characterized by high SiO2 content (SiO2≳75%) and low P2O5 contents (<0.08%). The HREE-rich granites are confined to highly dif- ferentiated (SiO2≳75%) muscovite granite and muscovite–biotite granite, which contain fine-grained fluorite and hydrothermal REE minerals formed during deuteric alteration. The most favorable REE source minerals for formation of ion-adsorption ores are REE fluorocarbonates, synchysite, and bastnäsite, because they release REE3+ during weathering and dissolve in low-pH soil water. Allanite and titanite are the subdominant sources of LREEs and HREEs, respectively. In contrast, monazite and xenotime are strongly resistant to chemical weathering. Apatite is altered and replaced by secondary phosphates including rhabdophane and florencite which immobilize REEs during weathering. These results indicate that high P2O5 contents in granites are not favorable for ion- adsorption ore formation. REE-bearing minerals in parent granites are decomposed by acidic soil water and the REEs migrate downward in the weathering profile. The REEs are immobilized by adsorption or incorporation into secondary minerals (mostly phosphates) due to pH increase, which results from the interaction of the soil water with various minerals or higher pH ground water. The REE-leached zone displays a positive Ce anomaly due to the immobile CeO2, which formed from oxidation of Ce3+ to Ce4+. In contrast, the accumulation zone, corresponding to the ion-adsorption ores, exhibits a negative Ce anomaly because the other REE3+ are preferentially adsorbed onto weathering products. The weathering profile of the deposits can be divided into a REE-leached zone with positive Ce anomalies in the upper part of the profile and a REE-accumulation zone with negative Ce anomalies in the lower part of the profile. Many studies suggest that REEs are transported as complexes with carbon- ate ions, bicarbonate ions, and humic substances and as REE3+ ions in soil and ground water. The predominant ligand is dependent on pH and concentration of the species. Previous studies suggest that REEs are most likely to adsorb onto the surface of kaolinite and halloysite in ion-adsorption ores because of their PZCs and abundances of these clays. The fractionation between LREEs and HREEs in ion-adsorption ores is constrained by the weatheringPDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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