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206 Handbook on the Physics and Chemistry of Rare Earths because more extraction reagent is required (Tian et al., 2013). Ion-adsorption ores generally contain lower amounts of Th and U than REE ores from other deposits. For example, ion-adsorption ores in Dingnan contain 29–37 ppm Th and 2.8–4.7 ppm U (Murakami and Ishihara, 2008). After a reaction with ammonium sulfate solution, 0.03–0.31 ppm Th and 0.25–0.71 ppm U were extracted from these ores (Sanematsu and Kon, 2013). Acidic solution during granite weathering indicates that ion-adsorption ores are not lateritic because the latter are typically formed in alkaline solution. Eventually, major compo- sitions of ion-adsorption ores (high SiO2 and low Fe2O3 and Al2O3 contents) do not correspond to lateritic compositions of low SiO2 and high Fe2O3 and Al2O3 contents. Generally, small sizes of ion-adsorption deposits are due to the constraint that ore bodies can only develop near the surface. Thicknesses of the weathering profiles on parent rocks generally range from 6 to 10 m on average, although they can reach up to 30 m (Bao and Zhao, 2008; Chen, 1996, 2013; Li and Li, 2010; Pan, 2011; Tong, 2013; Wang et al., 2013; Wu et al., 1990, 1996; Yang and Xiao, 2011; Yang et al., 1981; Zhang, 1990; Zhu, 1999), and locally up to 60 m (Bao and Zhao, 2008; Huo, 1992; Yang and Xiao, 2011). Parent rocks of ion-adsorption type deposits consist mostly of granitoids with minor felsic volcanic rocks and metamorphic rocks (Chi and Tian, 2009; Huo, 1992; Zhang, 1990). 4.2.2 REE Geochemistry of Parent Granites Rare earth contents of igneous rocks are generally dependent on both the degree of partial melting and fractional crystallization as previously explained in Section 2. The parent rocks of the deposits consist mainly of biotite granite, muscovite–biotite granite, muscovite granite (eg, Longnan deposits; Fig. 37) and lesser amounts of volcanic rocks such as rhyolite (Heling deposits; Fig. 38). Most REE deposits in the world are typically associated with per- alkaline rocks (eg, Linnen et al., 2014; Wall, 2014); however, ion-adsorption type deposits are sourced from metaluminous and peraluminous rocks, not from peralkaline rocks, based on a molar Al2O3/(Na2O+K2O) ratio and Al2O3/(CaO+Na2O+K2O) ratio (aluminum saturation index: ASI) as shown in Fig. 39. These granites show K2O contents approximately from 4.5% to 7% and Na2O+K2O contents from 7.5% to 10% (Bao and Zhao, 2008; Deng, 1988; Feng et al., 2012; Gu et al., 2006; Hua et al., 2007; Huang et al., 1989; Ishihara et al., 2008; Li et al., 2007; Ma et al., 1991; Yang and Xiao, 2011; Yang et al., 1981; Yu et al., 2004; Zhang, 1990), classified as calc-alkaline and alkali granites (Fig. 40A and B). Based on the classification of granitoid-series (Ishihara, 1977), the parent granites are primarily classified into ilmenite-series with some exceptions of magnetite-series granitoids in Dajishan and Qinghu (Table 13).PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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