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REE Mineralogy and Resources Chapter 279 203 4.2 Ion-Adsorption Type Deposits 4.2.1 Characteristics of Ion-Adsorption Type REE Deposits Minable ion-adsorption type REE deposits are confined almost exclusively to southern China (Xie et al., 2016) in a mild and humid climate although several deposits and prospects are known in Africa, South America, and Southeast Asia (Fig. 34). The first discovery of the ion-adsorption type deposit dates back to 1969 when HREE-rich granites were found in Longnan County of Jiangxi Prov- ince (Chen, 1996; Ding, 2012). Over 169 of ion-adsorption type REE deposits have been found in southern China (Chen, 1996; Ding, 2012; Wall, 2014; Wang et al., 2013; Xie et al., 2016; Zhang, 1990), and they are present in Jiangxi, Guangdong, Fujiang, Guangxi (Zhuang Autonomous Region), Hunan, and Yunnan Provinces (Fig. 35) listed in decreasing order of resources (Zhou et al., 2012). Some of the deposits were found in Heinan Province (Yuan et al., 2013). Ore reserves of the ion-adsorption type deposits are classified into large (>0.5 Mt), medium (0.05–0.5 Mt), and small (<0.01 Mt) (Zhu, 1999). Although specific data on the reserves are not available, the total resources of each deposit were reported by Xie et al. (2016). The deposits sizes are generally smaller than other REE deposits associated with carbonatites, peralkaline rocks, and hydrothermal veins which were previously mentioned in Section 4.1. Ion-adsorption type REE deposits are categorized as weathering deposits; however, this ore formation process contrasts to the other weathering REE depos- its such as placer deposits and weathered carbonatite deposits (Lottermoser, 1990; Verplanck et al., 2016), which are called residual deposits. The most significant difference in these deposit types is whether the REEs are adsorbed on the surfaces of weathering products or are incorporated in mineral structures. In residual deposits, REEs are contained in residual minerals such as monazite and zircon which are resistant to chemical weathering. If weathered rocks are mechanically eroded and these residual heavy minerals are selectively concentrated in basin by sedimentation, they may form placer deposits. In ion-adsorption ores, REEs are mostly present in ion-exchangeable phase of weathered granites, and blasting, crushing, grinding, or mineral processing is not needed. REEs are easily extracted by ion exchange using dilute electro- lyte solutions such as ammonium sulfate solution at ambient temperature (eg, Chi and Tian, 2009; Chi et al., 2012; Moldoveanu and Papangelakis, 2012, 2013; Tian et al., 2013; Wu et al., 1990) as illustrated in Fig. 36. The schematic of extraction by leaching is suggested by Chi and Tian (2009) as follows: Al2Si2O5ðOHÞ nREEðsÞ3+ +3nNH4+ðaqÞ 4m4+3+ 3+ , Al2Si2O5ðOHÞ4 m NH 3nðsÞ + nREEðaqÞ This bulk-leaching process enables mining low-grade ores. In recent years, ion-adsorption ore grades and extraction recoveries have both been decreas- ing, and therefore, extraction costs become more expensive than beforePDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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