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192 Handbook on the Physics and Chemistry of Rare Earths deposit type has been producing the majority of the REE resources in the world. The representative carbonatite deposits are Bayan Obo and Maoniuping in China, Mountain Pass in the United States, Mount Weld in Australia, and Araxa ́ and Catala ̃o in Brazil (Fig. 29). The major REE minerals are bastnäsite-(Ce), synchysite-(Ce), monazite-(Ce), and apatite, but so far bastnäsite-(Ce) ores have been preferentially processed due to the easiness of the REE extraction. Although carbonatite is magmatic in origin, most of the high-grade ores of this type are formed by subsequent hydrothermal activities (Smith et al., 2015; Wall et al., 2008; Williams-Jones et al., 2012; Xie et al., 2009). Some representative deposits (Bayan Obo, Mountain Pass, Mount Weld) possess large reserves (>1 Mt REO) and relatively high-grade ores (>4 wt.%) that can fully cover the world REE demand, which is 100–200 kt a year. These deposits are enriched in light REEs because of the genetic condition of the carbonatite magmas (low degree of partial melting of garnet-bearing amphibole lherzolite at depth >70 km), although the deposits with apatite ores are relatively enriched in HREEs (Table 11). Exceptions for the LREE-enriched nature in carbonatites are hydrothermal xenotime-(Y)-rich veins associated with carbonatite at Lofdal in Namibia (Wall et al., 2008), and Yen Phu, a small, and highly weathered carbonatite prospect in northern Vietnam, which is composed of goethite-rich ore containing xenotime-(Y), monazite-(Ce), samarskite-(Y), and fergusonite-(Y) above an intrusive xenotime- bearing dolomite-carbonatite body (Watanabe, 2014). Although the genetic conditions of the carbonatite magmas of these deposits have not been clarified, the low grades and enrichment of HREEs in the ores are similar to those of the deposits associated with peralkaline rocks, suggesting that these carbonatite magmas might have formed above the garnet-stable depth in the lithosphere (Fig. 14). The Bayan Obo deposit in Inner Mongolia of China is the largest and most productive REE deposit in the world (Fig. 31). It has estimated reserves of high-grade ores of 48 Mt at 6 wt.% REO, and lower grade resources of 750 Mt at 4.1 wt.% REO (Smith et al., 2015). The deposit also contains 1500 Mt of iron (at 35 wt.% Fe), and 2.2 Mt of Nb (0.13 wt.% Nb). Despite the large scale of the deposit and abundant scientific research conducted on the deposit, its genesis has been controversial because of the complex nature of the deposit (Wu, 2008). The Bayan Obo deposit is composed of three major ore bodies: Main, Western, and Eastern ore bodies, which are hosted in the sedimentary dolo- mite layer of the Proterozoic Bayan Obo Group. In the Main and Eastern ore bodies, the REE mineralization is associated with magnetite–hematite iron ores, whereas in the Western ore body it is hosted in dolomite. The Main and Eastern ore bodies have cores of massive magnetite, which are fringed with banded ores dominated by aegirine–augite, magnetite, fluorite, and hematite (Chao et al., 1992). Based on the published papers, Smith et al. (2015) compiled mineral paragenesis of the deposit, which is separated into five stages: Stage 1PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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