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REE Mineralogy and Resources Chapter 279 187 This low, but slightly different, cheralite component is characteristic of mona- zite in the Japanese granitic rocks, suggesting that the huttonite and cheralite components in monazite reflect the source of the granitic magmas. The REE minerals crystallized in a relatively young geologic setting such as the Japanese arc after a start of mantle convection in the history of earth evolution show the different geochemical trends for those before the Paleozoic era. Further studies are required in order to clarify if similar geo- chemical trends are observed for REE minerals from other island arcs and continental margins. Understanding the occurrences of a variety of REE minerals in different tectonic settings and different geologic ages will also contribute to effective exploration of REE deposits. 4 INTRODUCTION TO REE DEPOSITS 4.1 Classification of REE Deposits Rare earth deposits are formed by different geological processes in deep- seated plutonic environment, magmatic–hydrothermal environment, and sur- face weathering environment. The variable style of mineralization is due to the nature of REE concentration, which can be realized by the divergent processes in magmatic, hydrothermal, weathering, and sedimentary environ- ments. These processes commonly overlap with each other, as exemplified by the magmatic–hydrothermal REE mineralization related to carbonatite intrusions such as the Mount Weld deposit in Australia, in which REEs were concentrated by weathering. This chapter reviews the representative types of REE deposits, which are commonly categorized into carbonatite, peralkaline-rock associated, iron oxide–apatite (IOA), hydrothermal vein, sediment-hosted hydrothermal, ion-adsorption type, and placer deposit (Kanazawa and Kamitani, 2006; Linnen et al., 2014; Long et al., 2010; Wu et al., 1996). In addition to these deposit types, some mining operations try to produce REEs from bauxite and clay deposits as well as sedimentary phosphates (Fig. 29). Rare earth deposits are also classified in terms of the production style of the ores; main production or by-production, and LREE- and HREE-dominant deposits (Table 10 and Fig. 30). Because of (1) segregation of LREE and HREE groups by the natural magmatic and hydrothermal processes, (2) vari- able amounts and values of individual REEs, and (3) variable production costs of REE ores, mineable ore grades are significantly different by deposit types, REE mineralogy, and REE compositions in the ores. Thus, the types of REE deposits that actually produced or have been producing the ores are limited. 4.1.1 Carbonatite Deposits Carbonatite deposits are the most dominant and common REE deposit type. Since the mid-1960s, when the Mountain Pass deposit started production, thisPDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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