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REE Mineralogy and Resources Chapter 279 257 4.3.2.3 REE-Bearing Apatite in Sedimentary Deposits (Phosphorites) and Deep-Sea Mud In this chapter, several important studies concerning REE-bearing apatite in both sedimentary apatite deposits and deep-sea mud will be introduced. As mentioned earlier, sedimentary phosphate deposits range in age from Precam- brian to Recent. Thus, from the geochemical view, REE contents in sedimen- tary phosphates from worldwide occurrences are used as paleoenvironmental indicators. However, the recent REE crisis has sparked widespread attention, and sedimentary apatite rocks are also likely to become yet another important target for REE exploration. As mentioned earlier, the crystal–chemical charac- teristics of apatite prefer LREEs to HREEs in most natural environments. Because the current demand for REEs is increasing mainly for HREEs, HREE-rich apatite ores are much more appropriate for exploration and mining. From this perspective, Ihlen et al. (2014) investigated the sedimentary and igneous apatite rocks in Norway and evaluated their REE potential. The igne- ous apatite rocks contain 3000–10,000 ppm REEs (HREEs: 170–1000 ppm) and may have REE potential. On the other hand, the sedimentary apatite rocks contain very low REE contents, but geochemical data were not shown. Thus, the authors concluded that sedimentary apatite rocks do not have REE poten- tial. Alternatively, Emsbo et al. (2015) analyzed 23 sedimentary apatite depos- its (phosphorites) and found some of them contain relatively high concentration of REEs ($5000 ppm) and HREEs ($2000 ppm). Interestingly, their study also showed that REE contents in sedimentary apatite rocks are very heterogeneous over geologic age and sampling location. The geologic process controlling REE contents in sedimentary phosphate deposits are intensely debated and its further understanding is expected to facilitate more efficient exploration for HREE-rich apatite deposits. Besides these sedimentary apatite rocks, apatite in deep-sea mud is also proposed as HREE resource recently (Kato et al., 2011; Kon et al., 2014; Nakamura et al., 2015; Yasukawa et al., 2014). While their profitable exploi- tation depends on overcoming the technical and environmental obstacles associated with seafloor mining at depth of 2–6 km (Nakamura et al., 2015), REEs in such deep-sea mud are easily extractable by simple chemical treat- ments as well. The following paragraph will mention some important charac- teristics of REE-bearing apatite in deep-sea mud. The distribution of REEs in the deep-sea unsolidified sediments has been investigated by numerous researchers (eg, Bernat, 1975; Toyoda and Tokonami, 1990). Bernat (1975) made several important observations on average REE contents of separated constituent minerals in the deep-sea unso- lidified sediments of the Pacific. Deep-sea mud abundant in HREEs (REEs in the deep-sea mud at numerous sites throughout the eastern South and central North Pacific sea floor) was recently reported by Kato et al. (2011). The HREE-rich deep-sea mud contains low Th and U, which is a big advantage for HREE resource. On the other hand, REE-enriched deep-sea mud withPDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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