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West FIG. 32 Alluvium Lacustrne sediments Dolerite dike Carbonatite Residual apatite Ferruginous cap rocks Glimmerite Archean Greenstone REE Mineralogy and Resources Chapter 279 195 Depth (m) East 440 400 360 320 2 km Supergene zone Residual zone Carbonatite Depth of weathering Schematic cross section of the Mount Weld carbonatite. After Lottermoser, B.G., 1990. Rare-earth element mineralisation within the Mr. Weld carbonatite laterite, Western Australia. Lithos, 151–167. Some of the carbonatites (Mount Weld, Araxa ́, Zandkopsdrift) have elevated REE grades due to weathering. The Mount Weld carbonatite is covered by 40 m thick regolith, which was formed during the Mesozoic and is composed of a lower residual zone with abundant relict igneous minerals (apatite, magnetite, ilmenite, pyrochlore, monazite, and silicates) and an upper supergene zone of insoluble aluminous oxides, clays, aluminum phos- phate, and phosphate minerals (Fig. 32). The REE concentration is less than 0.2 wt.% REO in the fresh carbonatite, but 4.5 wt.% in the regolith (Lottermoser, 1990). Although higher ore grades are preferable for mining, these carbonatite ores have difficulty in mineral concentration and REE extraction, because of the complex and highly refractory nature of the second- ary REE mineralogy, represented by aluminum phosphate minerals (crandal- lite, florencite-(Ce)), and minute mineral size. In addition to this, abundant presence of iron oxides (goethite and hematite) in the weathered ores disturbs selective flotation of REE minerals. Therefore, special care is necessary to produce high-grade ore concentration from the weathered carbonatite ores, which were successfully achieved at Mount Weld by optimizing conditions such as flotation pH, modulus of sodium silicate, etc. (Qi et al., 1992). 4.1.2 Peralkaline Rock-Associated Deposits There are a variety of the REE deposits associated with peralkaline rocks, which are largely classified into the deposits associated with nepheline sye- nites, alkaline granites, and alkaline volcanic rocks. Because the REE deposits associated with peralkaline rocks are always lower in grade (<2 wt.% REO) than the carbonatite deposits, so far no project has been successfully devel- oped except the Lovozero deposit in Kola Peninsula of Russia, wherePDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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