FORMATION CONDITIONS OF LARGE AND RICH RARE-METAL DEPOSITS

The largest and richest magmatic deposits of oxyphile rare metals are confined to ancient platforms or Precambrian median massifs; the most important conditions for the formation of such deposits exist in the e arth's ancient crust. The largest and richest lithium and cesium depos its are generated by intracrustal granite magmatism, whereas those of all other oxyphile metals are genetically related to alkaline subcrust al magmatism. The largest and richest deposits of lithium are associat ed with albite-spodumene pegmatites; of cesium, with spodumene-microcl ine-albite pegmatites with cesium-biotite metasomatites and pollucite; of beryllium, with fluorite metasomatites with phenakite and bertrand ite; of yttrium, with yttrium lanthanides and tantalum with alkaline g ranites; of cerium lanthanides, with carbonatite alkaline of syenite m assifs; of niobium, with carbonatites of alkaline-ultramafic massifs; and of zirconium and hafnium, with eudialyte lujavrites of nepheline-p otassium feldspar massifs.