ENDOGENOUS RARE-METAL ORE FORMATIONS AND RARE-METAL METALLOGENY OF MONGOLIA

Rare metal mineralization in Mongolia is represented by occurrences of rare metals (Sn, W) and rare elements (Ta, Nb, Zr, Be, Li, Cs, Sr, an d REE) which can be divided into two groups (magmatic and hydrothermal ) according to their composition and genesis. The magmatic group consi sts of deposits whose origin is mainly a result of processes of igneou s concentration of rare elements and includes rare metal granites (e.g ., Ta-bearing lithium-fluorine granites and Nb-REE-Zr-bearing peralkal ine granites), pegmatites, ongonites, cesium glasses, beryllium tuffs, Nb-REE-Zr pantellerites and comendites, rare metal albite-bearing nep heline syenites, REE-bearing carbonatites and magnetite-apatite alkali ne volcano-plutonic rocks. The hydrothermal group comprises hydrotherm al deposits of Sn, W, and Mo with a significant enrichment of rare ele ments (Ta, Li, Be) and fluorine. Rare metal mineralization in both gro ups is closely related to igneous rocks of a relatively narrow composi tional range including peralkaline granites, lithium-fluorine granites , leucogranites, nepheline and pseudoleucite syenites, and correspondi ng volcanic rocks. Four metallogenic epochs of magmatism and associate d rare metal mineralization are distinguished: (1) middle Paleozoic, ( 2) late Paleozoic, (3) early Mesozoic, and (4) late Mesozoic. The midd le Paleozoic metallogenic epoch is marked by the creation of an active continental margin of the Andean type. Rare metal mineralization was emplaced at the rear of the marginal magmatic belt, in northwestern Mo ngolia, where local continental rifting gave rise to the presence of m assifs of REE-Nb-Zr alkali granitoids and their subvolcanic analogues, pantellerites. The late Paleozoic metallogenic epoch relates to an ac tive margin of the Californian type, represented by an extensive margi nal magmatic belt with numerous continental rift zones featuring basal t-comendite-pantellerite volcanism and massifs of alkali granites, leu cogranites, and Li-F granites. REE-Nb-Zr mineralization is associated with alkali granites and pantellerites, whereas Ta, Sn, and W ore occu rrences relate to Li-F granites and leucogranites. The early and late Mesozoic metallogenic epochs are related to the evolution of the Mongo l-Okhotsk collision belt. Igneous activity produced magmatic areas wit h a concentric zonal structure. Granodiorite-granite batholith cores h ave low rare metal mineralization potential, whereas the peripheral zo nes (zones of dispersed magmatism) contain numerous small massifs of l eucogranite, Li-F granite, and alkali granite. These massifs are produ ctive for magmatic and hydrothermal rare metal mineralization. Late Me sozoic rare metal carbonatite and apatite-magnetite alkali volcano-plu tonic complexes occur within the magmatic area formed above the South Hangay hot spot.