LATE MESOZOIC RARE-METAL GRANITES, PEGMATITES, AND GREISENS OF MONGOLIA: AGE, MINERALOGY, GEOCHEMISTRY, ORE POTENTIAL, AND PETROGENESIS

Comparative analysis of the earlier obtained and new geological data, age, mineral, petrological, and geochemical compositions of plutons of calc-alkalic granitoids and rare-metal Li-F granites shows their evolution in a wide time interval during the formation of Mesozoic areas of granitoid magmatism. Mineralogical and geochemical analysis of the evolution of plutons of palingenetic calc-alkalic granitoids (Baga Hentiyn (MZ1) and Ikh Narotiin = Hiid (MZ2)) and intrusions of rare-metal Li-F granites of Central and Eastern Mongolia revealed their petrological and geochemical differences. The closure of the Mongol-Okhotsk Basin with the formation of large plutons of calc-alkali & scy; granitoids, obviously related to collisional processes, did not cause significant enrichment of the late granite phases with lithophile and ore elements. Within the peripheral zones of & iecy;& rcy;& ucy; MZ1 and MZ2 magmatic areas, mineralization is often associated with Mongolian multiphase plutons and small intrusions of rare-metal Li-F granites. The rare-metal granites are characterized by a decrease in indicative K/Rb, Nb/Ta, and Zr/Hf values and a regular increase in F, Li, Rb, Cs, Sn, W, Be, Ta, and Nb contents during the evolution of Li-F magmas. Igneous and, particularly, metasomatic rocks in most intrusions of ore-bearing rare-metal Li-F granites are characterized by significant variations in Sn and W contents. At the magmatic stage, the pegmatoid varieties of amazonite-albite granites and pegmatites of the zonal Baga Gazriin (MZ1) and Barun Tsogto (MZ2) plutons are significantly enriched in both Sn and W. Maximum Sn and W enrichment has been established in greisenized granites and zoned greisen bodies (zwitters), which is due to the percolation of ore-bearing solutions into the upper horizons and the mineralization of ore elements in the late phases of intrusions and in metasomatites. The wide variations in the age (321-126 Ma) and trace element and isotope compositions of Mongolian rare-metal Li-F granites within various zones of large magmatic areas suggest the influence of mantle plume sources on the composition of rare-metal granitic magmas and on their ore potential in intermediate chambers in the continental crust.