Endogenous rare metal ore formations and rare metal metallogeny of Mongolia

Abstract

Rare metal mineralization in Mongolia is represented by occurrences of rare metals (Sn, W) and rare elements (Ta, Nb, Zr, Be, Li, Cs, Sr, and REE) which can be divided into two groups (magmatic and hydrothermal) according to their composition and genesis. The magmatic group consists of deposits whose origin is mainly a result of processes of igneous concentration of rare elements and includes rare metal granites (e.g., Ta-bearing lithium-fluorine granites and Nb-REE- Zr-bearing peralkaline granites), pegmatites, ongonites, cesium glasses, beryllium tuffs, Nb-REE-Zr pantellerites and comendites, rare metal albite-bearing nepheline syenites, REE-bearing carbonatites and magnetite- apatite alkaline volcano-plutonic rocks. The hydrothermal group comprises hydrothermal deposits of Sn, W, and Mo with a significant enrichment of rare elements (Ta, Li, Be) and fluorine. Rare metal mineralization in both groups is closely related to igneous rocks of a relatively narrow compositional range including peralkaline granites, lithium-fluorine granites, leucogranites, nepheline and pseudoleucite syenites, and corresponding volcanic rocks. Four metallogenic epochs of magmatism and associated rare metal mineralization are distinguished: (1) middle Paleozoic, (2) late Paleozoic, (3) early Mesozoic, and (4) late Mesozoic. The middle 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 Mongolia, where local continental rifting gave rise to the presence of massifs of REE-Nb-Zr alkali granitoids and their subvolcanic analogues, pantellerites. The late Paleozoic metallogenic epoch relates to an active margin of the Californian type, represented by an extensive marginal magmatic belt with numerous continental rift zones featuring basalt-comendite-pantellerite volcanism and massifs of alkali granites, leucogranites, and Li-F granites. REE-Nb-Zr mineralization is associated with alkali granites and pantellerites, whereas Ta, Sn, and W ore occurrences relate to Li-F granites and leucogranites...