Large-scale Permian-Jurassic rare-metal mineralization in the Altai Orogenic Belt: A review

The Altai Orogenic Belt, a key part of the Central Asian Orogenic Belt, is an important rare-metal (Li–Be–Nb–Ta–Sn–W) metallogenic province in the world, containing pegmatite deposits enriched in LCT (Li, Cs, Ta), granite-hosted Li-Rb deposits, ongonite Li deposits, greisen and quartz-vein hosted Sn-W deposits. The Altai provinde comprises three metallogenic belts in three mineralization periods: early Permain (297–272 Ma) Kalba-Narym belt, late Permian (274–253 Ma) South Altai belt and late Triassic–early Jurrasic (230–180 Ma) North Altai belt from south to north, and the largest number of deposits occurred in the last period North Altai belt. The rare metal mineralization is contemporaneous with granitic intrusions where some intrusions are barren, and the other are mineralized. Barren granites are less fractionated and large (>10 km² on the surface), and were emplaced at high temperature (700–850 °C), whereas the mineralized granites, related to the mineralization, are strongly fractionated and small (<5 km² on the surface), and were emplaced at low temperature (600–650 °C)_. Hf and Nd isotope data suggest that barren and mineralized magmas were derived from juvenile and ancient crust. The predominant rare-metal mineralzation is associated with granites derived from ancient crust. The magmatic fractionation model has been widely accepted for the genesis of the rare metal deposits, including LCT pegmatite deposits, granite-hosted Li-Rb deposits, greisen and quartz-vein hosted Sn-W deposits. Our data suggest melting of Devonian granite with enrichment in rare metals and magmatic fractionation explains the mineralization in the North Altai belt.

The mineralization in the Kalba-Narym and South Altai belts took place during the Permian collision between Siberia Craton and Kazakhstan-Junggar Block after the closure of the Irtysh-Zaisan Ocean. In contrast, the deposits in the North Altai belt formed during the Mesozoic intraplate extension. The sites for pegmatite and granite deposits coincide with the transitional zone of positive and negative gravity anomalies in the proximity to the regional fault, suggesting that this zone is favourable for the intrusion of the mineralized magma. On the other hand, the areas with negative gravity anomalies host greisen-quartz-vein Sn-W deposits, confirming that the Sn-W deposits are hosted by low-density rocks. Therefore, except for strongly magmatic fractionation, the existence of enriched Devonian granite, site of steep gravity anomaly gradient, derivation of magma from ancient crustal rocks and the tectonic setting from the contraction to extension in the Mesozoic played a key role in the formation of large-scale rare metal mineralzation in the North Altai belt.