Zircon U-Pb age, differentiation process, and its constraints on Rb mineralization of the ore-bearing granites of the Baitoushan rubidium deposit in the Beishan area

The Baitoushan rubidium deposit is located in the Beishan orogenic belt, which is a newly discovered super-large rubidium deposit.The garnet-bearing leucogranite is the important ore-bearing rock and is crucial for understanding the mechanism of rubidium enrichment and mineralization of the deposit.The Baitoushan garnet-bearing leucogranite is enriched in silicon(SiO₂=73.56%~75.60%), alkali(Na₂O+K₂O=8.84%~10.39%), and aluminum(Al₂O₃ 14.41%~15.01%), lower in Mg, Fe, Ca, P, and Ti.The Al saturation index is high(A/CNK=0.98~1.14).In terms of trace elements, the Baitoushan garnet-bearing leucogranite is enriched in Rb, Th, U, and Ta, relatively depleted in Ba, Sr, P, and Ti, with strong Eu negative anomalies(Eu/Eu^*=0.02~0.03).The petrological, mineralogical, and geochemical characteristics of the Baitoushan garnet-bearing leucogranite indicate that it is a highly fractionated S-type granite.Fractional crystallization of mica, plagioclase, and zircon may have occurred in this granitic system during magmatic evolution.The total amount of rare earth elements is low(REE=32.06×10^-6~45.33×10^-6), with an obvious tetrad effect(TE_1.3=1.28~1.31).Combined with the non-CHARAC(no radius-controlled) behavior of the characteristic element pairs(Zr/Hf, Nb/Ta, Y/Ho, K/Rb), it reflects that they also underwent a strong differentiation evolution with melt fluid interaction.The extensive magmatic differentiation of highly fractionated granites and F-rich fluid interaction are the key controlling factors for the enrichment and mineralization of rubidium and other rare metals.Zircon U-Pb dating shows that the crystallization age of the garnet-bearing leucogranite is 226±3.8 Ma.Combined with previous studies, it is suggested that the Triassic is an important period for the formation of highly fractionated granites and associated rare metal mineralization in the Eastern Tianshan-Beishan region, which will provide a new direction for prospecting rare metals in the study area.