Rare metal enrichment during magmatic-hydrothermal evolution in lithium-cesium-tantalum pegmatite: Evidence from in situ elemental and boron-isotopic variations in micas from the Shaliuquan pegmatite along the northern margin of the Qaidam Basin, China

Lithium-cesium-tantalum (LCT) pegmatites are a type of peraluminous pegmatites enriched in a variety of rare metal elements, such as Li, Be, Rb, Cs, Nb, Ta, Sn, and W, and they are generally considered to evolve from parental S-type granites through extreme fractionation. The mechanism underlying the enrichment of these rare metals in S-type granites and LCT pegmatites is not yet fully understood. We conducted a comprehensive textural, in situ elemental, and in situ boron (B)-isotopic analysis of primary and secondary muscovite from the Shaliuquan pegmatite in the Quanji Massif, located at the northern edge of the Qaidam Basin, China. The internally zoned pegmatite comprises four main distinct textural and lithological zones from border to core: a medium-grained muscovite-tourmaline (MT) zone, a muscovite-microcline (MM) zone, a quartz-muscovite (QM) zone, and a Li-muscovite (LM) zone. The increasing concentration of rare metal (Li, Rb, Cs, Nb, and Ta) and fluxing (Li, F, and B) elements—along with decreasing K/Cs ratios—in primary muscovite from the MT zone to the LM zone suggests that fractional crystallization played an important role in producing compositional variations during the magmatic stage. Modeling of elemental ratios (e.g., K/Rb and K/Cs) suggests that Rayleigh fractionation continued during this stage. However, the B-isotopic composition of primary muscovite decreases at the transition from the QM to LM zones, deviating from the expected Rayleigh fractionation trend. This suggests that the crystallization environment transitioned from a silicic, fluid-undersaturated melt to a fluid-saturated melt. Additionally, textural and chemical differences between primary and secondary muscovite indicate that fluid-crystal interactions were active throughout the magmatic-hydrothermal evolution. Specifically, secondary muscovite in the MM, QM, and LM zones exhibits lower concentrations of Rb, Nb, and Ta compared to its primary counterpart, suggesting its crystallization from a fluid-dominated system through fluid-crystal interactions. This study highlights that Li enrichment in LCT pegmatite systems can be explained by a combination of extreme fractional crystallization and fluid-crystal interactions. However, the depletion of Li in secondary muscovite from the LM zone, relative to primary muscovite, suggests that fluid-crystal interaction contributes not to Li enrichment, but rather to its redistribution.