Tracing magmatic and hydrothermal processes in rare-metal granites using zircon geochemistry: the Janchivlan pluton, Central Mongolia

Distinguishing magmatic from hydrothermal processes in rare-metal granite systems is critical for understanding ore formation; however, zircons in these rocks are commonly affected by fluid-mediated modification, complicating the interpretation of both geochronological and geochemical signatures. We investigated zircon crystals from the Janchivlan rare-metal granite complex (Central Asian Orogenic Belt, Mongolia), a highly fractionated peraluminous system evolving from biotite-granite through graphic granite, amazonite-bearing to albite-lepidolite granite, with potential for Sn, W, Ta, and Li mineralization, as well as associated pegmatites. These lithologies record successive stages of magmatic differentiation and increasing fluid involvement.

Zircon U-Pb dating yields concordant ages of 290 ± 2.1 Ma for pegmatite and 195 ± 2.1 Ma for biotite-granite, indicating that the pegmatites formed from a different magmatic event. Zircon single-grain ages from biotite and lepidolite granites define a discordia with lower intercepts at 220 ± 2.1 Ma and 195 ± 2.1 Ma, respectively, interpreted as Pb loss during hydrothermal alteration. This interval overlaps with the apatite U-Pb age of 213 ± 3.7 Ma, supporting hydrothermal activity at this time.

Zircon REE patterns show a systematic evolution from biotite granite, characterized by (1) high ΣREE, moderate Eu/Eu* (~0.1–0.2), through graphic and amazonite granites with variable REE distributions and weak tetrad effects, to (2) lepidolite granite marked by LREE depletion, very low Eu/Eu* (<0.05), and pronounced tetrad effects. These trends document progressive melt fractionation accompanied by increasing melt-fluid interaction. Whole-rock geochemistry shows element-specific decoupling from zircon fertility: biotite-granite displays high Ta–W–Li–Rb concentrations, reflecting accumulation in biotite and accessory phases before fluid exsolution; amazonite granite records Sn and Pb enrichment during an intermediate fractionation window; and albite-lepidolite granite exhibits extreme Li and Rb enrichment but no corresponding enrichment of Ta, W, or Sn despite representing the most evolved stage. This pattern indicates that exsolution of fluids selectively redistributed fluid-compatible metals, while Li and Rb were mostly retained in late-crystallizing mica phases, producing distinct metallogenic stages within the granite system. These findings show that rare-metal mineralization in highly fractionated granites results from a multi-stage process where magmatic differentiation establishes initial metal budgets, but subsequent fluid exsolution and melt-fluid partitioning govern the ultimate distribution and concentration of specific ore metals.