Ore-forming process of the rhythmic layered pegmatite imprinted in quartz: Implications for melt-melt-fluid immiscibility and pressure dependence

Abstract

Cathodoluminescence observations of quartz from the orebody of the Uljin Li-pegmatite reveal at least four distinct formation stages. Large pegmatitic quartz exhibits rhythmic layered textures, featuring interlayered banding with lepidolite and albitite. These quartz grains exhibit three distinct CL colors. The blue CL zone (BCZ), characterized by a coarse-grained, massive appearance, decreases in dominance from the outer to the inner core. Its high Li and Al content, combined with the lowest 518O values, suggest a silicic melt origin. The green CL zone (GCZ) consists of fine-grained quartz and displays sharp boundaries with the BCZ. The mosaic and fracturehealing textures indicate that pressure drops occurred before crystallization. The yellow CL zone (YCZ) is predominantly fine-grained and features fracture-filling textures, typically accompanied by mineral inclusions. Decreasing Al and Li content, along with an increase in K and Na content from GCZ to YCZ, indicates a peralkaline aqueous melt origin. Steady B composition in these pegmatitic quartz suggests non-isobaric segregation of melts, implying pressure change during melt immiscibility. In addition to large pegmatitic quartz, secondary hydrothermal quartz is associated with fine-grained muscovite and albite. Its CL intensities are similar to the YCZ of the pegmatitic quartz, and it has the highest 518O values. The reduced Na composition suggests that crystallization occurred after albitite formation, while the highest Ge composition indicates the lowest formation temperature, suggesting an aqueous fluid origin. Consistent increase in 518O throughout the evolution suggests limited external fluid input during pegmatite formation. These textural and geochemical findings of quartz from the Uljin Li-pegmaite highlight the significant role of the pressure release-build-up process during pegmatite melt evolution. This process triggers melt-melt-fluid immiscibility and results in the formation of rhythmic layered pegmatite cores.