Magmatic to aqueous phase transition in Li-pegmatite: microtextural and geochemical study of muscovite–lepidolite from Boam mine area, Uljin, South Korea
Woohyun Choi, Changyun Park, Chul-Ho Heo, Seok-Jun Yang, Il-Hwan Oh, Kyung Su Park, Sung Hwa Choi
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引用次数: 0
Abstract
This study investigated the Boam mine area, a prominent Li-pegmatite deposits located in South Korea, using Li-bearing micas to determine the magmatic–aqueous transition involved in rare-element pegmatite formation. Muscovite–lepidolite series micas from the layered pegmatite exhibited six textures, classified into three stages (early, intermediate, and late) based on compositions of major and trace elements. The substitution mechanisms of muscovite–lepidolite series micas follow lithium fixation (Si ↔ Li + Al) and phengitic substitution (Aliv + 2Alvi ↔ Li + (Fe2+, Mg2+, Mn2+) + Si) vectors. Early-stage micas displayed a large grain size due to rapid crystal growth due from low undercooling. Diffusional zonation of these micas with the higher Nb–Ta and lower Li concentrations compared with later-stage lepidolite indicate a lower degree of fractionation. These features suggest a silicic melt origin for early-stage micas. Intermediate-stage micas are distinctly separated from the early-stage type and feature erratic boundaries with higher Li composition. B enrichment reduced the melt viscosity and increased the H2O solubility, resulting in an increase in growth rate and retardation of mineralization. The inhibition of HFSE partitioning by B lead to a lower Nb–Ta concentration than the silicic melt, suggesting the existence of an aqueous melt. Fine-grained late-stage mica coexists with microcrystalline quartz, and is characterized by Cs enrichment and Nb–Ta depletion that exclusively occur in flux-rich aqueous fluids. Non-Rayleigh behavior of K-Rb-Cs indicates a deviation from fractional crystallization unlike melt phases, suggesting an aqueous fluid origin for late-stage micas. Consequently, the formation of Li-pegmatite in the deposit was predominantly controlled by the immiscibility of silicic melt–aqueous melt–aqueous fluid and fractional crystallization within each medium.
期刊介绍:
The journal Mineralium Deposita introduces new observations, principles, and interpretations from the field of economic geology, including nonmetallic mineral deposits, experimental and applied geochemistry, with emphasis on mineral deposits. It offers short and comprehensive articles, review papers, brief original papers, scientific discussions and news, as well as reports on meetings of importance to mineral research. The emphasis is on high-quality content and form for all articles and on international coverage of subject matter.