The SHRIMP zircon U-Pb geochronology and micro-structural study of the albite-spodumene pegmatite from the Boam Li deposit in Uljin, South Korea

The Boam Li deposit in Uljin, South Korea, is a hard-rock Li deposit composed of albite-spodumene pegmatite and lepidolite-elbaite greisen. The timing of Li mineralization in the Boam deposit is poorly constrained due to a wide range of the mica K-Ar ages of the greisen from 176 to 127 Ma. We investigated the microstructure, mineral composition, and U-Pb age of zircons separated from the albite-spodumene pegmatite of the Boam Li deposit. Zircons are found as inclusions in spodumene and discrete grains intergrown with secondary albite, muscovite, columbite group mineral (CGM), microlite, and apatite, indicating that the zircon was saturated during the intrusion of pegmatite melt and subsequent greisenization of the Boam deposit. Most zircons have altered porous domains in cores mantled by overgrown rims with contrasting microstructures and chemical compositions. The porous domains in zircon cores contain Ca, Al, and Fe- bearing zircon solid solution, quartz, K-feldspar, muscovite, CGM, and microlite, which indicates pervasive alteration during greisenization. Higher U contents and Zr/Hf ratios of the core than the rim suggest that the melt from which the zircon cores crystallized was U-rich and less evolved. The overgrown rim domain with weak oscillatory zoning occurs in association with albite, muscovite, and apatite. Based on the microstructure and zircon compositions, we suggest that the SHRIMP U-Pb age of the zircons from the pegmatite indicates the timing of greisenization and Li mineralization in the Boam deposit. The zircon core and rim U-Pb isotope data constitute a single discordia line, yielding a weighted mean age of 172.4 [+ or -] 1.1 Ma (MSWD = 2.3). The age is consistent with the oldest mica K-Ar age of the lepidolite-elbaite greisen in Uljin. Key words: albite-spodumene pegmatite, greisen, zircon U-Pb geochronology, Boam deposit, Uljin