Magma fractionation and the magmatic-meteoric fluids mixing drive the sequential deposition of cassiterite to lepidolite in the Uljin prospects, South Korea

Abstract

Several SnLi prospects in the Uljin area, which is in the eastern part of South Korea, including Dongseok, Yuchang, Buguk, Chomak, Hyundong, and Boam, contain sub-economic cassiterite and Li-bearing silicate minerals such as lepidolite and spodumene, and phosphate mineral such as amblygonite, along with accessory Nb-Ta-bearing minerals. Among these, Dongseok and Yuchang are Sn-dominant, Buguk, Hyundong, and Chomak exhibit mixed SnLi mineralization, while Boam (Western, Main, Eastern) is Li-rich. The SnLi mineralization is associated with a series of hydrothermal alterations, particularly greisenization and albitization. Two stages of greisenization have been identified, with albite alteration occurring between them. Cassiterite (± NbTa minerals) precipitated during early greisenization (Greisen I), whereas spodumene crystallized as a primary mineral in subsequent pegmatite intrusion, lepidolite and amblygonite formed during late-stage greisenization (Greisen II). SEM-CL analysis distinguishes two types of cassiterite: CL-brighter, oscillatory-zoned type I (commonly found in Dongseok) and CL-darker, massive type II (dominant in Yuchang, Buguk, and Chomak).

Muscovite ArAr dating constrains the timing of greisenization, placing Greisen I (cassiterite formation) at 168–174 Ma and Greisen II (lepidolite formation) at ~166 Ma. These overlapping ages suggest that the SnLi mineralization was derived from a fractionating magmatic batholith. Fluid inclusions in cassiterite and quartz contain CO₂, CH₄, and N₂, with CH₄/CO₂ ratios indicating a reducing environment during early cassiterite precipitation. A positive correlation between homogenization temperatures and salinities in the fluid inclusions suggests the mixing of magmatic and meteoric fluids during cassiterite and lepidolite precipitation. The transition from Sn to Li mineralization was driven by progressive oxidation and cooling, as reflected in CO₂/CH₄ ratios and cassiterite chemistry. Higher-temperature (>300 °C), lower-salinity fluids (>8 wt% NaCl eq.) formed cassiterite during Greisen I, while later, somewhat cooler (>250 °C), more saline fluids (>10 wt% NaCl eq.) facilitated lepidolite precipitation during Greisen II. This magmatic-hydrothermal evolution, potentially controlled by batholith-scale magma fractionation, resulted in these sequential Sn and Li mineralization in the Uljin area.