Metasomatic changes during periodic fluid flux recorded in grandite garnet from the Weondong W-skarn deposit, South Korea

Abstract

The Weondong skarn deposit primarily consists of grandite (andradite-grossular) garnets, which exhibit poikilitic/intergrowth textures and compositional zoning in metasomatism. The garnets can be classified into three main types based on their textural characteristics and rare earth element (REE) concentrations: (1) exoskarn garnet near quartz porphyry with a poikilitic texture and oscillatory zoning (core: And31–50, rim: And54–63) (T1 garnet), (2) exoskarn garnet near quartz porphyry with intergrowths (core: And55–83, mantle: And55–59, rim: And77–97) and poikilitic textures (T2 garnet), and (3) vein-hosted garnet with reverse zoning (core: And70–96, rim: And31–36) and epitaxial growth on distinct cores (T3 garnet). The textural features of the garnets are considered to have been caused by periodic fluctuations. The chondrite-normalized REE patterns of the garnet types show that the Al-rich garnets are HREE-enriched and formed at equilibrium with slow growth rates according to the kinetics of the garnet's growth. In contrast, Fe-rich garnets are slightly LREE-enriched and HREE-depleted. In terms of the kinetics of the garnet's growth, the Fe-rich garnets grew rapidly from externally derived fluids during infiltration metasomatism. The enriched W contents in the grandite garnets also indicate the type of growth mechanism, the infiltration process, and a specific skarn environment (such as advective metasomatism). Furthermore, each textural type of garnet was influenced differently by fluid/rock interactions, the magnitude of the fluid flow and the fluid compositions in the skarn system. Consequently, periodic fluid fluctuation in the skarn system affected the morphology and composition of individual garnet grains. Thus, the geochemical features of the grandite garnet from the Weondong polymetallic deposit provide key information to understand the evolution of the skarn deposit and its metasomatic history.