A new genetic model for the Triassic Yangyang iron-oxide-apatite deposit, South Korea: Constraints from in situ U-Pb and trace element analyses of accessory minerals

Abstract

The Yangyang iron-oxide-apatite deposit in South Korea has undergone multiple episodes of igneous activity, deformation, hydrothermal alteration, and iron-oxide-apatite (IOA) mineralization. The iron orebodies occur as concordant- to discordant-layered lenticular or massive magnetite and/or magnetite-pyrite ores. The iron mineralization occurs along a N-S-trending shear zone within the Yangyang syenite, which experienced both early ductile and later brittle deformations. Alteration was caused mainly by the injection of hydrothermal fluid through the shear zone, leading to Fe-P mineralization. We recognize multiple stages of alteration in the Yangyang deposit, based on a paragenesis that is defined by distinct mineral assemblages including Na-Ca-K alteration phases (e.g., albite, diopside, actinolite, and biotite) and accessory minerals containing high field strength elements (e.g., apatite, sphene, allanite, and monazite). The alteration around the magnetite ore body shows an evolutionary trend from Ca (-Na) alteration, through K to phyllic alterations. The Fe-P mineralization is associated with the Ca-K and K alteration products. The iron orebodies are hosted by deformed and altered syenite, which intruded the Paleoproterozoic gneiss complexes at 233. ±. 1. Ma (SHRIMP U-Pb zircon age) in a post-collisional tectonic setting. LA-ICP-MS U-Pb dating of REE-rich sphene and apatite from the iron ores and alteration products yields Fe mineralization ages of 216. ±. 3. Ma (sphene) and 212. ±. 13. Ma (apatite). This is the first time, which IOA-type mineralization in the Korean Peninsula was dated as Triassic age related to post-collisional magmatism within the Gyeonggi Massif, South Korea. The U-Pb system was subsequently reset (208. ±. 3. Ma-sphene and 151. ±. 13. Ma-apatite) by Jurassic and Cretaceous magmatism. This unique geological evolution was responsible for Mesozoic metal enrichment and remobilization into suitable structural traps in the Yangyang district.