Magnetite records high-δ18O fluid evolution in the Shinyemi Fe skarn deposit, South Korea

Carbonate-hosted Fe skarn deposits form through reactions between magmatic-hydrothermal fluids and carbonate wall rocks, but the effects of magmatic input, wall-rock interaction, and later hydrothermal modification are difficult to separate. Here we combine zircon U–Pb geochronology, phlogopite 40 Ar/ 39 Ar dating, magnetite trace-element chemistry, Ti + V-in-magnetite thermometry, and Fe–O isotopes to examine fluid evolution in the Shinyemi Fe skarn deposit. Zircon U–Pb ages indicate Late Cretaceous emplacement of quartz monzonite and granite at ca. 84–81 Ma, followed by emplacement of an intermediate dyke at 80.4 ± 0.8 Ma. A phlogopite 40 Ar/ 39 Ar weighted mean apparent age of 84.6 ± 2.5 Ma provides a constraint on hydrothermal cooling or alteration. Magnetite has low Ti–V contents and variable Mg, Mn, Al, Si, and Ca contents, consistent with skarn-type hydrothermal magnetite. Ti + V-in-magnetite thermometry yields a median temperature of 385 °C ( n = 304). Magnetite δ 56 Fe values range from − 0.12 to + 0.31‰, and δ 18 O values range from + 2.6 to + 4.4‰, whereas calculated apparent δ 18 O fluid values range from + 11.1 to + 12.8‰. Together, the trace-element and isotope data are consistent with magmatic-hydrothermal Fe input and subsequent oxygen-isotope modification during interaction with carbonate wall rocks and later hydrothermal processes.