Iron isotope fractionation during skarn-type metallogeny: A case study of Xinqiao Cu–S–Fe–Au deposit in the Middle–Lower Yangtze valley

Fe isotope compositions of mineral separates and bulk samples from Xinqiao Cu–S–Fe–Au skarn type deposit were investigated. An overall variation in δ57Fe values from − 1.22‰ to + 0.73‰ has been observed, which shows some regularity. The δ57Fe values of endoskarn and the earliest formed Fe-mineral phase magnetite are ca.1.2‰ and ca. 0.3‰ lower, respectively, relative to the quartz–monzodiorite stock, indicating that fluid exsolved from the stock is enriched in light Fe isotopes. Moreover, spatial and temporal variations in δ57Fe values are observed, which suggest iron isotope fractionation during fluid evolution. Precipitation of Fe-bearing minerals results in the Fe isotope composition of residual fluids evolving with time. Precipitation of Fe (III) minerals incorporating heavy iron isotopes preferentially leaves the remaining fluid enriched in light isotopes, while precipitation of Fe (II) minerals preferentially taking-up light iron isotopes, and makes the Fe isotopic composition of the fluid progressively heavier. The regularity of Fe isotope variations occurred during fluid exsolution and evolution indicates that the dominant Fe source of Xinqiao deposit is magmatic. Overall, this study demonstrates that Fe isotope composition has great potential in unraveling ore-forming processes, as well as constraining the metal sources of ore deposits.

Research Highlights
► δ57Fe values of Xinqiao skarn-type ore deposit varies from −1.22‰ to +0.73‰.
► Spatial and temporal variations in δ57Fe values have been observed.
► Fe isotopes fractionate during fluid exsolution and fluid enriches in light isotope.
► Fe isotope composition of the fluid evolves during mineralization.
► Fe in Xinqiao deposit is mainly from igneous source as constrained by Fe isotopes.