Fe isotope behaviours during sulfide-dominated skarn-type mineralisation

Abstract

Fe isotope compositions of mineral separates and bulk samples from Dongguashan and Fenghuangshan skarn type Cu–Fe deposits in Tongling District, Middle–Lower Yangtze Valley Cu–Fe–Au–Mo polymetallic belt have been investigated. Differences in δ57Fe values exist in co-precipitated bornite–chalcopyrite and pyrite–chalcopyrite mineral pairs. The bornite is enriched in light iron isotopes relative to coexisting chalcopyrite, whereas pyrite samples show a tendency of heavy isotopes enrichment relative to chalcopyrite. Moreover, the δ57Fe values of endoskarn and the earliest formed Fe-mineral phase magnetite of the three deposits investigated in Tongling district are consistently lower relative to ore-related igneous rocks, suggesting Fe isotopic fractionation occurs during fluid exsolution and exsolved fluid is enriched in light Fe isotopes relative to the stock. Furthermore, spatial and temporal variations in δ57Fe values within single mineral phases are observed, suggesting iron isotopic fractionation occurs between mineral and fluid phases as a result of precipitation of Fe-bearing minerals during fluid evolution. Overall, according to Fe isotope compositions of three representative skarn deposits formed under the same geological setting in Tongling district, this study indicates that Fe isotopes have the potential to unraveling ore-forming processes, as well as constraining the metal sources of skarn deposits. Combining with results of previous studies, a protocol for using Fe isotopes to trace skarn-type metallogeny is proposed.