Redox state of the Baogutu reduced porphyry Cu deposit in the Central Asian Orogenic belt

•The primary magma of Baogutu reduced porphyry Cu deposit is oxidized and its reduced features are secondary.•δ56Fe of less altered plutonic rocks plot into the field of arc magma, well consistent with the high oxygen fugacity of primary magma.•Plutonic rocks and wall rocks turn to be heavy Fe isotopes enrichment during the hydrothermal alteration.•This is the first time to report the Fe isotopic composition and fractionation of reduced porphyry Cu deposit.•Fe isotopes may be a potential tool to constrain the redox state of primary magma and reveal the origin of reduced porphyry Cu deposit.