Large Fe isotope fractionations in sulfide ores and ferruginous sedimentary rocks from the Kuroko volcanogenic massive sulfide deposits in the Hokuroku district, northeast Japan

•Fe isotopes were variable in ferruginous rocks during and after Kuroko formation.•Anoxic pools in Hokuroku Basin may have facilitated preservation of sulfide ores.•Prominent Eu anomalies in ferruginous rocks indicate association with VMS deposits.•Fe isotopes in Zn-rich black ores are negatively shifted due to a kinetic effect.•Duration of hydrothermal circulation is a primary control on ore grade and tonnage.