The Eastern Tianshan Orogenic Belt (ETOB) in NW China is composed of the Dananhu–Tousuquan arc belt, the Kanggurtag belt, the Aqishan–Yamansu belt and the Central Tianshan belt from north to south. These tectonic belts have formed through arc–continent or arc–arc collisions during the Paleozoic. A number of Fe(-Cu) deposits in the Aqishan–Yamansu belt, including the Heifengshan, Shuangfengshan and Shaquanzi Fe(-Cu) deposits, are associated with Carboniferous–Early Permian volcanic rocks and are composed of vein-type magnetite ores. Metallic minerals are dominated by magnetite and pyrite, with minor chalcopyrite. Calcite, chlorite, and epidote are the dominant gangue minerals. Pyrite separates of ores from those three deposits have relatively high and variable Re contents ranging from 3.7 to 184 ppb. All pyrite separates have very low common Os, allowing us calculation of single mineral model ages for each sample. Pyrite separates from the Heifengshan Fe deposit have an 187Re–187Os isochron age of 310 23 Ma (MSWD = 0.04) and a weighted mean model age of 302 5 Ma (MSWD = 0.17). Those from the Shuangfengshan Fe deposit have an isochron age of 295 7 Ma (MSWD = 0.28) and a weighted mean model age of 292 5 Ma (MSWD = 0.33). The Shaquanzi Fe-Cu deposit has pyrite with an isochron age of 295 7 Ma (MSWD = 0.26) and a weighted mean model age of 295 6 Ma (MSWD = 0.23). Pyrite separates from these Fe(-Cu) deposits have d34SCDT ranging from -0.41‰ to 4.7‰ except for two outliers. Calcite from the Heifengshan Fe deposit and Shaquanzi Fe-Cu deposit have similar C and O isotope compositions with d13CPDB and d18OSMOW ranging from -5.5‰ to -1.0‰ and from 10‰ to 12.7‰, respectively. These stable isotopic data suggest that S, C, and O are magmatichydrothermal in origin. The association of low-Ti magnetite and Fe/Cu-sulfides resembles those of Iron–Oxide–Copper–Gold (IOCG) deposits elsewhere. Our reliable Re–Os ages of pyrite suggest that the Fe(-Cu) deposits in the Aqishan–Yamansu belt formed at ~296 Ma, probably in a back-arc extensional environment.