Graphical abstract Highlights • Permian to Triassic Ni-Cu sulfide-bearing mafic-ultramafic intrusions in the CAOB formed from oxidized mafic magmas. • The magmas were derived from the mantle sources with f O 2 slightly higher than MORBs. • The magmas may become more oxidized with fractionation process. • The oxidized magmas could contain enough sulfur to form Ni-Cu sulfide deposits in the CAOB. Abstract A number of Permian to Triassic mafic-ultramafic intrusions along the central Asian orogenic belt (CAOB) host either economically important or potential Ni-Cu sulfide mineralization. These intrusions were proposed to have emplaced in a post-subduction, extensional setting. In this study, we chose rocks from the Poyi, Huangshanxi, Huangshandong, and the Hongqiling No. 2 intrusions in the CAOB that contain olivine and spinel pairs to calculate the f O 2 of magma using olivine-spinel equilibria. The obtained f O 2 of the magmas overall range from FMQ+0.3 to FMQ+2.6, much higher than those for MORBs (FMQ-1 to FMQ). In addition, the obtained magma f O 2 of the Poyi intrusion is progressively elevated with the decrease of Fo contents of olivine. The f O 2 of the mantle source of the Hongqiling No.2 intrusion in the eastern part of the CAOB was calculated to be ∼FMQ+0.5 with the compositions of olivine and spinel during partial melting of the mantle being simulated by an inverse model. The f O 2 of the mantle source of the Poyi intrusion in the western part of the CAOB was calculated to be ∼FMQ+0.3 using the ultramafic rocks containing Fo 90-91 olivine in the intrusion. In addition, the V/Sc of the mantle peridotite and basalts from Paleozoic (533–260 Ma) ophiolites in the CAOB yielded the f O 2 of FMQ-1.5 to FMQ, which can be taken as the f O 2 of the mantle either prior to or after the closure of the paleo-Asian ocean. The calculated f O 2 for the mantle sources of the two intrusions are slightly higher than the upper f O 2 limit of the mantle sources (FMQ-1.5 to FMQ) obtained from Paleozoic ophiolites in the CAOB and the mantle sources for MORBs (FMQ-2.5 to FMQ). Permian to Triassic Ni-Cu sulfide-bearing, mafic-ultramafic intrusions in the CAOB are therefore considered to have formed from relatively oxidized mafic magmas that may have been derived from the metasomatized mantle sources. The magmas may have become more oxidized with the fractionation process until they were reduced by organic-rich crustal components to reach sulfide saturation, forming economically important Ni-Cu sulfide deposits. [ABSTRACT FROM AUTHOR]