Magma oxygen fugacity of Permian to Triassic Ni-Cu sulfide-bearing mafic-ultramafic intrusions in the central Asian orogenic belt, North China

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 fO2 of magma using olivine-spinel equilibria. The obtained fO2 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 fO2 of the Poyi intrusion is progressively elevated with the decrease of Fo contents of olivine. The fO2 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 fO2 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 Fo90-91 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 fO2 of FMQ-1.5 to FMQ, which can be taken as the fO2 of the mantle either prior to or after the closure of the paleo-Asian ocean. The calculated fO2 for the mantle sources of the two intrusions are slightly higher than the upper fO2 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.