Abstract: Olivine is one of the earliest crystallization minerals from basaltic magma, and its composition can directly give the metallogenesis information on parent magma composition, processes of magmatic fractional crystallization, and sulfide segregation degree. In this study, the authors selected the Tudun mafic-ultramafic complex to conduct electron microprobe analysis of olivine minerals. According to the Fo and Ni values of olivine, he MgO content of parent magma, which contains the highest Fo olivine, was estimated to be 12.95%, indicating that the parent magma of the Tudun complex is a high-MgO basalt. The modeling results show that sulfide segregation and fractional crystallization almost took place at the same time. A slight sulfide segregation (about 0.2%) occurred during the early stage (0-2%) of olivine fractional crystallization process, and less sulfide segregation (about 0.01%) occurred during the later stage (6%-7%), which suggests that the metallogenic potential of the Tudun complex is limited. In addition, some data implying the negative relationship between Fo and Ni indicate that Fe-Ni exchange reaction happened between Fe-rich olivine and trapped sulfide liquid, which caused remarkable differences in olivine composition.