The nature of mantle source of the Xiarihamu Ni deposit is still under debate, with three possible source regions from asthenosphere, sub- continental lithospheric mantle or pyroxenite mantle. In this study, we collected orthopyroxenite and disseminated ore samples from the drill hole ZK3109, and carried out the in- situ analysis on olivine composition of the No. I intrusion to identify the nature of mantle source, and analyzed the in- situ S isotopic composition of sulfide and whole- rock chalcophile elements to portray the processes of sulfide saturation and segregation. Most olivine grains of orthopyroxenite and disseminated ores have low Mn/Zn ratio (＜13. 5), and most of those from orthopyroxenite have low 100×Mn/Fe ratio (＜1. 35), indicating a pyroxenite mantle component in the source origin. Moreover, the low Ca（＜300. 00×10-6） and high Li with an average value of ~9. 83×10-6 in olivine from orthopyroxenite and disseminated ores, and the positive correlation of Sc and Ni for olivine from orthopyroxenite, suggests that the pyroxenite mantle was formed by the metasomatism of sub- continental lithospheric mantle by materials from the subducted oceanic crust. On the other hand, the negative correlations of Co concentration with Ni concentration, and Fo value of olivine from orthopyroxenite and the ambiguous correlation of S and PGE indicate that the sulfide segregation occurred after the formation of orthopyroxenite. In contrast, the positive correlation of Co and Ni of olivine and the positive correlation of S and PGE of disseminated ores suggest that the olivine composition is mainly controlled by the sulfide segregation. Pentlandite, pyrrhotite and chalcopyrite of orthopyroxenite and disseminated ore have δ34S ranging from 2.28‰ to 6.11‰, distinctly higher than the mantle value (0±2‰), indicating that the S saturation of parental magma in the shallow magma chamber was triggered by the selective addition of crustal S. The modeling results suggest that the different chalcophile elemental compositions of disseminated ores and net- textured ores in the west and east segments of the No. I intrusion were ascribed to different R- values (the mass ratio of the silicate melt to sulfide melt) during sulfide segregation of the parental magma, ranging from 30~3000 and 3000~15000, respectively. Therefore, the combination of mineral, S isotopic and whole- rock chalcophile elemental compositions can be a comprehensive way to effectively identify the nature of mantle source and describe the processes of sulfide segregation of the Cu- Ni sulfide deposits.