Abstract: The Cihai iron deposit, located in the western Beishan tectonic-metallogenic belt, is mainly composed of Cihai ore block, in which the orebodies hosted by diabases in direct contact show stratiform and lentoid forms, whereas other orebodies in Cihai ore block are hosted by garnet-diopside-skarns.Based on the study of field geochemical characteristics and ore microscopy, the authors hold that the formation processes of the Cihai iron deposit involved two periods:① ore-magma period characterized by magnetite+clinopyrox-ene+hornblende formed almost simultaneously, and ② skarn period which contained garnet+diopside+magnetite formed earlier and tremolite/actinolite+quartz+pyrrhotine+chalcopyrite+pyrite formed at late stage.The geochemical and Fe-S-O isotopic data of main minerals in two periods show that the metallogenic iron was mainly derived from the upper mantle, and the ore-forming fluid in skarn period inherited that of the ore-magma period.In combination with previous studies, it is suggested that the iron mineralization of the Cihai iron deposit occurred in a setting of Permian mantle plum in Tarim.Targeted on iron ores in diabases and based on geological, geo-chemical and isotopic data and previous researches, the authors hold that the formation process of the Cihai deposit could be divided into immiscibility and injection of iron-rich silicate liquids(primary process)and hydrothermal metasomatic process.The iron mineraliza-tion occurred in a setting of Permian mantle plum in Tarim Basin and the metallogenic magma was a branch of mantle plume.