The Nixiong ore field, located in the Longgeer-Gongbujiangda fault uplift zone, is an important part of Coqen-Shenzha iron and copper metallogenic belt. The Gunjiu iron deposit lies in the northwest of the Nixiong ore field. The ore bodies mainly occur in the contact zone, which lie between the intrusive granodiorite and mon- zogranite and the Permian Dibucuo Formation, with some also located in the interlayer fracture zone of Dibucuo Formation. The main metallic minerals include magnetite, hematite and maghemite, with small amounts of limonite and goethite, whereas the skarn minerals such as grossular, andratite, diopside, sahlite, ferrosalite, serpentine, phlogopite, actinolite, epidote and chlorite are developed in the Gunjiu iron deposit. Among these minerals the phlogopite is closely associated with magnetite. Electron microprobe analysis and ⁴⁰Ar-³⁹Ar dating of phlogopite were conducted in this paper. Electron microprobe analyses show that the phlogopite from the Gunjiu iron deposit is rich in magnesium but low in iron, with Mg/(Fe +Mg+Mn+Ti) value being 0.90～0.94. The ⁴⁰Ar-³⁹Ar dating of phlogopite does not yield a plateau age or isochron age but a total age of 112.3 Ma. The value represents the time that magnetite started to precipitate and is well consistent with the published LA-ICP-MS zircon U-Pb age of 113.6±1.6 Ma and 112.6±1.6 Ma from granodiorite and monzogranite. The result suggests that the iron mineralization took place in late Early Cretaceous at about 113 Ma. Different from iron mineralization, the copper mineralization mainly occurred in early Late Cretaceous at about 87 Ma. Combined with regional geology and the published data, the authors hold that the Gunjiu iron deposit is related to the southward subduction of Bangong Co-Nujiang Ocean crust. In late Early Cretaceous (113 Ma), the Gangdise and the Qiangtang continental crust started to collide. The subducted oceanic crust of Bangong Co-Nujiang began to rotate under the diving inertia and/or gravity drag. Because of the rotation of the oceanic crust, the asthenospheric fluids were upwelling and caused partial melting of the lithospheric mantle and the overlying crust due to the tremendous heat. At last there formed the mixed magma, which intruded upward under the back-arc extensional condition, forming intrusive rocks and fractionated a large number of ore-forming fluids. Meanwhile, the ore-forming fluids met the stratigraphic formation. There occurred contact metasomatism and eventual formation of the magnetite deposit.