The north Gangdise Pb-Zn-Ag-Cu-Fe belt is one of the most important metallogenic belt in the Tibetan collisional orogenic belt. The Longmala deposit is located in the eastern part of the Gangdise metallogenic belt. Constraining the timing of mineralization of this deposit is crucial to the better understanding of the genesis of the deposit and the deep geodynamic processes which controlled the large-scale mineralization of this area. The electron microprobe analyses reveal that the altered mica is phlogopite. This paper presents the Ar-Ar dating results for magnetite mineralization conducted on phlogopites collected from the Longmala deposit. Phlogopite from the Longmala deposit yielded a well-defined ⁴⁰Ar/³⁹Ar plateau age of 56.32±0.48 Ma and an isochron age of 55.58±0.93 Ma. These ages suggest that the Longmala deposit was developed in the period of main collisional orogenic setting in Tibetan orogenic belt. Combined with the former research and the data of regional geology, the authors hold that the deep geodynamic process which controlled the formation of this deposit was the rollback of the Neo-Tethys slab.