From north to south, there are three metallogenic belts in the Gangdise metallogenic region of the Tibetan Plateau, in which the northern metallogenic belt has a more abundant mineral composition (Fe-Cu-Pb-Zn-Mo) than the other two belts. The developmental mechanism of polymetallic coexistence has become the key of regional metallogenetic regularity research. In this paper, the authors studied the Lietinggang-Leqingla Fe-Cu-Pb-Zn deposit. Based on the detailed study of muscovite in Fe ore-forming stage, the authors conducted a new exploration of this problem from the duration of ore-forming fluid evolution. The systematic field work, microscopic observation and electron microprobe analyses show that the magnetite-associated muscovite is developed in the Lietinggang-Leqingla deposit. Ar-Ar isotopic dating shows that the ⁴⁰Ar/³⁹Ar plateau age of muscovite is 51.00±0.38 Ma and the isochron age is 50.45±0.62 Ma. Systematic analysis suggests that this age represents the age of the muscovite crustallization during the oxide stage of the Lietinggang-Leqingla deposit after the closure of the Ar-Ar system. Compared with the molybdenite age (61.96±0.58Ma), the authors hold that the difference of ages may be attributed to the different closure temperatures of different isotopes in different minerals. It is believed that the hydrothermal evolution process lasted a long time. The occurrence of temperature gradients caused by this long hydrothermal evolution process might have aggravated the differentiation of polymetallic materials (such as Pb, Zn, Cu and Fe) in magmatic hydrothermal fluid, and provided an important condition for the coexistence of various metal minerals. From the perspective of the metallogenic belt, the Gangdise north mineralization belt had a longer magmatic evolution time and mineralization duration compared with the other belts, which is consistent with its rich mineral assemblage.