The studies, including optical petrography and mineragraphy, EPMA and XRD, show that four different generation phyllosilicate (micas) were distinguished petrographically in Lannigou gold deposit of Guizhou Province: clastic flakes of muscovite, diagenesis or metamorphic hydromuscovite, fine-grained matrix alteration illite, and vein-filling hydrothermal sericite.Of the four, the latter two are related with the hydrothermal alteration and gold mineralization.The 39Ar-40Ar dating of vein-filling hydrothermal sericites, formed during the fourth metallogenic stage, gives a plateau age of (194.6±2) Ma, which represents the age of the latest hydrothermal alteration during the mineralization period, that is, the upper limit of ore-forming time.