The Lesser Xing’an Range is located in the eastern segment of the Central Asian Orogenic Belt. It hosts an important polymetallic metallogenic belt that contains more than 20 large- to small-scale porphyry Mo, epithermal Au, and skarn Fe-polymetallic deposits. The Cuihongshan Fe-polymetallic deposit is one of the largest polymetallic deposits in northeastern China. To better understand the formation of the Cuihongshan Fe-polymetallic deposit, we investigated the geological characteristics of the Cuihongshan deposit and applied geochemistry and geochronology to constrain the timing of the mineralization, and characteristics of the magmas. Zircon U–Pb dating of the alkali-feldspar granite and monzogranite yielded weighted mean 206Pb/238U ages of 495 ± 1.6 and 203 ± 1 Ma, respectively. Re–Os dating on molybdenite yielded an isochron age of 203.2 ± 1.4 Ma, and 40Ar/39Ar dating on phlogopite yielded an age of 203.4 ± 1.3 Ma. These data suggest that mineralization occurred during the Late Triassic, and is closely related with the monzogranite emplacement. These rocks belong to the high-K calc-alkaline and subalkaline series, are enriched in Rb, U, and Th, are depleted in Nb, Ta, and Ti, and show strong Eu anomalies, implying that they are A-type post-orogenic rocks. The Cuihongshan Fe-polymetallic formation is possibly related to an extensional environment resulting from the final closure of the Paleo-Asian Ocean.