College of Earth Sciences��Jilin University��Changchun 130061��China

Erdaohezi lead- zinc deposit belongs to the Derbugan metallogenic belt lying on the northwestern Hailaer- Genhe Mesozoic volcanic basin�� located on the western slope of the Da Hinggan Mountains�� The deposit is considered as one of the hypabyssal low- temprature hydrothermal lead- zinc deposits associated with volca- nism�� In order to lay the foundation on studying its diagenesis and mineralization ages�� the detailed studies were carried out by dating the host rocks (i.e. rhyolitic lithic- crystal tuffs) using zircon LA- ICP- MS U- Pb method�� The dating results show three groups of ages�� The first group is the captured zircons (the weighted mean 206 Pb/ 238 U age as 175.6 ± 2.3 Ma��MSWD = 0.70��n = 3)�� The second group can be regarded as the rock- forming age (the weighted mean 206 Pb/ 238 U age as 165.3 ±1.9 Ma��MSWD =2.40��n =14)�� The third group should represent the late stage of the magmatic evolution (the weighted mean 206 Pb/ 238 U age as 161�� 0 ± 3.1 Ma��MSWD =0.86��n =4)�� According to the ages and the crystal form or CL image characteristics of zircons�� it is determined that the diagenesis occurred in the late Middle Jurassic�� Based on the regional geology and geo- chronological research��the acidic pyroclastic rocks are space accompaniment and time connection with the Tamu- langou Formation intermediate- mafic volcanic rocks�� Both of them constitute the host rocks of the deposit togeth- er�� The rock combination also provides favorable conditions for large- scale silver��lead and zinc mineralization in this area��