(U-Th)/He thermochronology of metallic ore deposits in the Liaodong Peninsula: Implications for orefield evolution in northeast China

The Liaodong Peninsula is an important region where polymetallic ores are mined in northeast China, but precise thermochronological data needed to effectively constrain the evolution of this orefield are presently lacking. For this study, fifteen samples of granitoid rocks were collected from the Qingchengzi and Wulong orefields located in the Liaodong Peninsula to undertake (U-Th)/He study of zircon and apatite. Single-grain ZHe and AHe ages from the Qingchengzi orefield range from 73 to 166 Ma and from 22 to 48 Ma, respectively, whereas single-grain ZHe and AHe ages from the Wulong orefield range from 42 to 88 Ma and from 21 to 56 Ma, respectively. Some samples yield dispersed ages. Comprehensive analyses indicate that the main causes of the dispersion in the ZHe ages are heterogeneous distribution of U and Th within the crystals, radiation damage and cooling rates, whereas the dispersion in the AHe ages is primarily due to the first two factors. Based on the helium ages, we calculate the average exhumation and cooling rates of the two orefields during different periods and the associated denudation, which are 4.3 km for the Qingchengzi orefield and 4.7 km for the Wulong orefield. The mineralization depths are estimated using homogenization temperature and the salinity of fluid inclusions, and are calculated to be 1.5-3.2 km and 1.0-2.2 km for the Qingchengzi Pb-Zn and Au-Ag deposits, respectively, and 2.1-3.1 km for the Wulong Au deposit. These results are much less than the total denudation, suggesting that the orebodies have been heavily eroded. In conjunction with the distribution of the helium ages, the modeled low-T thermal histories of the two orefields reveal that the Liaodong Peninsula experienced three stages of relatively rapid cooling during the Early Cretaceous (ca. 135-115 Ma), from the end of the Late Cretaceous to the Early Eocene (ca. 70-50 Ma) and from the Late Eocene to the Oligocene (ca. 38-26 Ma). The last stage of cooling affected both orefields and represents a period of relatively intense tectonic activity. Finally, these results are compared with those of previous studies from the adjacent Jiaodong Peninsula. This discussion shows that the cooling experienced by the orefields in the Liaodong Peninsula is largely consistent with that experienced by the Jiaodong Peninsula. These data provide valuable information for further studies of the evolution of orefields in northeast China.