LA-ICP-MS zircon U-Pb dating of dykes of Dajing tin-polymetallic deposit, Inner Mongolia, China, and its geological significance.

The Dajing deposit is an important tin-polymetallic in Inner Mongolia, North China. The hypabyssal dikes, which are closely coexisting with ore veins in deposit, are extensively developed. The previous chronology studies show that both the dikes and ore veins were generated in Mid-Late Yanshanian Period in this study area, so the deposit was regarded to be magmatic hydrothermal type deposit, which related to the activities of the subvolcanic dike swarms in this area. We obtain the zircon LA-ICP-MS U-Pb ages of dacite porphyry and felsite dikes, which are widely distributed in the deposit and were considered to be closely connected with the genesis of the deposit. The zircon U-Pb ages are 240Ma and 239Ma for the dacite porphyry of DJ-7 and YX-20 respectively, and 162 +/- 1Ma for felsite dike of DJ-1. These results implicate that the dikes were formed in Early Indosinian and Middle Yanshanian in the deposit area. Combining with the previous regional research, these results indicate that the dacite porphyry was formed in syn-orogenic to post-orogenic setting, and felsite was formed in post-orogenic setting. Because of the 140Ma ore-forming age, we concluded that the dikes in the ore areas have no obvious relationship with the deposit genesis. However, it provided the advantageous space for the formation of the ore veins.