Abstract: The Chenjiazhuang uranium deposit is a typical pegmatite-hosted ore deposit located in the middle part of the Danfeng triangular domain of the northern Qinling Orogen, China. The U-bearing ore bodies mostly occur in the contact zones between the biotite granitic pegmatite in the vicinity of the Caledonian granite and the Qinling Group biotite-plagioclase gneiss. To constrain the petrogenesis, tectonic setting and pegmatite-hosted uranium ore formation, we presented in this paper a detailed geochronological and geochemical study of granites and granitic pegmatites in the ore district. Zircon LA-ICP-MS U-Pb dating results yielded the crystallization ages of 446±3 Ma for the Huanglongmiao biotite granite, 419±2 Ma for the Chenjiazhuang biotite monzonite granite, 417±3 Ma for non-mineralized granitic pegmatite, 414±4 Ma for lean-ore granitic pegmatite and 416±3 Ma for mineralizing granitic pegmatite. The Huanglongmiao biotite granodiorite exhibited the affinity and adakitic characteristics of an I-type formed by partial melting of lower basaltic crust in a collisional setting. The Chenjiazhuang biotite monzonite granite also possessed the I-type properties but formed in a relatively shallow source zone in a post-collisional decompression setting. The age data revealed that granitic pegmatite formed in concurrence with the host Chenjiazhuang biotite monzonite granite and therefore they should have closely related origins. Uranium minerals and U-rich biotite all occur in granitic pegmatite dikes. The differences in petrological and geochemical characteristics among the non-mineralized, U-bearing and U-rich granitic pegmatites were attributed to various degrees of greisenization. The U-poor granitic pegmatites occur within the biotite monzonite, while the U-rich granitic pegmatite dikes always maintain contact with the Qinling metamorphic complex. This suggests that uranium enrichment in ore deposit is strongly associated with uranium extraction from the Qinling metamorphic complex by late stage fluids, and this association resulted in high degree of uranium concentration in the process of quartz and biotite rich greisenization. Therefore, the late stage metamorphic alteration of the Qinling complex is critical for uranium mineralization in the Chenjiazhuang uranium deposit.

Key words: Qinling Orogen, pegmatite-hosted type uranium deposit, petrogeochemistry, zircon U-Pb dating, Caledonian period