LA-ICP-MS zircon U-Pb ages of the two phases of magmatism in the Xiuwacu W-Mo deposit, northwest Yunnan, and their implications for ore genesis

Two phases of intrusive magmatism were developed in the Xiuwacu deposit, northwest Yunnan.This study focused on the geochronological and geochemical characteristics of these intrusive rocks so as to define the petrogenesis of the Xiuwacu deposit.The LA-ICP-MS zircon U-Pb dating results show that biotite granite from the east of Xiuwacu deposit was formed at 214.9±1.3 Ma, granite porphyry and monzogranite from the west of Xiuwacu deposit was formed at 87.47±0.51 Ma and 83.29±0.60 Ma.The results of the studies show that the molybdenite from the Xiuwacu deposit was formed at 82~86 Ma, corresponding to Late Yanshanian.The two phases of intrusions have similar geochemical characteristics, both belonging to the shoshonitic type and metaluminous to slight peraluminous series, with enrichment of LILE(Rb, Th, U)and depletion of HFSE(Nb, Sr, Zr, Hf).The Y-Nb and Yb-Ta discrimination diagrams indicate that the two phases of intrusions were formed in different tectonic environments.The Late Triassic intrusive rocks fall into the field of syn-collision granites while the Late Cretaceous intrusive rocks fall into the field of intra-plate granites.Based on comprehensive study with regional geology and acquired achievements in fluid inclusions, S and Pb isotopes as well as zircon Hf isotopes, the authors infer that the partial melting of thickened lower crust in the Late Cretaceous seems to have been the main metallogenic material source of the Xiuwacu deposit.The ore-bearing thermal fluid migrated along the fault-crack system in the Xiuwacu deposit, and then the W-Mo orebodies were formed at the favorable position along with the combined effect of both temperature decreasing and the mixing with meteoric water.