1.The present zircon U–Pb geochronological study indicates that SCH 51344 the crystallization ages for the Yangchang and Shabutai monzogranites are 137.4 ± 2.1 Ma and 138.4 ± 1.5 Ma, respectively. These results combined with previously reported data suggest that the Early Cretaceous (i.e., 130–140 Ma) is an important Mo mineralization stage in the Xilamulun district.2.Zircon Ce/Nd ratios from the mineralized intrusions from the Xilamulun district (i.e., ~ 20.7 for Chehugou, ~ 34.0 for Banlashan, ~ 9.8 for Aolunhua, ~ 27.9 for Yangchang, ~ 41.2 for Haisugou and ~ 29.5 for Shabutai) are dramatically higher than their cospatial barren intrusions (i.e., ~ 8.0 for Chehugou, ~ 8.6 for Banlashan, ~ 2.7 for Aolunhua and ~ 9.0 for Yangchang), indicating the former are characterized by higher oxygen fugacity than the latter.3.Zircon Hf and whole rock Sr–Nd–Pb isotopic compositions of the granitoids hosting the Mo deposits in the northern Xilamulun district are similar (εHf(t) = − 0.8 to + 10.0; εNd(t) = − 3.7 to + 1.6; (87Sr/86Sr)i = 0.70211 to 0.70743; (206Pb/204Pb)i = 18.03 to 18.88, (207Pb/204Pb)i = 15.48 to 15.58 and (208Pb/204Pb)i = 37.72 to 38.28) and altogether suggest that the magmas from which the Mo deposits were generated originated predominantly from a juvenile lower crust source derived from depleted mantle although diverse consequent processes have also occurred during their evolution.4.The Mo deposits in the Xilamulun district were formed in three stages. The Triassic Mo mineralization was closely related to the remelting of old crust material, the Late Jurassic Mo-causative magmas were generated by mixing of old crust material with depleted mantle component, whereas the Early Cretaceous Mo deposits were associated with juvenile mantle-derived magmas. The variation in the origin of these Mo-related intrusions indicates that the formation of Mo deposit lies not in the composition of magma sources. Other factors, including high oxygen fugacity of the magma inferred from high zircon Ce/Nd ratios, may be more important for Mo enrichment and subsequent mineralization.
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