Discriminating characters of ore-forming intrusions in the super-large Chalukou porphyry Mo deposit,NE China

The Chalukou porphyry Mo deposit, located in the Great Hinggan Range, is the largest Mo deposit in northeast China, although the age and genesis of the associated magmatic intrusions remain debated.Here we report zircon U-Pb ages and trace elements, whole rock geochemistry and Sre Nd isotope data with a view to understand the relationship between the magmatism and molybdenum mineralization.Zircon U-Pb analysis yield an age of 475 Ma for rhyolite in the older strata, 168 Ma for the premineralization monzogranite, and 154 Ma for the syn-mineralization granite porphyry. The granite porphyry and quartz porphyry are considered as the ore-forming intrusions. These rocks are peraluminous, alkali-calcic, and belong to high-K to shoshonitic series with a strong depletion of Eu. They also display characteristics of I-type granites. The rocks exhibit wide variations of(87 Sr/86 Sr)iin the range of 0.705426 -0.707363, and ε_(Nd)(t) of -3.7 to 0.93. Zircon REE distribution patterns show characteristics between crust and the mantle, implying magma genesis through crust-mantle interaction. The Fe_2O_3/FeO values(average 1) for the whole rock and EuN/Eu*Nvalues(average 0.45), Ce^(4+)/Ce^(3+) values(average 301)for zircon grains from the granite porphyry are higher than those from other lithologies. These features suggest that the ore-forming intrusions(syn-mineralization porphyry) had higher oxygen fugacity conditions than those of the pre-mineralization and post-mineralization rocks. The Chalukou Mo deposit formed in relation to the southward subduction of the Mongol-Okhotsk Ocean. Our study suggests that the subduction-related setting, crust-mantle interaction, and the large-scale magmatic intrusion were favorable factors to generate the super-large Mo deposits in this area.

Timming of Lithospheric Extension in Northeastern China:Evidence from the Late Mesozoic Nianzishan A-Type Granitoid Complex

New zircon U-Pb dates obtained by laser ablation inductively coupled plasma mass spectrometry（LA-ICP-MS）,whole-rock geochemical data and Sm-Nd and Rb-Sr isotopic data are presented for miarolitic alkaline granites,porphyritic syenite and rhyolites of the Nianzishan A-type granitoid complex（NAGC）in the Great Xing’an Range-Songliao Basin in Northeast（NE）China.New crystallization ages of 112.95±0.93 and 114.1±1.71 Ma for granite and 118.6±0.51 Ma for porphyritic syenite were determined by high-precision LA-ICP-MS.The εNd（t）of the rocks range from +1.85 to+2.06,with Nd model ages(TDM1)from 671 to 821 Ma,indicating that the NAGC originated from juvenile source rocks and exhibits geochemical characteristics of A1-and AA-type granite which formed in an extensional setting.We attribute the magmatism to regional extension and lithospheric thinning caused by the subduction of the western Pacific Plate about 120 to 100 Ma.