The Geological Characteristics And Tectonic Setting Of Qiancengbei Porphyry Molybdenum Deposits In Chengde,North Hebei

This thesis takes the Qiancengbei porphyry molybdenum deposit as the research object,on the basis of research in many ways,such as the field geology survey,petrography,U-Pb dating of zircon,geochemistry and the analysis of Hf isotope,we discuss the geological characteristics and tectonic background of the Qiancengbei porphyry molybdenum deposit.The porphyry molybdenum deposit is new-found deposit located in the middle of Jining block in the middle of the North China Craton.It is mainly controlled by the fault structure in the southwest.The ore body is stratoid,displaying a dome framework mineralization.The wall rock alteration include potassic,silicification,kaolinization,chloritization and sericitization,which has typical alteration characteristics of porphyry deposit.Ore occur in potassic belt,silicified belt and orthophyre,which show metasomatic textures,microscopic scaly textures,veinlet or reticulate structures,veinlet disseminated structures.The Qiancengbei porphyry ore-forming rock mass include quartz-orthophyre,syenogranite,monzogranite and syenite-porphyry.We determine the age of rocks on the basis of U-Pb dating of zircon and research achievement of predecessors,the results show that the formation age of quartz syenite is 120 Ma,syenite granite is 140.31+0.8Ma –133.12+0.79 Ma,monzonite is 123 Ma – 115 Ma,syenite porphyry is 134.30+0.86 Ma.Moreover,the diagenesis and mineralization of the Qiancengbei porphyry molybdenum deposit is a continuous geological activity,so the ore deposit should be formed in the early Cretaceous.Geochemical characteristics of the porphyry molybdenum deposit are high silica and high potassium,it belongs to shoshonite series or HKCA and peraluminous rock.Syenogranite,monzogranite and syenite-porphyry are relatively enriched with large ion lithophile elements of Rb,Th,K,but they are short of Ba,Sr,P,Ti and other high field strength elements,the chondrite-normalized REE patterns indicate that the rocks are enriched in light rare earth elements,and obvious negative Eu anomalies;Quartz-orthophyre are enriched with Ba,K,Th,but they are short of Rb,Sr,P,Ti and so on,the chondrite-normalized REE patterns indicate that the rocks are enriched in light rare earth elements,and inapparent positive Eu anomalies.Own rocks belong to the characteristics of A-type granites.Haker diagram and the ratios between incompatible trace elements imply the rocks have similar magma sources.But there differential degree imparity.The rocks are formed by crust-mantle mixing,which on the basis of research of Hf-isotope and Sr–Nd isotope.Combined with the tectonic discriminant diagrams and regional tectonic settings,we consider that the Qiancengbei porphyry molybdenum deposit was formed in the extensional environment after the collision between the Pacific and North China plates.