Origins Of The Ore-forming Materials And Enrichment Mechanism In The Xiaoqinling Goldfield,southern Margin Of The North China Craton

The Xiaoqinling goldfield,located along the southern margin of North China Craton(NCC),is the second largest gold producer in China.Although amount of studies have been done in this district,some key scientific issues,such as the sources of ore-forming materials,the age of gold mineralization and metallogenic mechanism,are still controversial.Taking Fancha gold deposit as the research object,this thesis makes a comprehensive and systematic analysis of the deposit geology,mineralogy,geochemistry and geochronology,and the main conclusions are summarized as the follows:1.The abundant Te-Bi mineral assemblages associated with gold was recognized,which has the distribution characteristics of "upper tellurium and lower bismuth" in the vertical.The pyrite in the Fancha gold deposit is As-free pyrite,and Au is positively correlated with Ag,Te and Bi.LA-ICP-MS mapping of gold-stage pyrite shows that Au,Ag,Te and Bi are unevenly distributed,and they are consistent with pyrite growth zones,indicating that gold mainly occurs as submicroscopic inclusions of Au-Ag-Te-Bi assemblages in the pyrite.Te-Bi melts,as an excellent gold scavenger,have played an important role in governing gold distribution patterns and leading to high concentrations during the mineralization process.The abnormal enrichment of Te-Bi minerals in the auriferous quartz veins is an signature related to the magmatism.2.Data of stable isotopes and noble gas isotopes in each ore-forming stage of the Fancha gold deposit show consistent signatures that the ore-forming fluids and metals are genetic to the mantle-derived magma,and the meteoric waters and crust materials were gradually added in the late ore-forming stage.3.The gold-bearing pyrite yield a Re-Os isochron age of 126-124 Ma.In-situ LA-ICP-MS dating of the monazite and rutile crystals yielded 206Pb/238U age of 127.5±0.7 Ma and 129.7±4.3 Ma,respectively.Rutile U-Pb dating in Xiaoqinling goldfield is the first time.Precise geochronology studies indicate that the gold mineralization took place in the Early Cretaceous,which is consistent with the large-scale lithospheric extension and magmatism of the NCC in time.4.The large-scale lithospheric extension and thinning coupled with the upwelling of the asthenosphere lead to extensive crust-mantle interactions and the tectono-magmatic activities,which provide fluid,metal,volatiles and sufficient heat source for gold mineralization.The crustal shear systems associated with the lithospheric extension provided spatial context for ore-forming fluids migration and accumulation.The large-scale lode gold mineralization in the Xiaoqinling goldfield is the material response of tectonic evolution and deep fluid process under the Early Cretaceous lithospheric extensional geotectonic setting of the eastern NCC.