Metallogenic Process And Potential Of The Silver-polymetallic System In Fengning, Northern Hebei Province: Constraint Of Geochronology, Geochemistry And Genetic Mineralogy

The northern Hebei silver polymetalic metallogenic belt,one segment of west pacific ring,is an important silver polymetallic metallogenic belt in China.Fengning silver polymetallic deposit is a typical continental volcanic type deposit related to volcanic and subvolcanic activity in this district.Based on systemic studies on genetic mineralogy,ore geology,petrochemistry and geochemistry,this thesis clarify the genetic relationship between magmatism and mineralization,define the metallogenic epoch,the properties of metallogenetic fluid,the sources of the metallogenic elements and the mechanism of migration and precipitation of silver,analyzed the metallogenic dynamic background of this deposit,establish the metallogenic model,and has carried on the forecast to the metallogenic prospect.Fengning silver polymetallic deposit,formed in the large tectonic setting of ithospheric thinning of the North China craton,includes Niujuan silver?gold?ore and Yingfang Pb-Zn ore.Surrounding rocks of ore deposits are mainly Mesozoic igneous rocks,including Baiyingou coarse-grained granite of early triassic and Erdaogou fine-grained granite of Late Jurassic.Baiyingou coarse-grained granite,containing inherited zircons with ages of Proterozoic,is emplaced at 244Ma and highly fractionated I-type granites.Combined with the Pb isotopic compositions,it is considered that Baiyingou coarse grained granite originates from partial melting of the thickened crust in post-collision extended environment.Erdaogou fine-grained granite,originated from partial melting of the thickened crust in extended environment from compression to extension,is emplaced at 152Ma and highly fractionated I-type granite.The ?³⁴S values indicate a magmatic source for sulfur,whereas the?¹⁸O?1.31 to4.15 per mil?and?D?-98.7 to-116.9 per mil?values suggest that the ore fluid is a mixture of magmatic water and meteoric water.Lead isotopes suggest the origin of ore materials and magmatism responsible for andesites and granites was partly attributed to mantle.Noble gas?He and Ar?isotopes demonstrate clear mixing trends between crustal and mantle-derived components.Fluid inclusions are mainly dominated by three distinct tyes,i.e.,L-V,V-L,pure liquid or gas inclusions.Micrometeoric data reveal that the Niujuan silver?gold?ore precipitated from moderate-temperature?250°C to 300°C?and low to moderate salinity?0.5 to 8 wt percent NaCl equivalent?hydrothermal fluids.As the proceding of mineralization,the hydrothermal system was characterized by subsequent cooling and decreasing of salinity.Sulfide samples from Niujuan silver?gold?ore yield a well defined Rb-Sr isochron age of 154.1±2.6 Ma and fluorite samples yield a well defined Sm-Nd isochron age of 139.2±3.8 Ma,which are indicative of the formation of alteration minerals.Sulfide samples from Yingfang Pb-Zn ore yield a well defined Rb-Sr isochron age of 135.7±4.1Ma,which is approximately synchronous with the formation age of fluorite and younger than the mineralization age of Niujuan silver?gold?ore.Based on the characteristics of the deposit,the geochemical halo of the ore body,the typomorphic characteristics of typical minerals,and the analysis of comprehensive information in field,we forecasts the deep mineralizing perspective.