An analysis of geological age and materials source of the Niujuan Ag-Au polymetallic deposit in Chengde

Abstract：On the basis of field reconnaissance, field observation, rock (ore）sample analyses and the application of such means as isotope geochemistry, REE, fluid inclusion homogenization temperature and composition, the authors hold that the ore-forming materials of the Niujuan Ag-Au deposit were derived from guanite. The analytical results of REE from rock (ore）show that the chondrite-normalized REE patterns of rocks and ores from the Niujuan Ag-Au deposit are similar to each other, both assuaming LREE enrichment, HREE depletion and obvious Eu negative anomalies. These characteristics demonstrate that the ore-forming materials of the Niujuan Ag-Au deposit were derived from granite. Researches on hydrogen and oxygen isotopes show that the ore-forming fluids were of multiple sources, being the mixture of magmatic water and circulated atmospheric precipitation. According to the composition of ore minerals, the authors used the Rb-Sr isochron method for monomineral sphalerite and paragenetic mineral assemblage of pyrite and galena to determine the ore-forming age, and the result shows that the age of ore mineral is (154.1±2.6)Ma, belonging to middle Yanshanian period. On such a basis, it is deduced that the formation of the deposit was closely related to Yanshanian magmatite intrusion. The results obtained by the authors provide a theoretical basis for the determination of the genetic type of the ore deposit.