Characteristics and geological significance of helium, argon, carbon, hydrogen, oxygen isotopes in ore-forming fluids of polymetallic deposits in Shihu area of western Hebei Province

The gold, silver and polymetallic deposits in Shihu area are located in the mid-northern Taihang Mountains and occur in Archean Fuping Group metamorphic rock series around Yanshanian Mapeng granitic intrusion. The gold, silver and polymetallic deposits in Shihu area have attracted the attention of many geologists, and their metallogenesis has long been in dispute. The authors chose pyrite and quartz samples from these deposits in Shihu area to analyze their helium, argon, carbon, hydrogen and oxygen isotopic composition of fluid inclusions and the hydrogen, oxygen and carbon isotopic composition of the quartz in the same ore sample. The analytical results show that 3He/4He ratios in pyrite range from 0.43 to 2.40 Ra, and 40Ar/36Ar ratios range from 477 to 879.The composition of helium and argon isotopic system suggests that the source of ore- forming hydrothermal fluids was the mixture of mantle- derived fluids and crust- derived fluids. It is estimated that the δDV- SMOW values of the ore- forming fluid responsible for the formation of the quartz are -62‰~-105‰, and the δ18OV-SMOW values of the fluid inclusions of the quartz are 9.6‰ ~13.8‰, showing that the ore-forming hydrothermal fluids were formed by the mixture of magmatic water and meteoric water. The δ13CPDB values of quartz vary in a relative narrow range of -3.5‰~-5.0‰, showing that the ore-forming hydrothermal fluids were derived from the mantle. The relationship between hydrogen, oxygen, carbon and helium and the argon isotopic systems of oreforming hydrothermal fluids show tracing consistency in gold, silver and polymetallic deposits of Shihu area. All the noble gas isotopic data, combined with stable isotopic data from five deposits, demonstrate that the ore-forming fluids were a mixture between mantle-derived fluids and crustal-derived fluids.