Ore-forming Fluid Evolution And Superimposed Mineralization Process Of Jinchangyu Gold Deposit In Eastern Hebei Province

Eastern Hebei province is located in the eastern part of the Yanshan platform fold belt on the northern margin of the North China Craton.The tectonic movement in the area is frequent,the magmatic activity is strong,the mineral resources are rich,and a large number of gold,iron,molybdenum and other metal deposits are developed.It is an important gold ore concentration area in China.The Jinchangyu gold deposit is located in the Jinchangyu synclinorium in the western part of eastern Hebei Province.It occurs in the metamorphic rock series of the Archean Zunhua Group in eastern Hebei Province.It is the largest and oldest gold deposit in EHP.On the basis of field work,the author summarized the previous research results,carried out the test analysis of petrography and geochemistry,further determined the metallogenic period and metallogenic stage,discussed the sources of ore-forming materials and ore-forming fluids,and summarized the multi-stage superimposed mineralization process.The results show that:（1）The metallogenic process of Jinchangyu gold deposit is two periods and five stages.The first stage is the early ductile shear mineralization period,which is the product of ductile shear in Paleoproterozoic,and is characterized by the development of disseminated ore and banded ore.The second stage（main mineralization period）is the magmatic hydrothermal mineralization period,which is the product of Mesozoic Indosinian magmatism.It is characterized by the development of quartz vein,quartz-albite vein,quartz-sulfide vein and altered rock type ore.（2）The characteristics of S-Pb isotope indicate that the ore-forming materials in the main metallogenic period have the characteristics of crust-mantle mixed source.The characteristics of H-O isotope indicate that with the process of mineralization and magmatic differentiation,there may be the addition of atmospheric precipitation,and the ore-forming fluid has evolved from magma to mixed fluid formed by magmatic water and atmospheric precipitation.（3）The characteristics of major elements in the whole rock,Haeker diagram,Fe O/（Fe O+Mg O）-Si O₂,Zr-10000Ga/Al and other granite type discrimination diagrams show that the ore-bearing dykes in the main metallogenic period are closely related to the highly differentiated I-type granite,that is,the initial metallogenic stage of the main metallogenic period involves water-rich melt,which is the product of post-collision extensional tectonic environment.The average homogenization temperature of fluid inclusions in the quartz-sulfide vein stage of the main metallogenic period is 262.8°C,and the salinity is 8.23%,which is a medium-temperature medium-low salinity fluid;according to the theory of Stringer’s hydrothermal synthesis experiment,combined with the variation characteristics of altered minerals from potassium feldspar-albite,quartz and carbonate minerals in different stages,it shows that the ore-forming fluid has undergone alkaline to acidic,and then from acidic to alkaline.According to the characteristics of gold in different stages,it shows that the change of PH value and the decrease of temperature are the main mechanism of gold precipitation and enrichment.（4）The average value ofδFe,δS,Co and Ni of pyrite in the early ductile shear metallogenic period of Jinchangyu gold deposit is-2.11,-1.14,0.46%and 0.10%,respectively.The averageδFe value of pyrite in magmatic hydrothermal mineralization period is 0.55,and theδS value is-1.23.The content of Co and Ni elements is low,which is below the detection limit.The characteristics ofδFe andδS values and the contents of Co,Ni elements and related diagrams show that the ore-forming fluids of pyrite in disseminated ores and banded ores in the early ductile shear mineralization period are mainly metamorphic hydrothermal fluids,which are the products of retrograde metamorphism of amphibolite.Pyrite ore-forming fluids in quartz veins,quartz-albite veins,quartz-sulfide veins and altered rock-type ores in the late magmatic hydrothermal mineralization period are mainly magmatic hydrothermal.The characteristics ofδFe,δS values and the contents of Co,Ni elements in the two stages of mineralization are obviously different.In the superimposed position,the Co/Ni value of pyrite is between that of magmatic and metamorphic hydrothermal fluids.（5）Based on the comprehensive study of the geological and geochemical characteristics of the deposit,the nature and source of the ore-forming fluid in the study area,it is considered that the area has experienced two stages of mineralization,the early stage is ductile shear mineralization,and the late stage is magmatic hydrothermal mineralization.The two stages of mineralization have superimposed transformation in space,and the grade is increased in the superimposed transformation position.According to the classification scheme of the deposit type by the theory and method of prospecting prediction in the exploration area,the genetic type of the deposit is a medium-low temperature magmatic hydrothermal gold deposit.