Occurrence And Migration Of Sliver In Hydrothermal Systems

Southern North China Craton contains numerous mineral resources（e.g.,Au,Mo,Ag,Pb,Zn,Cu,barite,fluorite）.Vein-type Ag-Pb-Zn deposit is an important constituent part of the Southern North China Craton metallogenic belt,and many large deposits affiliated to this type have been discovered in the past decade,such as Laoliwan,Shagou,Tieluping,Lengshuibeigou,and so on.Previous research has done detailed work through geological survey,distribution of mineralization and alteration,geochronology,ore-forming fluid compositions,and minerogenetic geotectonic setting.However,the migration and occurrence of sliver remain poorly understood.In this study,the Zhonghe vein-type Ag-Pb-Zn deposit from the Xiaoshan District,in the Southern North China Craton metallogenic belt have been picked up as our target.The distribution of mineralization and alteration,the paragenetic mineral assemblage,the occurrence of metals,the micro morphology of silver minerals,the source and evolution of ore-forming fluids,and the ore genesis from the Zhonghe deposit have been investigated to trace the migration and precipitation of metals in magmatic-hydrothermal systems,and reconstruct the enrichment and evolution history of silver.Ore bodies from the Zhonghe Ag-Pb-Zn ore field,controlled by NNW Faults（F113）occur as veins within the Paleoproterozoic Xiong’er Group or the Early Cretaceous porphyritic granite.The wall rocks near ore bodies are significant with alteration zonation.The mineralization is proximal to sericitization,silicification,and carbonation.The paragenetic mineral assemblage was obtained mainly from drill cores and microscopic observations,including the quartz-siderite-pyrite alteration（StageⅠ）,polymetallic sulfide precipitation（StageⅡ）,silver mineralization（StageⅢ）,and quartz-carbonate stage（StageⅣ）.More than 40 minerals have been identified by optical microscope,scanning electron microscope equipped with an energy dispersive spectrometer（SEM-EDS）,and electron microprobe（EMP）.The ore minerals consist of sulfide,telluride,and silver mineral.The sulfide minerals are composed of galena,sphalerite,pyrite,and chalcopyrite.The telluride minerals are scarce,which mainly constitute of tetradymite and joseite.The silver minerals are diverse,such as freibergite,famatinite-Ag,pyrargyrite,polybasite,argyrodite,acanthite,and native silver.The occurrence of sliver can be divided into nano-micro silver particles and independent silver minerals,which showed a process of continuous evolution.Nano-micro silver particles are composed of isomorphism,submicroscopic inclusion,and microscopic inclusion.The independent silver minerals include sulfosalts,sulfides,native metals,and tellurides.Silver exists in the form of isomorphism in Ag-bearing minerals by coupled substitution.Submicroscopic inclusion nucleated from Ag-bearing minerals by exsolution.The precipitation and growth of the microscopic inclusion and the independent silver minerals are dominated by the interface-coupled dissolution-reprecipitation reaction（ICDR）.Microthermometry of fluid inclusions and isotopic equilibrium temperature of pyrite and chalcopyrite indicate that the ore-forming temperatures of main stage（StageⅡandⅢ）vary from 157 to 271℃,and the salinity of ore-forming fluids vary from 4.1to 13.3 wt.%.The H-O isotope data indicate that ore-forming fluids are mainly composed of magmatic water with contamination of meteoric water in the late mineralization.In order to constrain the ore-forming components of the Zhonghe deposit,a combined in-situ analysis was conducted on represented sulfides from main stage.In-situδ³⁴S values of the analyzed sulfides,including the pyrite,chalcopyrite,sphalerite,and galena,display a relatively narrow range（0.90-5.00‰）,which is close to magmatic sulfur source.The²⁰⁶Pb/²⁰⁴Pb and²⁰⁷Pb/²⁰⁴Pb ratios show narrow range（17.140-17.360 and 15.385-15.490）,whereas the ²⁰⁸Pb/²⁰⁴Pb ratios exhibit a broad variation（36.601-37.943）,indicating a contamination of the Xiong’er Group.With a synthesis of geochronological and geochemical data from the Xiaoshan District,we contend that the ore-forming materials of the Zhonghe Ag-Pb-Zn deposit are originated from the deep-seated magmatism,which is related to the partial melting of the lower crust.The ore-forming fluids were dominated by H₂O-CO₂-Na Cl system with medium-low temperature,low f _O2,and neutral-weak acid.When felsic magma uprose from the deep crust,the metals dissolved out of melts and migrated to the hydrothermal fluids because of temperature and pressure decrease along with mineral crystallization.The chloride complexes were dominated at high temperature in hydrothermal fluids.Afterwards,hydrothermal fluids migrated along the F113 fault to the shallow crust of the Zhonghe deposit,and H₂S and HCl were decomposed into H⁺with a suddenly decrease in temperature and pressure.The alteration of wall rocks occurred,such as sericitization,silicification,kaolinization,pyritization,and so on.The metals were transported as hydrosulfide complexes and chloride complexes in hydrothermal fluids,and led to the precipitation of minerals in StageⅠ,including pyrite,quartz,sericite,kaolinite,and carbonate minerals through p H increase and temperature decrease.At the StageⅡ,metal complexes became a local supersaturation state and thus unstable and in hydrothermal fluids due to a wide range of temperature decrease along with local boiling,leading to the precipitation of Ag-bearing minerals with high concentration of metals.At the StageⅢ,plenty of independent silver minerals with various shapes precipitated from the hydrothermal fluids and crystallized slowly in the open space under stable physical and chemical conditions.At the StageⅣ,massive quartz and carbonate minerals began to precipitate with temperature decrease,which means the end of mineralization.Temperature decrease is the most important process during mineralization,water-rock interaction,boiling,and fluid mixing play an important role during different mineralization stages.The Zhonghe deposit has presumably been influenced by the large-scale lithospheric delamination of the eastern North China Craton due to the westward subduction of the Paleo-Pacific Plate during the Early Cretaceous.The ore-forming fluids and components had a closed relationship with the deep-seated magmatism during～130 Ma.The Zhonghe Ag-Pb-Zn deposit is considered as E-type intermediate sulfidation（IS）epithermal deposit.We propose a porphyry-skarn-IS epithermal-distal vein ore system in the Xiaoshan-Xiong’ershan District and the Zhonghe Ag-Pb-Zn deposit may be a potential exploration target for vein-type Au-Ag deposit and porphyry Mo deposit.