Study On Geological Characteristics And Metallogenic Mechanism Of Changba-Lijiagou Superlarge Lead-Zinc Deposit In West Qinling

The Changba-Lijiagou superlarge Pb-Zn deposit is located in the Xicheng ore cluster,West Qingling polymetallic metallogenic belt,and the ore genesis has been debated for years.In this dissertation,through the application of modern metallogenic theory,geochemistry and high-precision analytical and testing methods,the systematic research works on deposit geology,petrology,isotopic geochemistry,fluid inclusions,metallogenic chronology have been carried out on this deposit and magmatic rocks in the Xicheng ore cluster.Combined with the previous research progress,the metallogenic characteristics and regional tectonic background are revealed,the ore genesis is clarified and the metallogenic model is established.The main research achievements obtained are as follows:（1）The zircon U-Pb age of Huangzhuguan Changba rock mass are（229～215）Ma and（218～209）Ma respectively.TheirεHf values are（-14.33～-3.43）,which indicate that the magmatic activity is related to lithosphere delamination during post-collision stage of Triassic collisional orogeny in West Qinling,and the lithogenous material originates from the lower crust mixing with some mantle-drived components.（2）The metallogenic process of the Changba-Lijiagou Pb-Zn deposit can be divided into three stages,Ⅰ.Brown yellow sphalerite-porous pyrite-arsenopyrite stage,Ⅱ.Dark brown high Fe granular sphalerite-galena-nonporous pyrite stage;Ⅲ.Calcite-quartz-sulfides stage,and the first two stages are the main metallogenic stages.（3）The sulfur isotope values ofδ³⁴S_V-CDT in stage I,II,andⅢrange from 20.3‰to 29.0‰,from 15.1‰to 23.0‰,and from 13.1‰to 22.0‰,respectively.The sulfur mainly comes from sea water sulfate.In the middle and late stages,the magmatic hydrothermal fluid or metamorphic fluid mixed with the connate water.From early to late stage,the sulfur tends to come from magma or metamorphic fluids.The lead isotopic values of sulfide are concentrated in the range of lead of the subduction zone（magmatism）where the lower crust and mantle are mixed,close to the orogenic belt,indicating that the Pb of ore in this deposit is derived from magmatic activity of crust-mantle interaction.Theδ⁶⁶Zn values of three stages sphalerite show a wide range from 0.08‰to 0.37‰.FromⅠstage toⅢstage,theδ⁶⁶Zn values tend to get heavier,which may be due to the zinc source mixed with the composition of the wall rock in stageⅠand the magmatic fluid in stageⅡandⅢ.（4）The in-situ LA-ICP-MS U-Pb dating on two types of titanite yielded ages of（212.8±3.0）Ma in the ore and（218.0±9.3）Ma in the mineralized alteration host rock,which indicate that Changba-Lijiagou superlarge Pb-Zn deposit is formed during the Late Triassic,which are consistent with the time of large scale magmatic-hydrothermal activities in the Western Qinling Orogen（229～211）Ma.（5）There are mainly three kinds of fluid inclusions:small amount of the pure liquid inclusion、gas-liquid two-phase inclusion and pure gas inclusion.The gas compositions are mainly H₂O、CO₂、N₂、CH₄.The ratios of gas/liquid are between 10～30%.The homogenization temperatures of the main ore-forming stage have two peak values,250℃and 350℃.The range of salinities is from4.18 to 8.00%,with peak value of 6.74%.The range of pressures is from 12.1to 32.1MPa.The range of densities is from 0.73 to 0.93 g/cm³.Through the study of inclusion microthermometric measurements and Raman spectroscopianalyses,combined with the study of C-H-O isotopes,it is considered that the ore-forming fluid of the Changba Lijiagou Pb-Zn deposit is the mixture of magmatic water,metamorphic water and connate water.（6）The metallogenic model of the Changba-Lijiagou Pb-Zn deposit is established:During the Late Triassic,the Western Qinling orogen transformed into the extensional regime of the post collisional stage（229～209 Ma）.The intense crust-mantle interaction triggered the large-scale magmatic activity,and the metals in the basement rocks are reactivated during the magmatic activity.At the same time,dynamic metamorphism impelled the dehydration of regional strata to release metamorphic fluid.The ore-forming metals enter magmatic water and metamorphic water to form ore-forming fluids,then mixed with the connate water rich in SO₄^2-.The fluids mixing led to precipitation and mineralization in the tectonic dilational space.With the addition of magmatic fluid and metamorphic fluid increased,the content of connate water decreased in the shallow level.During the upwelling process,the ore-forming fluid precipitated rapidly by cooling and immiscibility.There are also some ore-forming fluid reacted with the carbonate formation and form the skarn type mineralization.After the main mineralization,vein orebodies are formed in the fracture system during tectonic uplift,which crosscut the banded and massive orebody formed in the early stages.