Ore-forming Control Factors,mineralization Enrichment Regularity And Prospecting Potential Evaluation Of The Xitieshan Pb-Zn Polymetallic Deposit,Qinghai Province

The North margin of Qaidam basin is located at the intersection of Kunlun Mountains,Qilian Mountains and Qinling Mountains.The formation and transformation of Xitieshan Pb-Zn deposit in the middle segment of North Qaidam basin margin is closely related to the tectonic evolution of North Qaidam basin margin.This research is focused on the Xitieshan Pb-Zn deposit.Based on the field observation,this research studied the ore forming material sources by restudying previous data,Pb isotope analysis and in-situ S isotope analysis.Then we discussed the ore forming fluids resources by analyzing hydrothermal calcite C-O isotope and fluid inclusion.The 3D geological model was built via collecting the pre-existing exploration data.Above all,this study summarized the rock deformation regularities,ore spatial distribution regularities and major metallogenic elements accumulation regularities and refreshed the exploration data,aiming to provide more references for the ore prospecting.Fields,geophysical and geochemical work has been carried out in the peripheral area of Xitieshan deposit.Combined with the available information,this study evaluated the potential of Northwest mining area and Hongshangang-Lvshigang mining area.The main results and achievements are as follows:1.Previous study set out a high-ultrahigh pressure metamorphic zone according to the exposed eclogite,garnet peridotite and related gneiss in the north Qaidam margin area.The ultrahigh pressure metamorphic zone might record the complete evolution history of this area from the beginning of oceanic subduction to continental subduction and collision.The Xitieshan Pb-Zn deposit was formed under the back-arc basin background during plate subduction rather than continental rift valley.Based the field survey,drill core record compiling and production database processing,it is confirmed that the post-mineralization was existed in Xitieshan Pb-Zn deposit.The vein-type Pb-Zn mineralization controlled by fracture is widely developed.This study identified three residual breccia pipes were existed in this area,including line 47 in Xitieshan valley,line29 in Wuming valley and line 07 in Zhongjian valley.It indicates that the Xitieshan Pb-Zn deposit has undergone two stage mineralization:the early stage of exhalative sedimentation mineralization and the late stage of tectonic-hydrothermal transformation mineralization.2.The mining area has been influenced by deep and shallow tectonic levels.The deep ductile and ductile-brittle deformation structure in the mining area also includes the extensional detachment fault between the a-1 petrofabric of Tanjianshan group and Dakendaban group.The shallow level brittle deformation should be formed after the sedimentation of cruentous pebbly sandstone from C petrofabric of Tanjianshan Group.After its formation,this area suffers strong regional tectonic compression,which lead to the formation of the main brittleness structure tracks in the mining area.It lays the present tectonic framework and is the main factors for locating the ore body position.3.The S isotope composition of xitieshan lead-zinc deposit can be divided into two ranges.The?³⁴S values of the ore from submarine breccia pipe phases were characterized by great variations,indicating the multiple sources of fluids.The?³⁴S values of the ore that may suffer the hydrothermally-rework varies in a tiny range,which is similar to that of the late hydrothermal vein type ore in other petrofabric group.It is also similar to that of the breccia pipe phases ore with high degree of deformation.Therefore,it can be inferred that these ore genesis may be related to the late deformation metamorphism and hydrothermal process.The sulfur isotope has reached the maximum of homogenization after deformation and metamorphism,especially the hydrothermal migration and infiltration processes.The Pb isotopes are of mixed origin in orogenic belt and upper crust.The ore and gangue minerals show similar synsedimentary features.The Sr isotope of them indicates that the ore forming materials are mainly crustal origin.The high Sr isotope contents of the marble in the a-2 formation of Tanjiashan group are related to the submarine exhalative sedimentation of back-arc basin.Based on the study of mineralization characteristics and sulfur isotopes of the deposit,the mineralization processes of the early stage exhalative sedimentation and the superposition transformation in the late stage are revealed.4.The study of fluid inclusions shows that the main peak of the homogenization temperature of the inclusions from the northwest Xitieshan mining area,the main mining area and the Lvshigang-hongshigang mining area is different.From west to east,the homogenization temperature increases gradually.This study shows that the homogenization temperatures of fluid inclusions vary in a wide range,suggesting the superposition of different hydrothermal mineralization period.The C-O isotope compositions of calcite that is closely associated with galena and sphalerite suggest that the ore-forming fluids of Xitieshan deposit mainly originates from deep mantle.These fluids interact with seawater during the exhalative sedimentation.The C-O isotopes of ore-hosting marble suggests a normal marine sedimentary origin.5.This research drew the Pb,An,Ag,Au grade contour map,ore body thickness contour map and marble thickness contour map based on the collecting data.After systematically summarized the mineralization enrichment regularity,it is suggested that the orebody and ore-host marble in Xitieshan mining area show SE lateral trending.The variation of strength in vertical direction suggest that the third mineralization enrichment zone may be existed in the deep.The Zhongjian valley-Duanceng valley mining area is the extension of the main mining area and the ore-host formation is also stable in this area.The orebodies also show SE lateral trending.According to the grade and thickness contour map,the orebody should still exist in the deep,suggesting a good potential of this region.6?This study proposed two prospecting target area and two prospecting prospect area by summarizing regional,geochemical and geophysical information.It is believed that there may be an ore-bearing marble belt under the footwall of the main ore belt of Xitieshan deposit.Besides,the third mineralization enrichment belt may be existed in the deep of the main mining area.These suggestions are favorable for the further ore-prospecting work.Besides,the orebody of Northwest mining area and Hongshigang-Lvshigang mining area are buried too deep to mining,further study is needed.