Characteristics And Metallogenic Genesis Of The Carbonate-hosted Lead-zinc Deposits In The Middle Part Of Sanjiang Area, Tibetan Plateau

Yushu and Changdu regions, located in the northeastern margin of Tibetan collisional orogenic belt, are the important parts of Southwest’Sanjiang’tectonic and magmatic belt. Thrust nappe structures formed in Cenozoic and carbonate-hosted Pb-Zn deposits are both abundant in the two regions in the middle part of Sanjiang area. The global features of these carbonate-hosted deposits are similar to MVT deposits word wild. However, they are obviously controlled by thrust nappe tectonic systems, with unique characters and wide distribution and become the focus of attention. Make an intensive study of these deposits will develop the Metallogenic theory of continental collision and sediment-hosted Pb-Zn deposits and improve the predication and prospecting efficiency of Pb-Zn ore deposits in China. Based on detailed geological mapping in mining areas and systematic sample tests in laboratories, the writer got seven cognitions, including ① basic geological characters of carbonate-hosted Pb-Zn deposits in the middle part of Sanjiang area, ② four mineralization patterns and two periods of regional Pb-Zn mineralization, ③ controlled forms of thrust nappe tectonic systems for Pb-Zn ore forming, ④ ages of these Pb-Zn deposits, ⑤ properties of ore-forming hydrothermal fluid and sources of the metallogenic material, ⑥ regional metallogenic model of sediment-hosted Pb-Zn ore deposits in Sanjiang area, and ⑦ prospecting methods for this kind of carbonate-sediment hosted Pb-Zn deposits in the region.According to detailed geological mapping in mining areas, it is recognized that sediment-hosted Pb-Zn ore deposits in the middle part of Sanjiang area are all hosted in carbonate rocks which are the tectonic slices and controlled by back-thrusts of the thrust nappe tectonic system formed in Cenozoic. Based on characters of ore-bearing formation and mineralizaion, carbonate-hosted Pb-Zn deposits in the middle part of Sanjiang area can be divided into three deposit patterns including Dongmozhazhua type, Mohailaheng type, Lalongla type and Zhaofayong type. Dongmozhazhua type and Mohailaheng type mainly formed in Yushu area. Sphalerite+galena+pyrite+marcasite ± chalcopyrite ± tetrahedrite+ dolomite+calcite+barite+quartz+fluorite are the main mineral assemblage. About Dongmozhazhua type, ore bodies are controlled by fractures formed in limestone interlayer and intensive dolomitization of the host rocks in the hanging wall of thrust fault, and deposited as stratiform-like. Dissemination is the main ore structure. About Mohailaheng type, ore bodies are controlled by fractures formed in folds’cores in the hanging wall of thrust fault and mainly produced as tabular cutting layers. Breccia and vein are the main ore structures. Lalongla and Zhaofayong types mainly formed in Changdu region. Galena+sphalerite +smithsonite+cerusite+sardinianite+dolomite+calcite+gypsum+pyrite+barite+willemite+ hydrozincite are the main mineral assemblage. About Lalongla type, ore bodies are controlled by facies transition between limestone and shale near thrust faults and mainly produced as lens. Breccia and mesh-vein are the main ore structures. About Zhaofayong type, ore bodies are controlled by preore soluton collapse breccias and paleokarst and mainly produced as tubular. Crust and breccia are the main ore structure.According to systematic observation of thin sections and dating works, two periods of regional Pb-Zn mineralization in the north and middle part of Sanjiang area are recognized. In the first period, coarse sulfides are main ore minerals and ore bodies formed in the fracture zone in interlayer and core of folds nearby the thrust faults. In Yushu region, the dating of Rb-Sr isotopic isochrone for sphalerite and Sm-Nd isotopic isochrone for fluorite limit Pb-Zn mineralization in this period forming in 35.0-31.8Ma. In Changdu region, the dating of Sm-Nd isotopic isochrone for calcite limits Pb-Zn mineralization in this period forming in 41.1-38.1Ma. In Tuotuohe region, geological characters limit the Pb-Zn mineralizaion in this period forming in 34-30Ma. In the second period, fine Pb-Zn carbonates are produced and ore bodies formed in the facies transilion between limestone and shale, preore solution-collapse breccias and paleokarst and fracture zones nearby the thrust faults. In Changdu region, geological characters and regional correlation limit the Pb-Zn mineralization in this period forming in 29~26Ma. In Tuotuohe region, the geological evidences limit the Pb-Zn mineralization in this period forming in 23-20Ma. All the dating information indicate that carbonate-hosted Pb-Zn deposits in the North and Middle part of Sanjiang aera formed in the unanimous space and time. The Pb-Zn mineralization mainly formed in a late-collisional transform structural setting and partly continued to a post-collisional crustal extension setting.Dating of Sm-Nd isotopic isochrone for calcite from thrust faults indicate the thrust faults in Mohailaheng deposit in the Middle part of Sanjiang area and Chaqupacha deposit in the North part of Sanjiang area formed at 34.6Ma and 35.3Ma, respectively, both of which were earlier than the Pb-Zn mineralization age of the deposits. So, it indicates Pb-Zn mineralization happened in the extension stage after the compression of the thrust faults. According to the observation of thin section under the microscope, the minerals produced in first period of Pb-Zn mineralization in Changdu region are all deformed by compression, which indicates the second Pb-Zn mineralization happed after the second regional compression. The two mineralized periods in the North and middle part of Sanjiang area both happed after the regional thrust nappe activities and formed in the stress relaxation stages after the regional compression.Microthermometry and component analysis of fluid inclusions show that the fluid inclusions of carbonate-hosted Pb-Zn deposits in the middle part of Sanjiang area mainly are L-V two-phase inclusions and three-phase enriching CO2 inclusions, belonging to a Ca2+-Mg2+-Na+-K--SO42--Cr--F- system. The fluid temperature, salinity and density peak values of sulfide precipitation period in Pb-Zn deposits, Yushu region are 110~150℃,16.0~27.0 wt% NaCl eq. and 0.98-1.14 g/cm3 respectively, while in Changdu region, these values are of 120~150℃,19~24 wt% NaCl eq. and 1.04-1.12 g/cm3. Fluids display different features before and after sulfide precipitation, with higher temperature, lower salinity and lower density. Fluids in different stages pump into the mining areas.The values of δ13Cv-PDB, δ18Ov-SMOW in calcite and δDV-SMOW, δ18 Ofulid in fluid are respectively-10.4‰~2.6‰,15.8‰~25.3‰,-140‰~-53‰ and-4.9‰~13.9‰ in Pb-Zn deposits, Yushu region, while these values are of-5.0‰~7.2‰,9.1‰~27.4‰,-147‰~-93‰ and-2.9‰~13.6‰ in Changdu region. There are two resources of ore-forming fluids in carbonate-hosted Pb-Zn deposit in the middle part of Sanjiang aera, including ①ow-temperature, high salinity basin brine descending from Paleogene basin, ②iddle-temperature, low-salinity regional fluid migrating along thrust fault, including the evaporated-concentrated seawater cocooned in strata, metamorphic water released by metamorphic rock and the cycling meteoric water. According to the C-O data, the writer proposes that the regional fluid undergoes water-rock exchange interaction with carbonate rocks during processes of migrating and mineralizing, and also functions with organic materials in deposition and organic gases in metamorphic water during evolutionary period.In Yushu region, the δ34S values of sulfide and barite in Pb-Zn deposits and regional paleogene gypsum are -30.0‰～5.9‰,11.8‰～24.2‰ and 13.1‰～18.3‰ respectively,18OV-SMOW value of barite is between 12.7‰～17.8‰; While in Changdu region,δ34S of sulfide, barite and regional paleogene gypse are 7‰～0.1‰,10.8‰～27.1‰ and 2‰～4.7‰, respectively, and δ18OV-SMOW value in barite varies between 10.5‰～13.2‰. Combined with the research of C isotope in calcite, the writer figures out that reducing S mainly came from biological sulfate reduction, with part contribution of organic matter thermal sulfate reduction. The sulfate basically came from the basinal brine descending from overlying paleogene basin and evaporated-concentrated seawater cocooned in strata. Mixture of these two kinds of fluid causes the deposition of mineralized minerals.The research of Sr-Pb radioactive isotopes indicates that the initial 87Sr/86Sr ratios of barite formed in mineralization period and regional strata of deposits in Yushu area are 0.708553～0.708715 and 0.706188～0.718789, respectively, while the ratios in Changdu area are of 0.709865～0.711809 and 0.709288～0.709827. Comprehensive contrast of Sr content in regional strata shows that Sr isotope comes from either limestone which is high in Sr content with low radiogenic Sr or clastic rock(including volcanic rock) which is low in Sr content with high radiogenic Sr. The range of 206Pb/204Pb,207Pb/204Pb and 208Pb/204Pb of galena and regional strata in Yushu area are (18.298～18.526,18.316～18.645) (15.401～15.598,15.461～15.543) and (38.234～38.476,38.338～38.692), respectively, proving that Pb isotope came from clastic rock, limestone and metamorphic rock..The regional metallogenic model of sediment-hosted Pb-Zn deposits in Sanjiang area is established on the basis of comprehensive analysis and contrast about metallization characteristics and processese in sedimenta-hosted Pb-Zn deposits in sanjiang area. The model can be explained as followings:Structural trap is formed in the carbonate formation by thrust nappe; The basinal brine in paleogene basin descends, accumulates accompanied by the sulfide biological reduction and generate the local fluid reservoir rich in H2S; Regional fluid released by the compression deformation migrates along the main detachment zone of thrust nappe, leaching ore-forming minerals in strata, generating allochthonous fluid rich in metallic elements; Regional fluid ascends to favorable space along faults shaping by final stretch of reverse thrust, then migrating with local fluid, which results in the uninstall mineralization of metal materials.Based on the summary of regional Pb-Zn mineralization characteristics, methods for mineral prospecting of sediment-hosted Pb-Zn deposits in Sanjiang area are proposed, including ① hunting for dolomitization and silication in a wide range, ② delineating pyritization in the range of dolomitization and silication region, also the peripheral domain nearby,③ figuring out the strongest parts of pyritization in the region definited on the previous step, and ④ judging the action central of mineralizing fluid according to the atomic ratio of S/Fe in pyrite and of S/Zn in sphalerite.