Tectonic-magmatic Evolution And Copper Polymetallic Mineralization In Yushu Area,Northern Sanjiang District

The study area is located in Yushu area,Qinghai Province,in the north part of Sanjiang metallogenic belt in Southwest China.The tectonic position is located in the joint of Jinsha River and Ganzi Litang suture zone in the middle of Qinghai Tibet Plateau,which is closely related to the evolution of ancient Tethys.There are a lot of Permian Triassic.Based on the long-term and detailed field geological survey,zircon U-Pb geochronology,whole rock geochemistry and Sr nd Hf isotope geochemistry of mafic rocks,intermediate acid intrusive rocks and volcanic rocks are studied in this paper to trace the ore-forming fluids and materials of copper polymetallic deposits,The relationship between diagenetic and metallogenic background and regional tectonic evolution is discussed,and the prospecting direction is pointed out.The main conclusions are as follows:Magmatic rocks in the study area are mainly composed of basic intrusive rocks,intermediate-acid intrusive rocks and volcanic rocks(basalt,andesite,and dacite).The LA-ICP-MS zircon U-Pb geochronology results of these magmatic rocks show that the206Pb/238U weighted average age of the Chayong gabbro is 283±3Ma,and the multicolored massive gabbro is 252±2Ma.The diabase dyke in the ditch is 254±4Ma,the diabase dyke in the east of Longmogou is 248±2Ma,and the middle-acid intrusive rock is 233±2Ma in the Maoyong quartz diorite.The ages of the Yonggranodiorite,Dangjiangrong granodiorite and its dark particle enclaves are 218±1Ma,217±2Ma,221±1Ma,and 222±1Ma,respectively.The syenite porphyry in Nari is 170±3Ma..Basic volcanic rock Chayong pillow-shaped basalt is 235±3Ma,Longmogou pillow-shaped basalt is 249±2Ma,Mizhana can be almond-shaped andesite 237±3Ma,Sanalongwa basalt is 221±3Ma,middle-acid volcanic rock,and most in the south Rirong layered dacite and Longmogou south layered dacite are 228±1Ma and 227±2Ma,respectively.The dacite and dacite porphyry in Galonggema east-west mining area are 223±2Ma and221±1Ma,respectively.Most of the intermediate-acid intrusive rocks and volcanic rocks were formed in the Late Triassic.The age distribution of the colorful-Chayong ophiolitic mélange has been distributed from the Early Permian to the Middle Triassic,while the basic volcanic rocks are mainly concentrated in the Middle Triassic.Stacked and early Late Triassic.The mafic rocks in the Zhiduo melange may be derived from partial melting of different mantle sources.The chayong gabbro is derived from the lithospheric enriched mantle source and is produced by partial melting of spinel lherzolite mantle.The multicolored gabbros and pillow basalts may come from the polluted or reformed spinel+garnet mantle source.GEOCHRONOLOGICAL and geochemical evidence shows that the West Jinshajiang Ganzi Litang paleo Tethys system lasted for a long time,indicating a mature ocean.The intermediate acid intrusive rocks have a relatively continuous compositional change trend,which is closely related to subduction.The magma may originate from the mixing of magma(basic endmember)produced by subducted slab fluid or sediment melt metasomatic mantle wedge and ancient lower crust melt(acid endmember).The fractional crystallization and chemical diffusion during the later magma rise also affect the chemical changes in the magma evolution process,The formation is related to the arc environment and the southwest subduction of the West Jinshajiang Ganzi Litang ocean.The intermediate acid Batang group volcanic rocks in Zhiduo,northern Qiangtang,were formed by partial melting of Cenozoic crustal source,which may be the product of partial melting of garnet amphibolite magma source area of Late Triassic under high temperature and low pressure environment.Diagenetic ages tend to be younger in Northeast China,indicating a model of southward subduction and retreat.The Permian Late Triassic Zhiduo melange(283-235 MA),Late Triassic intermediate acid intrusive rocks(222-217 MA),volcanic rocks(228-221 MA)and other rocks indicate that there is a complete submarine spreading and subduction in Litang Paleo Tethys Ocean from Jinsha West to Ganzi area Combined with the study of Songpan Ganzi block,it is suggested that there may be bidirectional subduction and retreat in the West Jinshajiang Ganzi Litang ocean.In Salanongwa deposit,the properties of the ore-forming fluid in main metallogenic stage are of low temperature(120??180?),low-middle salinity(6.0?12.4%Na Cl.eqv),and low-middle density.Besides,the values of?D and?¹⁸O are-104.8‰-82.3‰and-4.67‰?0.43‰,respectively.We speculate the source of the ore-forming fluid was the mixing fluid of seawater and magmatic fluid.As for ore-forming materials,?³⁴S has the feature of magmatic Sulphur,(-4.3‰?1.7‰),and the values of ²⁰⁶Pb/²⁰⁴Pb,²⁰⁷Pb/²⁰⁴Pb,²⁰⁸Pb/²⁰⁴Pb are 18.512?18.715,15.690?15.981 and 38.705?39.745.Thses S-Pb isotopic values are similar to volcanics of Batang Group,indicating that the ore-forming materials sources are from the activity of magma,and the lead isotope composition of ore is close to that of volcanic rocks in Batang group,and it is speculated that it mainly comes from deep part of magmatic activity at Late Triassic.Through the comparative study with typical VMS deposit in Sanjiang area and"black ore"in Japan,Sanarongwa deposit has great similarity with them in geological characteristics,ore-forming fluid,material source and metallogenic structure background.Therefore,the paper holds that the sanarongwa copper polymetallic deposit is VMS type,which is related to the background of the intergarc rift in Late Triassic.In Chayong deposit,quartz veins of molybdenum ore forming fluids uniform temperature is 167.2?381.1?(peak is 340?360?),freezing temperature is-5.5?-0.4?,salinity is 0.70?8.38wt%Na Cl(peak is 4.0?5.0wt%Na Cleq),?D_H2O is-110.7?-85‰,?¹⁸O_H2O of 9.5?10.9‰,?18O is 2.89?5.19‰.Dense disseminated copper polymetallic ore forming fluids uniform temperature is 224.5?365.8?(peak is 340?360?),freezing temperature is-6.2?-1.2?,salinity is 2.07?10.11wt%Na Cl(peak is3.0?4.0wt%Na Cleq),?D_H2O is-110.0?-77.6‰,?¹⁸O_H2O of 3.4?12.1‰,?¹⁸O is-2.19?5.89‰.There are similar sources of ore-forming fluids in the two types of ore,mainly in magmatic water,in the late evolution of mixed with atmospheric precipitation.?³⁴S_V-CDTof the molybdenite in the contains quartz veins of molybdenum ore is-3.6?-2.2‰,and have a feature of enrich?³²S and close to mantle sulfur.This indicates that S comes from deep magma.?³⁴S_V-CDTof the pyrite in the Dense disseminated copper polymetallic ore is-5.2?-4.8‰,and?³⁴S_V-CDTof the the chalcopyrite is in the ore is-6.5?-5.1‰.In the ores,pyrite and chalcopyrite are more?³⁴S than molybdenite,and due to the formation in the mining area contains peat components,suggested that the Batang Group strata in the investigation area provide some mineralization materials in the process of ore-forming fluid activity.²⁰⁸Pb/²⁰⁴Pb of the molybdenite in the contains quartz veins of molybdenum ore is 38.191?38.792,²⁰⁷Pb/²⁰⁴Pb is 15.51?15.618,²⁰⁸Pb/²⁰⁴Pb is 17.928?18.456.²⁰⁸Pb/²⁰⁴Pb of the pyrite and chalcopyrite in the Dense disseminated copper polymetallic ore is 38.918?39.051,²⁰⁷Pb/²⁰⁴Pb is 15.64?15.738,²⁰⁸Pb/²⁰⁴Pb is 18.55?18.585,has a good correlation with Galonggema deposit.It is shown that the mineralization of Chayong deposit is similar to the Galonggema deposit,which is derived from deep magmatic activity.Therefore,it is inferred that the mineralization of Chayong deposit has deep source.In Duorirong deposit,the homogeneous temperature of fluid inclusions in the multi-day velvet deposit is concentrated at 90?170?,fluid salinity is concentrated at 2-11%,density is 0.98 g/cm³,belongs to low-temperature,medium-low salinity,and low-density fluid.The range of calcite?¹⁸O_V-SMOW value is 13.8?18.8‰,and the range of calcite?¹⁸O_V-PDB value is-16.6?-11.7‰.The ore-forming fluid mainly comes from atmospheric precipitation and dissolves the carbonate rocks in the surrounding rocks to obtain the carbon.The pyrite?³⁴S_V-CDT value in the Dazinrong deposit is between-35.5 and 0.5‰,mainly concentrated at 0±3‰.which has the characteristics of deep source magmatic sulfur.The lead isotope composition of galena in the ore is ²⁰⁶Pb/²⁰⁴Pb value between 18.477?18.615;²⁰⁷Pb/²⁰⁴Pb value between 15.693?15.770;²⁰⁸Pb/²⁰⁴Pb value between 38.768?39.010.Lead isotopes in sulfide from Zygosanthemum,Zanthoma,and Sanarongwa have a good correlation,and have little relationship with lead isotopes of Mesozoic and Cenozoic.The ore-forming materials may be mainly derived from deep magma.A comparative study of the deposits suggests that the genesis type of the polyazine lead-zinc polymetallic deposit is a low-temperature hydrothermal lead-zinc polymetallic deposit.Combined with the evolution characteristics of the Paleo Tethys ocean and its indication to the tectonic setting of diagenesis and mineralization in the area,there are multi-stage composite mineralization related to Paleo Tethys evolution,middle Tethys closure and India Eurasia plate collision in Yushu area,with magmatic hydrothermal deposits,collisional porphyry deposits,MVT deposits and late hydrothermal superimposition related to volcanic activities.The distribution of ore deposits shows the rule of belt in northwest,cluster in Northeast and local group.In view of the present metallogenic potential,it is the main prospecting direction to look for VMS type deposits in Zhiduo volcanic belt and porphyry type deposits in Zhiduo suture zone and near deep faults.Based on the tectonic setting of diagenesis and mineralization,four metallogenic prospect areas are divided,namely,Galongema-Salalongwa volcanic belt,which is mainly for VMS type copper lead zinc deposits related to Paleo Tethys volcanism;The Chayong-Xiqueyong-Dangjiang deep structure and magmatic belt is mainly for Cu-Mo and Pb-Zn and Au-Ag hydrothermal deposits related to porphyry mineralization system;The Longmogou-Duorimong volcanic sedimentary rock belt is mainly for hydrothermal vein type lead-zinc deposits;The Zaduo-Mizannaneng carbonate basin is mainly for porphyry Cu-Mo deposits related to Cenozoic continental collision and MVT Pb-Zn deposits related to thrust nappe structure.