Fluid Inclusion,Mineralogical,Geochronological,and Geochemical Constrains On The Characteristics And Genesis Of The Chagele Pb-Zn-Mo-Cu Deposit In Tibet,China

The large Chagele Pb-Zn-Mo-Cu deposit is located in the western section of the Nyainqentanglula Pb-Zn-Ag-Fe-Mo-Sn metallogenic belt.However,the source and evolution of ore-forming fluid,the source of ore-forming material,the geochronology for ore formation,the origin and petrogenesis of the granite porphyries and their relationships to Pb-Zn-Mo mineralization are lacking,which seriously restricts the exploration and development of the next step and the prospecting work in the west of metallogenic belt.This paper systematically carried out research on the geological characteristics,geochemistry,chronology,mineralogy,fluid geochemistry and isotope geochemistry of the Chagele deposit.We made these conclusions as fellow.1.Identify the geological characteristics of the Chagele depositFrom north to south,the Chagele deposit consists of the Longgen Pb-Zn mining area,the Chabei Pb-Zn polymetallic mining area and the Chanan Mo mining area.The Longgen mining area is rich in Pb,Zn and Fe.The ore body is agglomerate,disseminated or veined in skarn,marble and nearby interbedded fracture zone.The Chabei mining area is rich in Pb,Zn,Ag,Cu,and the ore body is agglomerate,disseminated or veined in horn,skarn,limestone and marble.The Chanan mining areais rich in Mo,Fe,and a small amount of Cu.The ore body is mainly disseminated or irregularly veined in the quartz vein and silicified granite porphyry.The skarn has obvious zoning characteristics.The proximal garnet is reddish brown.It changes to light brown and green away from the rock mass,and the content of grossularite from the near rock mass to the far rock mass increases gradually.2.Limited the magmatism and mineralization ages of the Chagele deposit,and proposed the geodynamic setting of Palaeocene–Early Eocene magmatism and mineralizationThe ore-forming granite porphyries in the three mining areas of the Chagele deposit have similar geochemical characteristics,both of which are characterized by high contents of silicon and total alkalis,low contents of Ti,Mg and Ca.The trace elements are characterized by enrichment in large-ion lithophile elements such as K,Rb,Th and Nd and depletion in high field strength elements such as Nb,Ta,Sr and Ti.The rare earth element has the characteristics of enriching light rare earth elements and relatively depleted heavy rare earth elements,and exhibits a large slope right-inclined"V"type rare earth partitioning model.The Pb isotopic compositions of the ore-forming granite porphyries of the three mining areas are similar and similar to the upper crust.The ore-forming granite porphyries of the three mining areas have similarεHf（t）values（-8.53^-0.23）andεNd（t）values（-15.48^-5.24）.The Nd model ages（1.3¹.77 Ga）and Hf model ages（1.02-1.47 Ga）are similar to the formation of the Nyainqentanglula group.The mantle contribution of the granite porphyries source is 10-60%by the Sr-Nd-Hf mixing proportion.The ore-forming granite porphyries of each section of the Chagele deposit have the same magma source,and were likely originated form the partial melting of previously Mid-Proterozoic basement and significant amounts mantle contribution was involved during their generation.The magmatism and mineralization ages of the three mining areas of the Chagele deposit are similar,ranging from 59 to 64 Ma.The sphalerite Rb-Sr isochron age of59.1±1.1 Ma is closely consistent with the emplacement timing of 64.3±0.7 Ma of the Longgen granite porphyry.The muscovite ⁴⁰Ar/³⁹Ar age（62.75±0.63 Ma）and the molybdenite Re–Os isochron age（62.3±1.4 Ma）are coeval with the emplacement timing of 63.8±1.1 Ma of the Chabei granite porphyry and 63.9±0.9 Ma of the Chanan granite porphyry,respectively.The magmatism and mineralization were formed related to the slab rollback of the northward subduction of Neo-Tethyan oceanic slab and the initial collision of the India–Eurasia continents,it is the product of transition environment of late subduction and initial collision.3.To explore the relationship between the three mining areas and the genesis of the Chagele deposit,and consider that the Chagele deposit is a typical porphyry Mo+skarn type Pb-Zn polymetallic deposit.The three mining areas of the Chagele deposit are produced under the same structural system and exhibit mineralization zones from Mo,Mo-Cu,Cu-Pb-Zn to Pb-Zn.The ore-forming granite porphyries have the similar characteristics of the petrography,geochemistry,zircon U-Pb age,mineralization age and Sr-Nd-Pb-Hf isotope composition,indicate that they have a common magma source and similar evolutionary processes.Fluid inclusions and C-H-O isotopes indicate that the ore-forming fluids of the Chagele deposit are mainly derived from the magmatic hydrothermal system,and with the progress of mineralization gradually evolved to meteoric water.The sulfides of the Mo and Pb-Zn mineralization in the Chagele deposit have similar S and Pb isotopes,in addition to molybdenum Re and sphalerite Rb isotopes,indicating that these two mineralizations have similar sources of ore-forming materials,both mainly derived from magmatic hydrothermal.From the investigation of Chanan Mo,Chabei Pb-Zn polymetallic to Longgen Pb-Zn mining areas,the trace elements of pyrite and chalcopyrite,analyzed by LA-ICP-MS,showed regular variation.For example,pyrites from the Chanan porphyry Mo mining areaare rich in Sb,Mo,Mn and As and those from the Chabei mining areaare rich Cu and Zn,whereas pyrites from the Longgen mining areaare rich in Pb,Ag,Co and Ni.Most of the pyrites from the three mining areas have the Co/Ni≥1,with Au and As contents similar to the pyrites of porphyry hydrothermal deposits.The genetic analysis of pyrite,chalcopyrite,sphalerite and galena shows that they related to magmatic hydrothermal fluid.Therefore,we conclude that the three mining areas are formed under the same tectonic-magmatic event and belong to an integral porphyry-skarn Mo–Pb–Zn metallogenic system.4.Exploring the mineralization process of the Chagele depositThe study on the fluid inclusions and H-O isotope indicate that in the I stage,the ore-forming fluid associated with Pb–Zn mineralization is high temperature,moderate salinity NaCl-H₂O magmatic water,and it interacts with limestone at the depth of about1.2².1 km.The garnets in the Longgen mining areaare mainly andradite,and the minor pyroxene is mainly diopside and hedenbergite,indicate a relatively high oxidation conditions.The pyroxene in the Chabei mining areais mainly hedenbergite and johannsenite,and minor garnet is mainly grossularite,representing reducing conditions.The ore-forming fluid in the Chanan mining areahas the characteristics of high temperature,moderate-high salinity and weak reduction,forming potassic alteration,sericitization alteration and related veins.In the II stage,the temperature and salinity of the ore-forming fluid are further reduced.At this stage,minor meteoric water was added,and led to the precipitated of calcium aluminosilicate,magnetite,and quartz.The fluid boiling in the Longgen mining areacaused the magnetite to precipitate.The propylitization alteration was developed in the Chanan Mo mining areaand formed the altered minerals such as epidote,chlorite and quartz and dotted pyrite.In the III stage of the main mineralization period,the temperature and salinity of the fluids in the Chabei and Longgen mining areas are greatly reduced.The gradual transition of the ore-forming fluid to the transition from oxidation to the reducing environment,the fluid boiling and mixing of meteoric water lead to the precipitation of chalcopyrite and pyrite.In the Chanan mining area,the fluid boiling during the cooling and depressurization process,resulting in the precipitation of molybdenite,pyrite and a small amount of chalcopyrite.Subsequently,in the IV stage,as the proportion of meteoric water increased,the fluid temperature and salinity decreased significantly,leading to galena and sphalerite rapid precipitation in the Chabei and Longgen mining areas.A small amount of pyrite and chalcopyrite precipitated in the Chanan mining area.In the V stage,with the mixing of meteoric water,the fluid gradually evolved into a low temperature,low salinity fluid dominated by meteoric water,representing the waning or termination of the ore-forming hydrothermal process.5.Established the metallogenic model of the Chagele deposit.The collision between India and Eurasia started at⁶5Ma,and caused the slab rollback of the northward subduction of Neo-Tethyan oceanic slab.The upwelling of mantle material and the partial melting of the crystalline basement of the overlying Nyainqentanglha group formed mantle-crust migma.Large-scale magmas rose and emplaced to the shallow crust,continuously separated the supercritical fluid and finally formed the“island-arc”magmas.The supercritical fluid of the Chagele deposit evolved into two completely different types of hydrothermal fluids.The fluid dissolved in the Chabei and Longgen mining areas were transformed into a high temperature,moderate salinity,rich in ore-forming elements（Zn,Pb,Cu,Fe）NaCl-H₂O magmatic water system.The ascending fluid occured selective metasomatism at the contacts between the granite porphyry and the Xiala Formation limestone or in lithologic interface zones.In the Chanan Mo mining area,the dissolved fluid was transformed into a high temperature,moderate–high salinity rich in Mo and Fe fluid system,final precipitation to form porphyry Mo（Fe,Cu）mineralization.6.Discussed the differences in genesis and source area of ore-forming instrusion between skarn Pb–Zn,Fe and porphyry Mo deposits in the Nyainqentanglha metallogenic belt.It is believed that the difference source and magmas nature are the main factors controlling metal endowment.Three major types of mineralization system have been identified formed in the Paleocene-Early Eocene of Nyainqentanglha area:skarn Pb-Zn systems,skarn Fe systems and porphyry Mo systems.The source rocks play an important role in determining the intrusions associated with Pb-Zn and Fe mineralization formed during the initial stage of the India–Eurasia continental collision.Mantle material was critical for the development of the skarn Fe deposits and associated granitoids,and more contributions from old Lhasa continental crust materials played a role in producing the intense Pb–Zn mineralization and associated granitoids in the Nyainqentanglha polymetallic belt.However,instead of source rocks,it is likely that the addition of crustal material,magmatic oxygen fugacity and magmas fractionated degree play an important role in determining the differences mineralization between Mo and Pb-Zn,Fe mineralization.7.Summarize the ore-controlling conditions,the temporal and spatial distribution characteristics of the deposits,and indicate the prospecting potential area.The contact zones of the carbonate strata,such as the Yongzhu Formation,Loubadui Formation,Xiala Formation,Angjie Formation,Laga Formation and Langshan Formation,and the Paleocene-Early Eocene intermediate-acidic magmas are the favorable target region for skarn Pb–Zn deposit and Fe deposit in Nyainqentanglha area.It should pay much attention to looking for porphyry Mo deposits in the periphery and deep.