Metallogenic Age And Genesis Of Aqishan Pb-Zn Deposit In Eastern Tianshan,Xinjiang

The Jueluotage metallogenic belt in eastern Tianshan is located in the convergence area of Siberian plate and Tarim plate.The Aqishan Pb Zn deposit is situated in the southeast of Shanshan County,Turpan area,Xinjiang,while the tectonic setting is of the Aqishan-Yamansu island arc volcanic belt of Jueluotage metallogenic belt in eastern Tianshan.Scholars have researched its metallogenic geological characteristics,geochemistry of the deposit and the geochronology of granitic rocks in the mining area,since it was discovered in 2013.Based on the study of geological characteristics of the Aqishan Pb-Zn deposit,the major and trace elements and U-Pb isotopic compositions of garnet,which is closely related to the mineralization,were analyzed by means of electron probe microanalysis and in-situ LA-ICP-MS trace element analysis,respectively.Major,trace element analysis and zircon U-Pb dating of the granitic porphyry in the south part of the ore body were carried out to diagnose the genesis of deposit.Combined with the geochemical analysis data of volcanic rocks in the lower Carboniferous Yamansu formation and the tectonic environment,we discussed the genesis of the Aqishan Pb-Zn deposit and its garnet,and some results were inferred as follows:The lower Carboniferous Yamansu formation is an important ore-bearing rock of Aqishan Pb-Zn deposit,which is distributed between Kangguer shear zone and aqikekuduke fault.Tts lower part is mainly composed of chartreuse tuffaceous clastic rock,dacitic tuff and garnetizedandesitic tuff with a small amount of thin limestone,the middle part is characterized by gray black pebbly tuffaceous siltstone and dacitic tuff with limestone lens,while the upper is composed of celadon epidotized andesite,andesitic breccia crystal tuff with a small amount of thin basalt and limestone lens.The Aqishan lead-zinc ore body occurs in the early Carboniferous Yamansu Formation marine volcanic tuff.Geochemical analysis results show that the volcanic rocks of Yamansu formation in Aqishan area belongs to calc-alkaline series,with the features of LREE relatively richer compared to HREE,LILE(Rb,Ba,K,La)enriched and HFSE(Nb,Ta,P,Ti)depleted.They are typical island arc volcanic rocks.Granitic porphyry,with SiO₂ content of 55.63%?65.83%,K₂O content of1.35%?1.76%,K₂O+Na₂O content of 4.30%?8.88%,is mainly composed of medium potassium calc alkaline series and high potassium calc alkaline series rock.REE is consistent with right dip,and displays variable LREE-enriched and HREE-depleted patterns.Eu shows obvious negative anomalies,while Ce without obvious anomalies.As presented by the standardized spider diagrams of trace elements,these samples are characterized by relatively enriched LILE and depleted HFSE,which are similar to the trait of granitic porphyry formed in island arc environment.Zircon U-Pb dating of the granitic porphyry shows that the formation time of the granitic porphyry is 318±1.4Ma,which is consistent with age of Yamansu formation.However,there are no marbleization and skarnization can be observed in the limestone hosted in the ore bearing strata of the Yamansu formation,which is close to the Yamansu formation.There are 42 ore bodies on the surface of Aqishan lead-zinc mine,with the length of 80-3000 m and the thickness of 2?106 m.The average grade of Zn,Pb and Pb+Zn are 0.5%?1.21%,0.3%?0.9%,and 0.63%?1.77%,respectively.In the local drilling cores of carbonaceous mudstone,the grade of Pb,Zn and Cu is high,and the grade of Pb+Zn+Cu can reach medium to large scale deposit.The ore bodies are mainly layered,stratoid and lenticular.The ore structure is layered,stratoid and lenticular in the upper part with the SE dip of ore body and dip angle of 40°to 60°and is consistent with the occurance of wall rock of volcanic tuff,while the vein and network vein structures are developed in the lower part.The main ore minerals are chalcopyrite,pyrite,sphalerite,galena and magnetite.The wall rock alteration mainly includes silicification,sericitization,chloritization,epidotization,garnetization,gypsum and baritization.Garnetization occurs in the upper part of the ore body in layered and stratoid form,which is closely related to mineralization.Spatially,the Aqishan Pb-Zn deposit is associated with ferromanganese ore hosted in marine volcanic rocks of the Yamansu formation.The geological characteristics of the Aqishan Pb-Zn deposit can be compared with those of the typical VMS-type deposits in the world,i.e.the metallogenic system attribute of submarine VMS.Garnet skarn,which is closely related to mineralization,is mainly layered and stratoid,consistent with the occurrence of strata and ore bodies of Yamansu formation.Some ore bodies occur in garnet skarn.Garnet can be divided into two types:brown garnet and gray green garnet.Microscopically,the later is filled with a large amount of gold sulfide in the intergranular and internal voids.The results of EPMA show that the former is mainly andradite,while the later is almandine series.The contents of SiO₂ and Ca O are vary from 34.791%to 37.8%and 32.493%to 34.274%,respectively,which are positively correlated.The concentration of Fe O and Al₂O₃ are vary greatly from 8.454to 27.275%and 0.012 to 15.293%,and there is an obvious positive correlation between them.The content of TiO₂ is 0.013%?1.057%,and the content of Mn O is0.323%?1.413%.The results of ICP-MS analysis show that the REE is generally right dipping,total REE is relatively low(?REE=71.405-826.52×10^-6),LREE is relatively enriched,and HREE is relatively depleted(LREE/HREE=8.66?4157.75,La _N/Yb _N=23.51?984.34),which is similar to the REE characteristics of ore-bearing rhyolitic volcanic rocks.Obvious Eu positive anomaliesand Ce positive anomalies(?Ce=0.94?1.85),indicating these garnets were formed in an oxidizing environment,which is apparently related to submarine hydrothermal activity.LA-ICP-MS in-situ U-Pb isotope dating of garnet documented a weighted mean²⁰⁶Pb/²³⁸U age of 316.6±4.4 Ma,which is consistent with the formation time of Yamansu formation.In conclusion,the results of in situ U-Pb isotopic age of garnet and zircon U-Pb age of granitic porphyry show that the eruption and deposition of aqishan lead-zinc deposit and Yamansu Formation volcanic rocks and the intrusion of granitic porphyry are almost simultaneous,and formed in the island arc environment of continental margin,and are VMS-type deposits of hydrothermal exhalative sedimentation deposit.The results of the major elements and rare earth elements test show that garnet is formed in the oxidation environment of hot water activity on the sea floor,different from the garnet in skarn type deposits in collision orogenic belt.