Study On Geological And Geochemical Characeristics And Genesis Of Bangbule Pb-Zn-Cu-Fe Deposit,Western Gangdese,Tibet

The Banbule Pb-Zn-Cu-Fe polymetallic deposit is newly discovered in the western Gangdese-Nyainentanglha metallogenic belt selected as the research object in this paper.Based on detailed field geological survey and detailed observation and analysis indoor,we summarize the geological characteristics of Bangbule deposit and the ore-forming geological conditions for the first time.The U-Pb dating of zircons,Hf isotopes of zircons and Sr-Nd-Pb isotopes of total rock were analyzed with high accuracy for the ore-forming related quartz porphyry and rhyolitic porphyry;mineralograph and geochemistry were analyzed via Scanning Electron Microscope,Electronic Probe and other methods;and the genesis of Bangbule deposit was discussed by means of microscopic temperature measurement of fluid inclusions,C-H-O isotope analysis and S-Pb isotope analysis of metallic sulfide minerals.The results are as follows:1.The geological characteristics of Bangbule deposit are identified and reported for the first time.The orebodies of Bangbule deposit mainly occur in the contact zone between the late cretaceous quartz-porphyry and limestone,and in the interbedded interface between limestone and sandstone.Pb-Zn-Cu mineralization in the Banbule deposit is closely associated with skarns.The ore minerals are dominated by galena,sphalerite,chalcopyrite,and magnetite.The main alteration in Bangbule is skarnization developed near the contact zone,donminated by Calcium ferric garnet,diopside and actinolite.Three orebody cluster and concealed orebodies under 300 m were preliminarily controlled and confirmed by the prospecting trench and drilling projects.In addition,76 Lead-zinc orebodies,2 copper orebodies and 12 deep concealed orebodies are delineated in Bangbule area.The measured and indicated resources defined Bangbule is a large scale deposit including 0.33Mt@Pb,0.38Mt@Zn,170.9t@Ag and 1259t@Cu.This discovery provides an important evidence for the westward extension of the Gangdese-Nyainqentanglha metallogenic belt,which extends about 250 km westward again after the discovery of the Chagele deposit.2.The petrogenesis of quartz-porphyry is determined by petrogeochemical studyThe Bangbule ore-forming quartz-porphyry belongs to the high differentiation I-type granite,which characterized by high-K calc-alkaline and partial aluminum.And it also shows enrichment in light rare earth,and loss in heavy rare earth.In the mean time,there were significant negative anomalies such as Sr,Eu,Nb,Ta and P,indicating that the magmatic formation had undergone important crystallization differentiation.The zircon ?Hf(t)values of quartz porphyry mainly ranging from-7.92~-12.85.The values of(87Sr/86Sr)of total rock is ranging from 0.7148 to 0.7258,and the values of ?Nd(t)is ranging from-9.01 to-7.32.The lead isotopic analyses yield 206Pb/204 Pb of 18.686 to 18.781,207Pb/204 Pb of 15.699 to 15.762,and 208Pb/204 Pb of 39.131 to 39.344 respectively.All the lead isotope data are constrained around the growth curves of upper crust and orogenic belt according to the tectonic discrimination diagrams.It indicates that the quartz-porphyry in Bangbule area may form in the post-collisional extensional environment after the southern subduction of Bangong Lake-Nujiang Ocean,which is related to the partial melting of the Mesoproterozoic ancient basement in Lhasa block.3.The diagenesis and metallogenic ages are identified and defined,providing a new evidence to improve the diagenetic and metallogenic evolution sequence of the Gangdese belt in the Late Cretaceous(80~70 Ma)period.The zircon U-Pb ages of quartz-porphyry,rhyolite porphyry and diorite porphyrite in Bangbule are all about ca.77 Ma.Based on the regional research results,we believe that the magmatic activity in the Gangdese belt is continuous in the Late Cretaceous,and there is no "magmatic gap" in the period of 80~70 Ma as previously studies.During the period of 80~70 Ma,the magmatism intensity in the northern Gangdese were stronger than that in the southern Gangdese,and the emplacement time was gradually becoming new from north to south,which also showed that the eastern region was stronger than the western region.During this period,iron-copper and copper-gold mineralization dominated the northern Gangdese area,forming iron-copper mineralization represented by Ria and Gengnai in the east,while lead-zinc-iron-copper polymetallic mineralization dominated the Nyainqentanglha belt,forming a large skarn Pb-Zn-Cu-Fe deposit represented by Bangbule.4.Genesis of Bangbule deposit is discussed and the metallogenic model is preliminarily established.The ?34S values of sulfides define a narrow range of-0.8 to 4.7‰,indicating the single magmatic source for the ore-forming materials.The lead isotopes in ore and quartz-porphyry is relatively stable and has a relatively consistent source characteristics of the upper crust.The C-H-O isotope results of transparent minerals related to mineralization show that the ore-forming fluids mainly consist of magmatic fluids in the early stage and gradually transit to meteoric precipitation in the late stage.The metallogenic model can be briefly described as follows: during the Late Cretaceous(ca.77Ma±),the Qiangtang terrane and Lhasa terrane changed from collisional environment to post-collisional environment.The ancient basement partially melted forming into quartz-porphyry magma and carried a large amount of mineralizing material upward into the which formed into Skarnization zone at the contact zone of rock mass and surrounding rock after metasomatic reaction occured with limestone in strata.The ore-forming fluids in the early stage dominated by magmatic water have the characteristics of low acidity,high temperature,high salinity and high oxygen fugacity.During the quartz-sulfide period,the temperature and salinity of ore-forming fluids reduced obviouslydue to the mixing with a mass of meteoric water.Then the ore-bearing fluid in the vicinity of the contact zone between quartz porphyry and the limestone of the upper Permian Xiala formation,the interlayer fracture zone between the limestone of the upper Permian Xiala formation and the sand slate of the Laga formation or the widely developed joints and fractures in the strata of the Laga formation will reduce pressure and temperature.Subsequently,the ore will precipitate and accumulate,forming the skarn type Pb-Zn-Cu-Fe deposit.5.Suggestions for the regional prospecting are put forward.In the study,we suggest the magmatism induced by the local extension of postcollision of Qiangtang-Lhasa terrane during the period of ca.80~70Ma,intruded into the carbonate strata,which may be the main mechanism for the formation of skarn deposits in this period.It should be focused on prospecting skarn deposits in the Nyainqentanglha belt in the next stage according to the magmatism of this period.And considering the newly discovery of Bangbulelarge Pb-Zn deposit and the characteristics of clustering distribution of deposit,more exploration efforts should have been made in LongerNanmulin arcback fault uplift zone and the northern margin of Gangdese volcanic arc.