Genesis Of Polymetallic Deposits And Propspecting Potential In The Linzizong Area, Western Gangdese Belt,Tibet

Three typical deposits?Nuocang Pb-Zn-?Cu?,Beina Pb-Zn-Ag?Cu?,and Dajiacuo Ag-Pb-Zn-?Cu??located in the Linzizong volcanic area,western Gangdese have been selected as the study object in this thesis.Based on the field geological survey and detailed observation and analysis indoor,we summarize the geological characteristics of ore deposits and their ore-forming geological conditions in the Linzizong volcanic area of western Gangdese.A synthetical research methods of zircon U-Pb geochronology,rock geochemistry,rock Sr-Nd-Pb and zircon Hf isotopes,mineralography,mineral geochemicstry,fluid inclusion,H-O isotope of transparent minerals and S-Pb isotopes of sulfides and sulfides,Re-Os geochronology methods have been carried out,and the results are as follows:1.Discuss the difference of volcanic rock genesis in the eastern and western Gangdese,and confirm the difference of crustal properties is an important reason for the difference in genesis.Though the Linzizong volcanic rocks of Dianzhong Formation in the eastern and western Gangdese are both characterized by the marginal arc magmatic rock.In contrast to the calc-alkaline,quasi-aluminous andesited volcanic rocks of Dianzhong Formation in the eastern Gangdese,the acid volcanic and sub-volcanic rocks in the Linzizong voclcanic area of western Gangdese are typical of the high silicon,high K,peraluminous features.They were generated by the remelting of ancient Lhasa subterrane due to mantle-derived magma invading(negative?_Hf?t?values and old Hf model ages that from the late Archean to middle Proterozoic),and belong to high fractionated I-type magmas.The limited intermediate-basic volcanic rocks in the western Gangdese display the metaluminous and calc-alkaline continental margin arc magmatic features,which were originated from the partial melting of a depleted mantle wedge that had been metasomatized by subduction-related fluids followed by²0%ancient crustal contamination and fractionation crystallization.However,the difference is that the Dianzhong Formation volcanic rocks in the eastern Gangdese usually mixed with minor of newly lower crust and experienced a wide fractionation crystallization.These features not only confirmed the view that Hou et al.,?2015?proposed that the Middle part of southern Lhasa subterrane?84-88°E?exisit the ancient Lhasa subterrane,but also indicated that the difference of crustal properties in the western and eastern Gangdese is an important reason for the difference in the genesis of Linzizong volcanic rocks.2.Extend the diagenetic age of Linzizong volcanic rocks of DianzhongFormation in the western Gangdese to the late Cretaceous,constraint that the initial time of India-Eurasia collision was diachronism from the western to eastern Gangdese,and propose a new diagenetic and metallogenic geodynamic modelThe ages of Dianzhong Formation volcanic rocks in the western Gangdese are mianly concentrated in the⁸2 to 60Ma.The starting time of volcanic eruption in the western Gangdese is earlier than that of eastern Gangdese about 13 Ma,and also the lasting time that is much longer than that of eastern Gangdese,which reveal that there exist the difference on the rock-forming ages of Dianzhong Formation volcanic rocks in the western and eastern Gangdese.The age of Nianbo Formation volcanic rocks which could represent the initial time of India-Eurasia collision have been accepted by some scholars.Based on the recent reports of India-Eurasia collision and the ages of Nianbo Formation volcanic rocks,we restrict the initial time of India-Eurasia collision in the western Gangdese at⁶0 Ma,and that of in the eastern Gangdese at⁶0 Ma.The result indicate that the initial time of India-Eurasia collision in the western Gangdese is earlier than that of eastern Gangdese.Most of the ore-forming ages of Pb-Zn-Ag polymetallic deposits in the eastern Gangdese are consistence with the ages of Dianzhong Formation volcanic rocks that represent the mineralization in the late stage of the Neo-Tethys oceanic subduction,and minor of them are related to the Neo-Tethys oceanic slab break-off after the strong compression stage of India-Eurasia collision.However,the strong compression stage of India-Eurasia collision commonly is difficult to form the deposits.Meanwhile,combined with the diagenetic and metallogenic ages of Pb-Zn-Ag polymetallic deposits in the western Gangdese,We expand their ore-forming ages to the Late Cretaceous.The fact that the ages of Dianzhong Formation volcanic rocks in the western Gangdese decrease from north to the south possibly resulted from the rollback of Neo-Tethys oceanic subduction.3.Find out the genesis of three typical deposits selected from the Linzizongvolcanic area of western Gangdese,and discuss their metallogenic mechanismThe obvious zonings from the intrusion to the limestone,the similar geochemical features between the skarn and magmatic rocks?granite porphyry and rhyolite?,and the distinct Mn-riched features of the skarn minerals in the Nuocang deposit,collectively reveal that it belongs to the typical magmatic hydrothermal metasomatic of skarn type Pb-Zn deposit.The Zn/Cd-Cd,Fe-Mn,and Fe-Cd diagrams also suggest that it belongs to the skarn type Pb-Zn deposit.The S and Pb istopic features of sulfides in this deposit indicate the ore-forming materials were sourced from the magmatic source of upper crust.Both of the granite porphyry and rhyolite provided parts of ore-forming materials for the sulfides.The similar ages?71-74 Ma?of granite porphyry and rhyolite suggest that they belong to the same magmatic system.Thus,we proposed that the Nuocang deposit is a typical skarn type Pb-Zn deposit related to Linzizong volcanism.The results of fluid inclusion and H-O isotopes indicate the ore-forming fluids in the early stage were derived from the magatic hydrothermal.However,the temperature and salinity of ore-forming fluids gradually decrease with the continued meteoric water mixing,suggesting that the fluid mixing is the key to the precipitation ofore minerals.Majority of the orebodies in the Beina deposit showing as the vein develop in the fractured zone of volcanic rocks,whereas minor of mineralization occurring as thin vein fill in the fissures of granite porphyry.The main wallrock alterations are the siliconization,firefly,carbonation?calcite and ferromanganese?,and sericitization.S-H-O isotopic features indicate the ore-forming fluids in the early stage were sourced from the magmatic hydrothermal.However,they are dominated by the meteoric water in the later stage.The ore-forming fluids belong to the medium-loW temperature and medium-low low salinity series.The occurance of leaf-like calcite and boiling inclusion group reveal that fluid boiling plays an important role in the mineralization.Pb isotopic features of sufides in this deposit indicate that the ore-forming materials were sourced from the upper crust of ancient Lhasa subterrane,and they have the similar features sith those of granite porphyry and volcanic rocks.Since the ages of granite porphyry were a bit later than those of volcanic rocks,suggesting that they shall the same parental magmas.Therefore,the Beina deposit is a typical epithermal Pb-Zn-Ag-?Cu?deposit,and its formation were related to the sub-volcanic hydrothermal activity of Linzizong volcanism.The Pb-Zn-Cu mineralization is dominantly located in the skarn at the Dajiacuo deposit.Most of vein-like Ag-Pb-Zn orebodies develop in the fractured zone of volcanic rocks,with only one vein-like Ag-Pb-Zn orebody cutting through the remote Angjie slate.The wallrock alterations in the vein-like Ag-Pb-Zn orebodies are dominated by the siliconization,clayization,sericitization,carbonation and chlorination.S-H-O isotopic features indicate the ore-forming fluids in the early stage were sourced from the magmatic hydrothermal,with meteric water mixing in the later stage.The Pb isotopic features of sufides in this deposit indicate that the ore-forming materials Were sourced from the upper crust of ancient Lhasa subterrane,with minor of mantle-drived materials involved into.The Re-OS age of molybdenite indicate the ore-forming age is a little later than that of volcanic rocks,Which belong to the typical sub-volcnic hydrothermal mineralization.Therefore,the Dajiacuo deposit is a typical skarn type and epithermal type Ag-Pb-Zn-?Cu?deposit.4.Summarize the spatial and temporal distribution and genetic types of ore deposits,establishe regional metallogenic model,and analyze the prospecting potential in the Linzizong volcanic area of Western GangdeseThe discovered deposit types in Linzizong volcanic area are mainly composed of relatively high quantity of epithermal Ag-Pb-Zn-?Au?deposits?cryptoexplosive breccia type+hydrothermal vein type?,skarn Pb-Zn-Mo?Cu?deposits,and possible porphyry Mo deposits?Chagele porphyry Mo?.Therefore,we establish the metallogenic model as folloWs:Since from the late Cretaceous?⁸2 Ma?,the rollback of the subducted Neo-Tethys ocieanic slab caused the oceanic slab dehydration,leading to the partial melting of the mantle wedge.The underplated basaltic magmas induced the remelting of ancient Lhasa crust,which generated large amounts of parent magmas.Generally,the parent magmas provided the initial source for the Linzizong volcanism and subsequent mineralization.The ore-forming fluid generated by the magmatic crystallization and differentiation usually migrated along the deep fauts.Under the favorable conditions,the ore-forming fluid started to precipitate and formed the porphyry Mo deposit at the top of the subvolcanic rock?ultra-shallow intrusive rock?,or the the contact zone between the intrusion and the the wallrock.The ore-forming fluids continued to transport along the fault structures under the drive of thermal power and regional tectonic stress,and finally assembled at the leading edge of fractures due to the resistence of the overlying rocks.Subsequently,the cryptoexplosion occurred when the pressure decreased abruptly,and formed the cryptoexplosion breccia type Pb-Zn-Ag-?Au?deposits.Parts of the ore-froming fluids driven by temperature and pressure along migrated along the faults beside the volcano mechanis.When encountering Carbonate-Permian the carbonate,they would form the skarn Pb-Zn deposits.The contitued migrated ore-froming fluids would precipitate at the secondary fault structures?caldera,ring and radial fractures?when the metallogenic environment changed.Hence,they would formed the hydrothermal veined Pb-Zn-Ag-Sn?Au?deposits.Combined with the geological and exploration geochemical and geophysical characteristics,we analyze the prospecting potential in the Linzizong volcanic area of western Gangdese:western Gangdese contains much greater ancient continental crustal materials reflected in the formation of the porphyry Mo?W?,the skarn W-Mo-Pb-Zn-Ag,the epithermal Pb-Zn-Ag-Au deposits,which should guide regional exploration.