Research On Metallization Of The Dongzhongla Epithermal-type Lead-zinic Deposit In Tibet, China

Dongzhongla deposit, located in the Zhaxue-Jinda lead-zinic mining area , Nyainqentanglha Range,Tibet,is the new founded lead-zinc polymetallic deposit and the degree research of geology in this area is low.In this paper,we use did works with the mineral deposit,geochemistry,plate tectonics, collision orogen theory, we gain the innovation achievements and judgements:1)The shape of the orebody is layered,stratoid,lens,located in the contact zone between limestone (griotte)and calcareous slate in Luobadui formation.The ore structure include massive,impregnation,vein,breccia and ribbon;the texture include euhedral crystal,subhedron,contained,coterminal,metasomatic relict,solid solution separated.The metallic minerals are pyrrhotite, pyrites,chalcopyrite,galena,sphalerite, azurite and bornite.The rocky mineral are quartz, feldspar,garnet,chlorite,epidote and calcite.2)The metallogenic process of Dongzhongla deposit are two stage,which are epithermal ore forming stage and supergene stage.When it terms to epithermal ore forming stage,we divide it to three period also,which are pyrrhotite-chalcopyrite period , galena-sphalerite period and calcite period.So, epithermal ore forming stage is the most important metallogenic period3)allgovite in Dong zhongla area belong to the alkali basalts series.The major elements of the allgovite are characterized by high Al₂O₃, intermddiate TiO₂,poor MgO and K₂O , and Na₂O> K₂O.The allgovite have high content of∑REE, a weak negative Eu anomaly or no Eu anomaly(δEu = 0.58⁰.93, with a mean of 0.81),Chondrite-normalized REE diagrams show LREE-enriched patterns (LREE/HREE=3.76⁴.37,with a mean of 4).traceelements are characterized by enrichments in HFSE(Nb,Ta,Hf and Th) and depletments in LILE(Ba,Sr and K). They are similar to ocean island basalts(OIB),the magmacontent were likely from enrichment mantl and have not experienced crustal contamination.The porphyry granite belong to peraluminous calc-alkaline series and the result of the crust crustal thicking melting.4)The lead-znic mineral are mainly from strata; Pb isotope belong to normal lead,and mainly from supracrust;the sulfur isotope is mainly from the magma.The nobel gas isotope shows the fluid is from crust and mantl;the sulfide strontium show that the fluid is from crust.The C is mainly from crust.5)There are three types of fluid inclusions in Dongzhongla deposit,mainly are gas-liquid fluid inclusions. The fluid belong to low and medium temperature,low sality,low density and formed in a low pressureand shallow environment. The gas compositions is dominated by H₂O,the liguid ion is SO42-,Cl-,Na+,belong to Cl--SO42--K+-Na+ hydrochemical type. The pH is weakly acidic, shows decreasing tendency in the different epithermal ore period ;The fugacity also shows decreasing tendency.The the ore fluids may have been derived from subsurface airsatutated water prodorminent.6)The total lead and zinic activity in the fluid have the law that,form pyrrhotite-chalcopyrite period,galena-sphalerite period and calcite period drop sharply.In the zinic complex compound,the Zn(HS)3- and ZnS(H₂S)20 prodorminent in pyrrhotite-chalcopyrite period; Zn(HS)3- prodorminent in galena-sphalerite period.In the lead complex compound, the PbS(H₂S)20 prodorminent in pyrrhotite-chalcopyrite period; PbCl+å’ŒPbCl3- prodorminent in the galena-sphalerite period.7)The magmatic body are complex massif.the porphyry granite Zircon SHRIMP U-Pb age is 124.4±1.9 Ma, and the allgovite Zircon SHRIMP U-Pb age is117.1±1.0 Ma, in late stage of Yanshan.And the dike age is from 38.47 to 53.35Ma(K-Ar),in early Himalayan.The Laser microprobe 40Ar-39Ar geochronology of ore quartz age is 42.2±1.7 Ma,represent the metallogenic age.we firstly put forward that Dongzhongla lesd-zinic deposit were metallogenic during main Indian-Asian collision period(41⁶5Ma) with the magmatic movement in late Yanshan and early Himalayan.8)Based on the physical chemical condition of the fluid,systemly analyzing the metallgenesis ,and combined with the geological evidence and large testing data, contrast with the different typical type lead-zinic deposit,we conclude that the Dongzhongla lead-zinic deposit is low and medium temperature epithermal type ,and establish the collision metallogenic model.