| The Bangpu Mo-Cu polymetallic deposit is one of the typical large-scaled porphyry deposit,lies in eastern Gangdese metallogenic belt,Tibet. Its major composition elements are Mo and Cu,and its symbiotic elements are Pb and Zn.In this article,the author maily talking about the geological characteristics,ore structure,chemistry constituents,and regards fluid geochemistry and theory of mineralizing as guidance,studied the fluid source of mineralizing fluid, physical chemistry conditions of ore-forming,molybdenum and copper migration form and precipitating mechanism, which using inclusion and isotope geochemistry, thermodynamics research approach. The result of research shows as fllowing:1)The ore body maily be made up of Mo and Cu, ore body of Mo and Cu's mother rock is monzonite granite-porphyry which belongs to early Himalayan stage(ηγπ61) ,while the Pb and Zn yields in the contact zone and nearby where rock contact with limestone belongs to Luobadui Formation in Lower Permian(P1l).There are tubular, prismatical , vesiclar and irregular-sharped orebody existed in this deposit.2)The structure of ore are impregnation,string ,crumb and breccoated structure et al.The main testure are self-crystals to semi-self crystals,xenotopic,inclusion and coterminal texture et al.Dominanent metallic minerals are pyrite,molybdenum, copper,galena,secondary are sphalerite, borite, magnetite, kibdelophane , chalcocite, et al.Element of production-grade are Mo,Cu,Pb and Zn,while S,Au and Ag could be comprehensive utilizationed.3)According to the geological information and chronology data,we divided the metallogenic process of Bangpu deposit into three stage,which are magma-advanced atage,hydrotherm ore forming stage and supergene stage.When it terms to hydrotherm ore forming stage,we partitioning it to three period also,which are Mo-Cu sulfide period ,Pb-Zn period and gypsum-calcspar period.So, hydrotherm ore forming stage is the most important metallogenic period,and also, Mo-Cu sulfide period have much more significance when relevant to molybdenum-copper ore body.4)There are four types of fluid inclusions in this deposit,mainly are pure gasphase,liquid phase,gas-liquid phases and the multiphase fluid inclusions with NaCl and(or) metal sulfide daughter minerals.From micro thermometry of fluid inclusion, homogeneous temperature shows that,during the period of Mo and Cu,its temperature varying from 165.4 to 372.6℃,high temperature vary from 223 to372.6℃, while Pb and Zn is perhaps mid-high,vary from 133.7 to 238.3℃,when turns to the gypsum-calcspar period,the temperature changes form high to middle and low,vary from 133.7 to 238.3℃;during the three period,the pressure falling siginificantly,from 110.25×105Pa to 90.79×105Pa to 38.32×105Pa to 36.97×105Pa.5)From porphyrotopic quartz to Mo-Cu's vein quartz to Pb-Zn to gypsum- calcspar,salinity of this deposit showing its high to low's change.That is to say,in the porphyrotopic quartz,its salinity is most high,in 26.00 to 42.71wt%NaCl,while in the last period,the salinity is low,just 26.01～35.72wt%NaC,during the middle period,the salinity is vary from 23.96 to 41.07wt%NaCl.The density of the fluid changes in this way:from 1.0696 g/cm3 to 1.0848 g/cm3 to 1.0803 g/cm3 to 1.0849 g/cm3.All of this indicate that metallogenic fluid belongs to mid-high salinity and high-mid density fulid.6)Through numerical statement,from Cu-Mo period to Pb-Zn period to gypsum-calcspar period,the pH shows increasing tendency,from 5.580(±0.081)to 5.689,at last change to 6.004;while the Eh do not change like the same way,during the firt period is high(0.112±0.024),accompany by evolution of the fluid,it changes to 0.097 for the lower temperature Pb-Zn period,while at the last period, araise to 0.151.7)The total sulfur activity in the fluid have the law that,form Cu-Mo period to Pb-Zn period to gypsum-calcspar period, drop sharply,as from 2.2863～9.0707 mol/L to 4.6939mol/L,at last to 0.9225 mol/L.8)In the hydrotherm fluid,total molybdenum of activity is high in its Mo-Cu deposition process,is vary from 0.4367 mol/l to 5.4716 mol/L; from Mo-Cu period to Pb-Zn period to gypsum-calcspar period, activity of total copper changes from high to low,is from 0.0067～0.0599 mol/L to 0.0257 mol/L to 0.0050 mol/L。9)In hydrotherm fluid,the leading complex compounds of molybdenum and copper are HMoO4- and Cu(H2S)(HS)2-, secondary are Cu(HS)2-,CuCl2-和CuCl32-.It is the fluid boling and when fluid mixed with atmospheric water that molybdenum and copper deposited.
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