Study On Bangpu-type Molybdenum Polymetallic Deposit, Tibet And The Metallogenic Regularity Of Molybdenum Polymetallic Deposits In The East Section Of The Gangdese Metllogenic Belt

Bangpu Mo polymetallic ore deposit of porphyry-skarn type is another large ore deposit since the Sharang Mo deposit found in the east section of the Gangdese metallogenic belt. The ore deposit are composed of three ore bodies which are the Mo (Cu) ore body occurred in the monzonitic granite porphyry, Cu (Mo) ore body occurred in the diorite porphyrite and the Pb-Zn ore body occurred in the skarn and marble of the Luobadui Formation, respectively. Zonation of mineralizaion elements is very obvious. In this paper, the author regards the Bangpu ore deposit as the key study object and carries out study on metallogenic theory deeply, meanwhile, the author takes some study on the geological features of the other main Mo ore deposits in the east section of the Gangdese polymetallic belt. Based on researches of the main Mo polymetallic ore deposits, the author have arranged, and summarized the metallogenic regularity of Mo polymetallic ore deposits and perfected the sub-class metallogenic series in the east section of the Gangdese belt. All these studies have an important guiding significance for the regional prospecting.Based on the field geological investigation, geological logging of drill core and mine adit, identification of minerals with polarization microscope and so on, the author have made a systematic research on the occurrence feature of ore bodies, ore fabrics, wall rock alteration, features of veins and types of veins and so on.. Then, the metallogenic episodes and metallogenic stages of Bangpu ore deposit are identified. This paper analyzes the mineral species of some altered minerals for example the biotite, garnets, augites and amphiboles and discusses the physicochemical conditions in the process of mineral formation using the electron microprobe analysis. The author recognizes the spessartite and the ferrobustamite and emphasizes the possibility of the existence of Ag in the skarn type Pb-Zn ore deposit.The systematically diagenesis geochronology results show that the ore-forming monzonitic granite porphyry and the diorite porphyrite are formed in16.23Ma and15.16Ma, respectively. The forming ages of the biotite adamellite and the quartz monzonite porphyry which are have no relationship to the ore-forming are (60.6～70) Ma and15.43Ma, respectively. Study of metallogenic geochronology has shown that the ore-forming age of Bangpu deposit is14.09Ma, and it’s ore-forming event in the mid Miocene. The results of geochemistry of magmatic rocks show that besides lots of crust-derived materials, there is obvious characteristic of injection of mantle-derived materials in the source of diagenesisGeochemical results of Bangpu ore deposit, such as isotope, the inclusions and rare earth-trace elements of single mineral, indicate that the fluid resources of the main mineralization stage is stable, which is most of magma fluid of the crust source with a small amount of mantle-derived fluids involved in mineralization and mixed some meteoric water in the late metallogenic stage. The mineralization process experienced magma exsolution of volatile and fluid, formation of supercritical fluid, processes of fluids boiling after decompressed, fluids mixed and so on. Ore-forming minerals of porphyry-type ore bodies are derived from the contamination of mantle and crustal materials, and ore-forming materials of the lead-zinc ore body are significantly from the upper crustal materials, with almost no mantle-derived materials contamination. The differences of sources of ore-forming mineral are the main reasons for the lead-zinc-copper-molybdenum elements zoning, while thedifferences of propertfes of the ore-forming intrusive rocks and the different properties of fluid between the stages of molybdenum mineralization and copper mineralization are another important factors leading to the separation of molybdenum and copper. Based on the above research, a metallogenic model had been built in this paper.It’s the first time to identify mineralization of Early Cretaceous in the Gangdese Back-Arc Faulted Uplift Zones. The paper preliminary summaries the genetic types of the Mo polymetallic deposits, figures out five Mo mineralization events, and build the space and time framework of Mo polymetallic deposits in the eastern Gangdese belt, and discusses their metallogenic dynamics background respectively by using the Mo mineralization as the main line. Using the Hf and Sr isotope geochemistry of Mo mineralization intrusions and Pb isotope geochemistry of the ore sulfides, the author systematically elaborates the relations between characteristics of diagenesis and mineralization source and mineralization elements distribution zones among main Mo polymetallic deposits in the eastern Gangdese, and points out that in the eastern section of the Gangdese belt, from the south margin of the South Gangdese to the middle-northern part of the South Gangdese, then to the Back-Arc Faulted Uplift Zones, extent of mixture with mantle materials appears to be weak to enhanced and weakened again. The process makes the characteristics of mineralization elements association vary regularly among regional Mo polymentallic deposits.Some comparative and arrangement studies on the geological and geochemical characteristics of ore-forming magmatic rocks of porphyry and porphyry-skarn type Mo deposits, Mo (Cu) deposits, Cu (Mo) deposits and Mo polymetallic deposits have been carried out. On this basis, the paper has discussed the relationship between the properties of ore-forming magmatic rocks and the ore-forming elements association. Some supplement and perfection on the sub-class metallogenic series of the east section of the Gangdese belt has been made after the determination of the metallogenic ages of Hahaigan ore deposit, Maxionglang deposit and the Lietinggang deposit.