The Magmatism And Genesis Of Dulong Zn-Sn Polymetallic Deposit In Maguan,Yunnan Province

In this study,the famous Dulong Zn-Sn polymetallic deposit was selected as the research object.Based on the detailed field investigation as well as the relationship among granite,fluid,tectonic and mineralization,the geochemical characteristics of the magmatic rocks,the geological and the geochemical characteristics of the deposit in research area were systematically studied to explore the ore-forming material source,the genesis and mineralization mechanism of the Dulong Zn-Sn polymetallic deposit.Through this research work,obtained the following main understanding:(1)We studied zircon U-Pb ages,geochemical,isotope geochemical and petrological characterisitics for the Laojunshan granites and Nanwenhe granitic gneiss,respectively.The LA-ICP-MS zircon U-Pb ages of Laojunshan granite are 88-83 Ma.The geochemical characteristics of the Laojunshan granites are characterized by the S-type granite.Besides,all features indicate that the Laojunshan granites fromed in the late stage of syn-collisional tectonic environment and the protolith were possibly derived from metamorphic mudstone.The LA-ICP-MS zircon U-Pb ages of Nanwenhe granitic gneiss are 411±2 Ma.The geochemical characteristics belong to the S-tpye granite.Moreover,the Nanwenhe granitic geniss formed in the postcollision environment and the protolith was derived from metamorphic sandstone.(2)Ore bodies of the Dulong Zn-Sn polymetallic deposit Moreover are mainly output as layered,lenticular,vein and banded with the phenomenon of branch complex,sharp out,reappear,expansion,contraction and crossing.The alteration of country rocks were composed of skarnization,chloritizaion,epidotization,sericitization and carbonatation.The ore types can be divided into banded,disseminated,massive,taxitic and vein.The formation of deposit experienced skarn phase,thermal ore-forming phase and surface oxidation phase.(3)The study of electron microprobe on major skarn minerals indicated that the skarn minerals were mainly composed of calc-skarn and the diagenetic environment was transformed from weak oxidizing to reducing environment.In addition,the stratiform skarn was formed by the infiltration metasomatism with country rock in slip band or fracture zone.(4)The domiant types of fluid inclusions are liquid-rich inclusions,gas-rich twophase inclusions and daughter-minerals bearing polyphase inclusions,as evidenced by petrography characteristic of fluid inclusions in different stages of minerals.The temperatures,salinities and densities of fluid inclusions gradually decrease from skarn phase to surface oxidation phase.(5)In this study,we reported the U-Pb dating of cassitierite by the method of insitu LA-MC-ICP-MS from the Tongjie section of the Dulong Zn-Sn polymetallic deposit.The results showed that the 206Pb/207Pb-208U/207 Pb isochron ages of cassiterite sample DTS was 88±5 Ma and the 207Pb/206Pb-238U/206 Pb Concordia age was 89.4±1.4 Ma,indicating the Dulong deposit formed during the Late Cretaceous which similar to the Laojunshan granites.(6)According to the study on H-O isotope of quartz,calcite and magnetite at different stages of the Dulong Zn-Sn polymetallic deposit,These results indicate that the ore-forming fluids in the Dulong Zn-Sn polymetallci deposit were mainly derived from magmatic fluids,and with the evolution of ore-fluids,the percipiation of meteoric water is increasing.The results of C-O isotope reflect that the C isopote of ore-fluids derived from the Laojunshan granite.The ?34S values of major sulfides indicate that the sulfur from the magmatic source,with a minor part form the mantle source.Besides,the lead isotope research also shows that the ore-forming materrials of Dulong Zn-Sn polymetallic deposit derived from the source of Laojunshan granites.In summary,the Dulong Zn-Sn polymetallic deposit belong to magmatic hydrothermal skarn deposit,and the mineralization was most closely related to the second stage of Laojunshan granite.