| The Bangong Co-Nujiang metallogenic belt,stretching across the central of the Tibetan Plateau in a near east-west direction,has experienced complex tectonic-metallogenic evolution such as the formation,subduction and closure of the Bangong Co-Nujiang Ocean,with superior metallogenic geological conditions and great metallogenic potentials.The Bangong Co-Nujiang metallogenic belt can be subdivided into the Southern Qiangtang,the Bangong Co-Nujiang and the Northern Gangdese metallogenic sub-belts.In recent years,with the discovery of Tibet’s first world-class porphyry-epithermal copper-gold deposit—of south Tiegelong（Rongna）in the western segment of the Southern Qiangtang metallogenic sub-belt,the tectonism-magmatism-mineralization in the western segment of the Southern Qiangtang metallogenic sub-belt has become one of the research hotspots in the geosciences of the Tibetan Plateau.Compared with the progress made in the western segment of the Southern Qiangtang metallogenic sub-belt,the research level of its middle segment is relatively low in both geological prospecting and scientific theoretical study,so it is urgent to carry out in-depth research.In this thesis,the copper-gold deposits in Tajigang,Gaobaoyue and Narong in the middle segment of the Southern Qiangtang metallogenic sub-belt were selected as the study objects.Then we carried out detailed multi-technique metallogeny analysis based on zircon U-Pb chronology,whole-rock geochemistry,whole-rock Sr-Nd isotope and zircon in-situ Hf isotope to provide new constraints on the diagenetic age and petrogenesis of ore-forming related rock bodies of copper gold deposits in the area.Based on the above research,the crustal structure and distribution characteristics of the Southern Qiangtang metallogenic sub-belt were revealed through the isotopic comparison analysis of the Late Jurassic ore forming and ore-barren magmatic rocks.In addition,this work further conducted isotopic correlation research on the three-phase metallogenic related rock bodies in the Bangong Co-Nujiang metallogenic belt,and established relevant regional ore prospecting indicators in combination with the metallogenic geological conditions such as regional tectonics,stratigraphy and tectonic system,and further proposed the next prospecting direction.The main results obtained are as follows:（1）The tectonic-magmatic-metallogenic age of the Southern Qiangtang metallogenic sub-belt is determined.Zircon U-Pb dating results show that the diagenetic ages of ore-forming related rocks of Tajigang,Gaobaoyue and Narong copper-gold deposits are 120-118Ma,152-145Ma and 158-155Ma,respectively.Combined with the exisiting diagenesis and mineralization age data,the tectonism-magmatism-mineralization in the Southern Qiangtang metallogenic sub-belt is further divided into two periods—of Late Jurassic and middle-late Early Cretaceous.（2）The petrogenesis and magmatic source of ore-forming related rock bodies in the main mining areas are investigated.The granites in Tajigang Mining area display positiveεHf（t）values（+3.9～+7.2）with a relatively young crustal model age（TDC M=717～926 Ma）and show similar Zr/Hf（26.88～38.65）,Nb/Ta（7.01～13.61）,Sm/Nd（0.17～0.21）,and Nb/La（0.32～0.58）ratios to crust,indicating a magma source of juvenile lower crust.The rocks in Gaobaoyue area are characterized by high Sr(mostly>400×10-6),Al2O3（14.46～17.04%）,Mg O,Cr,Ni and Na2O contents,with depletions of heavy rare-earth elements(i.e.,Yb=1.05～1.45×10-6,Y=10.50～14.78×10-6),similar to the adakites which are formed by the partial melting of the subducting oceanic lithosphere.The rocks in the Narong area have high Sr(mostly>400×10-6),Al2O3（16.48～19.22%）contents,and high Sr/Y（15～82）,（La/Yb）N（17～34）ratios with depletions of heavy rare-earth elements(i.e.,Yb=1.05～1.45×10-6,Y=10.50～14.78×10-6).Additionally,the studied samples have relatively high Th content and high Th/U ratio and low Mg O,Cr and Ni contents.Most zirconεHf（t）values are positive,with few negative values.These features indicate that they are the adakitic rocks generated from partial melting of mafic lower crust,with minor pollutant from ancient crustal material.（3）Based on the comparative analysis of the isotopic composition of the Late Jurassic intermediate-felsic intrusive rocks in different regions of the Southern Qiangtang metallogenic sub-belt,we reveal the coexistence of ancient crust and new crust in the southern margin of the Southern Qiangtang terrane,which are spatially separated by the Bangong Co-Caima-Qingcaoshan-Renaco-Rongma zone.Among them,the northern part of the line belongs to the area of ancient crust(zirconεHf（t）value<0),while the south part that includes the Duolong ore-concentrating area,the Tajigang mining area,the Gaobaoyue ore-concentrating area,and the Narong mining area belong to the area of juvenile lower crust(zirconεHf（t）value>0).（4）The rocks related to mineralization in Tajigang mining area,the Gaobaoyue ore-concentrating area,and the Narong mining area have positiveεHf（t）values,which can be compared with the ore-bearing rocks of Duolong and Gaerqiong-Galale ore concentrating areas in the western section of the Bangong Co-Nujiang metallogenic belt(εHf（t）value>0),indicating that the middle segment of the Southern Qiangtang metallogenic sub-belt should has certain prospecting potential.Moreover,the rock bodies related to mineralization in the Gaobaoyue ore-concentrating area and the Narong mining area have high whole-rock Sr/Y and V/Sc ratios and zircon trace elements have high Ce/Ce*,Eu/Eu*,10000×（Eu/Eu*）/Y,（Ce/Nd）/Y and low Dy/Yb ratios.These features imply that they have good metallogenic prospects and which can be the key direction of regional copper-gold deposit exploration.However,the corresponding whole-rock and zircon trace elements of the granite in Tajigang mining area do not have ore-bearing characteristics,suggesting that the metallogenic potential is low. |
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