Structural Framework And The Relationship With Mineralization Of Jiama Copper-polymetallic Deposit,Southern Tibet

Jiama copper-polymetallic deposit located in the east of Miocene Gangdese porphyry Cu belt and belonged to a Cretaceous back-arc basin of the south of Gangdese magmatic arc,is a typical and super-large " collision-type" pophyry-skarn deposit.Despite scholars have done a great deal of research and gotten outstanding achievements of this deposit,there are still some weaknesses in the research of the deposit on important scientific questions,such as the relationship between Miocene metallogenesis of Jiama deposit and structure of Cretaceous back-arc basin,low-temperature thermochronology.With the method of filed geological survey,structure and mineralization on drill core record compiling,related laboratory study and integrating several theories,this study has carried out research in deposit structure and skarn metallogenesis,ore-bearing porphyry and metallogenesis,regional uplift and metallogenesis.Through detailed analysis,contrast and research,we established a structure-skarn metallogenetic model of Jiama deposit and discussed petrogenesis of ore-bearing porphyries and the relationship between magma and metallogenesis.Furthermore,this study simulated geothermal history of Jiama deposit and discussed the relationship between uplift of the eastern Gangdese and metallogenesis.Finally,we established tectonic evolution model of Eastern Gangdese and regional tectonic metallogenetic model which is of great significance for prospecting in Jiama and the region.Through this study we have achieved the following achievements:1.We have determined structural characteristics of Jiama deposit.90-62Ma,with northward subduction of the Neo-Tethys,Duodigou Formation and Linbuzong Formation experienced the south-directed sense of shear movement and developed Jiama decollemment(JMD or GD2).Jiama decollement and the above shovel type folds(Linbuzong Formation,Chumulong Formation,Takena Formation and Shexing Formaion)make up decollement-fold.Under the progressive shear strain,Duodigou Formation and Linbuzong Formation formed a series of isoclinal or recumbent folds with two kinds of foliations and developed complex structures,such as folds of vein,interlayered flowage folds and drag folds.The EBSD fabric result of quartz and calcite in mylonite showed Jiama decollement formed in middle-low temperature and simple shear structural environment.Jiama decollement was reconstructed into domed structure later.2.We have established a structure-skarn metallogenetic model of Jiama deposit.Ore-bearing hydrothermal fluid drived by tectonic stress and heat from magma migrated along the interformational faulted structure and metasomatized selectively marble and limestone of Duodigou Formation and slate and hornstone of Linbuzong Formation.Therefore,some alteration and mineralization looks like banded structure and fold shape.Eventually,stratiform skarn and skarn type copper polymetallic main orebody formed in interlayer structural expanding space.Massively thick skarn and skarn type copper polymetallic orebody formed in collapse locations of fold.Thick plate-like skarn and skarn type copper polymetallic orebody formed in the contact area of porphyry and marble.3.We have discussed petrogenesis of ore-bearing porphyries.Ore-bearing porphyries of Jiama deposit are mainly granite porphyry,monzonitic granite porphyry,granodiorite porphyry,quartz diorite porphyrite,gabbrodiorite porphyrite.The LA-ICP-MS zircon U-Pb ages of ore-bearing porphyry samples are 16.69-14.39Ma.Most samples show high Al2O3 contents but low-TiO2 contents,belonging to high-K calc-alkali basalt series.But a minority are calc-alkali basalt and shoshonite series.All the samples have similar chondrite normalized REE right distribution patterns characterized by enrichment in LREE and depletion in HREE.The primitive mantle-normalized trace element patterns show the enrichment in large ion lithophile elements(LILE,e.g.Rb,Ba and Sr)and Th,U elements,depletion in high field strength elements(HFSE,e.g.Nb,Ta,Ti and P).All samples have geochemical characteristics of typical adakitic rocks.Isotope geochemical characteristics and trace element ratios and diagrams show that the magma of granite porphyry,monzonitic granite porphyry,granodiorite porphyry and quartz diorite porphyrite were derived from the lower juvenile crust,but the magma of gabbrodiorite porphyrite was from enriched lithospheric mantle.4.We have established a magma-metallogenetic model of Jiama deposit.About 26Ma,owing to the removal of thickened Lhasa lithospheric root and upwelling of the asthenosphere,the adakitic magma with enrichment in sulphur and ore formed by moderate partial melting of the lower juvenile crust.Afterwards,18?13Ma,the Qinghai-Tibet plateau in the stage of tectonic transition from compression to extension or strike slip,the adakitic magma rised through vertical fractures and mixed with mid-upper crust,forming granite porphyry,monzonitic granite porphyry,granodiorite porphyry,quartz diorite porphyrite.However,the synchronous magma derived from lithospheric mantle formed gabbrodiorite porphyrite.Ore-bearing hydrothermal fluid drived by tectonic stress and heat from magma taked place at inter-bedded tectonic crush belt and collapse locations of fold,forming skarn type copper polymetallic main orebody.5.We have simulated geothermal history of Jiama deposit.Zircon and aptite(U/Th)/He(ZHe and AHe)and aptite fission track(AFT)dating of ore-bearing porphyries are 14.9Ma,14.7?14Ma and 15.5?13.3Ma respectively.The result of thermal history of Jiama deposit,Qulong deposit and Lakange deposit in the same ore concentration area indicates that these deposits experienced faster cooling in 18-13Ma.Gangdese was in the environment of rapid uplift in 26-12Ma;therefore,deposit metallogenetic events accompanied rapid uplift of Gangdese.Jiama deposit was exhumed between 14.7 Ma and 14 Ma(AHe)at an estimated rate of 0.16?0.17mm/y,which is consistent with previous estimates for other Gangdese Miocene porphyry deposits.6.We have established a regional tectonic metallogenetic model.18?13Ma,the Qinghai-Tibet plateau was in the stage of tectonic transition from compression to extension or strike slip and formed a series of N-S trending normal faults.Ore-bearing magma rised through vertical fractures and formed porphyries in intersections of NS-and EW-trending faults.Ore bearing hydrothermal fluid taked advantages of surrounding rock,such as limestone of Duodigou Formation and Yeba Formation,and formed skarn type copper polymetallic orebodies in interlayer structural expanding space and collapse locations of fold.Ore-bearing porphyry and skarn type copper polymetallic orebodies together make up the deposits(Jiama deposit,Qulong-Langmujiaguo-Zhibula deposit,Lakange deposit,Caisheng deposit and Xiangyang deposit)of the east of Miocene Gangdese porphyry Cu belt.