| In the Sanjiang region,the lack of porphyry Cu mineralization in Paleo-Tethyan arc magmatic rocks is mainly due to the low oxygen fugacity of arc magma.Under reduced conditions,early sulfide saturation and chalcophile metal loss,and lack of a focus for hydrothermal fluids,which hinders the formation of Porphyry and Skarn Cu deposit.Surprisingly,the giant Yangla Cu deposits developed in the Paleo-Tethyan orogenic belt.What are the redox state of magma-hydrothermal systems of its?If,will reduced arc magma background limit the formation of skarn Cu deposits?These are important scientific questions that need to be solved urgently.Furthermore,The ore bodies of the Yangla Cu deposit are mainly developed in the periphery of the granodiorite and between metamorphic sandstone and carbonate with straiform-like shapes.A lot of research has been carried out on the geological characteristics of the deposit,the genesis of granodiorite in the mining area and the mineralization age.However,there is controversy about its mineral deposit genetic type,and the genesis of quartz porphyry from the mining area is not well studied.Focus on scientific questions what mentioned above,in this paper,we investigate newly discovered quartz porphyry and minerals from different stages of hydrothermal evolution（e.g.,garnet,magnetite,scheelite）.The geochemical and chronological results of quartz porphyry reflect the timing and genesis of its formation and the characteristics of the source.The results of Zircon oxygen fugacity meter and Laser Raman studies of fluid inclusions which come from different stages pinpointed the redox state of magma and fluids,and revealed their formation environment.Garnet U-Pb chronology and the composition characteristics of magnetite further define the timing and genetic type of this deposit.The main insights gained are as follows:Major and trace elemental compositions for quartz porphyry showed that the SiO2 contents range from 70.8 to 77.3 wt.%with an average of 73.5 wt.%,and A/CNK values are in the range of 0.57～1.78 with an average of 0.92.On the Harker diagram,SiO2 has a positive correlation with P2O5 and a negative correlation with Pb,and it is considered that quartz porphyry is metaluminous and slightly peraluminous I-type granites.The quartz porphyry SIMS zircon U-Pb concordant age is approximately～227 Ma,which is consistent with the timing of granodiorite.Compared with the major and trace geochemical characteristics and Sr-Nd isotope of whole rock,and Hf-O isotope of zircons,it is concluded that they have similar mixed magmatic sources and formation processes,but quartz porphyry generates much later.The zircon from granodiorite associated with mineralization had a significant negative Eu anomaly（δEu=0.20～0.31,average 0.27）,and the oxygen fugacity estimation result wasΔFMQ=-0.83～0.38with an average of-0.19,indicating that the mineralizing magma was relatively reduced.The results of Laser Raman analysis showed that the gas phase components of fluid inclusions in garnet and pyroxene in the dry skarn stages included CH4,N2 and H2O.The results of in situ micro-analysis of single mineral showed that most of the garnets had positive Eu anomalies（0.62～13.7,average 2.78）.The Mo content of scheelites was low(17.4×10-6～149×10-6,average 57.7×10-6)and had positive Ce anomaly（1.05～1.27,average1.14）.In addition,the symbiosis of magnetite and pyrrhotite can be observed under the microscope,which also indicates that the metallogenic fluid is relatively reduced,and the fo2 is lower than the Mt-S-Po buffer line.The above geochemical and geological evidence comprehensively indicate that the magma-hydrothermal systems of Yangla is relatively reduced.Garnet U-Pb age and the compositions of garnet and magnetite were determined to further define the timing and genetic type of this deposit.The analysis showed that garnets have U,Th and Pb contents of 1.18×10-6～6.69×10-6,0.04×10-6～1.43×10-6,0.11×10-6～1.16×10-6,respectively,and yielded an intercept age of 231.0±5.3 Ma（2σ,n=32,MSWD=2.1）.This represents the timing of skarn formation coeval with the generation of granodiorite.In combination with magnetite showing compositions similar to skarn deposits around the world,it is confirmed that the Yangla is a typical Indosinian skarn Cu deposit.Under reducing conditions,the early sulfide saturation results in a relative loss of the dissolved magmatic fluid Cu,We speculate that sulfides accelerate precipitation in a confined space where magma is in contact with carbonate formations as magma-hydrothermal physicochemical conditions change.These factors contributed to the formation of the reduced Yangla Cu deposit.Compared with other oxidized Cu skarn deposits worldwide,garnets at Yangla contain significantly higher Sn(485×10-6～7433×10-6,average 3931×10-6)and W(0.20×10-6～736×10-6,average 156×10-6),and magnetites also have higher Sn(115×10-6～778×10-6,average 405×10-6).This is similar to the characteristics of W-Sn skarn deposits around the world.Combined with the above reasons,it is preliminarily proposed that W and Sn mineralization may have developed in the the ore field and the mineralization of W and Sn should be paid attention to in future prospecting exploration. |
PDF Full Text Request