Composition Characteristics Of Scheelite And Garnet From Xintianling Skarn-type Scheelite Deposit,South Hunan Province And Its Implications For The Tungsten Mineralization Process

The Xintianling deposit is one of the largest skarn-type scheelite deposits in south Hunan province,South China.This deposit is closely related to the Jurassic granite intrusion,and its formation has undergone complex magmatic and hydrothermal processes.Exploring the origin of the Xintianling deposit has great significance to understand the mineralization process of tungsten.The previous research on the genesis of Xintianling deposits was mainly based on the geochemical and chronological studies of granite intrusion,but there was little research on skarn minerals,and there was a lack of research on the changes in physical and chemical conditions in the mineralization process,which restricted the understanding of the mineralization process of Xintianling scheelite deposit.According,in this study,we takes scheelite and garnet in the Xintianling deposit as the research object.On the basis of field geological investigation and petrographic observation,the U-Pb age of garnet,the structure of scheelite and garnet,and the characteristics of main elements and trace elements were studied by using scanning electron microscope,EPMA and LA-ICP-MS,which revealed the physical and chemical change conditions in the mineralization process and provided a basis for the characterization of the mineralization processes.The results of this paper show that:(1)Based on the texture of scheelite under CL images,the scheelite in the Xintianling deposit can be divided into three generations:(1)earlier stage scheelite which has the darkest CL color(Sch1);(2)medium stage scheelite has darker CL color(Sch2),in which the earlier Sch2a was metasomatically transformed to Sch2b;(3)later stage scheelite has the brightest CL color(Sch3).(2)Based on the texture of garnet under BSE images,the garnet in the Xintianling deposit can be divided into three generations:(1)dark garnet(Grt1)in the earlier stage;(2)the middle stage is garnet(Grt2)with obvious oscillating rings,which can be divided into thin annular bands(Grt2-1)and thick annular bands(Grt2-2);(3)the later stage is bright garnet(Grt3).(3)Through the LA-ICP-MS U-Pb dating analysis of Grt2,it was determined that the age of mineralization was 159.5±5.0 Ma,which was basically consistent with the diagenetic age(164～157 Ma)of the fine-grained porphyry biotite granite,indicating that Xintianling tungsten mineralization was related to the granite in this period.(4)The rare earth element pattern and trace element composition of scheelite and garnet in the Xintianling deposit show that the mineralization of the Xintianling deposit underwent a multi-stage hydrothermal process.The oxygen fugacity and temperature of the fluid first increased and then decreased,and the pressure and p H value first decreased and then increased during the ore-forming processes.The mineralization of tungsten mainly occurs in the retrograde metamorphic stage,and the oxygen fugacity gradually decreases during the precipitation of scheelite,and the process of dissolution-reprecipitation may be experienced in the late stage of degeneration after the formation of earlier scheelite.(5)Based on the above results,the formation processes of the scheelite in the Xintianling deposit was established.In this study,combined with petrography and scanning electron microscopy,the macrotexture of minerals is studied in detail,and the generations of scheelite and garnet are divided.The trace element characteristics of scheelite and garnet reflect changes in the mineralization environment,revealing changes in oxygen fugacity,temperature,pressure,and p H value.The change of W content in garnet suggests that tungsten mineralization mainly occurs in the retrograde metamorphic stage,and it undergoes the process of dissolution-reprecipitation in the late retrograde metamorphic stage,and the change of p H value of metallogenic fluid caused by the intervention of H~+rich fluid may be the cause of the dissolution-reprecipitation of scheelite.