Mineral Compositional Characteristics Of Micas In The Xintianling Skarn Scheelite Deposit, Hunan Province Of South China: Insights Into The Tungsten (W) Mineralization During Magmatic-hydrothermal Processes

The Xintianling deposit,located in Chenzhou,Hunan Province,is the largest skarn-type scheelite deposits in Southern Hunan.The Xintianling deposit is spatially related to the Jurassic granite intrusions and experienced complex magmatic and hydrothermal events.Previous studies on this deposit mainly focused on the petrogeochemistry and geochronology of the ore-bearing granites,fluid inclusion analysis,and sources of metals and fluids.However,there is still a lack of research on the changes of physicochemical conditions and the evolution of volatile substances in the ore-forming process of the Xintianling deposit,which affects the understanding of geochemical behavior of tungsten in the magmatic-hydrothermal evolution process,as well as the fine characterization of the enrichment and precipitation mechanism of tungsten in the deposit and the ore-forming process.Base on this,taking the Xintianling deposit as an example,on that basi of field geological survey and detailed petrographic and petrographic studies,the systematic in-situ analysis of mica minerals in different magmatic-hydrothermal stage of the Xintianling deposit is carried out by using EPMA and LA-ICP-MS analysis methods in order to solve the key scientific problem of tungsten mineralization mechanism in magmatic hydrothermal process.The results show that:（1）The mica of different stages in the Xintianling deposit is all trioctahedral mica.（2）In the magmatic stage,the contents of SiO₂and Al₂O₃are gradually increased,while those of FeO^T,Mg O and Ti O₂are gradually decreased,and the incompatible elements such as W,Rb,Cs,Nb and Ta are gradually enriched,indicating that the biotite,plagioclase and Fe-Ti oxide undergo crystallization differentiation and form a tungsten-rich melt.The contents of Cs,Nb and Rb decrease in the skarn stage,reflecting the strong fluid-rock reaction process.（3）The crystallization temperatures of medium-coarse-grained porphyritic biotite granite,fine-grained porphyritic biotite granite,greisenization fine-grained biotite granite and skarn mica minerals are 692～735℃,649～718℃,220～270℃,and 180～220℃,respectively.From the magmatic-hydrothermal stage,the temperatures decrease gradually.（4）The accessory mineral assemblages in the magmatic stage are magnetite and ilmenite,while those in the hydrothermal stage are rutile and ilmenite,reflecting the reduction of oxygen fugacity in the fluids from magma to hydrothermal stage.According to the diagram of mica mineral Fe³⁺-Fe²⁺-Mg²⁺,it can be seen that the oxygen fugacity does not change in the magmatic stage,but it increases first and then decreases in the hydrothermal stage.（5）Log（fH₂O/fHF）^fluidshows an increasing trend from the magmatic stage to the hydrothermal stage,suggesting that the content of F shows an increasing trend in the melt-fluid stage,and the continuous enrichment of F indicates that the magmatic-hydrothermal evolution is more adequate.Based on the above results,the metallogenic model of the Xintianling deposit is established in this paper.The evolution degree of the Xintianling ore-forming granite is relatively high,and the oxygen fugacity basically remains unchanged during the evolution process,and W is gradually enriched in the residual melt.The enrichment of ore-forming elements is mainly controlled by fluid hydrothermal process.Fluid interaction results in the reduction of oxygen fugacity of the system and the increase of F content,thus reducing the W⁶⁺distribution coefficient between the melt and the metal mineral.With the decrease of temperature and the increase of p H value and the decrease of W solubility,W enrichment in hydrothermal solution is promoted,and then the fluid reacts with carbonate surrounding rock and precipitates,forming skarn scheelite deposit.