Mineralogical Characteristics And Mineralization Of Sandaozhuang Skarn Type MoW Deposit In East Qinling

Sandaozhuang superlarge skarn type Mo W deposit is located in the south margin of North China craton and in the famous East Qinling molybdenum ore belt.It is one of the typical skarn type Mo W deposits in China.Previous studies on the Sandaozhuang Mo W deposit mainly focused on the geological characteristics,metallogenic epoch,metallogenic fluid and geochemical characteristics of ore-forming rock mass.However,the research on the metallogenic mechanism of Sandaozhuang deposit,especially the control of volatile(F,Cl,S)and oxygen fugacity changes on massive Mo W precipitation in the process of mineralization,is relatively weak.Therefore,the main research objects of this study are rock-forming minerals(magmatic biotite and magmatic apatite),altered minerals in different hydrothermal alteration stages(hydrothermal biotite,hydrothermal apatite,garnet,diopside,actinolite)and metal minerals(magnetite,scheelite)in different occurrence and mineralization stages.The contents of major,trace and rare earth elements in different minerals were determined by means of electron microprobe analysis(EMPA)and LA-ICP-MS in-situ microanalysis.The changes of temperature,oxygen fugacity,volatile and other physicochemical conditions in the metallogenic system from magma to hydrothermal stage were discussed by means of mineral thermometer,and oxygen fugacity meter.The migration and precipitation mechanism of ore-forming materials,so as to fully understand the mineralization zoning and the controlling factors of metal precipitation of skarn type Mo W deposits,and provide a reference for the study of metallogenic mechanism and prospecting prediction of similar deposits in the future.According to the results of field geological survey and laboratory research,the diagenetic and metallogenic stages of Sandaozhuang skarn type Mo W deposit can be divided into six stages:(1)Diagenetic stage,that is the formation stage of granite porphyry related to mineralization.(2)Early skarn stage.(3)Quartz-potash feldspar-biotite-magnetite stage.(4)Late skarn stage.(5)Quartz-sulfide stage.(6)Quartz-carbonate stage.Among them,quartz sulfide stage is the main mineralization stage,and molybdenite is mainly veinlet and disseminated mineralization.The results show that the crystallization temperatures of phenocryst biotite and matrix biotite are623°C-662°C,546°C-579°C,respectively.The alteration temperature of hydrothermal biotite is355°C-435°C.It shows that the temperature of the whole ore-forming system decreases gradually from magma to potassium silicate alteration stage.The electron microprobe compositions of biotite and apatite in granite porphyry show that the contents of volatile F and S are higher and Cl is lower in the magmatic system.The higher contents of F and S in the early stage increase the solubility of W and Mo in each system.W and Mo may migrate in the fluid mainly in the form of Mo O₃F^-and H₃WO₄F.Electron microprobe analysis of hydrothermal biotite and apatite shows that the content of F in the fluid gradually decreases,while the content of Cl increases in the skarn stage,indicating that Mo O₂Cl₂^2-nand WO₂Cl₂^2-ncomplexes may be involved in the migration of Mo and W.The high S content of the initial magmatic fluid provides the material basis for the enrichment of molybdenite.The Fe³⁺/Fe²⁺(0.11-0.13)and X_Mgvalues(0.56-0.60)of magmatic biotite are lower than those of biotite Fe³⁺/Fe²⁺(0.12-0.17)and X_Mgvalues(0.60-0.64)in quartz-potassic feldspar vein,suggesting that the oxygen fugacity of fluid increases from magmatic stage to potassic stage.At the same time,LA-ICP-MS analysis results of magnetite and apatite in different occurrences show that the V contents of magnetite in granite porphyry,Quartz-potassium feldspar-biotite-magnetite vein and late skarn stage are787×10^-6-1274×10^-6(average value 921×10^-6),17.34×10^-6-215.51×10^-6(average value 105.76×10^-6)and2563×10^-6-3620×10^-6(average value 2911×10^-6),respectively,which are generally from ore-forming granite porphyry to quartz-potassium feldspar-biotite-magnetite vein,The V content in magnetite shows a weak downward trend,while the V content in magnetite shows an obvious upward trend from quartz-potassium feldspar-biotite-magnetite vein to late skarn stage;apatite in granite porphyry,skarn and quartz calcite veins?Eu were 0.21-0.32,0.54-0.67 and 0.34-0.65,respectively,which showed the characteristics of first increasing and then decreasing.These characteristics indicate that the fluid oxygen fugacity increases first and then decreases from magmatic stage to potassium silicate stage and then to late skarn stage.The early skarn stage is mainly composed of andradite and diopside,while the late skarn stage is composed of grossular and hedenbergite,which also indicates the decrease of oxygen fugacity.The serrated changes of Fe and Al contents in the early garnet ring indicate that episodic fluid action leads to the increase decrease of oxygen fugacity in the local mineralization stage.The results of EMPA show that the Mo contents of five types of scheelite are 10.75%-24.11%(sch1a),2.81%-11.75%(sch1b),0.23%-0.56%(sch2a),0.16%-0.47%(sch2b)and 0.11%-0.36%(sch3),respectively.The Mo content of scheelite has been decreasing from sch1a to sch3,and there is a good positive correlation between Mo and?Eu,these characteristics indicates that the oxygen fugacity decreases further,which is of great significance for W precipitation in scheelite form.To sum up,the early Sandaozhuang magmatic fluid is characterized by high temperature,oxygen fugacity and rich volatile F and S,and the composition of biotite,apatite and magnetite shows that the content of Mo and W in the early Sandaozhuang magmatic fluid is higher;With the evolution of magmatic hydrothermal solution,the oxygen fugacity increase slightly in skarn stage and potassic stage.The Mo and W contents of biotite,apatite and magnetite are consistent,indicating that the Mo and W contents in the fluid tend to increase.With the increasing reaction between ore-forming fluid and surrounding rock,a large number of skarn minerals are formed,accompanied by fluid boiling and mixing of atmospheric water,resulting in rapid transformation of fluid properties,resulting in large-scale precipitation of magnetite,scheelite and molybdenite.Therefore,the precipitation of W and Mo in Sandaozhuang deposit is affected by many mechanisms.