Characteristics Of Ore-forming Fluids And Metallogenic Depth Estimation In The Shiyaogou Porphyry Molybdenum Deposit, Eastern Qinling

The Shiyaogou molybdenum deposit is located in East Qinling metallogenic belt, which is another one of typical porphyry molybdenum deposits in this district. According to the detailed study of geological chracteristics of the deposit, Mineral petrography, fluid inclusion petrography, combined with microscopic thermodynamics, Laser Raman spectroscopy and stable isotope study were used to identify ore-forming fluid composition, salinity, temperature and other physical and chemical conditions. From the perspective of ―origin, migration and precipitation‖ of Mo, we discussed the hydrothermal mineralization. In the end, Metallogenic depth of Shiyaogou molybdenum deposit was estimated by the pressure of fluid inclusions.The ore is characterized by disseminated multistage stockworks. The hydrothermal ore-forming process at Shiyaogou contains three stages: I)Quartz+K-feldspar stage; II)Quartz+polymetallic sulfide stage; III)Quartz-carbonate stage. The hydrothermal alteration types of wall rocks in Shiyaogou molybdenum deposit include K-felspar alteration, biotitizaion, carbonatization, propylitization etc.The initial ore-forming fluid in Shiyaogou molybdenum deposit are charcteristics of medium-high temperature, medium to low salinities of H2O-Na Cl-CO2 system, and the late hydrothermal process is H2O-Na Cl system. The homogenization temperature and salinity about fluid inclusions gradually reduced all around the ore-forming evolution, from 191.6~437.2℃ and 191~484℃and 132~423℃to117~247℃, from 2.07~16.45% Na Cleqv and 3.55~57.62 % Na Cleqv and 3.23~50.03 % Na Cleqv to 2.74~8.9% Na Cleqv, respectively. In addition, fluid immiscibility results in high salinity partly in the ore-forming evolution.Hydrogen-oxygen isotopic compositions indicate that magma fluid mixing with meteoric water in various degrees at the main ore-forming fliud. Combined with regional stable isotopes and Re isotope in Shiyaogou molybdenum deposit, we hold that metallogenetic matters comes from crust-mantle, but mainly comes from the crust. Given that the ore-forming fluid is characteristic of medium-high temperature, medium to low salinities, the transportation of Mo element may as ion pair(KMn O4) or molybdenic acid form. The deposition of molybdenum resulted from fluid immiscibility, fluid mixing and oxidizing condition.Fluid boiling method and fluid inclusion homogenize by halite disappearance method were used to estimated the metallogenic depth of Shiyaogou molybdenum deposit, and the depth are 3.7~5.2 km and 2.81~4.48 km,respectively. Considered of the deviation during the calculation, the author supposed the depth of Shiyaogou molybdenum deposit may 3~5km.