The Ore-forming Fluids Of The Quanzigou Porphyry Molybdenum Deposit In Jining,Inner Mongolia

As a major producer of molybdenum,China has developed many porphyry molybdenum deposits belt.Among them,the Yanliao molybdenum deposit belt in the northern margin of the North China Craton has been emerging with new molybdenum deposits for many years.In this paper,a large-scale molybdenum deposit in quanzigou was studied,in order to better understand the metallogenic process and fluid evolution process of the porphyry molybdenum deposits which developed in the northern margin of the north China craton.based on the detailed field survey and careful observation under the microscope,the methods of qualitative and quantitative are combined in this paper to study the metallogenic fluid.Currently,the following results have been obtained:?1?Quanzigou molybdenum ore body mainly occurs in monzonitic porphyry and granite porphyry.According to the interpenetrating relationship of vein body,alteration characteristics and mineral combination,it can be divided into 4 stages:1^st stage quartz-pyrite,2^nd stage quartz-molybdenite±pyrite±magnetite,3^rd stage quartz-molybdenite±chalcopyrite±porphyrite±pyrite±hematite±zincite±sericite,4^thstage quartz±pyrite±sericite±fluorite±carbonate mineral.?2?Four types of inclusions were developed in the Quanzigou molybdenum deposit:liquid-rich type?LV type?,vapor-rich type?VL type?,carbon dioxide type?C type?,and daughter mineral-bearing type?LVS type?inclusions,In early mineralization stage the fluid system is H₂O-NaCl-CO₂.The freshly dissolved fluid has high oxygen fugacity and low salinity.The volatile substance is mainly H₂O and CO₂.It is rich in hematite-bearing fluid in the middle period,and occasionally contain some gypsum-bearing inclusions?The fluid-forming system Gradually changing into H₂O-NaCl system,a fluid with low temperature and oxygen fugacity.?3?Fractal studies have shown that the slope of the fitted straight line of the single inclusion log-log plots at various stages of the Quanzigou molybdenum deposit is generally between 0.48 and 0.57,and the peak and mean values of stage?are higher than those of stage?;the peak and mean values of stage III both decreased than stage?;the peak value in stage IV was slightly higher than that in stage III,and the mean value was slightly decreased.The intercept interval of straight line of P-A fractal fitting in different dimensions is mostly between 0.8 and 1.75.The peak value in stage II is higher than that in stage I and the mean value is lower;the peak value in stage III is significantly lower than that in stage II and the mean value is significantly higher;The change is not significant between stage III and stage IV.Under the condition of continuous decrease of temperature and pressure,the fluctuation of peak value and average value reflects that the complexity of the morphology of fluid inclusions is not only related to the temperature and pressure conditions,but also to the composition and immiscibility of the fluid.?4?LA-ICP-MS analysis results show that the main components of the ore-forming fluid are Na,K,Fe,Mn,Mg,and Al,and the Mo element content in the fluid fluctuates greatly after the middle period.In general,the content of elements in the first-stage and second-stage fluids has a decreasing trend such as Na,Mn,K,Cu,Mo,Sr,Pb,and Rb,which increases in the third and fourth stages.The growth rate in the late stage of the fourth stage increases but is not significant.The overall trend is consistent with the fractal law.?5?Strong water-rock reaction,fluid immiscibility?boiling?,changes in acidity and alkalinity,sudden pressure drop due to open fractures,temperature reduction,and oxidation-reducibility may all affect the precipitation of molybdenite to a certain extent.