Study On Metallogenic Epoch,Ore-forming Fluids And Mineralization Of The Bayandulan Copper Deposit,Inner Mongolia,Northeast China

The copper polymetallic deposit of Bayanduolan in Inner Mongolia is located in the Erenhot-East Wu Zhu Mu Qin Q metallogenic belt,which lies in the north margin of North China Craton and the south margin of Siberia Craton.Based on systematically field geological investigation,using the method of Scheelite Sm-Nd Isotopic Geochronology this papaer confirms the metallogenic age of the ore deposits.In the same time,it shows the source and evolution feature of metallogenic fluid with the help of fluid inclusions and C-H-O isotope characteristics.On this basis,my paper discusses the genesis type of deposits and metallogenic process combined with regional mineralization.The scheelite age that I determine in this paper is（314±15）Ma,indicating that the Bayanduolan metallogenic age is the late carboniferous period,which is basically consistent with the diagenetic age,formed in the post-orogenic tectonic background.Through the statistical analysis of the regional features of the diagenetic and metallogenic epoch,I find out that the mineralization of most deposits on the Erenhot-East Wu Zhu Mu Qin Q metallogenic belt which the Bayanduolan deposit is located in has a very close space-time and original relationship with the Late Carboniferous tectono-magmatic activities.After the research of field geological characteristics,the observation of mineral petrography microscope feature,the identification of fluid inclusions petrography,the analysis of the temperature measurement data,we divide the period of the Bayanduolan polymetallic deposit into four stages,early quartz-arsenopyrite-scheelite-pyrite metallogenic hydrothermal stage（390-450℃）,medium-term quartz-magnetic pyrite,chalcopyrite ore-forming hydrothermal stage（250-330℃）and late carbonate stage（180-260℃）.The epibiotic oxidation period is divided into one stage,namely the epibiotic oxidation stage.The analysis of H-O and C-O isotope test shows that the early stage of the fluid source is magmatic water,in the middle stage and the later stage the atmospheric precipitation flows into the fluid and finally form the characteristic of atmospheric-magmatic mix fluid.Through the fluid inclusion Laser Ramen spectroscopy analysis I figure out that there is more gaseous phase CH₄existing when no CH₄is found in the early stage and the late stage,which indicates that the middle period of minerogenic environment should be a reducing environment.From the early to late stage of the hydrothermal mineralization period,the salinity and the temperature of the fluid becomes lower and lower:In the early stage the fluid is mainly magmatic water,both the salinity and the temperature are in a high level,as the fluid gradually changes,atmospheric-magmatic mix fluid begins to blend in,which leads to the salinity and the temperature becomes lower compared to the early stage.In the late stage of carbonate,the ratio of the atmospheric-magmatic mixture increases,the salinity and the temperature of the fluid continue to decrease.The middle stage of hydrothermal fluid is the most important mineralization stage among all three of them,magmatic ore-forming fluid mixing with atmospheric-magmatic is the main mineral precipitation mechanism,which bring about the decreasing of salinity and temperature,the change of PH,and then causes the clathrate of the fluid which carries the metal resolve,resulting in the elements such as copper precipitate and accumulate,finally forms the mine.On the other hand,the CH₄content in the fluid of the middle stage fluid is commonly high,the minerogenic environment changes into reducing environment and forms the important oxidation and reduction barrier.CH₄ reduce the SO₂from the volatiles into S^2-,S^1-,then the low valence S ions react with metallic mineral ions and form sulphide mineral such as pyrite,chalcopyrite and magnetite-pyrite and so on,which results in a large-scale precipitation taking place in the useful metal of the fluid,leading to the enrichment and formation of the deposit.