Relationship Between Mesozoic Magmatism And Gold Mineralization In Jiaobei Terrane

Jiaodong area hosts the largest gold deposit in China.Linglong and Jiaojia superlarge gold deposits are located in Jiaobei terrane in the northwest of Jiaodong area.Granites and dikes are widely distributed in Jiaobei terrane,covering more than two thirds of bedrock outcrops.They are closely related to the time and space of gold deposits.Many achievements have been made in element and isotope geochemistry,but the role of magmatism in gold mineralization in Northwest Jiaodong is still unclear.Therefore,systematic study on the physical and chemical conditions of Mesozoic Magmatic rocks in Northwest Jiaodong can provide important constraints for further discussion on the relationship between granite and gold deposit.Based on petrographic observation and previous work,the oxygen fugacity,crystallization temperature,pressure and water content of Mesozoic amgmatic rocks in Northwest Jiaodong area were systematically investigated.Combined with the previous results of gold content of Mesozoic magmatic rocks in Northwest Jiaodong,this thesis discusses the oxygen fugacity,temperature,diagenetic pressure and other conditions of pre-and post-mineralization magmatic rocks(pre-mineralization Linglong and Guojialing magmatic rocks;post-mineralization Aishan magmatic rocks)and their significance for Au mineralization.Compared with post-mineralization magmatic rocks,oxygen fugacity of pre-mineralization magmatic rocks are relatively low(the average oxygen fugacity of Linglong magmatic rocks is calculated by zircon trace element data.The oxygen fugacity is about ?FMQ+0.43,and the average value of the Guojialing pluton is about?FMQ+2.31.The average value of the Aishan magmatic rocks is about ?FMQ+2.36.Based on the electron microprobe data of biotite and amphibole,oxygen fugacity of Linglong magmatic rocks is calculated at ?FMQ+0.98,and the average value of the Guojialing pluton is about ?FMQ+0.90,and the average value of the Aishan magmatic rocks is about ?FMQ+2.03).At the same time,the calculated pressure of Guojialing and Aishan rock are strongly negatively correlated with the oxidation state(the average formation pressure of Linglong rock calculated by biotite composition is about 1.83 kbar,that of Guojialing rock calculated by amphibole composition is about 4.14 kbar,and that of Aishan rock is about 1.51 kbar).The magmatic water content of Guojialing pluton is estimated to be 5.5 wt%,while the water content of Aishan pluton is slightly lower,which is about 4.6 wt%.Overall,there is a negative correlation between oxygen fugacity and pressure,which indicates that oxygen fugacity changed during magma evolution.Therefore,the oxygen fugacity of magma may not reflect the redox state of magma source area.Previous analysis of the gold content of granite shows that the overall gold content is less than 0.1 ppb,which is nearly an order of magnitude lower than the average continental content(1 ppb).It is speculated that the separation of sulfide melts occurred in the process of magmatization.On the one hand,the saturation of reduced species like Fe2+ and S2-in the magma resulted in the oxidation of the magma;On the other hand,these sulfide melts take away most of the gold in the magma,resulting in the loss of gold content in the analyzed rocks.These gold rich sulfides accumulated in the lower crust,and then oxidized and water rich basaltic magma underlay led to sulfide decomposition and gold mineralization.