Multistage Pyrite's In-situ Analysis And Its Constraints On Gold Metallogenic Fluid Evolution In Xinli Gold Deposit,Northwestern Jiaodong Penisula,Shandong Province

Pyrite is a dominant sulfide in Jiaodong gold deposits.The ore deposits such as Xinli gold deposit in northwest Jiaodong were sampled in stages,and the typical pyrite in Xinli gold deposit was divided into three stages from early to late(Py1,Py2,Py3)by microscopic observation and other literatures.With LA-ICP-MS technology,in-situ analysis of pyrite in different stages was performed to determine the content of trace elements in pyrite in different stages,the following conclusions were obtained:(1)No element was enriched in Py2,but the content of As was higher than that of Py1.The distribution of Py2 elements was dispersed to a large extent,indicating that the fluid was not in equilibrium when Py2was formed,and the fluid that formed gold particles and pyrite was not from the same source.Most trace elements in Py2's fluid were from different sources except As.Py3is rich in Au,As,Ag,Cu,Zn and gold-rich pyrite as well,indicating that the temperature in this stage is lower than that in Py2,which proportion of water in the fluid is higher,thus the main source of the fluid in this stage is meteoric precipitation.(2)According to the correlation matrix and regression line,it can be seen that the correlation of the three stages of significant changes in the typical element pair:Au-Ag,Pb-Bi,indicating that these elements in the three stages have different sources or occurrence;Au-Ag changed from strong positive correlation to non-correlation,as the characteristics of main mineralization stage were significantly different from those of common orogenic gold deposits.Pb-Bi changed from negative correlation to positive correlation.Co-Ni,Cu-Pb,Ag-Pb and Au-As have positive correlation in the first three stages,indicating that they are transported together in the ore-forming fluid or occurrence can be similar.(3)Combining with previous studies on the C-D-O-S-Sr isotope system of fluid inclusions,it is believed that the ore-forming fluid is a mixture of meteoric water and magmatic water,but the characteristics of hydrogen and oxygen isotopes are of multiple solutions.Therefore,the characteristics of hydrogen and oxygen isotopes cannot reflect the accurate source of ore-forming fluid,and even there are contradictions.(4)According to the Co/Sb-Se/As,Co-Ni plotting and other elementary classification,Py1 issimilar to magmatic deposit,whereas the main mineralization stages,including Py2 and Py3,are similar to low-temperature hydrothermal deposit,which present the sedimentary characteristics,reflecting the environment similar to the sedimentary pyriteand the addition of crust composition.But Tl concentration is lower than the Carlin type gold deposits.As a result,new primary gold mineralization stages of mineralization environment,process is more akin to hydrothermal deposit.(5)Gold does not tend to enter the lattice of Py2,which is the main mineralization stage,indicating that the solubility of gold in the fluid is unstable when Py2 is formed,and goldinclusion,crack hosted gold,and crystalline gap gold symbiotically with Py2 do not form completely at the same time as pyrite,which attributes non-Au[HS]~-complex.Gold complex instability and precipitation dominated mineralization.To sum up,the metallogenic mechanism of gold deposit in northwest Jiaodong is different from that of common orogenic gold deposits,a migration and enrichment mechanism other than Au[HS]~-complex do exist.