Formation of the Majiayao gold deposit, Jiaodong Peninsula, eastern China: Constraints from fluid inclusions, H-O-S-Pb isotopes, and pyrite Rb-Sr age

The Majiayao gold deposit at Qixia in Shandong Province is a typical gold deposit of medium-low temperature hydrothermal veins. This gold deposit is one of the most important types in the Jiaodong Peninsula. Four stages have been identified based on the cross-cutting relationships and mineralogical and textural characteristics. The study of quartz vein fluid inclusions show that the inclusions can be divided into three categories: pure liquid or pure gas phase inclusions (type S), two-phase gas and liquid inclusions (type W), and three-phase inclusions rich in CO2 (type C). The results of laser Raman probe analysis show that the fluid inclusions are generally rich in CO2, thus forming a typical CO2-H2O-NaCl system. The fluid temperature results show that in stage I, only the type W fluid inclusions are identified by microthermometry, the homogenized temperatures (to liquid) are 210-343 degrees C and mostly range from 260 to 280 degrees C, the range of salinity is 1.57-10.98 wt.% NaCl, with a peak value of 8-11 wt.% NaCl, and the density is 0.65-1.02 g/cm(3), with a peak value of 0.81-0.87 g/cm(3); in stages II and III, the primary fluid inclusion assemblages include both type W and C inclusions, which show similar phase-transition temperatures and are trapped between 190 and 300 degrees C, and the peak values of salinities are 4-10 wt.% NaCl equivalent; and in stage IV, the total homogenization temperatures (Th-TOT) into liquid range from 120 to 220 degrees C, and the range of salinities is 4.2-7.9 wt.% NaCl. These values indicate that temperature, salinity, and density gradually decreased from the early to late mineralization stage. The ore-forming fluid of the deposit was a thermal liquid system with medium-low temperature, medium-low salinity, and low density and was rich in reducing CO2. The H-O isotope shows that the ore-forming fluids were dominated by metamorphic fluid with only minor amounts of meteoric water involved in the ore-forming process. According to the S, Pb, and Sr isotopes analysis, it indicated that the source of ore-forming materials is mainly crustal fluid. The measured Rb-Sr isochron age of pyrite is 123.4 +/- 2.9 Ma, which indicates that the metallogenic age corresponded to the Early Cretaceous.