Re-Os isotope systematics and fluid inclusions of Xinqiao deposit in Tongling, the Middle-Lower Yangtze River Metallogenic Belt

Xinqiao Cu-Au-Fe-S deposit, located in Tongling Metallogenic province, Anhui, is a representative polymetallic sulfide deposit in the Middle-Lower Yangtze River Metallogenic Belt. In this study, pyrites were picked under binocular for Re-Os isotopic analysis. Two isochron ages were obtained for the first time; 138 +/- 26Ma and 393 +/- 40Ma, respectively. The initial Os isotopic compositions of the two groups of pyrites have some remarkable differences. The former are higher and have an affinity for crust source, while the later are lower and suggest a considerable mantle contribution. Furthermore, the age of 138 +/- 26Ma is in accordance with the age of intrusive quartz diorite (140.4 +/- 2.2Ma) in this region. Therefore, combining previous studies it can be implied that there were two mineralization events in Xinqiao deposit; one was magmatic hydrothermal mineralization in the Yashanian and the other was exhalative sedimentary mineralization in the Paleozoic. Fluid inclusions of intrusion and veins related to mineralization in Yanshanian were also analyzed in this study. The compositions of fluid inclusions are mainly brine and CO2-rich fluid inclusions were not found. These fluid inclusions homogenized between 140 and 432 degrees C with salinity ranging from 4.2% to 50.7% NaCl equivalent. These features indicate that ore-forming fluid is mainly magmatic aqueous fluid. The data also yield that fluid-boiling exists around 250 degrees C and fluid-mixing between magmatic fluid and meteoric waters, which indicates that both fluid-boiling and fluid-mixing play important roles in the Yashanian mineralization and the metallogenic temperature is around 250 degrees C.