Re-Os isotopic dating of pyrite and molybdenite from the iron deposits in Anshan-Benxi area of Liaoning Province and its geological significances

Abstract: Rhenium and osmium isotopes in a molybdenite sample and ten pyrite samples were used to determine the timing of mineralization by ICP-MS. Re-Os model age of the molybdenite sample is （2376±37）Ma, Re-Os ages of the ten pyrite samples yielded three kinds of ages : ① （2567±36）Ma—（2540±37）Ma (model age); ② （2237±112）Ma (model dating); ③ （1572±140）Ma (isochron age). These data suggest that the molybdenite of（2376±37）Ma was formed during early Proterozoic, and was the oldest Re-Os model age of molybdenite, the pyrite of（2237±112）Ma was formed during early Proterozoic, and both of them indicate that the molybdenite and the pyrite were derived from the crust and represented an important hydrothermal activity in 2.3 Ga; the pyrite of （2567±36）Ma—（2540±37）Ma was formed during new Archaeozoic period and yielded the oldest Re-Os model age, which indicates that the pyrite was formed with BIF in 2.5 Ga; the pyrite of（1572±140）Ma was formed during middle Proterozoic, which indicates that pyrite was derived from the crust and represented a hydrothermal activity. The Re-Os isotopic dating result provides a new proof for the existence of hydrothermal activity in iron deposits of the Anshan-Benxi area, and is also important for understanding the ore-forming processes and tectonic evolution in this area.