Re-Os dating of pyrite from Giant Bayan Obo REE-Nb-Fe deposit

Six pyrite samples from the giant Bayan Obo REE-Nb-Fe deposit are dated by Re-Os technique. Pyrite studied is associated with barite and separated from a vein cutting REE mineralization. Pyrites analyzed contain 16—30 ng Re and 0.10—0.16 ng Os, and yield a Caledonian isochron age of 439 ± 86 Ma. High Re/Os ratio, low Os concentration and highly radiogenic Os isotopic ratios of these samples suggest that they are of crustal origin. The northern margin of the North China Block was a passive continental margin, but not a subduction zone with enormous volcanic activities in the Early Paleozoic Era. Our Re-Os result provides new evidences, showing that Bayan Obo deposit experienced a thermal disturbance of crustal origin in the Caledonian time and some isotopic systems recorded only the time of the dis- turbance, not the mineralization age.