Himalayan Cu-Mo-Au mineralization in the eastern Indo-Asian collision zone: constraints from Re-Os dating of molybdenite

We present new Re-Os molybdenite age data on three porphyry Cu-Mo-Au deposits (Yulong, Machangqing, and Xifanping). These deposits are associated with the Himalayan adakitic magmatism that occurred in a continental collision environment, controlled by large-scale Cenozoic strike-slip faults in the eastern Indo-Asian collision zone. Three distinct episodes of Cu-Mo-Au mineralization are recognized. At Yulong, Re-Os isotopic data of four molybdenite samples from sulfide-quartz veins in the quartz-sericite alteration zone yield an isochron with an age of 40.1 +/- 1.8 Ma (2 sigma), coincident to a zircon sensitive high-mass resolution ion microprobe (SHRIMP) age of 40.9 +/- 0.1 Ma for the host monzogranite. The molybdenite Re-Os dates, together with K-Ar, Rb-Sr, U-Pb, and 40Ar/39Ar dates on the pre- and intra-ore porphyries, suggest that Cu-Mo-Au mineralization formed during the late stage (similar to 40 Ma) of regional porphyry magmatism, but hydrothermal activity probably lasted to at least similar to 36 Ma. At Machangqing, molybdenite Re-Os data from the K-silicate and quartz-sericite alteration zones yield an isochron with an age of 35.8 +/- 1.6 Ma (2 sigma), which is identical to the zircon SHRIMP and bulk-rock Rb-Sr ages (35 similar to 36 Ma) of the host granite, but older than bulk- rock K-Ar dates (31 similar to 32 Ma) for associated Au-bearing quartz syenite with advanced argillic alteration. At Xifanping, five molybdenite samples from the K-silicate alteration zone yield the youngest Re-Os isochron age in the area, at 32.1 +/- 1.6 Ma (2 sigma). The Re-Os molybdenite dates here are younger than K-Ar ages (33.5 similar to 34.6) for hydrothermal biotite and actinolite. There is a positive correlation between the absolute age of the deposits and their Cu and Au reserves in the eastern Indo-Asian collisional zone. Episodic stress relaxation probably caused multiple magmatic intrusions, which most likely resulted in three episodes of Cu-Mo-Au mineralization in the eastern Indo-Asian collision zone.