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The Himalayan Cu—Mo—Au Mineralization in the Eastern Indo—Asian Collision Zone: Constraints from Re-Os Dating of Molybdenite

ZENG Pusheng~ 1,2) , HOU Zengqian~ 2) , GAO Yongfeng~ 3) , DU Andao~ 4) 1) Department of Resources & Environment, Yunnan University of Fiance & Economics, Kunming, Yunnan, 650221 2) Institute of mineral Resource, Chinese Academy of Geological Sciences, Beijing, 100037 3) Shijiazhuang Economic College, Hebei, 65000 4) National Research Center of Geoanalysis, Chinese Academy of Geological Sciences, Beijing, 100037  
New Re-Os molybdenite dates are here presented from three porphyry deposits (Yulong, Machangqing and Xifanping), associated with the Himalayan adakitic porphyritic intrusives that occurred in a continental collision environment and were controlled by large-scale Cenozoic strike-slip faults in eastern margins of the Qinghai—Xizang(Tibet) plateau. The Re-Os isotopes in molybdenites have been used to determine mineralization time and constrain on duration of the porphyry hydrothermal systems. Three distinct mineralization episodes have been recognized for the porphyry Cu—Mo—Au deposits in the eastern Indo—Asian collision zone. At Yulong, Re-Os analyses of four molybdenite samples from sulfide—quartz veins in the quartz—sericite alteration zone yield an isochron with an age of 40.1±1.8Ma (2σ), coincident to zircon SHRIMP age of 40.9±0.1 Ma for the host monzogranite. The molybdenite Re-Os datings, together with K-Ar, Rb-Sr, U-Pb and ~ 40 Ar / ~ 39 Ar datings on the pre- and intra-mineral porphyries suggest that Cu—Mo mineralization took place at the late stage (~40Ma) of lifetime of porphyry magmatism, but hydrothermal system prolonged at least to ~36Ma. During the longevity of 4Ma, structurally controlled advanced argillic alteration and the overprinting of high-sulfidation mineralization over porphyry-type orebody were developed at Yulong. At the Machangqing deposit, molybdenite Re-Os data yield an eight-point isochron with an age of 35.8±1.6Ma (2σ), which is identical to the zircon SHRIMP and bulk-rock Rb-Sr ages (35~36Ma) of the host granites, but lower than bulk-rock K-Ar ages (31~32 Ma) for associated Au-bearing altered quartz syenite. This suggests a porphyry hydrothermal system longevity of ~4Ma, during which Cu-Mo introduction associated with the K silicate alteration at ~36Ma, while Au mineralization associated with advanced argillic alteration at the last period (31~32Ma) of the Machangqing hydrothermal system. At Xifanping, five molybdenite samples from the K silicate alteration zone yield the youngest isochron age of 32.1±1.6Ma (2σ) in the area. The Re-Os molybdenite ages are younger than K-Ar ages (33.5~34.6) of hydrothermal biotite and amphibole, which most likely reflects the cessation of the porphyry hydrothermal system at ~32 Ma. Such short duration of hydrothermal system probably explains the weakly sericitization and absence of the advanced argillic alteration in the district. Episodically stress relaxation during tectonically transforming from transpressional (55~40Ma) to transtensional (24~17Ma) regimes probably caused multiple magmatic intrusions, which most likely result in the protraction of the hydrothermal system and superimprosed mineralization in the eastern Indo—Asian collision zone.
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