Zircon U-Pb, Molybdenite Re-Os and K-feldspar Ar-40/Ar-39 Dating of the Bolong Porphyry Cu-Au Deposit, Tibet, China

摘要

The Bolong porphyry Cu-Au deposit is a newly discovered deposit in the central Tibetan Plateau, and is ranked as the second largest copper deposit discovered to date in the Bangong-Nujiang metallogenic belt in China. Three granodiorite porphyry phases occur within the Bolong porphyry Cu-Au deposit. Phyllic alteration is widespread on the surface of the deposit, and potassic alteration occurs at depth, associated with granodiorite porphyries. The copper and gold mineralization is clearly related to the potassic and phyllic alteration. Multiple chronometers were applied to constrain the timing of magmatic-hydrothermal activity at the Bolong deposit. Zircon U-Pb geochronology reveals that the granodiorite porphyry phases were emplaced at ca. 120 Ma. Re-Os data of four molybdenite samples from quartz-molybednite veinlets yielded an isochron age of 119.4 +/- 1.3 Ma. The plateau age of hydrothermal K-feldspar from the potassic alteration zone, analyzed by Ar-40/Ar-39 dating, is 118.3 +/- 0.6 Ma, with a similar reverse isochron age of 118.5 +/- 0.7 Ma. Therefore, the magmatic-hydrothermal activity occurred at ca. 120-118 Ma, which is similar in age to the neighboring Duobuza porphyry copper deposit. The period of 120-118 Ma is therefore important for the development of porphyry Cu-Au mineralization in the central Tibetan Plateau, and these porphyry deposits were formed during the final stages of the northward subduction of the Neo-Tethys Ocean.