The Gangdese belt contains four large, and three intermediate- and small-sized porphyry Cu–Mo deposits, and shows a continuous spectrum from typical porphyry-type Cu–Mo to skarn-type Zn–Pb mineralization. Twenty-three samples collected from four deposits yielded a limited range of molybdenite Re–Os isochron ages from 15.2 to 16.0 Ma, similar to previously-reported Re–Os ages (13.8 to 14.9 Ma) for other porphyry Cu–Mo deposits in the belt, suggesting that Cu–Mo mineralization in the belt occurred in a post-collisional extension environment.
The characteristics of porphyry Cu–Mo deposits in the collisional zone are similar in many aspects to those in arc settings, i.e., mineralization style, alteration zoning, and metal association. Preliminary fluid inclusion studies indicate that the early hydrothermal fluids are preserved as high temperature (≥ 450 °C), high salinity (40 to 47 wt. % NaCl equiv.) and high pressure (≥ 1100 to 1500 bar) fluid inclusions. These fluids are interpreted to represent an orthomagmatic fluid that boiled episodically due to fluid overpressuring and hydrofracturing, resulting in potassic alteration and precipitation of chalcopyrite–molybdenite assemblages.
On the basis of comparison with porphyry Cu deposits in arc settings, we proposed that melting of thickened, newly-formed basaltic crust resulted in generation of Cu-bearing magmatic systems in the Tibetan collision zone, and both orogen-transverse normal faults and orogen-parallel thrust zones during the Miocene post-collisional extension constrain the temporal and spatial localization of the porphyry Cu systems in the collision zone.