The Miocene Gangdese porphyry copper belt generated during post-collisional extension in the Tibetan Orogen

Abstract

The mid-Miocene Gangdese orogenic belt in southern Tibet contains porphyry Cu deposits that developed after the Indian–Asian continental collision in the early Cenozoic. Field work confirms that these porphyry Cu deposits are temporally and spatially associated with mid-Miocene monzogranitic and quartz monzonitic stocks, which intruded Mesozoic–Cenozoic Gangdese granitoid batholiths and surrounding Triassic–Tertiary volcano-sedimentary sequences in the Lhasa Terrane, and formed a 50-km wide, 400-km long E–W belt. Magmatism occurred during the interval 11.2 to 19.7 Ma, peaking at ∼ 16 Ma for the porphyry stocks; this is consistent with the age (10 to 18 Ma) of the east–west extension and subsequent NS-striking normal fault systems (≥ 13.5 Ma) in the Tibetan Orogen. The stocks typically occur as multiple small diameter intrusions (< 5 km), and have shallow emplacement depths (∼ 1 to 3 km). The Cu porphyries are shoshonitic and high-K calc-alkaline, showing geochemical affinities with adakites. The porphyries were derived from a thickened basaltic lower-crust source, rather than from the subducted oceanic-slab. Geochemical and geophysical data indicate that during the Paleocene–Oligocene mantle-derived mafic magmas created a newly-formed basaltic underplate at the bottom of the lower-crust, which undewent garnet amphibolite metamorphism under high pressure (> 50 km). Breakdown of hornblende in the juvenile lower-crust source during melting is regarded as fundamental for the formation of the fertile adakite and porphyry Cu systems in south Tibet. 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.