Sulfur and Lead Isotope Compositions of the Qulong Porphyry Copper Deposit, Tibet: Implications for the Sources of Plutons and Metals in the Deposit

The Qulong deposit is one of the representative porphyry copper deposits in the Gangdese metallogenic belt in the Tibetan continental collision orogen. S-and Pb-isotopic data for the bearing-copper porphyries and metals in the Qulong deposit are given in this paper. The S-isotope compositions from porphyries are similar to those from chalcopyrite. The δ 34 S values for the copper-bearing porphyries vary slightly, from-2.1‰ to-1.1‰, the δ 34 S values for chalcopyrite vary between-6.3‰ and-1.0‰ and the δ 34 S values for anhydrite vary from +12.5‰ to +14.4‰. The sulfur isotope fractionation in the ore fluid almost reaches the equilibrium, showing the typical signature of mantle S. The 206 Pb/ 204 Pb, 207 Pb/ 204 Pb and 208 Pb/ 204 Pb ratios for porphyries vary in ranges of 18.51044～18.6083, 15.59463～15.7329 and 38.6821～39.1531, respectively. The ranges of 206 Pb/ 204 Pb, 207 Pb/ 204 Pb and 208 Pb/ 204 Pb ratios for chalcopyrite are between 18.4426～18.5909, 15.5762～15.6145 and 38.5569～38.8568, respectively. The uniform Pb isotope compositions for the deposit imply that the Pb isotopes may have the same origin and similar developing history. The Pb isotopes are plotted mostly between the mantle and orogenic belt evolution curves in the plumbotectonic framework diagram, which indicates that the lead is mixed with those from the mantle and crust. The sulfur and lead isotopic features of the copper-bearing porphyries and ore minerals from the deposit suggest that ore-forming materials were mainly from magma and the ore-forming porphyry was formed due to complex mechanism involving partial melting of mafic materials in the thickened lower-crust under the Tibetan orogen and the input of enriched mantle components.