Sulfur and lead isotopic variations in the giant Yulong porphyry Cu (Mo Au) deposit from the eastern Tibetan Plateau: Implications for origins of S and Pb, and metal precipitation

The giant Yulong porphyry Cu (Mo Au) deposit, located in the eastern Tibetan Plateau, is a typical post-subduction porphyry Cu ± Mo ± Au deposit formed in an intracontinental strike-slip fault system, and it is also one of the largest porphyry Cu deposits in China with reserves of 6.5Mt Cu @ 0.62% and 0.41Mt Mo @ 0.042%, respectively. In this study, sulfur and lead isotopic compositions of sulfides and ore-related porphyry samples were determined to constrain the sources of ore-forming materials and ore-forming process. Sulfides (e.g., pyrite, chalcopyrite and molybdenite) from phyllic alteration zone, where the Cu and Mo mineralization mainly occurred, have uniform δ34S values ranging from −0.6 to +2.1‰ (average + 1.1‰), which indicates a magmatic origin of the sulfur. In situ Nano-SIMS sulfur isotope analysis further reveals that pyrites formed in potassic alteration stage have systematically negative δ34S values ranging from −0.2 to −9.7‰ (average − 4.5‰), whereas pyrites formed in phyllic alteration stage have systematically positive δ34S values ranging from +1.1 to +3.2‰ (average + 2.1‰). This regular variation is considered to be resulted from disproportionation of aqueous SO2 and subsequent sulfate reduction, further suggesting a magmatic origin of the sulfur. The sulfides have initial Pb isotopic compositions ((206Pb/204Pb)i = 18.616–18.812, (207Pb/204Pb)i = 15.670–15.740, (208Pb/204Pb)i = 38.956–39.080) deviating from those of the ore-related porphyries ((206Pb/204Pb)i = 18.549–18.718, (207Pb/204Pb)i = 15.647–15.662, (208Pb/204Pb)i = 38.794–39.888), but more close to those of the ore-hosting strata ((206Pb/204Pb)i = 18.423–18.761, (207Pb/204Pb)i = 15.654–15.713, (208Pb/204Pb)i = 38.867–39.062). The shift of Pb isotopic compositions of sulfide minerals toward those of ore-hosting strata indicates that the ore-hosting strata had provided a significant amount of Pb for ore formation, and most probably suggests fluid-rock interaction processes in the hydrothermal system. Our study represents a case study in which ore-hosting strata had provided a significant amount of ore-forming materials for porphyry Cu ± Mo ± Au formation in the post-subduction setting.