Timing and origin of the Shesuo skarn Cu-polymetallic deposit in the northern Lhasa subterrane, central Tibet: Implications for Early Cretaceous Cu(Mo) metallogenesis in the Bangong-Nujiang metallogenic belt

Abstract

The Bangong-Nujiang metallogenic belt is a newly discovered copper-polymetallic belt in the Tethyan domain. To date, the Shesuo skarn Cu-polymetallic deposit is the only productive Early Cretaceous Cu deposit in the middle segment of this belt. We conducted zircon U-Pb and molybdenite Re-Os dating of this deposit, and the results show that the Shesuo skarn Cu-polymetallic deposit formed at 117–112 Ma. Within the mining area, granodiorites and monzogranites are the main metallogenic granitic bodies. The petrogeochemical features imply that the granodiorites and monzogranites were derived from the partial melting of juvenile lower crust resulting from mantle-derived magmatic underplating, with the additional involvement of ancient crust in the magma chamber, whereas the monzogranites experienced significant fractional crystallization. The δ34S values of sulfide ores suggest that they were mainly sourced from the magma and slightly contaminated by surrounding strata. The Pb isotope compositions of sulfides and Re contents of molybdenite indicate that the metals were derived from the juvenile lower crust, with a small fraction from ancient crustal materials. Our study suggests that the generation of the Shesuo deposit was most likely triggered by slab breakoff during the southward subduction of the Bangong-Nujiang Ocean lithosphere. The Shesuo and Xiongmei deposits have low zircon Ce4+/Ce3+ and EuN/Eu* ratios and low whole-rock average V/Sc, Sr/Y and (EuN/Eu*)/YbN ratios. We propose that these values are due to the input of reduced components, leading to the generation of metallogenic magmas with relatively low magmatic oxidation states and water contents and rendering the magmas infertile for later porphyry-skarn Cu formation in the Xiongmei area.