Timing of formation and origin of the Tongchanggou porphyry–skarn deposit: Implications for Late Cretaceous Mo–Cu metallogenesis in the southern Yidun Terrane, SE Tibetan Plateau

Abstract

The newly-discovered Tongchanggou Mo–Cu porphyry–skarn deposit is located in the southern Yidun Terrane, SE Tibet, with more than 142.5metric ton resources (0.3metric ton @ 0.3% Mo, and 34kt @ 0.8% Cu) hosted in Permian to Triassic strata and Late Cretaceous granodiorite porphyry. Re–Os isotope analyses of molybdenite separates from skarn, granodiorite porphyry and the massive molybdenite orebody yield model ages ranging from 86.8 to 85.2Ma. LA-ICP-MS U–Pb dating of zircons from the ore-related granodiorite porphyry give 206Pb/238U ages ranging from 87.4 to 84.2Ma with a weighted mean 206Pb/238U age of 84.8±0.3Ma, indicating a temporal link between granitic magmatism and Mo–Cu mineralization. Geochemically, the ore-related granodiorite porphyries are metaluminous and have an adakite-like signature with relatively low MgO (1.32–1.58wt%), Cr (5.6–12.9ppm), Ni (3.79–10.81ppm), Mg# (43–52) values, and high Sr (304–844ppm), Sr/Y (21.2–50.8) and La/Yb ratios (37.0–60.1). They also display negative zircon εHf(t) values (−7.4 to −1.3) and negative whole-rock εNd(t) values (−5.3 to −4.2), as well as an older Hf–Nd model age, indicating that their magmas were derived from a thickened ancient lower crust within the garnet–amphibolite facies. Partial melting of ancient lower crust generated felsic magmas with high concentrations of Mo that accumulated within the ancient crustal components. Whereas, the copper mineralization is correlated to the remelting of residual sulfide cumulates in subduction-modified lower crust. The Late Cretaceous porphyry Mo–Cu metallogenesis in the southern Yidun Terrane is considered to be related to the transtensional faulting that triggered asthenospheric upwelling beneath the former sulfide-cumulated arc crust.