Metallogenesis and ore controls of Cenozoic porphyry Mo deposits in the Gangdese belt of southern Tibet

Abstract

The Gangdese is a newly explored porphyry copper ore belt in China. Except for the Cu (±Mo±Au) porphyry deposits there are some Mo (±Cu±W) porphyry and skarn deposits in this belt. Two pulses of molybdenite mineralization are recognized in the central Lhasa subterrane with ancient continent crust, including the Paleocene–Eocene (65–52Ma) porphyry Mo and skarn Mo–W deposits formed during the rollback of Neo-Tethyan oceanic slab in collisional setting, and Miocene (21–15Ma) porphyry Mo–Cu deposits generated in the postcollisional setting. The Gangdese copper deposits also occur during these two periods but are distributed in the southern Lhasa subterrane dominated by juvenile crust through mantle-derived magmatism resulting from Neo-Tethyan ocean lithosphere subduction. The intrusions related to molybdenite mineralization have relatively lower bulk-rock εNd(t) and higher (87Sr/86Sr)i values, lower zircon εHf(t) values and older Hf model ages, lower molybdenite Re contents, and more radiogenic Pb isotopes than the coeval copper related porphyries in the Gangdese belt. In addition, the Miocene Mo–Cu related intrusions generally have lower zircon Ce4+/Ce3+ ratios than the Miocene Cu (±Mo) related intrusions. These data indicate that more contributions from mature continental crustal materials play crucial role in the generation of Mo than Cu porphyry mineralization in the Gangdese belt.The Eocene Mo related intrusions are characterized by high SiO2 (68–76%) and K2O (5.1–5.9%) concentrations, enrichment of large-ion lithophile elements (LILE), depletion of high field strength elements (HFSE), and low Sr/Y (7.5–23.8) ratios. Their Sr–Nd isotope compositions (εNd(t)=−3.4 to −4.6; 87Sr/86Sr(i)=0.70622–0.70679) indicate that they are associated with partial melting of mantle wedge mixing with significant amounts (55%–65%) of middle-upper crustal materials. The Miocene Mo–Cu related intrusions have relatively high SiO2 (64%–73%) and K2O (1.9–4.8%) contents and are characterized by enrichment of LILE, depletion of HFSE, and high Sr/Y ratios (55–189). Their Sr–Nd isotope compositions (εNd(t)=−4.6 to −3.4; 87Sr/86Sr(i)=0.70622–0.70679) are indicative of derivation from remelting of previously subduction modified Tibetan lithosphere with significant amounts (50%–70%) of middle-upper crustal materials. For the Gangdese Mo (±Cu) porphyry mineralization, Mo metal was suggested to be derived from the middle–upper crustal materials, particularly the Carbonaceous–Permian marine sedimentary, whereas Cu metal to be sourced from the Gangdese Miocene Cu fertile magmas. The exploration targeting of the Gangdese molybdenite deposits should be focused on southern part of the central Lhasa subterrane where the Carbonaceous–Permian strata occur. The occurrence of Mo–Cu porphyry deposits could provide clue for targeting Cu–Mo porphyry deposits in the Gangdese belt. The Gangdese molybdenite deposits share many similarities with the East Qinling molybdenite deposits, such as type of ore deposits, geodynamic setting of their formation, variable Mo grades and high F contents.