Syn-collisional magmatism at the Longgen Pb–Zn deposit, western Nyainqentanglha belt, Tibet: Petrogenesis and implications for regional polymetallic metallogeny

Abstract

Syn-collisional orogeny generally lacks adequate fluids and has a low potential to form magmatic hydrothermal ore deposits. However, some syn-collisional Pb–Zn deposits have been recognized in the Nyainqentanglha polymetallic belt (NPB), central Lhasa subterrane and ore-related magma petrogenesis and the metallogenic geodynamic setting is limited. In this study, detailed petrology, geochemical and geochronological data of ore-related syn-collisional granite porphyries are evaluated to constrain the timing of mineralization and discuss the genesis and evolution of ore-related magma using the Longgen skarn Pb–Zn deposit as a case study. Zircon U–Pb dating of ore-related porphyries provides a weighted mean age of 61.2 ± 0.8 Ma, which is slightly earlier than the sphalerite Rb–Sr age of 59.1 ± 3.0 Ma, indicating that a Paleocene Pb–Zn mineralization event occurred during syn-collisional orogeny. The porphyries have high SiO2, K2O + Na2O, and are enriched in Rb, Th, U, Pb, LREEs, and A/CNK (0.94–1.15), but have low P2O5 and are depleted in Nb, Ta, and Ti. This suggests that the Longgen granite porphyries are metaluminous to weakly peraluminous high-K calc-alkaline “arc-like” granites. These porphyries have highly radiogenic isotopic compositions with (87Sr/86Sr)i 0.709513–0.713587, (207Pb/204Pb)i 15.657–15.713, and (208Pb/204Pb)i 38.964–39.163, but low εNd(t) values from − 6.7 to − 6.0, as well as negative zircon εHf(t) values from − 5.1 to − 3.2, corresponding to TDM2(Nd) ages of 1.37 to 1.42 Ga and zircon Hf TDMc ages of 1.30 to 1.42 Ga. This demonstrates that the Longgen granite porphyries may have been generated by the partial melting of Mesoproterozoic basement continental crust materials with significant amounts (30–50%) of mantle-derived melt input. Paleocene-Eocene (65–45 Ma) magmatic rocks associated with Pb–Zn and Fe–Cu deposits are compiled in the NPB. The Pb–Zn ore-related granitoids have relatively lower whole-rock εNd(t) and zircon εHf(t) values, higher (87Sr/86Sr)i values, and older TDMC ages than the coeval granitoids associated with Fe–Cu mineralization. This indicates that mantle material is more critical for the development of Fe–Cu ore-related granitoids than granitoids associated with Pb–Zn mineralization. The final results show that different sources of Paleocene–Eocene syn-collisional magma in the NPB are beneficial for different mineralization events during the syn-collisional orogeny of the Indian and Asian plates.