Timing and genetic link of porphyry Mo and skarn Pb-Zn mineralization in the Chagele deposit, Western Nyainqentanglha belt, Tibet

Abstract

Many new discoveries of both base and precious metal (Pb–Zn–Fe–Ag–Mo–Cu) deposits in the Nyainqentanglha polymetallic belt (NPB) have indicated this region as one of the most important Pb–Zn–Ag–Mo metallogenic belts of western China. However, the temporal and genetic relationship between Pb–Zn ± Ag and Mo ± Cu deposits is ambiguous and whether these diverse mineralization types formed in a magmatic-hydrothermal system or in response to separate events remains unknown. To address these questions, U–Pb, Ar–Ar, Re–Os dating, coupled with whole-rock chemistry, and Sr–Nd–Hf–Pb isotopes were evaluated to narrow the mineralization timing, and to establish a relationship between Pb–Zn ± Ag and Mo veins. The Chagele Pb–Zn–Ag–Cu–Mo deposit was used as case study. This deposit (with reserves of 1.12 Mt with 2% Pb, 3% Zn, 6.1 g/t Ag, 0.07% Mo, and 0.70% Cu), located in the western NPB, is characterized by diverse mineralization types, including southern porphyry Mo and northern skarn Pb–Zn ± Ag ± Cu veins. The northern Pb–Zn ± Ag ± Cu orebodies are hosted in the skarn and hornfels at the contact zone between granite porphyries and limestone and quartz sandstone, while the Mo mineralization in the southern mining area can be found at the top of granite porphyries. For granite porphyries, the magmatic event is recorded by robust zircon U–Pb ages of 62.7 ± 0.6 Ma in the north and 63.1 ± 0.3 Ma in the south. These ages are indistinguishable, within error, from the mean Re–Os age of 61.9 ± 0.4 Ma for Mo veins/veinlets hosted by porphyries and a 40Ar/39Ar plateau age of 62.8 ± 0.6 Ma as obtained for muscovite in Pb–Zn ± Ag ± Cu veins hosted in the skarn. Intrusions associated with skarn Pb–Zn ± Ag ± Cu and Mo orebodies have similar geochemical characteristics. These show high contents of SiO2, K2O, and CaO, low contents of TiO2 and P2O5, obvious light rare earth element enrichment, negative Eu anomalies, and peraluminous high-K cal-calkaline of granite. They have radiogenic isotopic compositions with (87Sr/86Sr)i of 0.71356–0.72333, εNd(t) from −15.5 to −5.2, (207Pb/204Pb)i of 15.633–15.713, and (208Pb/204Pb)i of 38.406–39.158. The positive zircon εHf(t) values range from −8.5 to −0.2, which corresponds to TDM2(Nd) and TDM2(Hf) model ages ranging from 2.09 Ga to 1.02 Ga. These results suggest that ore-related granitic magmatism was predominantly sourced from the partial melting of the previous Proterozoic basement, modified by mantle-derived melts during the continental collision of Indian and Asian plates. This result suggests that the Mo and Pb–Zn ± Ag ± Cu mineralization in the Chagele deposit constitute a porphyry–skarn metallogenic system, which has important implications for further local and regional exploration. Many Pb–Zn–Ag skarn deposits but extremely few porphyry Mo ± Cu deposits and epithermal Pb–Zn ± Ag ± Au deposits were discovered in the NPB. Targeting proximal porphyry Mo–Cu mineralization and distal epithermal deposits adjacent to, or superimposed on skarn Pb–Zn mineralization should be focused on in the future.