Two episodes of mineralization in the Mengya’a deposit and implications for the evolution and intensity of Pb–Zn–(Ag) mineralization in the Lhasa terrane, Tibet

Abstract

Mengya’a is a typical Pb–Zn–(Ag) deposit in the Lhasa terrane, which is located in the eastern part of the central Lhasa subterrane, Tibet. Two different types of Pb–Zn–(Ag) mineralization (skarn and porphyry-like styles) have been identified in the Mengya’a mining district. The skarn-type mineralization occurs as layered or lenticular units in the Laigu Formation and contains nearly all the Pb–Zn ore resources, whereas the porphyry-like mineralization exists mainly as veins in granite porphyry. 40Ar/39Ar dating of muscovite suggests that the skarn orebodies formed at 54.8±0.4Ma, whereas zircon U–Pb dating of granite porphyry (13.2±0.2Ma) indicates that the porphyry-like mineralization formed in the Miocene. As such, the primary mineralization event (skarn type) took place during the main Indo-Asia collision. The ore-forming mineralization and alteration characteristics of the Mengya’a deposit are consistent with other examples of regional Pb–Zn–(Ag) mineralization in the Lhasa terrane. The present results, combined with previous studies, indicate that multiple Pb–Zn–(Ag) mineralization events occurred in the Lhasa terrane, including during the Cretaceous, Paleocene, and Miocene, all of which were associated with felsic intrusions. Sulfur and lead isotopes suggest that the Pb–Zn–(Ag) mineralization events were mainly magmatic in origin, and were closely associated with the ancient basement of the central Lhasa subterrane. Whole rock major and trace elements, and zircon Hf isotope data of the ore-forming intrusions indicate that these intrusions are S-type granites, produced mainly by partial melting of the ancient crust, which underwent extensive fractionation. Therefore, the presence of moderately to strongly fractionated S-types granites, along with the ancient crustal magma source, were the key factors in generating the Pb–Zn–(Ag) mineralization in the central Lhasa subterrane. Crustal melting in the central Lhasa subterrane peaked in the Paleocene, and resulted in the most intense Pb–Zn–(Ag) mineralization during the initial stages of Indo-Asia continental collision (65–50 Ma). During the Cretaceous, the scale of crust melting was considerable, creating large Pb–Zn–(Ag) deposits. Limited Miocene crustal melting and magmatism in the central Lhasa subterrane resulted in the Miocene Pb–Zn–(Ag) mineralization being smaller in scale than that in the Paleocene and Cretaceous.