Discovery of the large-scale Eocene Xiwu Pb–Zn–Ag deposit in the Tethyan Himalaya: Geochronology, geochemistry, and C–H–O–S–Pb–Sr–Nd isotopes

Abstract

The known Pb–Zn–Sb–Au polymetallic deposits in the Himalaya mainly formed during the main-collisional convergence (65–41 Ma) and post-collisional crustal extension (25–0 Ma) tectonic settings. Here, we report the discovery and genesis of the large-scale Eocene Xiwu Pb–Zn–Ag deposit that formed during the late-collisional tectonic transition stage (40–26 Ma) in the eastern Himalaya. The surrounding ore-hosting rocks are mainly low-grade metamorphosed Jurassic clastic rocks and Early Cretaceous diabase. The most important ore-controlling structures are NE–SW thrust faults. The ore minerals include large amounts of galena, sphalerite, tetrahedrite and siderite, with minor chalcopyrite, pyrite, arsenopyrite, calcite and quartz. In this contribution, we analyzed the S–Pb isotopes of sulfides, fluid inclusions and the C–H–O–Pb–Sr–Nd isotopes of nonmetallic minerals. The results indicate that the ore-forming materials originated from deeper metamorphic rocks of the Greater Himalayan Crystalline Complex. This study infer that the source of ore-forming fluids was mainly metamorphic fluids with moderate temperatures and salinities, and S and C were mainly derived from microbial sulfate reduction and the decarboxylation of organic matter, respectively. The main ore-forming mechanism is fluid mixing between the deep ore-bearing metamorphic fluids and shallow formation waters. The Sm–Nd isotopic age of siderite is 37.4 ± 1.1 Ma, which is slightly younger than the zircon U–Pb age of the Zhunba leucogranite (38.9 ± 0.4 Ma) but is consistent with the main epoch of early prograde metamorphism (43–36 Ma) in the Himalaya. We propose a new metallogenic model for Himalayan Pb–Zn–Ag deposits that can greatly promote ore prospecting. The genetic type of the Xiwu deposit is a clastic-dominated subtype of sediment-hosted Pb–Zn deposits.