The Zhaxikang Vein‐type Pb‐Zn‐Ag‐Sb Deposit in Himalayan Orogen, Tibet: Product by Overprinting and Remobilization Processes during Post‐collisional Period

Abstract

Abstract The Zhaxikang Pb‐Zn‐Ag‐Sb deposit, the largest polymetallic deposit known in the Himalayan Orogen of southern Tibet, is characterized by vein‐type mineralization that hosts multiple mineral assemblages and complicated metal associations. The deposit consists of at least six steeply dipping vein‐type orebodies that are hosted by Early Jurassic black carbonaceous slates and are controlled by a Cenozoic N–S‐striking normal fault system. This deposit records multiple stages of mineralization that include an early period (A) of massive coarse‐grained galena–sphalerite deposition and a later period (B) of Sb‐bearing vein‐type mineralization. Period A is only associated with galena–sphalerite mineralization, whereas period B can be subdivided into ferrous rhodochrosite–sphalerite–pyrite, quartz–sulfosalt–sphalerite, calcite–pyrite, quartz–stibnite, and quartz‐only stages of mineralization. The formation of brecciated galena and sphalerite ores during period A implies reworking of pre‐existing Pb–Zn sulfides by Cenozoic tectonic deformation, whereas period B mineralization records extensive open‐space filling during ore formation. Fluid inclusion microthermometric data indicate that both periods A and B were associated with low–medium temperature (187–267°C) and low salinity (4.00–10.18% wt. NaCl equivalent) ore‐forming fluids, although variations in the physical–chemical nature of the period B fluids suggest that this phase of mineralization was characterized by variable water/rock ratios. Microprobe analyses indicate that Fe concentrations in sphalerite decrease from period A to period B , and can be divided into three groups with FeS concentrations of 8.999–9.577, 7.125–9.109, 5.438–1.460 mol.%. The concentrations of Zn, Sb, Pb, and Ag within orebodies in the study area are normally distributed in both lateral and vertical directions, and Pb, Sb, and/or Ag concentrations are positive correlation within the central part of these orebodies, but negatively correlate in the margins. Sulfide S isotope compositions are highly variable (4‰–13‰), varying from 4‰ to 11‰ in period A and 10‰ to 13‰ in period B . The Pb isotope within these samples is highly radiogenic and defines linear trends in 206 Pb/ 204 Pb vs. 207 Pb/ 204 Pb and 206 Pb/ 204 Pb vs. 208 Pb/ 204 Pb diagrams, respectively. The S and Pb isotopic characteristics indicate that the period B orebodies formed by mixing of Pb–Zn sulfides and regional Sb‐bearing fluids. These features are indicative of overprinting and remobilization of pre‐existing Pb–Zn sulfides by Sb‐bearing ore‐forming fluids during a post‐collisional period of the Himalayan Orogeny. The presence of similar ore types in the north Rhenish Massif that formed after the Variscan Orogeny suggests that Zhaxikang‐style mineralization may be present in other orogenic belts, suggesting that this deposit may guide Pb–Zn exploration in these areas.