A review of geological characteristics and selenium enrichment in Se-bearing volcanogenic massive sulfide deposits

Volcanogenic massive sulphide (VMS) deposits consist of upper stratabound massive-to-semimassive sulphide lenses and discordant stringer sulphides in the footwall, hosting significant base metals like Zn, Cu, and Pb, with some enriched in precious (Ag, Au) and semi-metals (like Se and Te). Selenium occurs at low concentrations (0.05 ppm) in Earth’s crust and is critical for renewable energy technologies. Here, we have combined global Se-bearing VMS deposits to illustrate selenium occurrence, enrichment conditions, sources, and depositional environments. Selenium generally occurs as by-products of Cu-rich ores in VMS Cu–Zn or Cu–Au systems; no independent VMS selenium deposit has been reported worldwide. Selenium mineralization is mainly associated with sulphide minerals formed at high-temperature (~300°C) hydrothermal stages as a function of fluid temperature and can occur as Se minerals under highly oxidizing conditions related to supergene geological processes. Selenium-bearing sulphide minerals (like chalcopyrite) are the result of isomorphic substitutions of S by Se; Se contents generally show a decreasing trend from galena, chalcopyrite, sphalerite, pyrite, to pyrrhotite. Other Se-bearing minerals are dominated by roquesite, tellurium-minerals, and tetrahedrite. Selenium minerals are not common but can precipitate at favourable conditions in VMS systems, including native Se, clausthalite (PbSe), naumannite (Ag2Se), bohdanowiczite (AgBiSe2), eucairite (AgCuSe), and tiemannite (HgSe). Co-existence of galena – clausthalite and chalcopyrite (like at the Xiaorequanzi deposit in China) is due to locally elevated H2Se activities of hydrothermal fluids, and the intergrowth of sulphate and selenide minerals (like in VMS deposits in the Urals in Russia), as well as oxide and selenide mineral assemblages (like at the Apliki deposit in Cyprus) is produced by secondary enrichment processes. Worldwide Se-bearing VMS deposits typically occur in arc settings related to the initial subduction of oceanic plates; the addition of magmatic volatiles into the circulated hydrothermal fluids provide the necessary selenium for anomalous Se enrichment. In some cases, leaching of thick Se-bearing sedimentary rocks by evolved seawater in basins is a critical process accounting for Se mineralization endowment.