Volcanogenic Massive Sulfide (VMS) and Sedimentary-Exhalative (SEDEX) Base Metal Deposits (Cu, Pb, Zn)

Abstract

Volcanogenic massive sulfide (VMS) and sedimentary-exhalative (SEDEX) ore deposits are important sources of base metals (Cu, Zn, and Pb) as well as silver, gold and other trace metals. They are stratiform accumulations of sulfide minerals that were precipitated on or near the seafloor at hydrothermal vents. Both deposit types are syngenetic, having formed at the same time or shortly after deposition of their volcanic and sedimentary host rocks. VMS deposits are typically massive lens-shaped bodies, underlain by discordant vein mineralization and altered volcanic rocks. SEDEX deposits are finely layered massive sulfides interbedded with shale, siltstone, or sandstone, typical of deep marine turbidites. The ores were deposited from reduced, seawater-dominated hydrothermal solutions that leached metals and sulfur out of the volcanic basement or from brines expelled from sedimentary basin formations. The discovery of modern hydrothermal vents at the seafloor validated many aspects of the genetic models, including magmatic-hydrothermal contributions and the role of chemosynthetic life forms. VMS and SEDEX deposits were widely preserved throughout the geological record because their host rocks became integral parts of continents during the closure of volcanic and sedimentary basins. However, many deposits were subjected to high pressures and temperatures during deformation and metamorphism.