Characteristics of VMS Deposits

Volcanogenic (Volcanic) massive sulphide (VMS) deposits, are major sources of zinc, copper, lead, silver and gold. They typically occur as lenses of polymetallic massive sulphide that form on the seafloor in submarine volcanic environments and are classified according to base metal content. They form from metal-enriched brines associated with seafloor hydrothermal convection. Their immediate host rocks can be either volcanic or sedimentary. Because of their polymetallic content, VMS deposits continue to be one of the most desirable deposit types for security against fluctuating prices of different metals. There are close to 850 known deposits worldwide.

The most common feature among all types of VMS deposits is that they are formed in extensional tectonic settings, including both oceanic seafloor spreading and back arc environments. Most, but not all, significant VMS mining districts are defined by deposit clusters formed within rifts or calderas. Their clustering is further attributed to a common heat source that triggers large-scale subseafloor fluid convection systems. These subvolcanic intrusions may also supply metals to the VMS hydrothermal systems through magmatic devolatilization. As a result of large-scale fluid flow, VMS mining districts are commonly characterized by extensive semi-conformable zones of hydrothermal alteration that intensifies into zones of discordant alteration in the immediate footwall and hanging wall of individual deposits. VMS camps can be further characterized by the presence of thin, but areally extensive, units of ferruginous chemical sediment formed from exhalation of fluids and distribution of hydrothermal particulates.

Most VMS deposits have two components (see Figure 1 below). There is typically a mound-shaped to tabular, stratabound body composed principally of massive (>40%) sulphide, quartz and subordinate phyllosilicates, and iron oxide minerals and altered silicate wall-rock. These stratabound bodies are typically underlain by discordant to semi- concordant stockwork veins and disseminated sulphides. The stock-work vein systems, or “pipes”, are enveloped in distinctive alteration halos, which may extend into the hanging-wall strata above the VMS deposit.

Figure 1: Cross section of a VMS deposit

Source: Volcanogenic Massive Sulphide Deposits, Alan G. Galley, Mark D. Hannington, And Ian R. Jonasson, 2007.