Volcanogenic massive sulfide (VMS) deposits are stratabound and sometimes stratiform lenses of polymetallic massive to semi-massive sulfides, sulfidic sediments, disseminated replacement ores, and stockwork sulfide-bearing veins that occur in submarine volcanic environments. They form at or near the seafloor by syngenetic precipitation from mainly seawater-derived and metal-enriched hydrothermal fluids. The fluids are moderate- to high-temperature driven by the accompanying volcanism.
VMS deposits are major global sources of Cu and Zn and, to a lesser extent, Pb, Ag, Au, Sn, Sb, and minor amounts of other elements, such as indium, germanium, and gallium. The are grouped according to base metal content, gold content, and host-rock lithology. The base metal classification used by Franklin et al. (1981) and refined by Large (1992) is perhaps the most common, although the host-rock classification used by Barrie and Hannington (1999), and later modified by Franklin et al. (2005) is gaining acceptance. According the base metal classification the deposits fall into two distinct groups, a Cu-Zn group and a Zn-Pb-Cu group. Deposits of the Cu-Zn group are associated by bimodal volcanic sequences, dominated by mafic rocks but with felsic rocks commonly occurring in close association with the deposits. In contrast, deposits of the Zn-Pb-Cu group are in stratigraphic sequences dominated either by felsic volcanic rocks or by subequal amounts of felsic volcanic rocks intercalated with sedimentary strata, such as turbidites, black shales or other deep-sea sedimentary rocks. Following the lithological classification, the VMS deposits are divided in five groups, i.e. mafic, bimodal-mafic, mafic-pelitic, bimodal-felsic, and felsic-siliciclastic. These groups correlate with different submarine tectonic settings, from the most primitive VMS environments, represented by ophiolite settings, through oceanic rifted arc, evolved rifted arcs, continental back-arc to sedimented back-arc. Sub-seafloor hydrothermal activity and seafloor sulfide deposits occur in any tectonic setting where there is seafloor volcanism.
The Corta Tiberio operated by Compania Minera Sotiel Coronada between end of the 19th century till beginning of the 20th century, Sotiel Coronda, Calanas, Andalucia, Spain. Roman workings are abundant on this VMS deposit. The total geologic reserve is about 75.2 Mt @ 0.56 % Cu, 3.16 % Zn, 1.34 % Pb, 0.21 g/t Au, and 24 g/t Ag.
The formation of VMS deposits span the whole geologic history with examples date back to the Archean about 3.4 billion years ago in the Pilbara of Western Australia and in the Barberton and Murchison greenstone belts of South Africa to actively-forming deposits in modern seafloor spreading and oceanic arc terranes (i.e. black smokers).
There are more than 800 known VMS deposits worlwide, ranging in size from 200,000 t to giant deposits containing a few hundreds of millions of tonnes. All VMS deposits contain predominantly iron sulfides, i.e. pyrite and to a lesser extent pyrrhotite. Chalcopyrite is the main Cu-bearing mineral and sphalerite and galena are the main ore minerals of Zn and Pb. Gold, where present, is existent in small grains of native gold and may be variably at highest concentrations either with the highest concentrations of chalcopyrite or with sphalerite and galena. Magnetite and hematite are minor components of many ores. Quartz is the main gangue mineral, which is ubiquitously disseminated in the massive sulfide bodies and the stockwork ores. Barite and other sulfate minerals are common especially in the upper levels of the body.
The grade is highly variable and range from massive pyritic or pyrrhotite-rich bodies with very low base metal contents (up to a few percent combined Cu+Zn+Pb) to very rich deposits. As the metals are leached out of the host-rocks, high grades of Zn and Pb are most commonly associated with felsic volcanic and sedimentary successions, whereas Cu-rich pryitic deposits are commonly associated with mafic volcanic-dominated successions, and some Cu-rich deposits may have derived their metals directly from a felsic subvolcanic magma.
Geometric mean concentrations of metals in VMS ores according to host-rock type (from Franklin, Gibson, Jonasson, and Galley, 2005, Volcanogenic massive sulfide deposits)
La Poderosas open pit, El Campillo, Huelva, Andalucia, Spain. This mine was operated from 1864-1924 by Compania James Hit. The host-rock in the hanging-wall is siliceous, sericite- and chlorite-bearing volcanic tuff (blocky structure). The VMS deposit has two lenses about 150-175 m long and the width is about 7 m and 3 m respectively. The total production is estimated to be 0.6 Mt @ 1.5-2.0 % Cu.
In general, a VMS system consists of two main parts, (1) a typically mound-shaped to tabular, stratabound body composed mainly of massive to semi-massive sulfide, quartz, minor phyllosilicates, and iron oxides minerals. This bodies are underlain by a (2) vertically extensive discordant to semi-discordant stockwork of Cu-rich veins and disseminated sulfides, referred to as the "feeder zone" or "stringer zone". These two ore types are accompanied with considerable and intense hydrothermal alteration and the host-rocks that encompass the feeder or stringer zone are called the "alteration pipe". This zone can be very irregular in shape and the alteration pipes are enveloped in distinctive alteration halos, which may extend into the hanging-wall strata above the VMS deposit.
The ores of VMS deposits consists of > 60 % sulfide minerals. In general, most VMS deposits show an internal metal zonation, with Cu-sulfides occurring dominantly at the base and in the stringer zone. Zn- and Pb-rich sulfides occur at the top of the massive sulfide body or at the outer margins, reflecting temperature-dependent solubilities of the ore minerals in cooling hydrothermal fluids discharged onto the seafloor.
Schematic cross-section of a typical volcanogenic massive sulfide deposit. Adapted from Lydon (1984) and Hannington (2014).
Above - Sharp hanging-wall contact between ore to siliceous and sericite- and chlorite-bearing shales, San Platon open pit, Almonaster la Real, Huelva, Andalucia, Spain. Current reserves are 2.4 Mt @ 1.5 % Cu, 12.3 % Zn, 0.5 % Pb, 2.05 g/t Au, and 69 g/t Ag.
Left - Sharp contact between ore (sulphate-bearing massive sulfide) and felsic volcanic rocks (rhyolite), San Telmo, Corta Santa Barbara, Almonaster la Real, Huelva, Andalucia, Spain.
Cu-bearing stockwork in felsic volcanic rocks (rhyolite and tuffs) with drifts, San Dionisio ore body, Corta Atalaya, Riotinto, Almonaster la Real, Huelva, Andalucia, Spain. The stockwork extends for a distance of 700-800 m in an east-west direction, is 200 m thick and 600 m deep. It is composed of pyrite, chalcopyrite, sphalerite, galena, and magnetite. The reserves of the stockwork is about 17.2 Mt @ 1.45 % Cu, the reserve of the massive sulfide is about 45 Mt @ 0.88 % Cu, 2.13 % Zn, 1.07 % Pb, 0.6 g/t Au, and 64.3 g/t Ag.
Oxidized stockwork with veins of goethite and limonite in brecciated and chlorite-bearing felsic volcanic rocks (rhyolite) and beginning formation of gossan in the upper right-hand side, Cerro Colorado, Riotinto, Almonaster la Real, Huelva, Andalucia, Spain.
VMS deposits form in extensional tectonic settings, including both oceanic seafloor spreading and arc environments. Although modern VMS deposits such as submarine hydrothermal systems with black and white smokers occur in oceanic spreading ridge and arc environments, VMS deposits that are still preserved in the geological record formed mainly in oceanic and continental nascent-arc, rifted arc, and back-arc settings.
Most of the VMS deposits worlwide occur in clusters, that are restricted by linear rifts or calderas. These are
features of extensional tectonics as well as crustal thinning, mantle depressurization and the generation of basaltic melts. These mafic melts pool at the base of the crust. Partial melting of the lithosphere results in generation of granitoid melts. These anhydrous, high temperature melts may quickly rise to a sub-seafloor level, where their heat may initiate and sustain convective hydrothermal cells which form VMS deposits.
A schematic illustrating the relationship between subvolcanic intrusions, subseafloor alteration, synvolcanic faulting and the generation of VMS deposits. Not to scale. Modified after Galley 1993, Franklin et al. 2005, and Herrington.