Volcanogenic massive sulfide (VMS) deposits share the following characteristics (Gifkins et al., 2005):
They are hosted by submarine volcanic and sedimentary rocks deposited in extensional tectonic settings (Fig. 22).
VMS deposits form from metal-enriched fluids associated with seafloor hydrothermal convection ("black smokers"). Therefore, they are the same age as the host rocks.
They are a major source of Zn, Cu, Pb, Ag and Au, and a significant source of Co, Sn, Se, Mn, Cs, In, Bi, Te, Ga and Ge.
Economic orebodies range from 200 000 tonnes to more than 150 million tonnes in size.
Typical grades are on the order of 2 to 5% Zn, 1 to 2% Cu, 1 to 2% Pb, 30 to 60 g/t Ag and 1 to 2 g/t Au (Galley et al. 2007).
Principal ore minerals are sphalerite, galena, chalcopyrite, pyrite and pyrrhotite.
Stringer-stockwork zones commonly underlie massive sulfides, and may also be a significant source of ore.
Ore metals can be vertically zoned from iron and copper sulfides at the base of an ore lens through to lead and zinc sulfides on the periphery. Some ore lenses carry significant barite with or above the Pb-Zn sulfides.
Massive sulfides can grade laterally into distal exhalites characterized by cryptocrystalline quartz, iron oxides, jasper, manganese oxide and elevated (but usually non-economic) metal concentrations.
VMS deposits occur above extensive footwall alteration zones that form by hydrolysis of feldspar. Primary alteration minerals include sericite, quartz, pyrite, and chlorite. In systems with highly acid fluids, kaolinite, pyrophyllite and even dickite may occur. These minerals are zoned in a systematic fashion from zones of high fluid flux outwards into less-altered host rocks. In metamorphosed VMS deposits, aluminous alteration minerals metamorphose to cordierite, andalusite, or kyanite.
The geometry of the footwall alteration zone depends on the competency of the host rocks. In sequences dominated by flows and domes, fluid flow is focused by sub-vertical synvolcanic faults, and the alteration zones are pipe-like. In contrast, strata bound alteration (mineralized) zones are more commonly developed in permeable rocks such as tuffs, breccias and sediments, particularly under impermeable cap-rocks such as sills (Gifkins et al., 2005).
According to the lithological classification system of VMS deposits (Galley et al., 2007), the Jalisco VMS deposits are best described as "siliciclastic-felsic" (Fig. 23). These types of VMS deposits form in the back-arc regions of continental arcs (Fig. 22). The most diagnostic characteristic of these types of VMS deposits are the occurrence of black carbonaceous argillite (Fig. 19). Continental back-arc setting include some of the world's most economically important VMS districts, including the Ordovician Bathurst camp of New Brunswick and the Devono-Mississippian Iberian Pyrite Belt (Galley et al., 2007).
Galley, A.G., Hannington, M.D., and Jonasson, I.R., 2007, Volcanogenic massive sulfide deposits in Goodfellow, W.D. ed., Mineral Deposits of Canada: A Synthesis of Major Deposit Types, District Metallogeny, the Evolution of Geological Provinces and Exploration Methods: Geological Association of Canada, Mineral Deposits Division, Special Publication No. 5, p. 141-161.
Gifkins, C., Herrmann, W., Large, R., 2005, Altered Volcanic Rocks; a guide to description and interpretation: Center for Ore Deposits Research, University of Tasmania, Australia, 275 p.