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Abstract

Volcanogenic massive sulfide deposits (VMS) are grouped into five lithostratigraphic types, using sequence boundaries defined by major time-stratigraphic breaks, faults, or major (subvolcanic) intrusions: (1) bimodalmafic settings (e.g., Noranda, Urals) occur in incipient-rifted suprasubduction oceanic arcs, typified by flows and <25 percent felsic strata; (2) mafic settings (e.g., Cyprus, Oman) occur in primitive oceanic backarcs, typified by ophiolite sequences with <10 percent sediment; (3) pelite-mafic (e.g., Windy Craggy, Besshi) settings occur in mature oceanic backarcs, typified by subequal amounts of pelite and basalt (including mafic sills); (4) bimodal-felsic (e.g., Skellefte, Tasmania) settings occur in incipient-rifted suprasubduction epicontinental arcs, typified by 35 to 70 percent felsic volcaniclastic strata; and (5) siliciclastic-felsic settings (e.g., Iberia, Bathurst) occur in mature epicontinental backarcs, typified by continent-derived sedimentary and volcaniclastic strata. Deposits in the first three types are predominantly Cu-Zn, whereas the last two also contain significant Pb. Each of these five may be further divided on the basis of the predominant lithofacies into flow-, volcaniclasticor sediment-dominated settings.

Ancient VMS deposits formed in collisional environments (ocean-ocean or ocean-continent convergence) during periods of extension and rifting. As the result of rifting, subsidence, and thinning of the crust accompanied by the rise of hot asthenospheric mantle into the base of the crust caused bimodal mantle-derived mafic and crustal-derived felsic volcanism. Magmatism associated with rifting, which manifests itself by the emplacement of cogenetic intrusions at shallow and mid-crustal levels, caused heating and modification of entrapped seawater within adjacent volcanic and/or sedimentary strata. Extensional arc environments are recognized by the change from a sequence of VMS-prospective primitive arc basalt and high silica rhyolite, intruded by tonalite-trondjhemite sills and dike swarms, to an overlying succession of MORB basalt-dominated terrane in oceanic back-arc basins, or alkaline basalt and MORB in mature continental back-arc basins.

Heat-induced water-rock reactions resulted in metal leaching and the formation of hydrothermal convection systems within the lower semiconformable alteration zones of VMS deposits. Long-lived systems ultimately discharged fluid from deep-penetrating, synvolcanic faults onto the sea floor or into permeable strata immediately below the sea floor, to form VMS deposits. In addition, in a few districts some of the metals may have been obtained directly from subvolcanic magmas (e.g., Cu, Au, and Sn).

The metal content of a deposit is controlled by the temperature, aS, and pH of fluids in the reaction zone, adiabatic cooling of the fluid during its ascent (related to water depth), and the amount of subsea-floor fluid mixing, and zone refining. Fluids formed by reaction with basalt typically have a maximum temperature of 350° to 400°C and produce Zn-Cu deposits with minimal Pb. Fluids formed by the reaction with sedimentary or felsic volcaniclastic strata may have been of lower temperature and produced Zn ±Pb ± Cu deposits, usually with higher (Zn + Pb)/Cu ratios than the former. The gold content of deposits in any setting is controlled by temperature, aS, boiling (related to water depth), and precipitation mechanisms, as well as redistribution (zone refining), plus input from magmatic sources. Subsea-floor replacement provides a more efficient mechanism to trap a higher proportion of metals and may be responsible for forming larger, more tabular VMS deposits. Some components of the hydrothermal fluid escape to be trapped in hanging-wall sediments and sea-floor precipitates. Silica (as chert) and conserved elements (Mn, Eu, P, Tl, base and precious metals) all accumulate in these sediments, forming useful vectors to potential ore.

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