Metallic mineralization affiliated to subaerial volcanism: a review

Abstract

Synopsis A considerable number of metallic ore deposits are genetically related to subaerial volcanism. Here four ore types are reviewed: epithermal precious- and base-metal deposits are usually of vein type, are associated with either propylitic (± adularia) or advanced argillic alteration, and cut mid- to late Caenozoic volcanic sequences, commonly of andesitic composition. Examples from western Mexico, central America, the central Andes, the western U.S.A., Japan, the North Island of New Zealand, the Philippines, Fiji and Spain are described. They are commonly generated less than 3 m.y. after their host volcanic rocks, and are localized at depths shallower than 1000 m by pre-existing faults; in the San Juan Mountains, Colorado, in particular, earlier fractures related to caldera collapse were utilized. Epithermal ores were deposited from low-temperature (200–300°C), low-salinity (normally < 12 equivalent wt % NaCl for base-metal ores and < 2 equiv. wt % NaCl for precious-metal ores) meteoric water, but isolated δD values suggest a magmatic contribution to the ore fluid. Various lines of evidence suggest subjacent stocks as a principal, but not exclusive, source of ore components. Manto-type copper deposits are best known from Chile and southern Peru. They consist of stratiform concentrations of disseminated and vesicle-filling chalcocite-bornite-chalcopyrite in the tops of Jurassic to early Tertiary andesite flows and felsic ash-flow tuffs, and in associated volcaniclastics and organic-rich limestones. Deposits were generated by the circulation of meteoric water through hot volcanic rocks immediately following their accumulation. Copper and subordinate silver were leached from the volcanic rocks and deposited in volcanics or related sediments in restricted lagoons, perhaps aided by bacterial action. Volcanogenic tin deposits are minor accumulations of cassiterite, wood tin and hematite in felsic volcanic rocks (+ possible rhyolite plugs) in western Mexico, the western U.S.A. and Bolivia and adjoining northwest Argentina. Their origin may be similar to that of the manto-type copper deposits. Volcanogenic magnetite (+ hematite, apatite and actinolite) deposits occur as flows, plugs and pyroclastic beds at Cerro de Mercado, Mexico, and El Laco, Chile. The magnetite magma separated from its calc-alkaline parent because of liquid immiscibility and was erupted because of a high volatile content and a lowering of its melting point by the contained phosphorus. These ore deposits of subaerial volcanic affiliation were generated at convergent plate boundaries, many of them while subduction was active. Volcanogenic tin and some epithermal precious-metal deposits in the western U.S.A. and an epithermal gold deposit in Fiji, however, were emplaced following the cessation of subduction, in extensional tectonic regimes, characterized in the western U.S.A. by a bimodal basalt-rhyolite suite and in Fiji by a basaltic suite of ocean-island affinity. Nevertheless, metallic mineralization appears to be absent from subaerial volcanic rocks accumulated in extensional environments at incipient divergent plate boundaries, as in the East African rift system and the Scottish Hebrides.