Volcanogenic sulfide deposits may be described as stratabound, lenticular bodies of massive pyritic mineralization, containing variable amounts of chalcopyrite, sphalerite, and galena in layered volcanic rocks. Often they are found to be immediately overlain by thin-bedded siliceous and iron-rich sedimentary rocks, and they are commonly underlain by extensive zones of altered, sulfide-impregnated lava. They are believed to have formed subaqueously by volcanic-fumarolic activity which occurred periodically during volcanism. Three distinct varieties of such deposits can be distinguished by their compositions, relative and absolute ages, and rock associations. Pyrite-sphalerite-chalcopyrite bodies are found in differentiated, mafic-to-felsic volcanic rocks; pyrite-galena-sphalerite-chalco-pyrite bodies occur in more felsic, calc-alkaline volcanic rocks, and pyrite-chalcopyrite bodies occur in mafic, ophiolitic volcanic rocks. The time-tectonic- stratigraphic interrelationships of these varieties can be related to evolutionary processes of crustal development. Deposits of the pyrite-sphalerite-chalcopyrite variety are numerous, important, and best developed in Archean greenstones, suggesting that they were generated under conditions of thin proto-crust,possibly by degassing of as yet poorly differentiated proto-mantle. Although they recur in younger volcanic successions, they become scarcer and smaller in later geologic time. However, their place is taken by the two other varieties which are notably rare or absent in the Archean. Thus, in Proterozoic volcanic rocks the pyrite-galena-sphalerite-chalcopyrite type appears. In Phanerozoic orogens, pyrite-chalcopyrite bodies are common in the ophiolites that typify an early stage of orogenic activity and are probably generated in oceanic ridge-rift environments during the initial stages of separation of continental crustal blocks. Both of the earlier varieties reappear in Phanerozoic belts; the pyrite-sphalerite-chalcopyrite type in early stages of subduction along continent margins, and the pyrite-galena-sphalerite-chalcopyrite type in later, more felsic, calc-alkaline volcanics that characterize somewhat later tectonism. In addition to their obvious application in mineral exploration, these concepts may have certain scientific applicability. If, on a very broad or general basis, mineral deposits (like fauna) are products of evolutionary change, then in the absence of fossils or other correlation aids they might be used as gross-scale indicators for correlation or age determination purposes. This could be particularly applicable to highly metamorphosed Precambrian terrane throughout the world. © 1973 Society of Economic Geologists, Inc.
