Hydrothermal Alteration and Mineralization of Oceanic Crust<subtitle>Mineralogy, Geochemistry, and Processes</subtitle>

Volcanic-associated massive sulfide deposits (VMS) are predominantly stratiform accumulations of sulfide minerals that precipitate from hydrothermal fluids at or below the sea floor, in a wide range of ancient and modern geological settings (Figs. 1, 2). They occur within volcanosedimentary stratigraphic successions, and are commonly coeval and coincident with volcanic rocks. As a class, they represent a significant source of the world's Cu, Zn, Pb, Au, and Ag ores, with Co, Sn, Ba, S, Se, Mn, Cd, In, Bi, Te, Ga, and Ge as co- or by-products.The understanding of ancient, land-based VMS deposits has been heavily influenced by the discovery and study of active, metal-precipitating hydrothermal vents on the sea floor. During the last three decades, excellent descriptions of sea-floor sulfides and related vent fluids and hydrothermal plumes have provided modern analogs for the landbased VMS deposits (Rona, 1988; Rona and Scott, 1993; Hannington et al., 1995). Conversely, the geology and mineralogy of land-based deposits have provided insight into the plumbing systems and sulfide mineral paragenesis of sulfide deposits relevant to sea-floor hydrothermal systems.This volume capitalizes on the complementary nature of ancient, land-based VMS deposits and active, metal-precipitating hydrothermal systems on the sea floor, much as the Reviews in Economic Geology Volume 2 (Berger and Bethke, eds., 1985) did with epithermal deposits and active, subaerial geothermal systems, and draws equally from land-based and sea-floor VMS research. This volume attempts to provide a balanced view of VMS systems, with descriptions of the processes involved in VMS formation and of important examples representing a variety of VMS deposits and districts, in modern and ancient settings. It is not meant to be a comprehensive review; rather, it presents a spectrum of current ideas based on research since the benchmark paper of Franklin et al. (1981). The contributions are divided into two parts. In Part I, reviews of the most significant geological, physical, and chemical processes involved in the formation of landbased and sea-floor VMS deposits are presented. These include: the volcanology of subaqueous settings and the relationship between volcanology and VMS systems by Gibson et al. (1999); structural aspects of magmatism and hydrothermal circulation in ocean floor and ophiolitic settings by Harper (1999); the relationship between magma chemistry and hydrothermal venting, with emphasis on the thickened oceanic crust in the Galapagos area by Perfit et al.(1999), and more generally in bimodal volcanic settings by Barrett and MacLean (1999); hydrothermal alteration of the oceanic crust by Alt (1999); fluid-rock interactions in VMS systems as recorded by stable isotope systematics by Huston (1999); the metal transport capabilities of hydrothermal fluids by Seyfried et al. (1999); precious metal enrichment associations and processes in VMS systems by Hannington et al. (1999); and heat and fluid flow in VMS systems by Barrie et al. (1999a).