14. Geochemical Characteristics By W. Ian Ridley 14 of 21 Volcanogenic Massive Sulfide Occurrence Model Scientific Investigations Report 2010–5070–C U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior KEN SALAZAR, Secretary U.S. Geological Survey Marcia K. McNutt, Director U.S. Geological Survey, Reston, Virginia: 2012 For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment, visit http://www.usgs.gov or call 1–888–ASK–USGS. For an overview of USGS information products, including maps, imagery, and publications, visit http://www.usgs.gov/pubprod To order this and other USGS information products, visit http://store.usgs.gov Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. 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Ian, 2012, Geochemical characteristics in volcanogenic massive sulfide occurrence model: U.S. Geological Survey Scientific Investigations Report 2010–5070 –C, chap. 14, 18 p. 205 Contents Trace Elements and Element Associations ...........................................................................................207 Zoning Patterns ..........................................................................................................................................207 Fluid-Inclusion Thermometry and Geochemistry .................................................................................211 Stable Isotope Geochemistry ..................................................................................................................215 δ18O and δD .........................................................................................................................................215 δ34S .......................................................................................................................................................216 δ11B, δ64Cu, δ66Zn, δ57Fe, δ82Se ..........................................................................................................216 Radiogenic Isotope Geochemistry ..........................................................................................................220 References Cited........................................................................................................................................222 Figures 14–1. Idealized cross sections through the two main stages of growth of a massive sulfide chimney .........................................................................................................209 14–2. Idealized cross section through the Trans-Atlantic Geothermal hydrothermal field based on Alvin dive observations and Ocean Drilling Program drilling during Leg 158 ..............................................................................................................210 Tables 14–1. Modern and ancient volcanogenic massive sulfide deposits with high sulfidation state minerals ........................................................................................................208 14–2. Fluid inclusion thermometry and chemical compositions for selected volcanogenic massive sulfide deposits ................................................................................212 14–3. Some transitional metal isotope ratios in volcanogenic massive sulfide deposits and related rocks ......................................................................................................218 14–4. Lead isotopic composition of selected volcanogenic massive sulfide deposits ...........221 14. Geochemical Characteristics By W. Ian Ridley Trace Elements and Element systems, high sulfidation fluids are associated with island-arc settings, and in ancient VMS deposits they occur in siliciclas- Associations tic-felsic and felsic lithologic associations (Sillitoe and others, 1996). A list of deposits with high sulfidation characteristics is Volcanogenic massive sulfide deposits are important shown in table 14–1. In many cases, the precipitation of exotic resources of base and precious metals and a variety of rare minerals that represent only trace constituents is parageneti- elements. The precious and rare metals usually are present at cally late and spatially associated with barite-rich lithologies, trace concentrations, but natural refining of such metals can consistent with an increase in sulfidation state with decreasing determine the economic viability of a deposit. The list of trace temperature. The close spatial association with advanced argil- elements includes the following: Co, Ni, Ga, Ge, As, Se, Mo, lic and alunitic alteration indicates that the hydrothermal fluids Ag, Cd, In, Sn, Sb, W, Au, Hg, Tl, and Bi. The proportions of were highly acidic and corrosive, and the high sulfidation state metals and their concentrations are largely a function of the of the fluids may indicate the involvement of magmatic fluids overall rock associations and fluid chemistry. For instance, in and gases. This also may be the source of some of the trace Canadian VMS deposits, felsic-dominated deposits have high elements associated with the high sulfidation minerals. Pb and Ag, whereas the bimodal-felsic deposits contain the highest Au concentrations. In many cases, the trace-element associations are determined by the overall chemistry of the Zoning Patterns various lithologies that are encountered by fluid during hydro- thermal circulation, and they may be determined by lithologies All VMS deposits show some degree of geochemical/ that are distal to the deposit. For instance, the Mount Read mineralogical zoning that is a function of fluid composi- deposits in Tasmania carry anomalously high concentrations tion, fluid mixing, temperature, and porosity/permeability. In of Au that appear to have been sourced from distal high-Ti ancient VMS deposits, the geochemical/mineralogical evolu- basalts (Stolz and Large, 1992). Volcanogenic massive sulfide tion of deposit growth is difficult to follow, as the later stages deposits, associated primarily with mafic and ultramafic of sulfide precipitation tend to overprint earlier stages and assemblages, tend to have high Co, Ni, and Se concentrations (Galley and others, 2007). there commonly have been several stages of ore mineraliza- Trace elements may precipitate during mixing of reduced tion. However, studies of modern black smoker chimneys, for hydrothermal fluids with heated or ambient seawater as a example, those at 21°N and 9°50’N on the East Pacific Rise function of the sulfidation state of the fluid. Modern and and at the TAG active hydrothermal field at the Mid-Atlantic ancient VMS deposits cover a wide range of sulfidation states Ridge, provide insights into the evolution of sulfide growth. based on sulfide mineral assemblages and associated altera- To some degree, these patterns are microcosms of the larger tion assemblages (Sillitoe and others, 1996). In the presence scale zonation observed at the deposit scale because in both of low to intermediate sulfidation minerals (pyrrhotite, pyrite, cases the mineralogic zonation is a function of fluid chemistry chalcopyrite, arsenopyrite, tennantite, high-Fe sphalerite, (mixing between hydrothermal fluid and seawater) and a steep galena), trace metals may precipitate as native elements (Bi, temperature gradient (hydrothermal fluid at up to 400 °C and Ag, Hg, Sb, Sn). These conditions appear to be mostly met in ambient seawater at 1–2 °C). In modern chimney growth, the Cu-Zn deposits associated with mafic assemblages and minor critical initial stage is the precipitation of anhydrite [CaSO4] felsic components. In the presence of intermediate to very ± caminite [Mg7(SO4)5(OH)4•H2O] from conductively heated high sulfidation minerals (pyrite, bornite, digenite, enargite, seawater as a consequence of the reverse solubility of these low-Fe sphalerite), trace elements tend to be fixed in sulfides minerals. It is the outward and upward precipitation of anhy- and sulfosalts, such as famantinite (Cu3SbS4), cinnabar (HgS), drite at the seawater interface that stabilizes chimney growth berthierite (FeSb2S4), bismuthinite (Bi2S3), stibnite (Sb2S3), and provides a matrix for precipitation of sulfide phases argentite (Ag2S), and tennantite ([Cu,Fe]12As4S13) but may also (Haymon, 1983). The chimney advances upward and outward be associated with native Au. In active marine hydrothermal by anhydrite growth. Hot hydrothermal fluid that circulates 208 14. Table 14–1. Modern and ancient volcanogenic massive sulfide deposits with high sulfidation state minerals (after Sillitoe and others, 1996). Characteristics Geochemical [Ag, silver; As, arsenic; Au, gold; Bi, bismuth; Cu, copper; Zn, zinc; al, alunite; and, andalusite; aspy, arsenopyrite; ba, barite; bn, bornite; cass, cassiterite; cc, chalcocite; cinn, cinnabar; cor, corumdum; cord, cordierite; cp, chalcopyrite; cv, covellite; dia, diaspore; dig, digenite; en, enargite; gal, galena; gyp, gypsum; id, idiate; kao, kaolinite; ky, kyanite; luz, luzonite; marc, marcasite; mont, montmorillonite; musc, muscovite; ns, native sulfur; po, pyrrhotite; py, pyrite; pyro, pyrophyllite; qz, quartz; rut, rutile; ser, sericite; sp, sphalerite; sulf, sulfosalts; tenn, tennantite; tetra, tetrahedrite; top, topaz; zir, zircon; Mt, mil- lion metric tons; g/t, gram