ALBERT

Description: THE hydrothermal circulation of sea water through permeable ocean crust results in rock–water interactions that lead to the formation of massive sulphide deposits. These are the modern analogues of many ancient ophiolite-hosted deposits1–4, such as those exposed in Cyprus. Here we report results obtained from drilling a series of holes into an actively forming sulphide deposit on the Mid-Atlantic Ridge. A complex assemblage of sulphide–anhydrite–silica breccias provides striking evidence that such hydrothermal mounds do not grow simply by the accumulation of sulphides on the sea floor. Indeed, the deposit grows largely as an in situ breccia pile, as successive episodes of hydrothermal activity each form new hydrothermal precipitates and cement earlier deposits. During inactive periods, the collapse of sulphide chimneys, dissolution of anhydrite, and disruption by faulting cause brecciation of the deposit. The abundance of anhydrite beneath the present region of focused hydrothermal venting reflects the high temperatures ( 〉 150 °C) currently maintained within the mound, and implies substantial entrainment of cold sea water into the interior of the deposit. These observations demonstrate the important role of anhydrite in the growth of massive sulphide deposits, despite its absence in those preserved on land.

Description: Petrologic and geochemical studies of vent solids from the Main Endeavour Field (MEF) and the High Rise Field (HRF), Juan de Fuca Ridge, demonstrate that the steep‐sided vent structures characteristic of these sites form dominantly by flange growth, combined with diffuse flow through sealed portions of structures, and incorporation of flanges into structures. Geochemical calculations suggest that the prevalence of amorphous silica and flanges in Endeavour deposits is the result of conductive cooling of vent fluids that have high concentrations of ammonia. At Endeavour, as the temperature of vent fluid decreases, ammonia‐ammonium equilibrium buffers pH and allows more efficient deposition of sulfide minerals and silica from fluids that have a higher pH than conductively cooled ammonia‐poor fluids present at most other unsedimented mid‐ocean ridge vent sites. Deposition of silica stabilizes flanges and allows structures to attain large size. It also leads to diffuse flow and further conductive cooling by reducing the permeability and porosity of the structures and of feeder zones, thus decreasing entrainment of seawater. Most inactive vent samples recovered from areas peripheral to the HRF and MEF are similar to barite + silica rich samples from the Explorer Ridge and Axial Seamount and likely formed from precipitation of silica and barite on a biological substrate. Active white smoker chimneys from the Clam Bed Field, located south of the HRF, are pyrrhotite rich and likely formed from vent fluids that are depleted in Zn and Cd and enriched in Pb and Ba relative to fluids exiting trans‐Atlantic geotraverse (TAG) and Cleft Segment white smoker chimneys.

Description: Mineralogical, textural, chemical, and isotopic features of a vertical section through the active Trans-Atlantic Geotraverse (TAG) hydrothermal mound reveal the nature of subsurface mineralization. The multistage growth and evolution of the TAG mound occurs by the following processes: (1) near-surface (〈10 m depth) hydrothermal precipitation of porous Fe-Cu-Zn sulfide and Si-Fe-oxyhydroxides; (2) modification of surface material within the mound (〉20 m depth) by sequential overgrowth, recrystallization and mineral dissolution; (3) hydrothermal mineralization within the mound, forming Fe-Cu sulfides, anhydrite and quartz; and (4) alteration and mineralization of basalt basement beneath the mound. During the long history of hydrothermal activity, these processes have driven the TAG mound toward a mineralogy dominated by pyrite and depleted in Cu, Zn, and trace elements. The basement beneath the mound is ultimately altered to pyrite-quartz. Sulfur-isotope composition of sulfides in the range +4.4‰ to +8.9‰ requires a deep hydrothermal source with elevated d34S to generate an end-member fluid with estimated d34S of +5.5‰. Vein-related sulfide mineralization is isotopically light, whereas sulfide disseminated in altered basalt is isotopically heavy. The systematic variations between sulfide generations and a general increase with depth are a result of sulfate reduction in a shallow seawater-hydrothermal circulation system developed around the hydrothermal feeder zone. This generates hydrothermal fluid and sulfide mineralization with a maximum d34S of +8.9‰. Mixing between this shallow circulated fluid and the end-member hydrothermal component would explain the variations of up to 3‰ observed between different sulfide generations in the mound.

Description: Representative samples of drill core were collected from each of the five main areas drilled on the TAG (Trans-Atlantic Geotraverse) mound during Leg 158 (Humphris, Herzig, Miller, et al., 1996). In this report, we present the results of chemical analyses of 66 samples previously analyzed for Cu, Fe, Zn, Pb, Ag, and Cd by atomic absorption during Leg 158. Data are presented for an additional 38 elements plus total sulfur, loss on ignition, and the rare earth elements by a combination of optical emission spectrometry (ICP-ES), mass spectrometry (ICP-MS), and neutron activation (INAA). These data are discussed in detail in other chapters in this volume.

Description: Eighty-five bulk samples consisting of varying proportions of pyrite, silica, and anhydrite and 82 mineral separates (pyrite, chalcopyrite) from the TAG hydrothermal mound were analyzed using Neutron Activation Analyses (INAA), Inductively Coupled Plasma Emission Spectrometry (ICP-ES), Inductively Coupled Plasma Mass Spectrometry (ICP-MS), and sulfur-isotopic methods. The samples were collected from five different areas of the Trans-Atlantic Geotraverse (TAG) mound during Ocean Drilling Program Leg 158. The chemistry of the bulk samples is dominated by high Fe (average 30.6 wt%, n = 57) and S concentrations (average 42.0 wt%, n = 50), reflecting the high amount of pyrite in these rocks. High Ca (up to 11.5 wt%, n = 57) and SiO2 values (up to 49.8 wt%, n = 50) indicate the presence of anhydrite-rich zones in the center of the mound, and pyritesilica breccias, silicified wallrock breccias, and paragonitized basalt breccias deeper in the system. The Cu and Zn concentrations vary from 〈0.01 to 12.2 wt% Cu (average 2.4 wt%, n = 57) and from 〈0.01 to 4.1 wt% Zn (average 0.4 wt%, n = 57), with highest values commonly occurring in the uppermost 20 m of the mound. Most trace-element concentrations are relatively low compared to other mid-ocean ridge hydrothermal sites and average 0.5 ppm Au, 43 ppm As, 234 ppm Co, 2 ppm Sb, 14 ppm Se (n = 85), 9 ppm Ag, 11 ppm Cd, and 59 ppm Pb (n = 57). Gold, Ag, Cd, Pb, and Sb behave similarly to Cu and Zn and are enriched close to the surface of the mound. This is interpreted as evidence for zone refining, a process in which elements that are mobilized from previously deposited sulfides in the interior of the mound by later hydrothermal fluids are transported to the surface, where they reprecipitate as a result of mixing with ambient seawater. The trace-element composition of pyrite and chalcopyrite separates is similar to the bulk geochemistry. However, down to about 50 mbsf, Au, As, Sb, and Mo values in pyrite separates are generally higher than in bulk samples and chalcopyrite separates. Below this depth, these elements appear to be enriched in chalcopyrite separates. Cobalt is typically more enriched in pyrite than in chalcopyrite throughout. A major difference between pyrite and chalcopyrite separates is the strong enrichment of Se in chalcopyrite at the top of the mound, whereas pyrite separates show a moderate increase of Se with depth. Sulfur-isotopic values for bulk sulfides from the interior of the TAG mound vary from +4.6‰ to +8.2‰, with an average of +6.4 ‰ d34S (n = 49). These values do not change significantly downhole, but samples collected from the top of the mound appear to have somewhat lower d34S values than samples from the interior. The average d34S value for TAG sulfides is about 3‰ higher than for most other sulfides generated at sediment-free mid-ocean ridges (average 3.2‰, n = 501). This is largely attributed to thermochemical sulfate (anhydrite) reduction by hightemperature hydrothermal fluids upwelling through the interior of the TAG mound.