Petrology of hydrothermal mineralization: a vertical section through the TAG mound

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 (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

0. RADIOGENIC ISOTOPE RATIOS AND INITIAL SEAFLOOR ALTERATION IN SUBMARINE SEROCKI VOLCANO BASALTS

ABSTRACT Basalts from ODP Site 648, recovered from Serocki Volcano, a near-axis volcano on the Mid-Atlantic Ridge, are normal mid-ocean ridge basalts, depleted in K, Rb, and LREE. They have homogeneous source regions wit

Physicochemical conditions and timing of rodingite formation: evidence from rodingite-hosted fluid inclusions in the JM Asbestos mine, Asbestos, Québec

Fluid inclusions and geological relationships indicate that rodingite formation in the Asbestos ophiolite, Québec, occurred in two, or possibly three, separate episodes during thrusting of the ophiolite onto the Laurentian margin, and that it involved three fluids. The first episode of rodingitization, which affected diorite, occurred at temperatures of between 290 and 360°C and pressures of 2.5 to 4.5 kbar, and the second episode, which affected granite and slate, occurred at temperatures of between 325 and 400°C and pressures less than 3 kbar. The fluids responsible for these episodes of alteration were moderately to strongly saline (~1.5 to 6.3 m eq. NaCl), rich in divalent cations and contained appreciable methane. A possible third episode of alteration is suggested by primary fluid inclusions in vesuvianite-rich bodies and secondary inclusions in other types of rodingite, with significantly lower trapping temperatures, salinity and methane content. The association of the aqueous fluids with hydrocarbon-rich fluids containing CH4 and higher order alkanes, but no CO2, suggests strongly that the former originated from the serpentinites. The similarities in the composition of the fluids in all rock types indicate that the ophiolite had already been thrust onto the slates when rodingitization occurred

Accretion, structure and hydrology of intermediate spreading-rate oceanic crust from drillhole experiments and seafloor observations

Downhole measurements recorded in the context of the Ocean Drilling Program in Hole 504B, the deepest hole drilled yet into the oceanic crust, are analyzed in terms of accretion processes of the upper oceanic crust at intermediate spreading-rate. The upper part of the crust is found to support the non steady-state models of crustal accretion developed from seafloor observations (Kappel and Ryan, 1986; Gente, 1987). The continuous and vertical nature of borehole measurements provides stratigraphic and structural data that cannot be obtained solely from seafloor studies and, in turn, these models define a framework to analyze the structural, hydrological, and mineralogical observations made in the hole over the past decade.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43190/1/11001_2005_Article_BF01204282.pd

Geochemistry on pryite concretions from the Romanche Trench

Pyrite concretions a few centimeters in diameter were recovered from within the east-west valley of the Romanche fracture zone (equatorial Atlantic). Unconsolidated sediments are scarce or absent in this area of the Romanche Valley. Some of the sulfide concretions were transformed into Fe hydroxides by the action of seawater. The major element, trace metal, rare earth element and sulfur isotopic chemistry of these concretions suggest that they were deposited from Fe- and S-bearing hydrothermal solutions at or beneath the seafloor. These findings support the hypothesis that oceanic fracture zones are the locus of metallogenesis. Heat flow patterns suggest deep sub-seafloor hydrothermal circulation in the highly fractured offset zones. Metals and sulfur can be extracted by the hydrothermal waters from rocks present beneath the fracture zone (basalt, gabbro and serpentinite) and can also be supplied by mantle volatile sources. Important metal deposits appear to be aligned along the predrift land extension of some oceanic fracture zones