ALBERT

Description: The active Trans-Atlantic Geotraverse (TAG) hydrothermal mound is a mature submarine massive sulfide deposit at the slow-spreading Mid-Atlantic Ridge at 26°N. Fluid inclusion measurements were conducted on quartz and anhydrite from six boreholes drilled in different areas of the mound to characterize the fluids responsible for the deposition of sulfide-silica breccias and anhydrite and to investigate the vertical and horizontal temperature zonation within an actively forming hydrothermal system. Fluid inclusions in both host minerals are generally two phase liquid/vapor inclusions that homogenize into the liquid phase. Trapping temperatures for quartz and anhydrite from the TAG mound range from 212° to 390°C. Salinities vary from 1.9 to 6.2 wt% NaCl equivalent for anhydrite and from 4.0 to 6.0 wt% NaCl equivalent for quartz. This salinity variation is probably best explained by supercritical phase separation at temperatures above 450°C with subsequent remixing of the liquid and the vapor phase during ascent. A zone of anhydrite-rich precipitates recovered at 20 to 35 mbsf below the central Black Smoker Complex (TAG-1) is characterized by trapping temperatures averaging 348°C for anhydrite and 358°C for quartz, which is slightly below the exit temperature of hydrothermal fluids presently venting at the Black Smoker Complex (360°-366°C). Breccias in the stockwork zone underlying the anhydrite zone were formed at slightly higher temperatures ranging from 327°- 381°C for quartz and from 349° to 384°C for anhydrite. Trapping temperatures vary strongly between different areas of the mound. Fluid inclusions in quartz and anhydrite from the central Black Smoker Complex are characterized by a narrow range of trapping temperatures, whereas other areas drilled on the mound were influenced by lower temperature hydrothermal fluids percolating through the mound or by local entrainment of seawater into the mound. White smokers venting on the southeastern side of the TAG mound are characterized by exit temperatures of 270°-300°C, (Kremlin area, TAG-2). Fluid inclusion measurements in quartz and anhydrite from this area give trapping temperatures in the range of 266°-375°C with a distinct peak around 340°C, only somewhat lower than results for the Black Smoker Complex. Trapping temperatures in anhydrite-hosted fluid inclusions in this area show a strong vertical temperature increase. The west side of the mound (TAG-4) is characterized by trapping temperatures ranging from 212° to 390°C showing evidence for seawater entrainment or overprinting by lower temperature hydrothermal events at the sulfide/basalt interface. Samples from the northern side of the mound (TAG-5) exhibit trapping temperatures in the range from 258°-383°C with a strong vertical temperature increase, indicating additional hightemperature upflow at the northern margin of the mound outside the central Black Smoker Complex.

Description: Drilling of the active Trans-Atlantic Geotraverse (TAG) deposit indicates that the size of the mound-stockwork complex is approximately 3.9 million t, including 2.7 million t of massive and semi-massive sulfide (~2% Cu) at the seafloor and 1.2 million t of mineralized breccias (~1% Cu) in a subseafloor stockwork. Quartz-pyrite veining in the stockwork zone extends from about 40 meters below seafloor (mbsf) to a depth of 95 mbsf. Siliceous wallrock breccias in the lower part of the stockwork grade abruptly into chloritized basalt breccias at the margins of the mineralized zone, and massive sulfides at the flanks of the deposit onlap relatively unaltered, partially hematized basalts. The pipe-like dimensions of the stockwork zone do not exceed the diameter of the sulfide mound. Comparisons with samples collected during earlier dive series confirm that the vent complexes at the surface of the mound are not representative of the bulk composition of the deposit. Steep vertical metal zonation within the mound suggests that a long history of hydrothermal reworking has effectively stripped the constituents that are soluble at lower temperatures from the massive sulfides and concentrated them at the top of the deposit through a process of zone refining. The bulk of the mound is composed of massive pyrite and anhydrite-cemented breccias. The massive anhydrite (~165,000 t) occupies a high-temperature zone, immediately beneath the central Black Smoker Complex and above the quartz-rich stockwork. Fracturing in the underlying quartz-pyrite stockwork also has resulted in anhydrite veining at considerable depths in the stockwork zone. Despite the abundance of anhydrite in the mound, the amount of seawater penetrating the region of hightemperature upflow is small in comparison to the total mass flux of hydrothermal fluid. The anhydrite has been deposited by conductive heating of a small amount of entrained seawater at the margins of high-temperature conduits, and little or no mixing has occurred with the end-member fluids. Collapse of the anhydrite-supported portion of the mound following major episodes of hydrothermal upflow has caused extensive in situ brecciation of the mound and is an important mechanism for the formation of “breccia ores” in the deposit. Although anhydrite is not well preserved in the geologic record, given its retrograde solubility, it has likely played an important role in the development of similar ore types in ancient massive sulfides. The morphology, size, and bulk composition of the TAG mound-stockwork complex is identical to that of some of the largest Cyprus-type massive sulfide deposits in the Troodos ophiolite. Typical Cyprus-type deposits comprise massive brecciated pyrite ores, underlain by a vertically extensive quartz-pyrite-chlorite stockwork. Sandy pyrite or conglomeratic ore, similar to that found in the TAG mound, is characteristic of the upper parts of Cyprus-type deposits. Textures in these ores, previously attributed to seafloor weathering and erosion, are most likely the result of anhydrite dissolution. Massive, granular pyrite (hard, compact ore), with abundant vuggy cavities lined by idiomorphic pyrite and quartz, occur below the conglomeratic ores and closely resemble sections of massive pyrite and pyrite-silica breccias from the TAG mound. At TAG, seafloor oxidation of the sulfides is currently taking place, even as the deposit is forming. Fe-oxide gossans have developed at the surface of the mound as a result of weathering of chimney debris. These deposits are modern analogs of the extensive ochers that typically overlie the massive sulfide deposits in Cyprus. By analogy with TAG, a number of the weathering features of Cyprus-type deposits (e.g., red clays, leached lavas), previously thought to be products of acid alteration by meteoric groundwaters, may have formed while the deposits were still on the seafloor. Low-temperature venting through this material has locally produced distinctive red cherts (silicified Fe oxides). This material is common within the mound and in the underlying basalts and closely resembles the red jaspers found throughout the pillow lava sections in Cyprus. Silicification in the upper part of the TAG mound also has produced a cherty, sulfide carapace at the top of the deposit that inhibits further degradation of the mound by seafloor weathering. This may have important implications for the long-term preservation of the deposit, although dissection of the mound along active fault scarps may eventually expose its interior to seafloor oxidation. An estimated growth rate for the TAG deposit, based on a total accumulation of 2.7 million t of massive sulfides and a cumulative venting history of 5 to 10 k.y., is between 500 and 1,000 t per yr. This is consistent with observed growth rates for the central Black Smoker Complex and with estimates of mass fluxes from heat and fluid flow at black smoker vents on the East Pacific Rise. Although TAG is among the largest of the known mid-ocean ridge deposits, grade-tonnage models for Cyprus-type massive sulfides world-wide suggest that much larger deposits are likely forming elsewhere on the mid-ocean ridges and at similar, slow-spreading centers in extensional back-arc basins.

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.