Detailed sulfur–isotope investigation of the TAG hydrothermal mound and stockwork zone, 26°N, Mid-Atlantic Ridge

Abstract

Drilling of the Trans-Atlantic Geotraverse (TAG) hydrothermal mound and underlying stockwork zone has revealed a complex internal stratigraphy consisting of, with increasing depth, massive pyrite and pyrite breccias, with significant chalcopyrite and sphalerite in places, pyrite-anhydrite breccias, pyrite-silica breccias, silicified wallrock breccias, and chloritized basalt breccias. Several stages of quartz pyrite ± chalcopyrite veins occur in the stockwork zone and lower portions of the mound whereas anhydrite ± pyrite ± chalcopyrite veins are common within the central and upper parts of the mound.A detailed sulfur-isotope investigation of sulfides and sulfates within mound and the underlying stockwork zone has revealed that the overall range of sulfide δ 34 S analyses is from 0.35‰ to 10.27‰, with a mean of 7.20‰. Anhydrite has a mean δ 34 S value of 21.10‰ within a tight range of 20.55‰-21.56‰.There are distinct differences in δ 34 S values between the different textural types of pyrite (massive sulfide, breccia clasts, disseminations associated with alteration, and veins) within the hydrothermal mound and stockwork zone. The massive sulfide and breccia clasts have a similar distribution of isotope values (δ 34 S = 6‰-8‰), however the δ 34 S values of the disseminated pyrite associated with the alteration are distinctly heavier (δ 34 S = 8‰-10‰). Vein sulfides have the lightest δ 34 S values (δ 34 S = 5‰-7‰) at TAG. The sulfur-isotope values measured at TAG are, in general, the heaviest reported for unsedimented mid-ocean ridge deposits.A sulfur-isotope model is proposed to account for the heavy δ 34 S signature (compared to other sediment-free hydrothermal systems), the distribution of δ 34 S values from the various textural styles and the spatial distribution of the δ 34 S values both laterally and vertically throughout the hydrothermal mound and underlying stockwork zone. The two initial sources of sulfur during the life of the TAG hydrothermal system are seawater sulfate (δ 34 S = 21‰) and mid-ocean ridge basalt (MORB) derived sulfur (δ 34 S = 0‰-1‰). Variations in δ 34 S values at TAG can be explained in a model where totally to partially reduced seawater sulfate of shallow origin mixes with a deep hydrothermal fluid dominated by MORB sulfur and interacts with previously formed sulfide and sulfate minerals in the upper parts of the stockwork zone and within the mound. Deep subseafloor processes cause the initial hydrothermal fluids entering the TAG system at depth to have a δ 34 S value of approximately 0‰-1‰. This fluid mixes with locally entrained partially reduced seawater in the upper parts of the subseafloor stockwork system (created by local hydrothermal convection though the porous and permeable mound and stockwork zone) and creates a modified hydrothermal fluid with a δ 34 S value of approximately 6‰−7‰. The fluid rises to the seafloor, precipitating pyrite in the quartz-pyrite veins (δ 34 S = 6‰−7‰) in the upper parts of the stockwork, and mixes with cold seawater, which causes rapid pr...

Subsurface processes at the lucky strike hydrothermal field, Mid-Atlantic ridge: evidence from sulfur, selenium, and iron isotopes

“…The present (2) Snake Pit (Kase et al, 1990), (3) Broken Spur (Duckworth et al, 1995), (4) TAG (Chiba et al, 1998;Gemmell and Sharpe, 1998;Knott et al, 1998), (5) Galapagos Rift (Skirrow and Coleman, 1982;, (6) EPR 21°N (Arnold and Sheppard, 1981;Styrt et al, 1981;Kerridge et al, 1983;Alt, 1988;Woodruff and Shanks, 1988), (7) EPR 11-13°N (Bluth and Ohmoto, 1988), (8) Juan de Fuca (Shanks and Seyfried, 1987).…”

Sulfur and lead isotopic compositions of massive sulfides from deep-sea hydrothermal systems: Implications for ore genesis and fluid circulation

“…3). A single value of 21.5‰ for the sulfate sample at the EHF falls within the overall range of sulfate samples from other seafloor hydrothermal fields (from 19 to 25‰; n = 288) (data are from Shanks and Niemitz, 1982;Kerridge et al, 1983;Zierenberg et al, 1984Zierenberg et al, , 1993Koski et al, 1985;Shanks and Seyfried, 1987;Hannington and Scott, 1988;Shanks, 1988, 1994;Kusakabe et al, 1990;Blum and Puchelt, 1991;Iizasa et al, 1992;Peter and Shanks, 1992;Lein et al, 1993;Chiba et al, 1998;Gemmell and Sharpe, 1998;Herzig et al, 1998a, b;Knott et al, 1998;Marumo and Hattori, 1999;Zeng et al, 2002a, b;de Ronde et al, 2003;Kim et al, 2004;. Sulfide minerals from the EPR near 1-2°S have much higher sulfur isotope ratios (pyrite δ 34 S values from 3.0 to 5.8‰, avg.…”

Sulfur isotope characteristics of the Permian VHMS deposits in Tasik Chini district, Central Belt of Peninsular Malaysia

“…Source of data: Permian VHMS deposits; Afterthought and Bully Hill, California-Gustin (1990), and Eastoe and Gustin (1996); Yanahara, Japan- Yamamoto et al (1968), and Kajiwara and Date (1971); Red Ledge, Idaho-Fifarek et al (1984), andFifarek (1985); Mount Chalmers, QueenslandHuston (1999), andHunns (2001); Permian seawater -Claypool et al (1980), and Kampschulte and Strauss (2004). Modern VHMS deposits; back-arc/arc-hosted deposits; Okinawa Trough, Japan- Halbach et al (1989); Manus Basin- Lein et al (1993); Mariana Trough- Kusakabe et al (1990);Brothers Volcano, Kermadec Tonga-de Ronde et al (2005); MORB-hosted deposits (unsedimented ridges); Southern Juan de Fuca Ridge (SJFR)- Shanks and Seyfried (1987); Galapagos Rift- Skirrow and Coleman (1982), and Knott et al (1995); Axial Seamount- Hannington and Scott (1988);Broken Spur-Duckworth et al (1995); Snakepit- Kase et al (1990);TAG-Herzig et al (1998), Chiba et al (1998), and Gemmell and Sharpe (1998; East Pacific Rise (EPR)-McConachy (1988), Bluth and Ohmoto (1988), Stuart et al (1994), Hekinian et al (1980), Arnold and Sheppard (1981), Styrt et al (1981), Kerridge et al (1983), Zierenberg et al (1984), Woodruff and Shanks (1988), and Marchig et al (1990); MORB-hosted deposits (sedimented ridges); Escanaba Trough- Koski et al (1988), Zierenberg et al (1993), and Böhlke and Shanks (1994); Guayamas Basin- Peter and Shanks (1992), and Shanks et...…”