5. DETAILED SULFUR-ISOTOPE INVESTIGATION OF THE TAG HYDROTHERMAL MOUND AND STOCKWORK ZONE, 26°N, MID-ATLANTIC RIDGE1J. Bruce Gemmell2 and Robina Sharpe2 |
ABSTRACTDrilling 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 delta34S analyses is from 0.35‰ to 10.27‰, with a mean of 7.20‰. Anhydrite has a mean delta34S value of 21.10‰ within a tight range of 20.55‰–21.56‰.There are distinct differences in d34S 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 (delta34S = 6‰–8‰), however the delta34S values of the disseminated pyrite associated with the alteration are distinctly heavier (delta34S = 8‰–10‰). Vein sulfides have the lightest delta34S values (delta34S = 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 delta34S signature (compared to other sediment-free hydrothermal systems), the distribution of delta34S values from the various textural styles and the spatial distribution of the delta34S 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 (delta34S = 21‰) and mid-ocean ridge basalt (MORB) derived sulfur (delta34S = 0‰–1‰). Variations in d34S 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 delta34S 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 d34S value of approximately 6‰-7‰. The fluid rises to the seafloor, precipitating pyrite in the quartz-pyrite veins (delta34S = 6‰-7‰) in the upper parts of the stockwork, and mixes with cold seawater, which causes rapid precipitation of the massive pyrite ± chalcopyrite (delta34S = 6‰-8‰) found at the top of the mound. Anhydrite (delta34S = 20‰-21‰) is formed from the heating of entrained seawater circulating within the massive sulfide or upper parts of the stockwork zone. During periods of little or no high-temperature hydrothermal upflow, thermal collapse is accompanied by infiltration of cold seawater through the TAG mound and upper parts of the stockwork. Anhydrite that was within the mound and overlying chimneys dissolves creating clasts and fragments of pyrite ± chalcopyrite ± sphalerite within and on top of the mound. When the hydrothermal system resumes high-temperature upflow, similar processes as those described above in the initial high-temperature phase take place, precipitating massive pyrite ± chalcopyrite massive sulfide (delta34S = 6‰-8‰) present at the top of the mound and the pyrite and quartz veins in the stockwork zone (vein Stages 2-4) and anhydrite ± pyrite ± chalcopyrite veins (Stage 5) within the mound (delta34S = 5‰-7‰). Anhydrite in these veins, primarily Stage 5, has delta34S values of 21‰, which reflects rapid heating of entrained seawater that is infiltrating the mound. Hydrothermal fluid leaks out from the veins though the porous and permeable mound and reacts with previously formed sulfides and sulfates causing the hydrothermal fluid to become slightly enriched in delta34S. This fluid is responsible for precipitation of disseminated pyrite, the heaviest sulfur-isotope values in the TAG system (delta34S = 8‰-10‰), associated with alteration in the stockwork zone and lower portions of the mound. |
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