Strontium and oxygen isotopic constraints on fluid mixing, alteration and mineralization in the TAG hydrothermal deposit

Strontium- and oxygen-isotopic measurements of samples recovered from the Trans-Atlantic Geotraverse (TAG) hydrothermal mound during Leg 158 of the Ocean Drilling Program provide important constraints on the nature of fluid-rock interactions during basalt alteration and mineralization within an active hydrothermal deposit. Fresh Mid-Ocean Ridge Basalt (MORB), with a Sr-87/Sr-86 of 0.7026, from the basement beneath the TAG mound was altered at both low and high temperatures by seawater and altered at high temperature by near end-member black smoker fluids. Pillow breccias occurring beneath the margins of the mound are locally recrystallized to chlorite by interaction with large volumes of conductively heated seawater (> 200 degrees C). The development of a silicified, sulfide-mineralized stockwork within the basaltic basement follows a simple paragenetic sequence of chloritization followed by mineralization and the development of a quartz + pyrite + paragonite stockwork cut by quartz-pyrite veins. Initial alteration involved the development of chloritic alteration halos around basalt clasts by reaction with a Mg-bearing mixture of upwelling, high-temperature (> 300 degrees C), black smoker-type fluid with a minor (< 10%) proportion of seawater. Continued high-temperature (> 300 degrees C) interaction between the wallrock and these Mg-bearing fluids results in the complete recrystallization of the wallrock to chlorite + quartz + pyrite. The quartz + pyrite + paragonite assemblage replaces the chloritized basalts, and developed by reaction at 250-360 degrees C with end-member hydrothermal fluids having Sr-87/Sr-86 approximate to 0.7038, similar to present-day vent fluids. The uniformity of the Sr-87/Sr-86 ratios of hydrothermal assemblages throughout the mound and stockwork requires that the Sr-87/Sr-86 ratio of end-member hydrothermal fluids has remained relatively constant for a time period longer than that required to change the interior thermal structure and plumbing network of the mound and underlying stockwork. Precipitation of anhydrite in breccias and as late-stage veins throughout most of the mound and stockwork, down to at least 125 mbsf, records extensive entrainment of seawater into the hydrothermal deposit. Sr-87/Sr-86 ratios indicate that most of the anhydrite formed from approximate to 2:1 mixture of seawater and black smoker fluids (65% +/- 15% seawater). Oxygen-isotopic compositions imply that anhydrite precipitated at temperatures between 147 degrees C and 270 degrees C and require that seawater was conductively heated to between 100 degrees C and 180 degrees C before mixing and precipitation occurred. Anhydrite from the TAG mound has a Sr-Ca partition coefficient K-d = 0.60 +/- 0.28 (2 sigma). This value is in agreement with the range of experimentally determined partition coefficients (K-d approximate to 0.27-0.73) and is similar to those calculated for anhydrite from active black smoker chimneys from 21 degrees N on the East Pacific Rise. The delta(18)O(SO4) of TAG anhydrite brackets the value of seawater sulfate oxygen (approximate to 9.5 parts per thousand). Dissolution of anhydrite back into the oceans during episodes of hydrothermal quiescence provides a mechanism of buffering seawater sulfate oxygen to an isotopically light composition, in addition to the precipitation and dissolution of anhydrite within the oceanic basement during hydrothermal recharge at the mid-ocean ridges. (C) 1998 Elsevier Science B.V. All rights reserved.