Dissecting an active hydrothermal deposit: the strontium and oxygen isotopic anatomy of the TAG hydrothermal mound - whole rock and silicate minerals

Abstract

Recent drilling during Leg 158 of the Ocean Drilling Program penetrated the Trans-Atlantic Geotraverse (TAG) Hydrothermal Mound and recovered a unique suite of samples that allow the description of the subsurface anatomy of an active hydrothermal deposit. Strontium- and oxygen-isotopic compositions of whole-rock samples and quartz separates, combined with petrographic observations, provide important constraints on the physical and chemical nature of fluid-rock interactions, and the compositions of fluids, during alteration of basalt and sulfide mineralization in the active TAG hydrothermal deposit. Fresh mid-ocean ridge basalt (MORB), with a 87Sr/86Sr of 0.7026, from the basement beneath of the TAG mound was altered at both low and high temperatures by seawater and altered at high temperature by near end-member hydrothermal fluids. Pillow breccias occurring beneath the margins of the hydrothermal mound are completely recrystallized to chlorite by interaction with large volumes of conductively heated seawater (>200°C). The development of a silicified, sulfide-mineralized stockwork within the basaltic basement follows a simple paragenetic sequence of initial chloritization by hot, Mg-bearing hydrothermal fluids, followed by mineralization by Mg-depleted black smoker-type fluids and the development of the paragonite + quartz + pyrite stockwork cut by quartz-pyrite veins. The initial high-temperature interaction and alteration in the TAG basement involved the development of chloritic alteration halos around the rims of basaltic clasts. The fluid responsible for this interaction was Mg-bearing and most probably a mixture of upwelling, high-temperature (>300°C), black smoker-type fluid with a minor (≪10%) proportion of seawater. Continued high-temperature (>300°C) interaction between the wallrock and initially Mg-bearing hydrothermal fluids results in the complete recrystallization of the wallrock to chlorite + quartz + pyrite. The hydrothermal assemblage of paragonite + quartz + pyrite overprints the chloritized basalts. This assemblage developed because of reaction at high temperatures (250°-360°C) with magnesium-depleted, hydrothermal fluids. End-member hydrothermal fluids had 87Sr/86Sr=0.7038, similar to present-day vent fluids. The uniformity of the 87Sr/86Sr ratios of hydrothermal assemblages throughout the mound and stockwork requires that the 87Sr/86Sr 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. The 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 and mixing with hydrothermal fluids. In contrast, the silicification and sulfide mineralization that dominate the stockwork and interior of the mound occurred by interaction of end-member black smoker fluids with basalt and early hydrothermally altered rocks, with limited conductive cooling. The dilution of hydrothermal fluids by mixing with seawater played only a minor role in the development of the main quartz + paragonite + pyrite stockwork.