Active and relict sea-floor hydrothermal mineralization at the TAG hydrothermal field, Mid-Atlantic Ridge

TLDR

The TAG hydrothermal field on the Mid‑Atlantic Ridge at 26°N is a major site of both active and relict volcanic‑hosted hydrothermal mineralization, with its axial high and fault‑intersection volcanic centers marking the principal loci of present magmatic intrusions. The field comprises an actively venting high‑temperature sulfide mound, two former high‑temperature vent areas, and a low‑temperature Fe/Mn oxide venting zone, with fault intersections serving as conduits for magmatic intrusions and hydrothermal upwelling. Radiometric dating reveals at least 140,000 years of episodic hydrothermal activity driven by intrusions, with sulfide deposits formed by long residence times near axial heat sources and mass wasting, and high‑temperature reworking of a relict zone recrystallizing sulfides and concentrating the first visible primary gold at an oceanic ridge.

Abstract

The TAG hydrothermal field is a site of major active and inactive volcanic-hosted hydrothermal mineralization in the rift valley of the slow-spreading Mid-Atlantic Ridge at 26[degree]N. The axial high is the principal locus of present magmatic intrusions. The TAG field contains three main areas of present and past hydrothermal activity: (1) an actively venting high-temperature sulfide mound; (2) two former high-temperature vent areas; (3) a zone of low-temperature venting and precipitation of Fe and Mn oxide deposits. The volcanic centers occur at the intersections between ridge axis-parallel normal faults and projected axis-transverse transfer faults. The intersections of these active fault systems may act as conduits both for magmatic intrusions from sources beneath the axial high that build the volcanic centers and for hydrothermal upwelling that taps the heat sources. Radiometric dating of sulfide samples and manganese crusts in the hydrothermal zones and dating of sediments intercalated with pillow lava flows in the volcanic center adjacent to the active sulfide mound indicate multiple episodes of hydrothermal activity throughout the field driven by heat supplied by episodic intrusions over a period of at least 140 [times] 10[sup 3] yr. The sulfide deposits are built by juxtaposition and superposition during relatively long residence times near episodic axial heat sources counterbalanced by mass wasting in the tectonically active rift valley of the slow-spreading oceanic ridge. Hydrothermal reworking of a relict hydrothermal zone by high-temperature hydrothermal episodes has recrystallized sulfides and concentrated the first visible primary gold reported in a deposit at an oceanic ridge.