ACTIVE AND RELICT SEA-FLOOR HYDROTHERMAL MINERALIZATION AT THE TAG HYDROTHERMAL FIELD, MID-ATLANTIC RIDGE

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-degrees-N. The TAG field occupies an asymmetric area, at least 5 X 5 km, of the floor and wall of the rift valley between water depths of 2,300 and 4,000 m situated between 2 and 8 km east of an axial high along the center of the spreading segment. 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 200 m in diameter by 35 m high at a water depth of 3,670 m on the floor of the rift valley near the base of the east wall; (2) two former high-temperature vent areas known as the Mir zone and the Alvin zone containing multiple sulfide bodies that are undergoing deformation and mass wasting during uplift on fault blocks on the adjacent section of the lower east wall between water depths of 3,400 and 3,600 m; (3) a zone of low-temperature venting and precipitation of Fe and Mn oxide deposits farther from the axial high and higher on the east wall between water depths of 2,300 and 3,100 m. The active sulfide mound and the Mir and Alvin zones are situated near the margins of discrete volcanic centers. 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. Convective heat transfer from a central black smoker vent complex (365-degrees-C; calculated convective heat flux 225 +/- 25 X 10(6) W), white smoker vents (less-than-or-equal-to 300-degrees-C), and widespread diffuse flow predominate on the active sulfide mound. Values of conductive beat flow measured at the margins of the active mound and the Mir zone are similar and appear to vary inversely with distance from adjacent volcanic centers, supporting the inference that episodic intrusions at the centers have driven hydrothermal circulation at the hydrothermal zones. A low in magnetic intensity coincides with the entire TAG field. The low is modeled as the combined effect of alteration pipes beneath the high-temperature hydrothermal zones and thermal degmagnetization of a still hot but largely solid intrusion beneath the axial high. 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 X 10(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. Supergene reactions of older sulfides with seawater produces secondary gold enrichment. Preservation of the recrystallized sulfides is favored by silicification or an armoring of oxides.