Mn–Ba–Hg mineralization at shallow submarine hydrothermal vents in Bahía Concepción, Baja California Sur, Mexico

Coastal submarine hydrothermal venting occurs on the west shore of the fault-bounded bay of Bahia Concepcion, along a stretch of about 700 m of rocky shoreline. Diffuse hydrothermal seepage of water and continuous gas bubbling (mainly CO2 and N2) take place through the sediment-covered seafloor at shallow depth (5 to 15 m). In addition, at about 500 m SE, a cluster of hot springs occurs within the intertidal zone. The temperature and pH of the hydrothermal fluids varies from 72 to 87 °C and 5.95 to 6.02 in the submarine diffuse venting area and reaches 62 °C and pH of 6.68 in the intertidal hot springs. Mn–Ba–Hg mineralization is presently forming as a result of the hydrothermal venting. At the intertidal hot springs, moss-like crusts of manganese oxides and structureless detrital aggregates cemented by opal-A, barite and calcite occur around the main discharge conduits. These aggregates are overlaid by silica-carbonate fine-layered stromatolitic aggregates that extend some meters from the hot springs forming crusts and coalescing rims over volcanic pebbles and boulders. Manganese crusts are made up of barium rich, X-ray-amorphous todorokite-like and romanechite phases, and represent a rare example of a manganese deposit formed in a shallow submarine hydrothermal environment. In the submarine diffuse venting area the hydrothermal precipitates are less conspicuous and consist in millimeter-thick iron oxyhydroxide coatings on volcanic cobbles and boulders. These coatings are composed of poorly crystallized six-line ferrihydrite, accompanied by minor cinnabar and fine-grained colloform pyrite. Textures and fabrics of the mineral assemblages suggest microbial mediation for mineral deposition. The vent precipitates display a significant enrichment of Hg, As and Sb. Arsenic is probably adsorbed onto Mn- and Fe-oxide substrates. The NASC (North American Shale Composite)-normalized REE patterns of the vent precipitates show a pronounced positive Eu anomaly that is probably inherited from the hydrothermal fluid.