MN-BA-HG MINERALIZATION RELATED TO THE SHALLOW SUBMARINE HYDROTHERMAL VENTS OF BAHÍA CONCEPCIÓN, BAJA CALIFORNIA SUR, MEXICO

Coastal gaseohydrothermal venting occurs along 500 m of a volcanic rocky shoreline along the western margin of the fault-bound bay of Bahía Concepción, Baja California Sur, Mexico. Diffuse hydrothermal discharge takes place in an area made conspicuous by continuous gas bubbling at shallow depth (5 to 15 m), and a group of hot springs are located within the intertidal zone. The temperature of the vent fluids varies from 72ºC to 82ºC. Mn-Ba-Hg mineralization occurs as a result of the submarine hydrothermal venting. X-ray diffraction analyses reveal the amorphous nature of the main components of the hydrothermal deposits. Millimeter-thick iron oxyhydroxide coatings precipitate covering volcanic cobbles and boulders around the diffuse vent area. These Fe-oxyhydroxide coatings are composed by poorly crystallized six-line ferrihydrite, accompanied by minor cinnabar and fine-layered colloform pyrite. In the intertidal hot springs moss-like manganese crusts, opal-A cemented aggregates and stromatolitic calcite occur, as well as a chalcedony-calcite banded vein that can be interpreted as a result of a previous stage of the hydrothermal system. Manganese crusts are made up of an X-ray amorphous manganese oxide, and represent a rare example of manganese deposit formed in a shallow submarine hydrothermal environment. Textural characteristics of the mineral assemblages suggest that many of the mineralization processes may be mediated by microbial activity. The vent precipitates display a significant enrichment of a suite of elements, including Hg, As, Sb. Arsenic is adsorbed onto Mn- and Fe- oxide substrates. The NASC normalized REE patterns of the vent precipitates show a pronounced positive Eu anomaly that is probably inherited from the hydrothermal fluid.