VENT FLUID CHEMISTRY IN BAHIA CONCEPCION COASTAL SUBMARINE HYDROTHERMAL SYSTEM, BAJA CALIFORNIA, MEXICO

Shallow submarine hydrothermal activity has been observed in Bahía Concepción along faults likely related to extensional tectonics of the Gulf of California region. Diffuse venting of hydrothermal fluids and gas occurs in the intertidal and shallow subtidal areas to depths of 15m. In the shallower hot springs, calcite is depositing as a result of the hydrothermal discharge. In the zone with diffuse venting iron oxides precipitate as millimeter-thick coatings over volcanic cobbles and boulders. Fluids are discharged at temperatures from 40 to more than 70ºC. Vent fluids and gas were collected along a NW-SE trending onshore-offshore fault at 5 m water depth, and at an intertidal hot spring. Samples of hydrothermal fluids and gas were obtained using a large funnel connected to a specially designed 1 litre cylindrical bottle equipped with two lines controlled by stop-cocks, which allowed gas and thermal fluids to completely displace seawater. Chemical analyses revealed that the vent fluids are more dilute than seawater and are enriched in Ca, As, Hg, Mn, Ba, I, Cs and Si. The beach hot spring also exhibited enrichment in the same elements and indicate that minor mixing with seawater occurs subtidally. The composition of the thermal end-member was calculated for Mg=0 using the chemistry data of the submarine samples. A linear regression of the concentration values vs. magnesium content was used to determine the concentration of the end-member thermal water. The results indicate that the thermal water is probably of meteoric origin and heated by the high local thermal gradient, similar to other coastal hydrothermal systems in Mexico at Punta Mita and Punta Banda.