VENT FLUID COMPOSITION IN BAHÍA CONCEPCIÓN COASTAL SUBMARINE HYDROTHERMAL SYSTEM, BAJA CALIFORNIA SUR, MEXICO

Shallow submarine hydrothermal activity has been observed in Bahía Concepción, at the Gulf coast of the Baja California Peninsula, along faults probably related to the extensional tectonics of the Gulf of California region. The bay surroundings are mostly made up of volcanic rocks (andesite lavas, breccias and basalt flows) of the Oligocene-Miocene Comondú Group, which unconformably overlies a Cretaceous granitic and schistose basement. Diffuse and focused venting of hydrothermal water and gas occurs in the intertidal and shallow subtidal areas down to depths of 15m along a NW-SE trending onshore-offshore fault. Fluids are being discharged at temperatures from 40 to more than 70ºC. Vent water was collected from two vent sites located, one at 5 m water depth and the other within the intertidal zone. Samples of hydrothermal water were obtained using a large funnel connected to a specially designed 1 liter cylindrical bottle equipped with two lines controlled by stop-cocks. Chemical analyses revealed that the thermal water is more dilute than seawater and is enriched in Ca, As, Hg, Mn, Ba, I, Cs, Fe and Si. Mixing of thermal fluids with seawater is more important in the 5-metres deep vents than in the intertidal hot springs. The composition of the thermal end-member was calculated for Mg=0 using the chemistry data of the submarine samples. Stable isotopic oxygen and hydrogen analyses show that both intertidal and 5-metres deep submarine hydrothermal fluids are very close to the local meteoric water. However, the different degree of mixing with seawater originates significant H and O isotopic variations between the two studied sites. Thus, the thermal water is of meteoric origin that is heated by the high local thermal gradient, similarly to other coastal hydrothermal systems reported in Western Mexico.