Geochemical processes in an active shallow submarine hydrothermal system, Bahia Concepcion, Mexico: mixing or boiling?

摘要

Hydrothermal activity at Bahia Concepcion, on the western coast of the Gulf of California, is not linked to present volcanic activity. This site is a potential energy source;;however, geothermal modelling of the system is needed in order to determine the processes that generated this activity. Two processes might lead to the formation of the secondary fluids that were sampled in this submarine hydrothermal system: the thermal endmember might be undergoing either boiling or mixing with a fluid trapped within the sedimentary strata. However, boiling was ruled out as this process would not produce a highly saline fluid within the temperature range determined for the Bahia Concepcion system (~87°C). We modelled the mixing of a highly saline fluid with a geothermal fluid of meteoric origin. Our results suggest that the composition of the thermal water representing the hot endmember at Bahia Concepcion consists of 20-30% highly saline fluid and 70-80% aqueous fluid of meteoric origin. The computed fluid has a chemical composition similar to that determined for the submarine vents. Its derived contents of calcium, chloride, and silica are similar to those we measured for the vent discharge. Our hypothesis of saline water addition is supported by the mineralizing fluid characteristics determined from fluid inclusion microthermometric study of a fossil intertidal system that discharges in the vicinity of the vent areas. This intertidal spring fluid at ~40°C, according to the geochemical model, would be supersaturated in opaline silica and calcite, consistent with the mineralogy of the stromatolitic sinter encircling the discharges.