Geochemical processes and vent fluid origin in the Punta Mita coastal submarine hydrothermal system

The Punta Mita shallow hydrothermal vents, located on the central Pacific coast of Mexico, present characteristics distinctive of different fluid venting geological scenarios. For that reason, an integration of geochemical and mineralogical data is needed to explain their occurrence. The Punta Mita vents could be classified as hydrothermal vents based on the temperature and chemistry of the water; however, the gases discharged by the vents are more typical of cold seeps as methane and nitrogen are the predominant gases. Trace element concentrations match those observed in deep submarine hydrothermal vents; however, isotopic composition indicates that the water has largely a meteoric origin and mixes with seawater in a later process, just before to be discharged. Chemical and isotopic analyses of vent fluids also suggest that the vent water interacts with the local sedimentary and basaltic rocks, reaching equilibrium at approximately 150°C.
Minerals deposited directly from the vent fluid, forming small hydrothermal mounds, include calcite and pyrite, with subordinate barite, cinnabar and carlinite. Calcite also forms from seawater carbonate, producing micritic cement. Stable isotopes in calcite and pyrite points to a microbial oxidation of methane coupled with seawater sulfate reduction. These results suggest that microbially-mediated processes based on anaerobic methane oxidation prevail in the submarine hydrothermal environment in Punta Mita.
The study of suspended particulate matter from vent discharges in Punta Mita also provides information about the mineralizing processes. Thus, the morphology of particulate pyrite crystals suggests large variations in the physicochemical conditions in short periods of time. Mixing with seawater takes place within the unconsolidated sediments, where a reducing local environment, with CH4 saturation, favours sulphide deposition.