NEW INSIGHTS INTO AUTHIGENIC AND HYDROTHERMAL MINERALIZATION PROCESSES OF THE GUAYMAS BASIN (GULF OF CALIFORNIA)

The Guaymas Basin is a unique geological setting where magmatic intrusions drive hydrothermal circulation and the alteration of a thick cover of organic-rich sediments. This originates complex physical, chemical and biological interactions that control mineralization processes under diverse redox and environmental conditions. The nature and characteristics of the authigenic-hydrothermal deposits of this basin remain largely unknown. In this study, authigenic fragments related to cold seeps and hydrothermal chimneys (white smokers) collected in Central Guaymas Basin during the Expedition AT37-06 have been studied combining petrographic and geochemical tools with the aim of shed light on the redox conditions and processes triggering mineralization. According to XRD analysis and SEM observations, authigenic fragments are characterized by a fine-grained matrix composed of opal nanospheres (opal-A). They are also composed of aragonite (87 wt%), barite (8–96 wt%), pyrrhotite (4 wt%) and framboidal pyrite (<1 wt%; 6–8 µm in diameter). Skeletal components comprise abundant diatoms and varying amounts of sponge spicules, planktonic foraminifera, gastropods, radiolarians and pellets. ICP-MS analyses reveal a gold content between 2 and 48 ppb. Calcite carbon and oxygen isotope composition (δ¹³C_carb and δ¹⁸O, V-PDB) ranges from -48 to -46.3 ‰ and from +3.5 to +4‰, respectively. These data suggest precipitation under oxygen-depleted conditions from normal marine water, mediated by anaerobic oxidation of methane and microbial sulfate reduction. On the other hand, the white smokers consist predominantly of calcite (50–96 wt%) and barite (1–22 wt%), and minor sulfides (pyrite, sphalerite and galena) and gold crystals (3–52 ppb). The δ¹³C_carb varies between -12.9 and -9.2 ‰ and the δ¹⁸O varies between -23.2 and -18.7 ‰. These data indicate precipitation with a minor participation of bacterial activity, from high temperature and/or ¹⁸O-depleted fluids, likely meteoric water or pore solutions after gas hydrate crystallization. These results will contribute to improving the knowledge of the environmental conditions and the complex chemical-microbial processes observed in active hydrothermal systems.