Cobalt distribution in the Boleo stratiform sediment-hosted ore deposit: a transtension rift basin in the central Baja California peninsula, Mexico

Abstract. The Miocene Santa Rosalia Basin, located in Baja California, Mexico, is a unique basin that contains the Boleo copper deposit. This basin provides a remarkable opportunity to study the early stages of tectonic evolution, volcanic activity, ore genesis, and sedimentary deposition in the incipient Gulf of California. Previous research suggested the presence of Co in areas between Cu and Zn, but the factors responsible for cobalt distribution in the basin remained unclear. This study utilized multiple methods, including historical geologic maps, core drilling, field measurements, XRF readings, petrographic analysis, and mineral composition analysis, to comprehend the Co mineralization of the Boleo Formation. Our research found Co anomalies in five localities, with the highest values situated at the base of a slump block in manto 3 near the Neptuno mine. The presence of elevated cobalt readings in areas with uplifted and eroded gypsum suggests the possibility of uplift and dissolution of earlier gypsum deposits, and an association with brine waters. Moreover, the result of the study indicates that gypsum interfingers with both the clastic facies and ore mineralization in mantos 1, 2, and 3, signifying a north-to-south expansion of the basin. The study also identified manganese oxides and Cu-Fe sulfides, including villamaninite, along laminated gypsum beds. The findings of this study suggest that cobalt enrichment is influenced by structural factors and occurs near subsurface evaporitic deposits. This supports the notion that cobalt mineralization forms near the redox front and through sulfate reduction processes. The study's findings offer valuable insights for further exploration and understanding of cobalt mineralization in similar geological settings.