CONTROLS ON COBALT METAL DISTRIBUTION IN AN EVAPORITIC BASIN: AN EXAMPLE FROM THE BOLEO DEPOSITS OF BAJA CALIFORNIA PENINSULA OF MEXICO

Cobalt demand is likely to exceed production in the future due to the projected increased use of Co in electric vehicles. Cobalt metal is usually extracted as a byproduct of Cu or Ni processing, although the transport and depositional conditions of cobalt minerals likely vary from those controlling the distribution of copper minerals in the same deposit. The Boleo deposit in the Baja California Peninsula is the largest Cu-Co deposit in Mexico, but little is known about the metal zonation of cobalt except that Co is concentrated in areas between zones of Cu and Zn enrichment. To understand the distribution of cobalt within the Boleo deposit, elemental geochemical data from recent mining exploration drillholes and mapping by the USGS in the 1950’s were georeferenced in GIS and modeled in Leapfrog software to create a 3D map of Co metal distribution. In addition, stratigraphic sections of gypsum outcrops from the Upper Miocene Boleo Formation were measured, described, and sampled for XRD and SEM analyses. Cobalt values from drillhole data show a distinctive ore enrichment within and in proximity to the evaporitic units and along the main faulting of the Boleo region. Trace mineralogy of the evaporites include strontianite, tenorite and Fe, Mn and Cu-Co oxides, and fine-grained sulfide grains, including a Cu-Fe sulfide, villamaninite ((Cu,Fe,Co,Zn)S₂), and an unidentified Co-S phase with minor Cu and Fe. Cobalt-enrichment values located in proximity to the evaporite unit and main faulting suggests an influx of metal-bearing fluids into the evaporitic environment with periodic reducing conditions. The evaporitic basin created the suitable environment as a chloride-rich metal transporter and supplier of sulfur via bacterial and thermochemical sulfate reduction when these fluids interacted with mineral-rich brine fluid flows. Finally, Co-enriched metal distribution suggests that the ore deposit is controlled by the basin hydrological conductivity and attributable to the physicochemical properties and presence of the evaporites beds.