DEVELOPMENT OF A COBALT EXPLORATION TOOL FOR DOLOMITE-BEARING HYDROTHERMAL ORE DEPOSITS

Cobalt (Co) is critical for carbon-free energy usage and, thus, Co production has increased by 400% over the last 20 years. Close to 80% of Co deposits are classified as hydrothermal deposits and associated with carbonate minerals. The southeast Missouri MVT district has been identified as one such system with Co commonly associated with deeper Cu-Ni rich zones. Dolomite, CaMg(CO₃)₂, is a common carbonate mineral in these systems and is often spatially and temporally related to ore deposition. Saline hydrothermal fluids are hypothesized to transport significant quantities of Co and precipitate Co-bearing sulfide or carbonate minerals such as siegenite or spherocobaltite, respectively. The dearth of data on Co in hydrothermal fluid-carbonate systems, and specifically within dolomite, hampers exploration efforts. To elucidate this relationship, experiments (n=42) were performed with dolomite and a hydrothermal fluid with Co compositions of 100, 500, 1000, or 10000 μg/g and a total salinity fixed to 16 wt.% NaCl_eq at 127, 177, and 227 °C. At the conclusion of each experiment, SEM-EDS was used for phase mapping and XRF for the concentration of Co in dolomite. XRF and ICP-MS were used to determine the composition of the quenched fluids. These data demonstrate that Co partitions into dolomite relative to the fluid and that Co concentrations within dolomite increase directly with the Co concentration of the hydrothermal fluid. Experiments conducted with fluid compositions of 1000 and 10000 μg/g Co produced Co-rich dolomite crystals, ~1 wt.% Co, and a Co-bearing carbonate phase, spherocobaltite. These data indicate an immiscibility field exists between spherocobaltite and Co-bearing dolomite, a trend noted for the Co-rich deposits of the Democratic Republic of the Congo (DRC). Thus, the Co concentration of dolomite can be used to estimate the Co concentration of ancient fluids and be used to vector exploration efforts. The presence of spherocobaltite can also help trace subsurface hydrothermal fluids with high concentrations of Co.