Frank, Mark R.
M.S. (Master of Science)
Department of Earth, Atmosphere and Environment
Cobalt mineralization has been documented in low temperature Mississippi Valley-type (MVT) deposits in southeast Missouri. Lead and zinc are the most economically important metals in these deposits and are contained within galena (PbS) and sphalerite (ZnS), respectively. Cobalt is also mined in some of the deposits with cobalt-bearing ores, e.g., siegenite ((Ni,Co)3S4) and fletcherite (Cu,(Ni,Co)2S4), occurring stratigraphically below main-stage galena and sphalerite. The hydrothermal fluids responsible for cobalt mineralization are thought to be chemically distinct from the predominant MVT ore-bearing fluids and represent a distinct mineralizing event. Cobalt is likely transported as CoCl2 in hydrothermal fluids and sourced from structures related to the Reelfoot Rift. The extent and potential of cobalt enrichment and mineralization within southeast Missouri MVT deposits is largely unknown.
Experiments were conducted to explore a cobalt-bearing portion of the dolomite-hydrothermal fluid system by using PARR 4746 acid digestion vessels at 127, 177, and 227 °C. The pressures of the experiments corresponded to that of saturated water-vapor pressure in an NaCl-H2O solution. Solid dolomite was equilibrated with experimental hydrothermal fluids of differing cobalt compositions to evaluate cobalt concentration in dolomite as a function of temperature and cobalt concentration of the fluid at equilibrium. The experimental fluids contained NaCl and variable concentrations of CoCl2 (100, 500, 1000, or 10000 μg/g cobalt) to maintain a total salinity of 16 wt.% NaCleq. A total of 42 experiments were completed. Solid run products were analyzed using SEM-EDS and XRD to identify new phases that may have formed during the experiment. Solid and fluid run products were collected at the conclusion of each experiment and elemental concentrations of cobalt, calcium, and magnesium in these products were determined by using XRF. Compositional data were used to calculate partition coefficients and equilibrium constants that can be used to model the behavior of cobalt in low-temperature hydrothermal fluids.
Experiments conducted at 127 °C had average cobalt concentrations within dolomite run products of 19.5(±0.2)x102 μg/g, 83.4(±1.0)x102 μg/g, 184.4(±5.4)x102 μg/g, and 178.5(±0.8)x102 μg/g for starting fluids with 100, 500, 1000, and 10000 μg/g cobalt, respectively. Experiments conducted at 177 °C had average cobalt concentrations of 9.5(±0.1)x102 μg/g, 46.6(±0.2)x102 μg/g, 90.7(±0.7)x102 μg/g, and 213.6(±1.3)x102 μg/g over the same starting fluid cobalt compositions. Partition coefficients for experiments with starting fluid compositions of 100, 500, and 1000 μg/g cobalt averaged 4.69(±0.11) for 127 °C experiments and 2.15(±0.02) for 177 °C experiments. These data demonstrate that cobalt concentrations within dolomite increase as a function of increasing cobalt concentration of the hydrothermal fluid and that cobalt preferentially partitioned into the solid dolomite phase. Experiments conducted with fluid compositions of 1000 and 10000 μg/g cobalt produced dolomite crystals with a maximum of 3.9 wt.% cobalt and contained a newly formed cobalt-bearing carbonate phase, spherocobaltite. These data indicate a field of immiscibility exists between spherocobaltite and Co-bearing dolomite that can be used to trace subsurface hydrothermal fluids with high concentrations of cobalt.
Coursey, Seth Kurtis, "Cobalt Partitioning and Solubility in Low-Temperature Carbonate-Hydrothermal Systems" (2020). Graduate Research Theses & Dissertations. 6948.
Northern Illinois University
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