Power, Ian M

Mineral feedstocks, including alkaline mine wastes, can sequester CO2 as a dissolved phase (e.g., HCO3-) or a solid carbonate via enhanced rock weathering (ERW). For this thesis, the release of easily accessible Ca and Mg from non-carbonate sources was determined for kimberlite residues from several diamond mines and commonly proposed ERW rock types, including wollastonite and olivine. Batch leaches determined the CO2 sequestration potentials of kimberlites to be in the range of 3–12 kg CO2/t, which was exceeded by most ERW feedstocks. Leaches also assessed the release of Ni and Cr, elements of concern in ERW settings, and P and K, which benefit agricultural soils. Year-long leaching columns were deployed using kimberlite from the Gahcho Kué and Venetia diamond mines, wollastonite skarn, and olivine sand from the initial assessment. The kimberlite residues sequestered 0.03 kg CO2/t as dissolved inorganic carbon and 0.6 kg CO2/t as solid total inorganic carbon. Weathering of wollastonite skarn resulted in CO2 removal rates via mineral trapping of CO2 of 6.31 kg CO2/t, while the olivine sand yielded rates of 0.5 kg CO2/t via solubility trapping. Both methodologies used in this study demonstrated value in the prediction and monitoring of drainage chemistry as it relates to ERW and CO2 mineralization. Implementation of these strategies can progress ERW efforts by providing confidence in feedstock selection and the verification of carbon offsets.

Author Keywords: CO2 mineralization, Drainage chemistry, Enhanced weathering, Mine wastes, Mineral trapping, Solubility trapping

Mafic and ultramafic mine wastes have the potential to sequester atmospheric carbon dioxide (CO2) through enhanced weathering and CO2 mineralization. In this study, kimberlite residues from South African diamond mines were investigated to understand how weathering of these wastes leads to the formation of secondary carbonate minerals, a stable sink for CO2. Residues from Venetia Diamond Mine were fine-grained with high surface areas, and contained major abundances of lizardite, diopside, and clinochlore providing a maximum CO2 sequestration capacity of 3–6% of the mines emissions. Experiments utilized flux chambers to measure CO2 drawdown within residues and unweathered kimberlite exhibited greater negative fluxes (-790 g CO2/m2/year) compared to residues previously exposed to process waters (-190 g CO2/m2/year). Long-term weathering of kimberlite residues was explored using automated wet-dry cycles (4/day) over one year. Increases in the δ13C and δ18O values of carbonate minerals and unchanged amount of inorganic carbon indicate CO2 cycling as opposed to a net increase in carbon. Kimberlite collected at Voorspoed Diamond Mine contained twice as much carbonate in yellow ground (weathered) compared to blue ground, demonstrating the ability of kimberlite to store CO2 through prolonged weathering. This research is contributing towards the utilization of kimberlite residues and waste rock for CO2 sequestration.

Author Keywords: CO2 fluxes, CO2 mineralization, CO2 sequestration, Enhanced weathering, Kimberlite, Passive carbonation