CARBON MINERALIZATION IN MINE TAILINGS AND IMPLICATIONS FOR GEOSEQUESTRATION

Carbon dioxide (CO2) is trapped within magnesium carbonate minerals at four ultramafic-hosted mine sites in Canada and Australia. Carbon mineralization occurs by natural weathering of mine tailings at inactive mines and as a by-product of mineral processing and tailings deposition at active mines. Carbon isotope fingerprinting indicates that the dominant source of carbon is the atmosphere, although carbon from mine organic waste and mine bedrock waste is also mineralized. Carbon is fixed by the precipitation of the Mg-carbonate minerals nesquehonite, dypingite and hydromagnesite. Other Mg, Ca and Na carbonate mineral efflorescences are present, but only in minor abundance. Quantitative phase analysis with X-ray powder-diffraction data is used to determine the modal abundance of mineral hosts for trapped CO2 and to provide an estimate of the amount of CO2 fixed in tailings. Carbon fixation rates vary between mine sites from less than 1,000 tonnes per year to more that 50,000 tonnes per year. Fixation rate appears to depend strongly on local climate and on tailings handling practices. The rates of fixation are broadly consistent with predictions from batch-reactor geochemical models that (1) assume silicate mineral dissolution is rate limiting and (2) that use experimentally determined mineral dissolution rate laws. These same models are not consistent with observed rates of mineralization if field-based mineral dissolution kinetic rate laws are employed. Mineral dissolution rate laws derived from laboratory experiments therefore appear to be relevant to the reaction and weathering of man-made materials such as finely ground mine waste. Incorporation of these rate laws into geochemical models for geosequestration in ultramafic-hosted aquifers provides estimates of the rates of carbon mineralization in the subsurface that are significant but remain untested.