Mineral traps for greenhouse gases in mine tailings : a protocol for verifying and quantifying CO₂ sequestration in ultramafic mines

Mineralization of CO₂ in ultramafic mine tailings can occur on a scale that is significant relative to the greenhouse gas emissions of a mine. Consequently, some active mining operations may be able to take advantage of carbon mineralization within their tailings to offset part of their greenhouse gas emissions. The secondary Mg-carbonate minerals that form in mine tailings are safe and durable traps for carbon and their presence can represent substantial disposal of atmospheric CO₂. Hydrated Mg-carbonate minerals precipitate within mine tailings from the Diavik Diamond Mine, Northwest Territories, Canada, and the Mount Keith Nickel Mine, Western Australia, Australia. An improved understanding of the carbon cycle in mine tailings, and the contribution of mineralogical and geochemical strategies for assessing carbon mineralization in ultramafic mine tailings, are achieved by studying these sites. Quantitative mineralogical procedures, which use X-ray powder diffraction data, are developed for quantifying low abundances of mineral traps for CO₂ within mine tailings. Quantitative mineralogical results are used to assess the amount of CO₂ stored within hydrated Mg-carbonate minerals at both mine sites, and to assist in determining which gangue minerals are the primary sources for Mg in these minerals. Radiocarbon and stable isotopes of carbon and oxygen are used to identify the sources for carbon in secondary Mg-carbonate minerals. Isotopic analogue experiments are used to study the fractionation of stable carbon isotopes during precipitation of dypingite, a hydrated Mg-carbonate mineral, under conditions that simulate those in the tailings storage facilities at Mount Keith. The results of these experiments suggest that hydrated Mg-carbonate minerals may be precipitating out of isotopic equilibrium with the atmosphere. A carbon isotopic fractionation factor obtained for dypingite, and computational models for isotopic mixing scenarios, are used to interpret stable isotope and radiocarbon data for carbonate minerals. Although models for mixing scenarios can provide convincing fits to stable isotopic data, they are commonly inconsistent with field observations, trends in quantitative mineralogical data, and radiocarbon results. Ultimately, radiocarbon data are used to determine that most of the carbon trapped and stored within hydrated Mg-carbonate minerals at Diavik and Mount Keith is sourced from the modern atmosphere.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat