The hydrated Mg-carbonate mineral, hydromagnesite [Mg 5 (CO 3) 4 (OH) 2 •4H 2 O], precipitates within mine tailings at the Mount Keith Nickel Mine, Western Australia as a direct result of mining operations. We have used quantitative mineralogical data and  13 C,  18 O and F 14 C isotopic data to quantify the amount of CO 2 fixation and identify carbon sources. Our radiocarbon results indicate that at least 80% of carbon in hydromagnesite is sourced from the modern atmosphere. Stable isotopic results indicate that dissolution of atmospheric CO 2 into mine tailings water is kinetically limited, which suggests that the passive rate of carbon mineralization could be accelerated. Reactive transport modeling is used to describe the observed variation in tailings mineralogy and to estimate rates of CO 2 fixation. Based on our assessment, approximately 39 800 t/yr of atmospheric CO 2 are being trapped and stored in tailings at Mount Keith. This represents an offsetting of approximately 11% of the mine's annual greenhouse gas emissions. Thus, passive sequestration via enhanced weathering of mineral waste can capture and store a significant amount of CO 2. Implementation of geoengineering strategies that accelerate the uptake of CO 2 into tailings water could further reduce or completely offset the net greenhouse gas emissions at Mount Keith and many other mines.

Dypingite, a hydrated Mg-carbonate mineral, was precipitated from high-pH, high salinity solutions to investigate controls on carbon fixation and to identify the isotopic characteristics of mineral sequestration in mine tailings. δ(13)C values of dissolved inorganic carbon content and synthetic dypingite are significantly more negative than those predicted for equilibrium exchange of CO(2) gas between the atmosphere and solution. The measured δ(13)C of aqueous carbonate species is consistent with a kinetic fractionation that results from a slow diffusion of atmospheric CO(2) into solution. During dypingite precipitation, dissolved inorganic carbon concentrations decrease and δ(13)C values become more negative, indicating that the rate of CO(2) uptake into solution was outpaced by the rate of carbon fixation within the precipitate. This implies that CO(2) gas uptake is rate-limiting to CO(2) fixation. δ(13)C of carbonate mineral precipitates in mine tailings and of DIC in mine process waters display similar (13)C-depletions that are inconsistent with equilibrium fractionation. Thus, the rate of carbon fixation in mine tailings may also be limited by supply of CO(2). Carbon sequestration could be accelerated by increasing the partial pressure of CO(2) in tailings ponds or by using chemicals that enhance the uptake of gaseous CO(2) into aqueous solution.

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The mineral waste from some mines has the capacity to trap and store CO(2) within secondary carbonate minerals via the process of silicate weathering. Nesquehonite [MgCO(3)·3H(2)O] forms by weathering of Mg-silicate minerals in kimberlitic mine tailings at the Diavik Diamond Mine, Northwest Territories, Canada. Less abundant Na- and Ca-carbonate minerals precipitate from sewage treatment effluent deposited in the tailings storage facility. Radiocarbon and stable carbon and oxygen isotopes are used to assess the ability of mine tailings to trap and store modern CO(2) within these minerals in the arid, subarctic climate at Diavik. Stable isotopic data cannot always uniquely identify the source of carbon stored within minerals in this setting; however, radiocarbon isotopic data provide a reliable quantitative estimate for sequestration of modern carbon. At least 89% of the carbon trapped within secondary carbonate minerals at Diavik is derived from a modern source, either by direct uptake of atmospheric CO(2) or indirect uptake though the biosphere. Silicate weathering at Diavik is trapping 102-114 g C/m(2)/y within nesquehonite, which corresponds to a 2 orders of magnitude increase over the background rate of CO(2) uptake predicted from arctic and subarctic river catchment data.