Carbon dioxide (CO2) is sequestered through the weathering and subsequent mineralization of the chrysotile mine tailings at Clinton Creek, Yukon Territory, and Cassiar, British Columbia, Canada. Accelerated weathering is attributed to a dramatic increase in surface area, which occurs during the milling of ore. We provide a detailed account of the natural process of carbon trapping and storage as it occurs at Clinton Creek and Cassiar, including mineralogy, modes of occurrence, methods of formation for carbonate alteration, light stable isotope geochemistry, and radiocarbon analysis. Powder X-ray diffraction data were used to identify weathering products as the hydrated magnesium carbonate minerals nesquehonite, and less commonly lansfordite [MgCO3⋅5H2O]. Textural relationships suggest that carbonate precipitates formed in situ after milling and deposition of tailings. Samples of efflorescent nesquehonite are characterized by δ 13 C values between 6.52 and 14.36 per mil, δ 18 O values between 20.93 and 26.62 per mil, and F 14 C values (fraction of modern carbon) between 1.072 and 1.114, values which are consistent with temperature-dependent fractionation of modern atmospheric CO2 during mineralization. Samples of dypingite ± hydromagnesite collected from within 0.2 m of the tailings surface give δ 13 C values between -1.51 and +10.02 per mil, δ 18 O values between +17.53 and +28.40 per mil, and F 14 C values between 1.026 and 1.146, which suggests precipitation from modern atmospheric CO2 in a soil-like environment. Field observations and isotopic data suggest that hydrated magnesium carbonate minerals formed in two environments. Nesquehonite formed in an evaporative environment on the surface of tailings piles, and dypingite and hydromagnesite formed in the subsurface environment with characteristics similar to soil carbonate. In both cases, these minerals †

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39 potassium-argon age determinations from a progressively metamorphosed greywacke-schist sequence in southern New Zealand (prehnite-pumpellyite to greenschist facies), range from 199 to 154 Ma (early to late Jurassic). They show an inverse correlation of age with metamorphic grade. Concordant ages from varied lithologies at all metamorphic grades show that degassing of detrital potassium-bearing minerals was completed during metamorphism. The lowest grade metamorphic rocks, which yield the oldest ages, about 200Ma, have only just exceeded the argon closure temperature and hence their ages date closely their time of metamorphism in the earliest Jurassic. Thus it is shown that the Jurassic-Cretaceous, Rangitata Orogeny in New Zealand commenced earlier, is more complex, and of longer duration than previously supposed.The remaining ages, from pumpellyiteactinolite and lower greenschist rocks, range uniformly through the Jurassic and are interpreted as cooling ages. From these data, a long-term uplift rate of 0.23 mm/yr is calculated for the Jurassic-Cretaceous.

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