Evidence for passive mineral carbonation from carbon isotope geochemistry of interstitial air in mine wastes from the Dumont Nickel Project (Abitibi, Quebec).

Natural weathering of ultramafic rocks in mine tailings allows the sequestration of atmospheric CO₂ through the formation of magnesium carbonates. The Dumont Nickel Project (DNP) is being studied to estimate the CO₂ sequestration potential of future mining residues and to evaluate the impact of mineral carbonation on the quality of mine waste drainage water. For this purpose, experimental cells were built and instrumented in 2011. The first was constructed using milling waste and the second with mining waste. Laboratory characterization of residues and field observations will be combined to propose a quantitative model of mineral carbonation and metal leaching.

A decrease of CO₂ concentration in the mining waste cell, from atmospheric concentrations (~390 ppmv) near the surface of the cell to ~100 ppmv near the bottom, reflects active CO₂ consumption by the residues. This cell contains mining waste with a large grain size distribution ranging from blocks (<40cm) to silt-size grains. Magnesium-rich minerals such as lizardite, chrysotile and brucite are the major minerals in the residues. Mineralogical analyses (XRD, SEM and EPMA) reveal precipitation of brugnatellite and hydromagnesite, with a lamellar texture on the surface of serpentine grains. In order to better identify the different processes involved in carbonation, the carbon isotopic composition of the interstitial gases was analysed in-situ with a WS-CRDS instrument. An increase of d¹³C_(air) from -8‰ to ~2 ‰ is correlated with the decrease in CO₂ concentration within the cell, and can be explained by dissolution of atmospheric CO₂ in interstitial water (D_co2-DIC 11‰) in the DNP mining residues. As gas advection is slow, CO₂ supply driven by diffusion is the limiting step in the experimental cell. CO₂ dissolution in interstitial water under this limited CO₂ supply condition enriches ¹³C in residual CO2 in interstitial air. Optimized mineral carbonation reactions in DNP mining waste will require an unconstrained CO₂ supply.