Evidence for Passive Mineral Carbonation from Carbon Isotope Geochemistry 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) will generate 1.7 Gt of ultramafic tailings and waste rocks. The DNP is studied to estimate the CO₂ sequestration potential of future mining operations and to evaluate the impact of mineral carbonation on the quality of mine waste drainage water. For this purpose, two experimental cells were built and instrumented in 2011. The first cell (EC-1) contains heterogeneous mining waste ranging in size from blocks to silt. The second (EC-2) is constructed from homogeneous milling waste (tailings) with a grain size near 175 μm. Both cells contain interstitial air sampling ports at different depths. Cell EC-2 is also equipped with two different probes to measure the water content and the dielectric potential of water. Laboratory characterization of mining wastes and field observations will be combined to propose a quantitative model of mineral carbonation and mine drainage from the two types of mine waste materials.

A decrease of the interstitial CO₂ concentration in both cells, from atmospheric values (~390 ppmv) near the surface of the cells to ~100 ppmv near the bottom, reflects active CO₂ consumption by the mining wastes. Magnesium-rich minerals such as lizardite, chrysotile and brucite are the major minerals in the mining wastes. White crusts are observed on the surface of waste rock in the EC-1. On cell EC-2 scattered white crusts on the indurated milling wastes are visible. To detect the position of the carbonation front in EC-2, cores were sampled and split into 5 to 7 sub-samples. Total carbon analyses were performed on the white crusts and sub-samples cores. The total carbon content ranges, in cores, from ~0.7 wt%, at the surface, to ~0.2 wt% C, at the bottom of the cell. In the white crusts samples, the total carbon content varies between ~1 wt% and ~3.5 wt% C. Mineralogical analyses (XRD, SEM and EPMA) reveal precipitation of seven different magnesium carbonates.

In order to better identify the different processes involved in carbonation, the carbon isotopic composition of the interstitial gases was analyzed in-situ with a WS-CRDS instrument. An in-crease of d¹³C_(air) from -8‰ to ~2‰ is correlated with the decrease in CO₂ concentration within the cells. This trend can be explained by dissolution of atmospheric CO₂ in interstitial water (D_co2-DIC 11‰) within the DNP mining residues. As gas advection is slow, CO₂ input is driven by diffusion in the two cells. CO₂ dissolution in interstitial water under these limited CO₂ input conditions enriches ¹³C in interstitial air. These observations are used to constrain a conceptual model of the carbonation reaction in the DNP residues.