Carbon accounting of mined landscapes, and deployment of a geochemical treatment system for enhanced weathering at Woodsreef Chrysotile Mine, NSW, Australia

摘要

Carbonation of ultramafic mine tailings has the potential to offset greenhouse gas emissions from mining by trapping CO2 within the crystal structures of Mg-carbonate minerals and hydrotalcite supergroup minerals, which form as weathering products in tailings storage facilities. Here, we present a detailed geochemical and mineralogical assessment of tailings from the Woodsreef Chrysotile Mine, New South Wales, Australia, demonstrating that coupling mineralogical and elemental datasets improves the accuracy of carbon accounting in mined landscapes. Detailed analysis of tailings mineralogy using quantitative X-ray diffraction (XRD) and total carbon analyses reveals that previous assessments of passive mineral carbonation at Woodsreef have been underestimated. Maximum values for the abundance of total carbon (up to 0.4 wt%), as well as the abundances of secondary carbonate minerals (i.e., up to 1.9 wt% hydromagnesite and up to 2.6 wt% pyroaurite, measured with XRD) are observed between approximately 2 cm and 30 cm depth in profiles collected within experimental plots. However, an amorphous Mg-carbonate phase, that cannot be detected using XRD, is also present at comparably high abundances to depths of at least 1 m. This phase is readily observed using scanning electron microscopy, it contributes a measured carbon content of approximately 0.2 wt% at up to 1 m depth, and it has a predominantly atmospheric carbon isotopic signature ((FC)-C-14 > 0.80). We find that using only XRD data results in the sequestered CO2 being underestimated by nearly four times compared to estimates incorporating total carbon measurements, highlighting the important role of amorphous Mg-carbonates in the carbon cycles of mines. Combining XRD and total carbon data, we provide an estimate for passive carbon sequestration by both crystalline and amorphous carbonates in the Woodsreef tailings (11.7 kg CO2/m(2), considering the upper 1 m(3)) and suggest that future studies should employ both XRD and total carbon measurements for carbon accounting.