Mining and the carbon cycle

We document fixation of the greenhouse gas carbon dioxide (CO2) in tailings from active and abandoned mining operations. The hydrated magnesium carbonate minerals nesquehonite, hydromagnesite, dypingite, and lansfordite form during processing and weathering of ultramafic mine tailings. The rate of silicate dissolution is greatly accelerated in the tailings environment as a direct result of the reduction in grain size, and corresponding increase in reactive surface area, afforded by mining and milling. Once CO2 has been incorporated into the crystal structure of a mineral, it is potentially trapped on a geologic timescale. We use stable and radiogenic isotope tracers to assess the sources of carbon bound in minerals. Potential carbon reservoirs include atmospheric, bedrock, and industrial sources. Using isotopic tracers, we are able to distinguish between these reservoirs to identify which minerals are hosts for atmospheric and industrial CO2. Quantitative phase analysis with X-ray powder-diffraction data is used to determine the modal abundance of mineral hosts for trapped CO2 and to provide an estimate of the amount of CO2 stored in tailings. By combining tracer studies with quantitative phase analysis, we are able to accurately assess the role that mineral processing and accelerated weathering play in reducing the greenhouse gas content of the atmosphere.