Grain-scale microanalysis of minerals in mine waste provides important insight into the mineralogical hosting of potentially toxic metals and helps predict whether metals are released through weathering or attenuated in secondary phases. Until recently, the number of target grains examined by microanalysis was limited by time and cost, and target selection was likely to be influenced by observer bias. By using automated mineralogy software associated with scanning electron microscopy (SEM) such Mineral Liberation Analyser (MLA) or QEMSCAN, hundreds of thousands of particles in a single thin section can be characterized, and detailed information acquired on abundance, variability, chemical composition, and physical characteristics.. The technique can be applied to rocks, tailings, sediments and soils. Careful sample preparation is critical to prevent particle agglomeration or density separation that can skew results. Minerals that are rare (e.g. 4 of 100,000 total particles), but host environmentally-significant metal concentrations (1000s of mg/kg) can be located and analysed using this software. The relative proportions of multiple metal-hosting phases can be quantified which is particularly useful if these phases vary in terms of solubility or bioaccessibility.
We have applied SEM-MLA to the following mine waste problems: (1) determination of acid potential and neutralization potential of mine tailings, with results that compare favourably with static testing, (2) comparing the modal mineralogy of mine-impacted stream sediments based on SEM-MLA, bulk-XRD, and X-ray absorption spectroscopy, (3) predicting the fate of Hg, As, Cd and other elements in tailings from a complex, oxidized orebody (4) calculating the relative proportions of anthropogenic and natural As-bearing grains in lake sediments and (5) explaining variations in Pb bioaccessibility in tailings samples.
Mineralogical characterization of mine waste benefits from the application of multiple analytical techniques. Quantitative, automated mineralogy provides a new approach for environmental geochemists to quantify modal analysis, particle size, and mineral association.