Alteration of sulfides within an open air waste-rock dump: application of synchrotron µ-XRD, µ-XRF, and µ-XANES analyses

We have applied a combination of synchrotron-based techniques (μ-XRD, μ-XRF, and µ-XANES) to determine the mineralogy and the elemental distribution of metals in partially altered sulphide-mineralization fragments deposited within an open-air waste-rock dump (Libiola mine, eastern Liguria, Italy). In this dump AMD processes are active and intense and sulphide-mineralized clasts progressively undergo oxidation originating centimetric-thick hardpans cemented by secondary iron oxides and oxyhydroxides. Selected samples, containing the transition from unaltered sulphides to secondary oxidation products have been analyzed along several millimetric transects. The results evidenced that sulphides (pyrite + chalcopyrite + sphalerite) oxidation starts from the crystals rims or from intra-grain microfractures. Sulphide-S firstly oxidizes to sulphate and then is rapidly leached out from the system. The altered layers are composed almost exclusively of Fe-oxides (hematite) and -oxyhydroxides (goethite and minor bernalite) that replace sulphides and fill intra- and inter-grain interstices. Elemental maps and μ-XRF transects evidenced that these secondary minerals efficiently and selectively scavenge many of the elements released during sulphides (e.g. Cu, Zn, As) and gangue minerals (e.g. Ni, Cr, Al) alteration.