Mineralogical and chemical variations in a sulphide waste-rock dump (Libiola mine, Liguria)

With this work we have studied the mineralogical, chemical, and NAPP (Net Acid Producing Potential) variations occurring on a 3.5 ha sulphide waste-rock dump built over a period of about 100 years by the dumping, in a vertical sequence, of both non-valuable mineralisations and non-mineralised rocks coming from underground and open-pit excavations. This dump, as well as the overall Libiola mine area, is characterised by active AMD (Acid Mine Drainage) processes triggered by the diffuse oxidation of sulphide minerals [1 and references therein]. Twenty-one sampling sites were choosed by defining a grid throughout the overall dump surface and the sampling points were selected within every cell where well-exposed vertical cuts occur. Qualitative and quantitative mineralogical analyses were performed by transmitted- and reflected-light optical microscopy and by SEM-EDS. Bulk chemistry was assessed by ICP-AES and XRF analyses whereas the NAPP evaluation was based on the "AMIRA P387A" procedure [2]. The studied samples are generally incoherent or weakly cemented by iron-oxide and -oxyhydroxides and vary from gravel-dominated to sandy-gravel sediments with a uniform particle size distribution in the range 2-64 mm. They are composed by variable amount (10-26 wt %) of sulphide mineralisation fragments (pyrite  chalcopyrite  sphalerite) showing various degree of oxidation. Mafic and ultramafic clasts (basalt, serpentinite and ophiolitic breccias) vary from 35 to 80 wt% and they are generally unaltered (serpentinite) or weakly altered. Secondary minerals are almost exclusively represented by Fe-oxyhydroxides (goethite) and -oxides (hematite) and they occur as cement filling interclast voids, homogeneous crusts (hardpan layers), and pseudomorphic replacement of sulphide mineralisation. They vary from 10 to 50 wt % and their abundance is strictly correlated to the sulphide content of the sample. The bulk chemistry (major, minor, and trace elements) appears to be controlled either by the primary unaltered minerals and by the different alteration products. In fact, as evidenced by several authors [3 and references therein], Fe-oxydes and -oxyhydroxides have a good affinity for several transition- and heavy-metals, that they effectively scavenge by the circulating solutions. The distribution of metals of environmental concern on the waste dump has been further plotted on contour maps. These statistical interpolations allow to evidence significant spatial variations throughout the dump that reflect either the composition of the dumped materials and the evolutive stage of the AMD processes. With the exception of few sites, all sampling points evidenced positive NAPP values which mean that AMD processes are still active and presumably should persist for long time, due to the high sulphide contents and to the complete absence of potentially neutralising mineral phases.