In this work, we investigated a 3 ha sulphidebearing waste-rock dump (Libiola Mine, Italy) using mineralogical, geochemical, and geostatistical analyses. The dumped materials were highly heterogeneous in grain size and lithology and varied both laterally and vertically. Other than the host rock of the ore, basalts and serpentinites, the dumped materials contained high amounts of low-grade chalcopyriteand pyrite-rich mineralisations. Due to these characteristics and to the absence of minerals able to neutralise acidity, this waste-rock dump can be classified as an acid mine drainage (AMD) producer. The study confirms that AMD is still active and, in the best scenario, can persist for up to 6.17 9 10 3 years. The consequences of this process are of serious environmental concern as it involves strong acidification of the circulating waters, the release of potentially toxic metals into the soil, streams and rivers and the precipitation of huge quantities of secondary Fe-oxides and Fe-oxyhydroxides.
The mineralogical and chemical variations of ochreous precipitates forming from acid sulphate waters discharged from the lowest mine adit (“Sf. Cruci din Orlea”) of the Roşia Montană Gold Mine (Romania) were investigated by a multianalytical approach (XRPD, IR, TEM, ICP) applied to surface precipitates and associated waters. The mineralogy of the precipitates changed significantly as a consequence of the variations in the chemical parameters of the circulating solutions (mainly pH, Eh, and sulphate concentrations) which were mainly controlled by mixing with unpolluted waters of Roşia River. Ochreous precipitates are characterized by high concentrations of potentially toxic elements (PTEs; in particular Cr, Co, Ni, Cu, Zn, As, Cd, and Pb) and consist of a mixture, in variable proportion, of jarosite and schwertmannite, which represent the stable secondary minerals along the investigated transect of Roşia River. Particular regard is given to the ability of authigenic phases to selectively scavenge selected PTEs from contaminated solutions during their genesis and minerogenetic evolution. Furthermore, laboratory kinetic batch experiments on natural heterogeneous samples of ochreous precipitates were carried out to investigate the release processes involving PTEs and to verify the type and the amount of elements that can be temporarily/permanently trapped by the solid phase from the contaminated solutions. The comparative analysis of the precipitates and waters of the Roşia Montană mining area indicated that the role of secondary minerals as “mitigating agents” can be limited because even minor pH–Eh oscillations would cause mineralogical transformations that could lead to trace elements mobilization in the environment
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