Metal distribution and environmental problems related to sulfide oxidation in the Libiola copper mine area (Ligurian Apennines, Italy)

Abstract

Metal mobility and distribution in water and sediments in the surroundings of the pyrite–chalcopyrite–sphalerite mine of Libiola, in the Gromolo Valley near Sestri Levante (Genova), are investigated to evaluate the environmental hazard in the area. The mineralisation, related to Apennine ophiolites, was already known in the Bronze Age (about 2500 yr bc), but economic exploitation started in the XVII century. Open galleries, waste dumps and acid drainage testified the past mining activities which ended in 1965. Mine waste material, covering an area of about 0.5 km 2, is reddish–yellowish, generally coarse-grained and stratified. The waste is a mixture of basalt, ultramafic-derived and iron-rich phases, with relatively high concentrations of S (mean 0.3% S) and Cu (mean 0.3% Cu), and significant contents of Zn, As, Mo and Se. Acid drainage, with pH values as low as 2.5, is quite rich in dissolved Fe, Al, Cu, Zn, Mn, Ni. Repeated sampling revealed marked chemical variations, particularly in the mine waste area, depending on water flow conditions. Where low-pH water mixes with normal (pH 8) surface water, reddish amorphous iron phases (e.g. ferrihydrite, schwertmannite) precipitate, favouring the scavenging of many elements carried by the acid waters and providing a natural way of remediation for the stream waters. The rapid flocculation causes anomalous contents of Fe, Cu, Zn, Co, Sc, Y, La, Ce, and to a lesser extent of Cr and Ni, in the sediments. Although the ochreous colloids are important components of the Gromolo River sediments for about 8 km from the mine, preliminary results indicate that, unless natural conditions change, remobilisation of heavy metals should not be expected.