The formation-dissolution-precipitation cycle of melanterite at the abandoned pyrite mine of Genna Luas in Sardinia, Italy: environmental implications

At the abandoned pyrite mine at Genna Luas (Sardinia, Italy), melanterite i s the most environmentally important secondary phase associated with pyrite oxidation. A complete cycle, including formation on pyrite surface, dissol ution in water, precipitation from water and subsequent alteration can be o bserved. The oxidation of pyrite to melanterite is related mainly to the 'i ntrinsic' features of the Genna Luas pyrite, such as its cryptocrystalline pseudo-colloidal texture. During melanterite formation on pyrite there is t ransient storage of Fe2+, SO42- and acidity. This is controlled mainly by w etting-drying seasonal cycles. Laboratory experiments have shown that the s imple dissolution of melanterite is an acidity-generating process which aff ects the geochemistry of water and, indirectly, contributes to the supply o f the Fe3+ required for the aqueous oxidation of pyrite. According to this geochemical model, the interaction between rainwaters and ground pyritic ma terials accumulated in heaps generates hyperacidic saline waters (pH = 0.6; TDS = 300 gill with very high SO42- contents (203 g/l), Fe2+ contents (73 g/l), Zn contents (11 g/l), and numerous potentially toxic elements. Melant erite precipitates from these waters (SIMelanterite = 0.15, calculated usin g PHRQPITZ software), giving rise to attractive emerald green crystal aggre gates which, when exposed to air, alter to a fine white powder composed mai nly of melanterite, and minor amounts of rozenite and szomolnokite.