Fe-sulphate-rich evaporative mineral precipitates from the Río Tinto, southwest Spain

Abstract The soluble metal sulphate salts melanterite, rozenite, rhomboclase, szomolnokite, copiapite, coquimbite, hexahydrite and halotrichite, together with gypsum, have been identified, some for the first time, on the banks of the Río Tinto, SW Spain. Secondary Fe-sulphate minerals can form directly from evaporating, acid, sulphate-rich solutions as a result of pyrite oxidation. Chemical analyses of mixtures of these salt minerals indicate concentrations of Fe (up to 31 wt.%), Mg (up to 4 wt.%), Cu (up to 2 wt.%) and Zn (up to 1 wt.%). These minerals are shown to act as transient storage for metals and can store on average up to 10% (9.5-11%) and 22% (20-23%), Zn and Cu respectively, of the total discharge of the Río Tinto during the summer period. Melanterite and rozenite precipitates at Río Tinto are only found in association with very acidic drainage waters (pH <1.0) draining directly from pyritic waste piles. Copiapite precipitates abundantly on the banks of the Río Tinto by (1) direct evaporation of the river water; or (2) as part of a paragenetic sequence with the inclusion of minor halotrichite, indicating natural dehydration and decomposition. The natural occurrences are comparable with the process of paragenesis from the evaporation of Río Tinto river water under laboratory experiments resulting in the formation of aluminocopiapite, halotrichite, coquimbite, voltaite and gypsum.