THE ROLE OF EFFLORESCENT SULFATES IN THE STORAGE OF TRACE ELEMENTS IN STREAM WATERS POLLUTED BY ACID MINE-DRAINAGE: THE CASE OF PEÑA DEL HIERRO, SOUTHWESTERN SPAIN

We have investigated the mineralogy and geochemistry of efflorescent sulfates precipitated in the banks of streams polluted by acid mine-drainage in the abandoned mine-site Peña del Hierro, in the Iberian Pyrite Belt. Several streams emerging from the waste dumps of the mine were sampled for water and efflorescences at the same stations. Stream water is characterized by low values of pH (0.7–3.5) and very high concentrations of Fetot (up to 30 g/L) and trace elements (72 mg/L As, 0.85 mg/L Cd, 26 mg/L Co, 160 mg/L Cu, 140 mg/L Zn). The precipitates consist of Fe, Mg and Al sulfates, rich in trace elements (e.g., >1000 ppm Cu and Zn, >500 ppm As and Co, up to 28 ppm Cd). The mineralogy is dominated by coquimbite, magnesiocopiapite and pickeringite, and occasionally, rhomboclase, alpersite, epsomite, hexahydrite and starkeyite. Concerning the storage of trace elements, coquimbite is the most important phase retaining As. Alpersite and epsomite can store up to 1–2 wt.% of Zn and Cd. Magnesiocopiapite and pickeringite may be important in the retention of Ni, Co and Zn. Some unidentified accessory minerals play an important role as scavengers of trace elements like Cd, Zn and Cu, and the accumulation of REE, such as Ce, La and Nd. The mineralogy of the efflorescences is strongly influenced by the geochemistry of the water. Epsomite, hexahydrite and starkeyite, which form a dehydration sequence, are associated with Mg-rich waters. The occurrence of alpersite, magnesiocopiapite and pickeringite instead of melanterite, copiapite and halotrichite reflects Fe2+-poor waters. Coquimbite and rhomboclase are associated with the low-pH and Fe3+-rich waters. Chemical data on the water in spring and summer times are consistent with the sulfate minerals observed along the edge of the streams.