Presented paper

IMWA2019 Students work

Mineralogical Attenuation Processes Associated With The Evolution Of Acid Mine Drainage In Sulfide-Rich Mine Wastes

Valente, Teresa (1); Gomes, Patricia (1); Sequeira Braga, Maria Amália (1); Moreno, Filipa (1); Ríos Reyes, Carlos Alberto (2); Pamplona, Jorge (1); Antunes, Isabel Margarida (1)
1: Universidade do Minho, Portugal; 2: School of Geology, Universidad Industrial de Santander, Colombia

Abandoned mines are important environmental focus of impact in regions with significant mining tradition, like the Northern Portugal. In sulfide-rich waste dumps there are typical water-rock interaction processes that cause acid mine drainage (AMD). In the presence of abundant pyrite and arsenopyrite, AMD may contribute to the mobility of toxic elements, being responsible for water, soil and sediment contamination, over a great distance from the mines. However, geochemical and mineralogical evolution of mining wastes can lead to the development of natural attenuation processes. Newly formed phases and assemblages can actually act as mineralogical control of the arsenic and metals mobility. The study of these processes and the role of these supergenic structures have strong environmental relevance. Therefore, their behavior should be considered during further remediation procedures planned for these sites.

This study was focused in a sulfide-rich waste dump in the North of Portugal (Penedono mine). This mine has been exploited for gold and arsenic until the 1990s. Presently, the mine is abandoned, even though the waste dumps do remain without environmental rehabilitation. Nevertheless, some natural attenuation processes are underway. Therefore, the present study identifies the newly formed phases while evaluating their role in fixing toxic elements in the waste dumps. The methodology included sampling of salt efflorescences and hard crusts as well as AMD. Mineralogical studies were performed by x-ray diffraction, scanning and transmission electron microscopy. Water samples were analyzed for pH, electric conductivity, redox potential, acidity, sulfate, metals, and arsenic. Polished sections of the hard crusts were also observed through optical microscopy and scanning microscopy.

This study allowed producing an inventory of newly formed phases, mainly composed by sulfates, arsenates and oxyhydroxysulfates. Results showed that gypsum, rozenite, rhomboclase, epsomite and minerals from halotrichite group are among the most common sulfates found. Jarosite is rather abundant as pure phase and also acting as cement in the hard crusts. Scorodite is the best represented arsenate. However, other very low crystalline or amorphous iron arsenates were identified. Since sulfate efflorescences are metastable, actually they only retain contaminants temporarily. On the other hand, adsorption on oxyhydroxysufates surfaces as well as development of hard crusts cemented by jarosite and scorodite are among the most efficient processes aimed for limiting the mobility of arsenic.

A future remediation project will be planned for Penedono. Such a procedure should be able to consider the hard crusts, particularly their physical and chemical stability during the intervention.