AMD-precipitates formed in an arsenic-rich environment – the case of Penedono mine waste-dumps (N Portugal)

Resum

This work presents the AMD-precipitates from supergenic neoformation in a mine waste-dumps located in the North of Portugal (Penedono mine). The waste dumps resulted from exploitation of gold occurring in quartz veins with sulfides, mainly pyrite and arsenopyrite. As a result, the mine wastes are composed by detrital minerals, mainly quartz, feldspar, micas, and pyrite that coexist with supergenic phases, formed by evolution of the acid mine drainage solutions. Mineral-water interaction lead to neoformation processes, including salinization (formation of soluble salts efflorescences) and crustification, consisting of the layering of crusts of minerals deposited successively. Jarosite, sco-rodite, and several soluble salts, such as gypsum and rhomboclase were identified as the most abundant precipitates. In addition, mineralogical analyses revealed the abundant occurrence of poorly crystalline iron-oxyhydroxides as well as amorphous nanoprecipitates, mainly composed by iron and arsenic. Mobilization or fixation of toxic elements can be strongly regulated by these neoformation processes. The obtained results indicated that arsenic is retained either by crystalline phases, such as scorodite, or by amorphous iron arsenates. Jarosite also play an important role in controlling mobilization of metals and arsenic by assuring crustification of mine wastes.