Master thesis presented by Teresa Maria F. Valente, 1997

Abstract
The mine waste dump derived from the mining works on the Valdarcas ore deposit, located at the "Domo de Covas" -Vila Nova de Cerveira, can be considered as a "key site", appropriate to the study of the environmental unbalance generated by the accumulation of sulphide bearing mine wastes.

The tungsten mineralization ofthe Valdarcas deposit, which occurs in coexistence with metallic sulphide minerals, calcium silicates and carbonates, confers to this deposit the special characteristics that lead to the generation of an environmental impact focus, related to the sulphide tails accumulation.

The rnine waste dump and its impact, represent the target study subject of this thesis.

The mining works are de activated for more than ten years, and after this time, their effects on the environment, still persist. They result from the re equilibrium reactions that occur at the waste dump, as response to the meteoric processes that cause the alteration of the primary minerals.

The water-mineral interaction leads to the generation of acid mine drainage ("AMD"), and to the development of a new mineralogy in the waste dump surface. Consequently occur the contamination of the Coura river, which is characterised by low pH values, high metallic concentrations and strongly ochreous colouring.

The new mineralogy paragenese includes iron crusts, soluble salts and clay minerals. They reflect the ore deposit geology and the occurrence of wet-dry cycles, that are related to the seasonal variations of the rainfall, and to the drainage conditions, that characterise the waste dump.

The soluble salt minerals and the iron crusts were treated with special detail in this study.

The soluble salts are represented especially by gypsum and several iron sulphates, with a low stability field, related to their high solubility. Therefore they occur especially during the Summer.

The iron crusts consist of aggregate structures that represent a terminal evolution of the tail's transformation. They include in their composition, the primary minerals inherited from the deposit, and the new minerals resulting from their supergenic alteration.

The salt minerals and the iron crusts have different capacities and mechanisms to fixate the chemical patcntial mobilised by the exploitation. These differences, are reflected in the "AMD" composition, and consequently in the magnitude of the Coura river contamination.

The instability of the salt minerals leads only to a temporary fixation of the contaminant elements. On the other hand, the iron crusts are capable to fixate efficiently these elements, because of the stability of their mineralogy, especially the iron oxides and the clay minerals. They isolate the reactive minerals from the weathering agents, and consequently limit the dispersion area of the "AMD" environmental impact.