A detailed study of two adjacent mine tailings impoundments comparing mineralogical and geochemical analyses of the solids with geochemical analysis of the pore-water and -gas was carried out in the Monte Romero sulfide mining district (Iberian Pyrite Belt, SW Spain). Since 1978, both facilities have been exposed to weathering in a semi-arid climate with dry-warm and wet-rainy seasons. In these impoundments, sulfide oxidation releases sulfate and contaminants to the pore-water that are transported and concentrated by evaporation in the top surface of the tailings, causing precipitation of soluble iron sulfate salts in dry seasons. These salts act as temporary sinks for minor elements such as Cd, Co, Zn, Ni, Cu and Mn, which are again released during rainfall events in wet seasons. Stored flotation tailings have an average pyrite content of 30. wt.% although average amounts up to 80% are present in the upper 50. cm of one of the impoundments. According to the results of the current study, this difference is key to control the oxidation model in both tailings. In the impoundment containing 30. wt.% of pyrite, pore-water pH increases in wet periods to 3.1 average by rainwater dilution, causing supersaturation and precipitation of secondary Fe(III) phases such as jarosite, goethite and schwertmannite. Consequently, this impoundment shows a 20-25. cm thick yellowish oxidation layer in the top part. In contrast, in the impoundment containing 80. wt.% of pyrite in the top part extremely acidic pore-waters (pH. <. 0.5) are present in both dry and wet seasons, and no precipitation of secondary Fe(III) phases occurs in the oxidation zone. The absence of secondary precipitates explains the apparent unweathered aspect of these tailings, in spite of its much higher remobilization of contaminants to the surface runoff or groundwaters. © 2013 Elsevier B.V.