Controls on acid mine water composition from the Iberian Pyrite Belt (SW Spain)

In the Iberian Pyrite Belt (IPB), especially within the Tinto and Odiel River basins, there is a large number of sources of acid mine drainages (AMD), characterised by very extreme pH conditions and pollution levels. This paper studies the main factors governing the composition of the different leachates, based on ionic relationships. Statistically there are no differences between leachates from adits and shafts and other types of AMD sources. The most extreme conditions (pH values near 1, 35 g/L of Fe and 104 mg/L of As) are found in an area of Riotinto covered by milled pyrite wastes, used for the production of copper through leaching. The Cu and Zn ore grades deduced from the leachates are in line with the usual values of the IPB, which indicate a geological control in the composition of the acid mine waters. The lowest SO4/Cd and Zn/Cu ratios in the acid leachates are found in the NE of the study area, whereas the minimum values of SO4/Ni ratios occur in the center and SW. This seems to be related with the type of deposit: shale-hosted at the southern part of the IPB and felsic volcanoclastic-hosted at the northern part, although there are exceptions to this general pattern. Likewise, the proportions of Fe, Al, Mg, Co, Ni and other elements in the leachates are influenced by the mineralogy and the degree of hydrothermal alteration of each deposit, the latter related with Co/Ni ratio.