Chemical characteristics of groundwater around two massive sulphide deposits in an area of previous mining contamination, Iberian Pyrite Belt, Spain Hélene Pauwels a,*, Marie-Lou Tercier-Waeber b, Miguel Arenas e, Ricardo Castroviejo d, Yves Deschamps a, Arnault Lassin a, Flavio Graziottin e, Francisco-Javier Elorza d aCentre Scientifique et Technique, BRGM Research Division, B.P. 6009, 3 Avenue Claude Guillemin, 45060 Orleans Cedex 2, France bCABE, Department of Inorganic, Analytical and Applied Chemistry, University of Geneva Sciences IL 30 quai Ansermet, 1211 Geneva 4, Switzerland clnima S.A., Arturo Soria No. 336, 6a Planta, 28033 Madrid, Spain dETS 1 Minas, u.P. Madrid, Rios Rosas No. 21, 28003 Madrid, Spain eldronaut Srl, Via Monte Amiata 10, 20047 Brugherio (Mi), Italy Abstract A detailed chemical study of groundwater was carried out to elucidate the processes controlling the oxidation and dissolution of sulphide minerals at two massive sulphide deposits in the Iberian Pyrite Belt (IPB), i.e. the mined La Zarza deposit and the unmined Masa Valverde deposit. It was found that major-element compositions varied according to the hydrological regime, La Zarza being in a relatively high area with groundwater recharge (and disturbance due to the human factor) and Masa Valverde being in a relatively low area with groundwater discharge. The variations mainly concem pH, Eh, S04 and Na concentrations. Metal concentrations were determined (a) by ICP-MS after filtration, and (b) in sorne cases by voltammetric measurement of Cu, Pb, Zn, Cd and Mn using the Voltammetric In situ Profiling (VIP) System, which allows detection of only the mobile fractions of trace elements (i.e., free metal ions and small labile complexes a few nanometers in size). If one compares the results obtained by each of the two methods, it would appear that the groundwater shows significant enhancement of metal solubility through complexing with organic matter andlor adsorption onto colloids andlor small particles. In areas of sulphide oxidation, however, this solubility enhancement decreases according to Cu> Zn>Cd>Pb. Dnder very low redox conditions, the attained metal concentrations can be several orders of magnitude (up to 10 8 -1 0 9 for Cu and 10 2 -10 3 for Pb) larger than those expected from equilibrium with respect to sulphide minerals as calculated with the EQ3NR geochemical code; Zn concentrations, however, are close to equilibrium with respect to sphalerite. The implication of these results is discussed with respect both to mineral exploration and to environmental issues.