Tracking multiple Sr sources through variations in 87Sr/86Sr ratios of surface waters from the Aljustrel massive sulphide mining area: Geological versus anthropogenic inputs

Abstract This study presents major and trace element data and 87 Sr/ 86 Sr ratios for surface waters from a small watershed draining the Aljustrel sulphide mining area (South Portugal). The watershed is located in a geologically complex area comprising two main compartments: the northern compartment is dominated by Cenozoic formations and does not bear any mining activity, whilst the southern compartment is underlain by rocks from both the Cenozoic sedimentary cover and the mineralized sequences of the Variscan Paleozoic basement of the South Portuguese Zone. Based on the available geochemical and isotopic data, the analysed water samples can be roughly divided into five main groups: (a) group 1 corresponds to AMD-contaminated water drainages from the southern part of the catchment, showing pH  87 Sr/ 86 Sr ratios ranging from 0.7101 to 0.7126; (b) group 2 is represented by unpolluted stream waters from the northern compartment, having pH > 8, marked depletions in SO 4 and heavy metals and displaying Sr isotopic compositions similar to rainwater ( 87 Sr/ 86 Sr = 0.7097–0.7098); (c) group 3 includes mixed-type stream waters with pH and elemental/isotopic signatures intermediate between those of groups 1 and 2; (d) group 4 comprises the samples collected at two lime-treated water dams from the mining area, that are clearly distinguished from the impacted waters of group 1 by their elevated Ca, Mg and Sr concentrations and low 87 Sr/ 86 Sr ratios ( 87 Sr/ 86 Sr = 0.7085–0.7091); (e) group 5 includes clean waters from two dams located upstream of the ore processing site, which are dominantly supplied by direct precipitation and surface runoff and have pH > 6, very low SO 4 and metal concentrations and 87 Sr/ 86 Sr ratios varying between 0.7094 and 0.7108. Variations in the 87 Sr/ 86 Sr ratios and Sr concentrations of the analysed waters are attributed to mixing of two main end-members: atmospheric meteoric waters (local rainfall/runoff) with low Sr contents and 87 Sr/ 86 Sr ratios ≈ 0.7095 and more radiogenic waters derived from the Aljustrel mining area. The highly radiogenic signatures of the southern tributaries appear to have been controlled by weathering of plagioclase from the acid volcanic rocks of the Paleozoic basement, enhanced by mining activities (pyrite oxidation), although released strontium from gangue carbonates could have locally contributed to the fluctuations of Sr contents and 87 Sr/ 86 Sr ratios observed in some of these water samples. By contrast, the low 87 Sr/ 86 Sr ratios and relatively high Sr (Cl and Na) concentrations shown by the surface waters draining the northern compartment suggest that these tributaries correspond to meteoric waters that have gained Cl, Na and Sr through wash-out of Cenozoic deposits containing finely disseminated halite layers. Finally, the decrease in the 87 Sr/ 86 Sr ratios and elevation of Sr contents observed in the samples from the lime-treated water dams support the involvement of an anthropogenic source of Sr with 87 Sr/ 86 Sr ratios lower than that of rainfall.