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Item type:Publication, Seasonal fluctuations and geochemical modeling of acid mine drainage in the semi-arid Puna region: the Pan de Azúcar Pb–Ag–Zn mine, Argentina(Elsevier, 2021-08-01)Pan de Azúcar is an inactive Pb–Ag–Zn mine in the semi-arid Puna region of NW Argentina at 3600 m above sea-level. The mine is situated in Los Pozuelos basin, which is a UNESCO Biosphere Reserve. Substantial pyrite and few carbonate minerals are present in the tailings. The generated acid mine drainage (AMD) has low pH (1.92–4.06) and is mainly Fe–Zn–SO4 type water with high concentrations of metals that in order of relative abundance vary as: Al ≫ Cd > As(T) > Ni > V > Cu > Co > Sb > Cr > Pb > Sn > Ag. The strong annual cycle of dry and wet seasons in the Puna generates a significant influence on AMD geochemistry. In the dry season, there is no runoff and AMD drains mainly in the form of seepages with low pH (1.92–2.21), high concentrations of metals, and the formation of metal-rich soluble efflorescent salts is favored by strong evaporation. At the beginning of the wet season, the efflorescent salts rapidly dissolve and metals concentrations in AMD increase significantly, creating the most hazardous period for the surrounding fluvial environment. At the end of the wet season the pH increases (3.44–4.06) and the concentration of metals decreases, especially arsenic associated with ferric iron precipitates. As a consequence of the DC3 dam restoration, a large volume of AMD is retained forming an acid pond, in which processes of AMD mixing, secondary minerals precipitation, and sulfide oxidation were quantified by inverse modeling with mixing. Jarosite and schwertmannite precipitates in the surface of the pond and favors the attenuation of As and Pb. However, subaqueous oxidation of sulfide minerals, hydrolysis of silicates, and mixing with upstream AMD seepage increases the concentrations of SO42-, Fe(II), and the other metals in the seepage of the pond that migrates downstream. The results of this study indicate the necessity to improve the remediation methods of Pan de Azúcar mine. It is expected that other sulfide mines in the Puna region will have a similar variability of the AMD geochemistry, which should be considered for mine closure, remediation, and monitoring of water quality in mining. - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Reconstructing a pre-mining geochemical baseline using a proximal natural analog and geochemical modeling: Los Bronces and Yerba Loca, Central Chile(Elsevier BV, 2025-10-01)• ARD of analog site helps to reconstruct pre-mining geochemical baseline. • Schwertmannite and hydrobasaluminite control pH along flow path below 5. • Coprecipitation and sorption processes control trace element concentrations. • ARD source freezing and snow- and glaciermelt control annual element cycling. • Pre-mining ARD had pH between 2.5 and 5 with important copper concentration. Due to the lack of pre-mining mineralogical and geochemical data at historic mine sites, evaluation of the pre-mining geochemical baseline condition is required. For this purpose, we reconstructed the pre-mining geochemical baseline for Los Bronces mine (Central Chile) using the Yerba Loca Nature Sanctuary, located 6 km south, as a proximal natural analog where in similar altitudes (4000–4200 m) than in Los Bronces (3600–4000 m), mineralized breccias are currently undergoing natural weathering, leading to acid rock drainage (ARD) with pH values as low as 2.57 with SO 4 at 2893 mg/L, Fe at 285 mg/L, Al at 64.4 mg/L, and Cu at 54.4 mg/L during summer. The stream's pH, initially controlled by jarosite and later by schwertmannite, remains below 3.94, sustaining elevated Cu concentrations. In winter, pH rises towards neutral due to frozen ARD sources at high elevation (>3600 m asl) combined with rainfall-driven dilution in the lower catchment, decreasing metal loads in the streams. Archival data indicates that pre-mining breccias at Los Bronces, with a Cu grade of 1.43 wt%, were composed of 1.43 wt% pyrite, 0.92 wt% chalcopyrite, and other sulfide minerals, leading to significant ARD when weathered. Based on similarities with Yerba Loca and geochemical modeling results, it is inferred that pre-mining drainage at Los Bronces had a pH range of 2.5 to 5 during summer, with likely higher Cu concentrations but similar trace element levels, suggesting a similar annual cycle as today observed at Yerba Loca with a potential increase towards neutral pH in winter.1