Reconstructing a pre-mining geochemical baseline using a proximal natural analog and geochemical modeling: Los Bronces and Yerba Loca, Central Chile

• 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.