Macla nº 24. Junio ’19 ! revista de la sociedad española de mineralogía Quantifying the potential seepage from the Quillayes porphyry Cu tailing dam using stable isotopes (Chile) Dídac Navarro-Ciurana (1*), Albert Soler (1), Agnés Saleta-Daví (1), Neus Otero (1,2), Carlos Quintana Sotomayor (3), Diego San Miguel Cornejo (3), Mónica Musalem Jara (3), Guillibert Novoa Godoy (4), Cristóbal Carrasco Jaramillo (4), Evelin Aguirre-Dueñas (5), Manuel Alexis Escudero Vargas (5) (1) Grup MAiMA, SGR Mineralogia Aplicada, Geoquímica i Geomicrobiologia, Dpt. Mineralogia, Petrologia i Geologia Aplicada, Facultat Ciències de la Terra, Universitat de Barcelona (UB). C/Martí i Franquès s/n, 08028 Barcelona, Spain. (2) Serra Húnter Fellowship, Generalitat de Catalunya, Spain. (3) Dpt. de Conservación y Protección de Recursos Hídricos. Dirección General de Aguas, Chile. Compañía de Jesús 1390, piso 4, of. 415, Santiago, Chile. (4) Dpt. Depósitos de Relave, Servicio Nacional de Geología y Minería. Santa Lucia N° 360, Piso 2, Santiago, Chile. (5) Lab. de Isótopos Ambientales, Dpt. de Tecnologías Nucleares, División de Investigación y Tecnologías Nucleares, Comisión Chilena de Energía Nuclear. Nueva Bilbao 12501, Las Condes, Santiago, Chile. *corresponding author: didac.navarro@uab.edu Palabras Clave: Isotopos, infiltraciones, balsas de lodos, pórfidos cupríferos, Chile. ! Key Words: Isotopes, seepage, tailings dams, porphyry Cu deposits, Chile. I N T R O D U C T I O N Chile is extraordinarily enriched in porphyry-style copper deposits and is the largest Cu producer worldwide. Mining activities of these ores, which are characterized by low Cu grades and high tonnages, produce several environmental footprints, as large amounts of tailing production and disposal in dams, and seepage of tailing dam waters towards the neighboring aquifers. The tailing dam managements usually includes different strategies to minimize seepage towards groundwater, such as drainage ditches and pumping wells installed as hydraulic barriers. Nevertheless, these strategies are not always sufficient to avoid seepage waters, and there is a growing interest to develop tools to trace and quantify the impact of tailings in groundwater. Mine tailings are characterized by high contents of dissolved sulfate, and isotopic studies have been used to trace the sources and processes of SO4 2- in tailing waters (e.g., Dold and Spangenberg, 2005; Spangenberg et al., 2007), due to their potential environmental impact. However, less studies are focused to the use of isotope geochemistry as a monitoring tool to determine the tailings seepage towards the aquifers. In the present study, isotope data (! 2 H- ! 18 O-H2O and ! 34 S-! 18 O-SO4 2- ) from the hydric systems related to the Quillayes tailing dam (from the Pelambres porphyry Cu deposit, Chile) have been studied to illustrate their potential as tool for quantifying tailings seepage contribution to groundwater. M I N E T A I L I N G S D A M Figure 1 shows the location of the Quillayes mine tailings from the Los Pelambres porphyry copper mine, as well as the situation and types of studied water samples. It is worth mentioning that the water used in the mine flotation plant (~1,600 masl) is from the Quillayes dam surrounding waters (~1,300 masl). F i g 1 . Location of the Quillayes tailing dam and studied water samples. ! 2 H- ! 18 O OF WATER The isotope signature of the Quillayes tailing dam waters projects far from the local meteoric waters, suggesting that waters are affected by evaporation (Fig. 2). The isotopic composition of groundwater samples collected downstream from the Quillayes dam plotted in the 91