Environmental Aspects of a Major ARD Source at El Indio Au-Cu-As District, North-Central Chile

El Indio, an Au-Cu-As deposit with outstanding gold grades, was mined in the Andes of the Coquimbo region, Chile, between 1975 and 2002. Sediment and water sampling of the rivers in the 2000 s found exceptional As and metal levels in modern and old sediments. The studies also revealed that acid rock drainage (ARD) has been present in the district for nearly 10,000 years and the effect that the mining of the district had in terms of geochemical anomalies. The convergence of mineralogical, structural, and hydrologic conditions has generated a metal-rich ARD, a process followed by transference of metals to the fine sediments. In this context, the study also deals with the risk of metal and metalloid transfers from the sediments to the river waters as a consequence of eventual physical-chemical changes, due for example, to climatic-driven conditions. Water and sediment samplings were carried out to provide materials for selective extraction tests under acidic, acid-reducing, and acid-oxidizing conditions. The different behavior of metals and metalloids was revealed and highlighted the refractory character of As. Additionally, the study included the characterization of the sediment’s mineralogy, and allowed the detection of new geochemical anomalies of Cu, Zn, Co, and Y in the Incaguaz River, along with high dissolved Li concentrations in the Toro and Turbio rivers.