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Most studies on the weathering of mine waste rock focus on the generation of acidic drainage with high metal concentrations, whereas metal(loid) release under neutral-rock drainage (NRD) conditions has received limited attention. Here, we present geochemical and mineralogical data from a long-term (>10 years) kinetic testing program with 50 waste-rock field barrels at the polymetallic Antamina mine in Peru. The weathering of most rock lithologies in the field experiments generated circumneutral to alkaline drainage (6 < pH < 9) but with concentrations of the oxyanion-forming metal(loid)s As, Mo, Se, and Sb in the mg/L range. The mobilization of As and Sb was particularly efficient from intrusive, marble and hornfels rocks that contained labile As- and Sb-sulfides, irrespective of bulk elemental content or waste-rock reactivity. High-alkalinity drainage from these materials sustained neutral-pH conditions that are unfavorable to oxyanion adsorption onto Fe-(oxyhydr)oxides and, therefore, enhanced As and Sb leaching. The release of Mo and Se from sulfidic skarn and intrusive waste rock was more proportional to elemental content but equally enhanced by pH-inhibited adsorption and negligible secondary mineral precipitation under NRD conditions. Our results demonstrate that oxyanion concentrations of environmental concern may be conveyed by neutral- to alkaline-pH waste-rock drainage and should be a focus of mine wastewater monitoring programs.

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Abstract. The Antamina mine, located in the Peruvian Andes, is one of the world's largest operating copper-zinc mines. A comprehensive geochemical characterization program of waste rock and low grade/marginal grade ore has been underway to fulfill a number of objectives. These objectives focus on developing an understanding of potential operational and post-closure impacts to the environment, assisting with identification and implementation of prevention/mitigation measures, and refinement of the current waste rock management program. The geochemical characterization program follows a multi-faceted approach, including extensive long-term laboratory and field testing and operational monitoring. This presentation will provide an overview of the geochemical characterization program at Antamina, including relationships between results from laboratory testing, field testing and operational monitoring, and water quality modeling.Additional Key Words: metal leaching, neutral drainage, waste rock, sulfide

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