THE ANTAMINA WASTE ROCK PROJECT: A MULTI-SCALE STUDY OF MINERAL RELEASE, TRANSPORT, AND ATTENUATION IN WASTE ROCK UNDER NEUTRAL DRAINAGE CONDITIONS

A multi-scale study of mineral release, solute transport, and attenuation mechanisms in waste rock under neutral drainage conditions is underway at the open-pit Cu-Zn-Mo Antamina mine in the Peruvian Andes. The objectives of the project are to better understand these mechanisms through controlled and/or monitored hydrological, geochemical, and microbial conditions, to assist in refining the current waste rock classification scheme, to develop methods for on-site waste rock classification, and to assess the use of field- and laboratory-derived scale-up factors and reactive transport modeling to predict waste rock drainage quality. The project includes microbial, surface area, and mineral availability studies at the μm scale; humidity cells, batch tests, column experiments, sequential leaches, and hydrological material characteristics tests at the cm-dm scale; field cells and large laboratory columns at the 1-m scale; and instrumented and monitored experimental waste rock piles at the 10-m scale. Experimental pile data to date suggest that drainage quality is dominated by waste rock solid phase composition and flow regimes, both of which are heterogeneous within and among waste rock types, resulting in highly spatially- and temporally-variable solute loadings across all scales. Laboratory studies suggest that, under neutral conditions, powellite and wulfenite precipitation limit Mo mobility, and that sorption may reduce Zn mobility. Preliminary results from field cells with layered mixtures of different waste rock types provide further evidence of these attenuation mechanisms. Microbial studies show that neutrophilic and acidophilic bacteria co-exist under circum-neutral pore water conditions, suggesting the possible presence of acidic micro-environments. Pore gas concentrations suggest that sulfide mineral oxidation may become O₂-limited in fine-grained reactive material, and that coarse-grained materials may provide O₂ pathways into large volumes of waste rock.