This study investigates the transport and sequestration mechanisms of Cu at an abandoned mine tailings site in Manitoba, Canada which has been partially remediated by organic- and carbonate-rich soils. The weathering of chalcopyrite in the Central Manitoba Gold Mine tailings produced acid mine drainage and thus in the enhanced release of Cu into the environment. The capping of two tailings ponds with organic- and Ca-Mg-carbonate-rich soils resulted in a near-neutral runoff enriched in dissolved and colloidal organic matter (DOM), which partially neutralized uncovered tailings in the surroundings. The objectives of this study were to understand mineralogical and geochemical changes induced by the runoff, with a focus on the speciation of Cu in the solute and colloidal fraction of the tailings. For this purpose, multi-analytical characterizations included (a) efflorescent salts collected along a transect, (b) tailings from one surface site and three depth profiles, two of which were partially neutralized, and (c) leachates that were collected in column experiments with tailings from various depths and sites. The results of this study showed that an increase in pH in the surface layers resulted in lower leaching rates of Cu in column experiments, higher abundance of unaltered chalcopyrite, and occurrence of Cu-bearing efflorescent salts at the base of the neutralized layer and thus in the immobilization of Cu and lower weathering rates of chalcopyrite within the neutralized surface layers. As the neutralization of the surface layers depleted the infiltrating solutions in carbonates, the pH in the subsurface layers remained relatively constant. Element enrichments in layers beneath the neutralized surface layers, leaching factors and proportions of organic silica-bearing colloids associated with detectable Cu were used to assess the mobility of Cu within the tailings columns. These factors indicated that the mobilization of Cu within the tailings columns varied from site to site, most likely due to different infiltration rates of colloids and aqueous species from the nearby capping soils. Here, the colloidal transport of Cu is strongly controlled by the presence of silica-bearing organic colloids as in their absence, the Cu concentrations in the identified colloids do not significantly contribute to the total concentrations of Cu in solution. The implications of these findings are that remediations with organic- and carbonate-rich soils results in the infiltration of carbonate-and organic-rich runoff into the tailings and thus in the immobilization of metals in the neutralized surficial tailings, but also in their enhanced mobilization in the layers beneath.