Geochemistry and mineralogy of Cu and Co in mine tailings at the Copperbelt, Zambia

Abstract

Two sulfidic mine tailings within the Zambian Copperbelt in the north of Zambia have been studied: Chambishi, representing an old site (age about 40 y) and Mindolo, which represents a relatively recent site (age less than 10 y). The neutralization capacity based on solid phase carbonates at both sites remains high, thus neutral to alkaline conditions (paste pH up to 8.5 at Chambishi and up to 6.9 at Mindolo) predominate. Pore water at Chambishi has 568 mg/l of Ca and 1820 mg/l of sulfate, but concentrations of Fe and Mn are below 0.1 mg/l and concentrations of Cu and Co are below 0.05 mg/l. The principal secondary minerals at both sites are gypsum, poorly crystalline Fe(III) phases and hematite. Secondary Fe(III) phases are found as mineral coatings or completely replaced primary sulfides like pyrite and chalcopyrite and include large quantities of copper and cobalt in surface rims (up to 7.0 wt.% of Cu and up to 2.0 wt.% of Co). The presence of Fe(III) phases is marked by red color of mine tailings material, which is observed even below the expected penetration of the sulfide oxidation front. This may be explained by reductive dissolution of Fe(III) phases caused by flooding of tailings and temporarily reducing conditions during rainy period, when dissolved iron is transported by infiltrating water to the deeper zone of mine tailings, where it re-precipitates later. At the Chambishi site, precipitation of secondary minerals resulted in an early stage of hardpan formation at 0.6–0.9 m depth, composed mostly of gypsum and hematite. This zone also corresponds to maximum solid phase contents of Cu and Co. No such hardpans were found at the relatively young Mindolo site, where red tailings material, which includes poorly crystalline Fe(III) phases and hematite, is present only in discrete banded zones at several depth levels. Based on geochemical modeling results at the Mindolo site, precipitation of secondary Cu phases such as brochantite and malachite is likely in the zone of evaporation enrichment close to the mine tailings surface. At both the Chambishi and Mindolo sites, there does not seem to be a threat of acid mine drainage formation even over the long-term. Furthermore, the Cu and Co incorporated in hematite seem to be immobilized within the mine tailings.