Identification of processes in Cu-ore heap leaching using Cu isotopes and leachate chemistry at Tschudi mine, northern Namibia

Abstract

Copper isotopic fractionation (in δ(65)Cu) and leachate characterization were studied in the context of heap leaching at the Tschudi copper mine in northern Namibia. The leached solution is of Mg-SO4 type with high Al and Fe concentrations. The source of Mg and Al in the leachate can be from the alteration of micas such as Mg-bearing muscovite confirmed by X-ray diffraction and scanning electron microscopy; however, the source of Mg cannot be determined with certainty. The principal secondary minerals identified in the leached ore are gypsum and jarosite. The value of pH in leachate is approx. 1.21 and the concentration of dissolved Cu, occurring mostly as CuSO4(0) and Cu(2+), is about 2 g/L. In comparison with unleached ore (avg. δ(65)Cu -1.47 parts per thousand), leached ore exhibits lighter isotopic values (avg. δ(65)Cu -6.01 parts per thousand) with apparent isotopic fractionation Δ(65)Cu(leached ore-unleached ore) of about -4.54 parts per thousand. In contrast, there is isotopic enrichment of leachate in heavier (65)Cu isotope (leachate δ(65)Cu 0.34 parts per thousand) with apparent isotopic fractionation Δ(65)Cu(leachate-unleached ore) value of +1.81 parts per thousand. These results are in good agreement with Cu isotopic fractionation and depletion in heavier (65)Cu isotope reported for dissolution experiments in laboratory and groundwater linked to the porphyry copper ore deposits around the world. The leaching of heaps can be considered an analogy of upper part of gossans, but here the supergene enrichment zone is missing due to extremely low pH and oxidizing conditions.