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

Copper isotopic fractionation (in δ⁶⁵Cu) 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 ∼1.21 and the concentration of dissolved Cu, occurring mostly as CuSO₄⁰ and Cu²⁺, is about 2 g/L. In comparison with unleached ore (avg. δ⁶⁵Cu −1.47 ‰), leached ore exhibits lighter isotopic values (avg. δ⁶⁵Cu −6.01 ‰) with apparent isotopic fractionation Δ⁶⁵Cu_{leached ore-unleached ore} of about −4.54 ‰. In contrast, there is isotopic enrichment of leachate in heavier ⁶⁵Cu isotope (leachate δ⁶⁵Cu 0.34 ‰) with apparent isotopic fractionation Δ⁶⁵Cu_{leachate-unleached ore} value of +1.81 ‰. These results are in good agreement with Cu isotopic fractionation and depletion in heavier ⁶⁵Cu 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.