An assessment of sulphide oxidation in abandoned base-metal tailings, Te Aroha, New Zealand

Tailings from the Tui base-metal mine were characterized using a variety of techniques including scanning electron microscopy (SEM) and flame atomic absorption spectroscopy (FAAS), to assess their potential for use as a plant growth medium. With the notable exception of Pb (10 568 mg kg−1), ‘total’ metal concentrations in the surface tailings (0–200mm) were relatively low (Cu, 113; Fe, 3660; Zn, 486 mg kg−1). The theoretical acid generating potential (TAGP) and ‘total’ concentrations of Cu, Fe and Zn of the tailings, were found to increase greatly with depth, reflecting an increase in the abundance of chalcopyrite (CuFeS2), pyrite (FeS2) and sphalerite (ZnS), as detected by X-ray diffraction (XRD) analysis. SEM micrographs indicate that the distribution of sulphide minerals in the tailings was originally uniform with depth. The depletion of Cu, Fe and Zn in the surface tailings is considered to be a result of sulphide oxidation, as evidenced by the craggy and highly irregular morphology of the sulphide particles and the high hydrogen ion activity (pH 2.3–4.0) in this zone. The persistence of high concentrations of acid-generating sulphide minerals between 200 and 600 mm has important implications in determining strategies for revegetating the tailings.