Geochemical and mineralogical controls on mine tailings rehabilitation and vegetation, Otago Schist, New Zealand
Large areas (square kilometre scale) of mine tailings have been deposited from placer gold mines in Central Otago, and are being deposited at Macraes orogenic gold mine in east Otago. Establishment of vegetation on these tailings involves at least some provision of plant nutrients from the rock. Phosphorus is the principal limiting nutrient, as the c. 1000 mg/kg P in accessory apatite, most abundant in micaceous schist, is only sparingly bioavailable on timescales of weeks to months. Nitrogen is an important limiting nutrient but schist, especially micaceous schist, typically contains 500–1000 mg/kg N, and this nitrogen is readily leachable with water on timescales of weeks to months. Arsenic uptake from tailings by pasture species is significant (< 90 mg/kg dry weight), but elevated As in tailings substrates (c. 1500 mg/kg) does not adversely affect plant health. Capping of tailings with variably oxidised schist is the most effective way of facilitating revegetation, and some addition of phosphatic fertiliser is desirable but other nutrients, including nitrogen, are adequately bioavailable in a schist cap and underlying tailings.