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.

奥塔哥中部的砂金矿山（placer gold mine）已堆积了大量平方公里级尺度的矿山尾矿（mine tailings），东奥塔哥的麦克雷斯造山型金矿（Macraes orogenic gold mine）目前仍在持续堆积尾矿。在这类尾矿上建立植被群落，至少需要从岩石中获取部分植物养分。磷是主要的限制养分（limiting nutrient）：赋存于副生磷灰石（accessory apatite）中的磷含量约为1000 mg/kg，这类磷灰石在云母片岩（micaceous schist）中最为富集，但在数周至数月的时间尺度下，其生物可利用性极低。氮也是重要的限制养分，但片岩（schist，尤其是云母片岩）通常含有500~1000 mg/kg的氮，且该类氮在数周至数月的时间尺度内可通过水体快速淋溶析出。牧草物种（pasture species）从尾矿中吸收的砷含量显著，以干重（dry weight）计浓度低于90 mg/kg，但尾矿基质（tailings substrates）中砷含量升高至约1500 mg/kg，并未对植物健康造成不利影响。采用不同氧化程度的片岩覆盖尾矿是促进植被重建（revegetation）的最有效手段，适量施加磷肥（phosphatic fertiliser）是较为可取的，但包括氮在内的其他养分在片岩覆盖层及下方尾矿中均具有充足的生物可利用性。