ABSTRACT

Evaporative mineral encrustations have formed on bare substrates at three different sites in the Maniototo basin. Two sites, at Patearoa and Belmont have salts derived primarily from marine aerosols. The Patearoa site has developed on clay-rich pans formed on a terrace eroded by the Taieri River into clay-altered schist basement, and dissolution of albite from altered schist has resulted in precipitation of sodium carbonate with halite, at pH∼10. The Belmont site lies on the flat distal part of an active alluvial fan, and halite-dominated salt encrustations develop on thin dry crusts through capillary action from below. At the nearby Hamiltons historic placer gold mine, excavations exposed clay-altered schist basement beneath Eocene auriferous sediments. Highly fractured and hydrothermally altered fault zones dominate basement, with secondary ankeritic carbonate and pyrite. Water-rock interaction of shallow groundwater in the basement leads to enrichment in dissolved Mg and sulphate that overshadows the marine aerosol components, and evaporative encrustations are dominated by Mg-rich minerals including brucite and epsomite. The halite-dominated sites host rare halophyte ecosystems, but progressive colonisation by adventive species will eventually cover the bare saline substrates. Similar colonisation at the Hamiltons site will result in natural rehabilitation of the abandoned mine site.

摘要

在马尼奥托托盆地的三个不同地点的裸露基质上形成了蒸发矿物结壳。Patearoa 和 Belmont 两个地点的盐分主要来自海洋气溶胶。Patearoa 遗址在富含粘土的盆地上发育而成，该阶地被 Taieri 河侵蚀成粘土蚀变的片岩基底，蚀变片岩中的钠长石溶解导致碳酸钠和石盐在 pH ~ 10 时沉淀。贝尔蒙特遗址位于一个活跃的冲积扇的平坦远端，岩盐为主的盐结壳通过下方的毛细管作用在薄的干燥地壳上形成。在附近的汉密尔顿历史悠久的砂金矿中，挖掘工作暴露了始新世含金沉积物下的粘土蚀变片岩基底。高度断裂和热液蚀变断层带主导地下室，含次生铁白云母碳酸盐和黄铁矿。地下室浅层地下水的水岩相互作用导致溶解镁和硫酸盐的富集，使海洋气溶胶成分黯然失色，蒸发结壳主要是水镁石和镁镁石等富镁矿物。以岩盐为主的地点拥有稀有的盐生植物生态系统，但外来物种的逐步殖民最终将覆盖裸露的盐碱基质。汉密尔顿遗址的类似殖民化将导致废弃矿区的自然恢复。以岩盐为主的地点拥有稀有的盐生植物生态系统，但外来物种的逐步殖民最终将覆盖裸露的盐碱基质。汉密尔顿遗址的类似殖民化将导致废弃矿区的自然恢复。以岩盐为主的地点拥有稀有的盐生植物生态系统，但外来物种的逐步殖民最终将覆盖裸露的盐碱基质。汉密尔顿遗址的类似殖民化将导致废弃矿区的自然恢复。