ABSTRACT Groundwater-driven clay alteration of gold-mineralised Otago Schist has caused leaching and redistribution of arsenic (As). At Macraes mine in East Otago, there is a zone of alteration up to 50 m thick beneath a Cretaceous-Cenozoic regional unconformity where hydrothermal As concentrations >1000 ppm have decreased to <150 ppm. Where periglacial aeolian silt (loess) overlies mineralised Otago Schist, As has migrated in near-surface groundwater to form geochemical enrichment anomalies in loess and colluvium, with As concentrations locally as high as 500 ppm. Most As in the clay-altered basement and Pleistocene cover is closely associated with iron oxyhydroxide (HFO) and occurs as adsorbed As(V) on the HFO or as ferric arsenate minerals. However, initial As mobility was most effective as more soluble As(III) during oxidative transition from original sulphides to arsenate. Upward As migration into Pleistocene debris on the metre scale can facilitate location of gold-mineralised rocks that have been obscured by the young cover.

中文翻译：

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新西兰东奥塔哥金矿化带上的地表砷再分布

摘要 地下水驱动的金矿化奥塔哥片岩粘土蚀变导致了砷 (As) 的浸出和重新分布。在东奥塔哥的 Macraes 矿，在白垩纪-新生代区域不整合面下方有一个厚达 50 m 的蚀变带，其中热液砷浓度 >1000 ppm 已降至 <150 ppm。在冰缘风成粉砂（黄土）覆盖在矿化奥塔哥片岩上的地方，砷在近地表地下水中迁移，在黄土和崩积层中形成地球化学富集异常，局部砷浓度高达 500 ppm。粘土变质基底和更新世覆盖层中的大多数 As 与羟基氧化铁 (HFO) 密切相关，并以吸附在 HFO 上的 As(V) 或砷酸铁矿物形式出现。然而，在从原始硫化物到砷酸盐的氧化转变过程中，初始 As 迁移率最有效，因为更易溶解 As(III)。向上迁移到米尺度的更新世碎片可以促进被年轻覆盖物遮挡的金矿化岩石的定位。