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Chemical evolution and evaporation of shallow groundwaters discharging from a gold mine, southern New Zealand
Applied Geochemistry ( IF 3.4 ) Pub Date : 2020-11-01 , DOI: 10.1016/j.apgeochem.2020.104766
Erin Weightman , Dave Craw , Cathy Rufaut , Gemma Kerr , James Scott

Abstract Waste rock piles at the Macraes gold mine in southern New Zealand expose abundant fresh schist to alteration reactions that principally involve pyrite, chlorite, and calcite, yielding alkaline waters (pH~8) with elevated dissolved loads. Pyrite oxidation has resulted in dissolved sulfate concentrations rising up to ~3000 mg/L over 10 years, and this high sulfate is a potential environmental issue in waters leaving the site. Up to 600 mg/L dissolved Mg2+ results from chlorite dissolution. The site has a semi-arid climate and evaporative precipitation of sulfate minerals could contribute to an engineered water treatment system. However, the evolved waters are saturated or supersaturated with respect to carbonate minerals and spontaneous precipitation of aragonite is widespread, thereby limiting Ca-sulfate (gypsum) precipitation. Localised precipitation of Mg-sulfate (epsomite) also occurs, but this redissolves during rain events. Compositionally different waters seeping from the mine tailings impoundment have >2500 mg/L dissolved sulfate, and elevated Na+, K+ and Cl− from addition of reagents in the processing system. These seepage waters have also undergone interaction with waste rocks in the impoundment walls. The seep waters are approximately saturated with respect to carbonate minerals, but no spontaneous carbonate precipitation has occurred over two years of observations. Ephemeral evaporative epsomite and Na–Mg sulfate (bloedite) precipitates form locally. Geochemical modelling suggests that evaporative precipitation of gypsum has potential to remove sulfate from both types of water, especially if carbonate precipitation was suppressed by lowering the water pH to ~6 in an engineered treatment system that involves substantial evaporation.

中文翻译:

新西兰南部金矿排放浅层地下水的化学演化和蒸发

摘要 新西兰南部 Macraes 金矿的废石堆使大量新鲜片岩暴露于主要涉及黄铁矿、绿泥石和方解石的蚀变反应中,产生溶解负荷升高的碱性水 (pH~8)。黄铁矿氧化导致溶解的硫酸盐浓度在 10 年内上升至约 3000 毫克/升,而这种高浓度的硫酸盐是离开现场的水域的潜在环境问题。亚氯酸盐溶解产生高达 600 mg/L 的溶解 Mg2+。该场地属于半干旱气候,硫酸盐矿物质的蒸发沉淀有助于设计水处理系统。然而,就碳酸盐矿物而言,演化出的水是饱和或过饱和的,并且文石的自发沉淀很普遍,从而限制了硫酸钙(石膏)沉淀。还会发生硫酸镁(epsomite)的局部沉淀,但在降雨期间会重新溶解。从尾矿库中渗出的成分不同的水具有 >2500 mg/L 的溶解硫酸盐,并且由于在处理系统中添加试剂而导致 Na+、K+ 和 Cl- 升高。这些渗水还与蓄水墙中的废石发生相互作用。渗出水的碳酸盐矿物接近饱和,但在两年的观察中没有发生自发的碳酸盐沉淀。局部形成短暂的蒸发epsomite和Na-Mg硫酸盐(bloedite)沉淀。地球化学模型表明,石膏的蒸发沉淀有可能从两种类型的水中去除硫酸盐,
更新日期:2020-11-01
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