Time-series monitoring of water–rock interactions in mine wastes, Macraes gold mine, New Zealand

The main tailings impoundment at the Macraes mine was active for 23 years until it was decommissioned in 2013. Water compositions in the tailings complex were closely monitored for 20 years, providing a time series of compositional changes during the interaction between mine waters, sulphidic ore (pyrite and arsenopyrite) and greenschist facies schist host rocks. Most waters had circumneutral pH because of abundant (2–10%) calcite in host rocks. Relict Fe2+ caused a temporary pH decrease before re-neutralisation. Chlorite, albite and muscovite partially altered to kaolinite, smectites and illite. Dissolved sulphate reached as high as 8000 mg/L, but this was partially attenuated by the precipitation of gypsum in the tailings complex, while the alkalinity (as dissolved ) rose by three orders of magnitude in parallel with the increase in pH. Dissolved As decreased by six orders of magnitude, to < 0.01 mg/L, between the processing plant and the groundwater system below the tailings complex. Dissolved sulphate plumes emanate from tailings and waste rock piles, but are diluted downstream.

麦克雷斯矿（Macraes mine）的主尾矿库（tailings impoundment）在2013年退役前已持续运行23年。研究团队对该尾矿综合体的水体成分开展了长达20年的密切监测，获取了矿坑水、硫化矿石（黄铁矿（pyrite）与毒砂（arsenopyrite））以及绿片岩相片岩围岩（greenschist facies schist host rocks）相互作用过程中水体成分变化的时间序列数据集。由于围岩中含有2%~10%的丰富方解石（calcite），多数水体呈近中性pH。残留的亚铁离子（Fe2+）会在发生再中和前引发pH的暂时性下降。绿泥石（chlorite）、钠长石（albite）与白云母（muscovite）会部分蚀变为高岭石（kaolinite）、蒙脱石类（smectites）与伊利石（illite）。溶解态硫酸盐（dissolved sulphate）的最高浓度可达8000 mg/L，但尾矿复合体内石膏（gypsum）的沉淀作用会部分削弱其浓度；与此同时，碱度（以溶解态计）随pH升高提升了三个数量级。从选矿厂（processing plant）到尾矿库下方的地下水系统（groundwater system）之间，溶解态砷（dissolved As）浓度下降了六个数量级，降至<0.01 mg/L。尾矿与废石堆（waste rock piles）会释放溶解态硫酸盐羽流（plumes），但此类羽流在下游过程中会被水体稀释。