Discriminating between background and mine-impacted groundwater at the Phoenix mine, Nevada USA

摘要

Differentiating between mineralized and non-mineralized background groundwater chemistry at a mine site can be challenging if there is an overprint of past and/or current mining on naturally mineralized conditions. At the Phoenix mine in the Copper Canyon mining district of Nevada, quantile–quantile H+/SO4 plots were used to segregate four wells clearly impacted by historical mining activity. The mineralogy of rock at the elevation of the well screen interval was used to partition the 53 remaining wells into mineralized and non-mineralized populations. For each class, groundwater chemistry was examined to identify if SO4 and H+ trends were stable (unimpacted) or increasing (impacted). Then each well was mapped as one of four resulting groundwater types across the mine site, defining the spatial extent of the different groups. Several groundwater regions were identified. A group of mineralized, mine-impacted wells (Type II) are located in Philadelphia Canyon adjacent to the Cu leach facility, with the anthropogenically impacted area bounded by several hydrologically downgradient, mineralized, unimpacted wells (Type I) to the south and east. There is a set of non-mineralized, impacted wells (Type III) downgradient from the tailings pond facility, where a historical release of SO4 is apparent in the well record. However, in some downgradient wells the tailings pond pump-back mitigation system has resulted in recovery of the groundwater quality to a non-mineralized background condition. Finally, in the vicinity of the Reona heap leach pad, there is a group of non-mineralized, unimpacted wells (Type IV). Not surprisingly, most mineralized wells (Types I and II) are located in or near mined areas, while non-mineralized wells (Types III and IV) tend to be in the southern portion of the facility in the alluvia of Buffalo and Reese River valleys. Once the well population was categorized, Phx-F was identified with extremely mineralized background groundwater (pH = 3.8; SO4 = 4600 mg/L; As = 0.056 mg/L; Cu = 72 mg/L; Zn = 61 mg/L), which required further analysis. The elevation of the Phx-F screen interval had up to 5% jarosite, with mineralized fracture zones mapping directly to cavities in the adjacent Iron Canyon mine that originally contained a saturated ferrous sulfate solution, explaining the anomalous chemistry in this well. This study demonstrates that an empirical approach using mining history, deposit geology, hydrogeology, well screen elevation mineralogy, groundwater chemistry trends, statistical analysis, and geochemical modeling provides a technique to distinguish among different classes of groundwater at the operating Phoenix mine site. Such an approach (or a derivative) may prove useful at other mines with different mineralogies and hydrogeologic settings.