The closure of the Century Pb-Zn mine open-pit workings brings with it the need for an understanding of the environmental impact of the mine and also an opportunity for renewed mineral exploration in the surrounding tenements. A novel combined approach to these questions is presented here using high-resolution Pb and Sr isotopic analyses of groundwaters surrounding the mine. A study of this nature has never been attempted before.
Pb isotopic signatures obtained from the 21 accessible groundwater bores in 2014, when compared to the Century (206Pb/204Pb = 16.441) (and nearby Silver King, 206Pb/204Pb = 16.337) ore bodies, show lower Pb isotope ratios proximal to the pit and higher Pb isotope ratios with increasing distance from the pit (A_GW05 206Pb/204Pb = 18.322). Mixing calculations, however, indicate only very small contributions from the bulk Century ore to the groundwater even in those samples measured closest to the pit. Sr isotope ratios are relatively uniform across the region. The mixing calculations comparing the contribution of both Pb and Sr from the two aquifers (Thorntonia Limestone Aquifer and Lawn Hill Formation (shale) Aquifer) to the groundwater geochemistry indicate that the groundwater receives a greater contribution from the Thorntonia Limestone than the Lawn Hill Formation. Overall, the mixing calculations indicate there is no significant Pb contamination from the mining operation to the surrounding groundwater, likely as a result of the low solubility of Pb complexes in this setting.
Despite the very low Pb concentrations in the groundwater, Pb isotopic compositions appear to faithfully mirror the compositions of local sources, thus signatures adjacent to known prospects reflect those of the prospects, demonstrating the potential of the groundwater to reveal mineralisation signatures. The results of this study show promise in the use of Pb isotopes as tracers of environmental contamination as well as vectors for identifying potential mineralisation, although the sensitivity of the method in specific settings will depend upon groundwater chemistry.