Regional isotopic investigation of evaporation and water-rock interaction in mine pit lakes in Nevada, USA

Abstract

The isotopic geochemistry of mine-impacted waters may reveal water-budget relationships and geochemical mechanisms of water-rock interaction. In this study, stable isotopic analysis of water (δ2H and δ18OH2O) and sulfate (δ34S and δ18OSO4) were completed and cataloged in 15 mine pit lakes in Nevada, USA, representing the most spatially distributed and largest dataset related to the isotopic composition of pit lakes yet presented in the literature. The isotopic composition of groundwater and surface springs was also included at several sites to more fully characterize the connected groundwater-surface water systems. Results of δ2H and δ18OH2O analysis indicate that the pit lakes are highly evaporated (more so than natural lakes in the same region), although the strength of evaporative fractionation differs spatially. Pit lakes also vary in the degree to which they are enriched in heavy isotopes relative to influent groundwater, which gives insight into the basin-scale water budget for these areas. Isotopes of sulfate (δ34S and δ18OSO4) and water (δ18OH2O) were used to test several commonly-held models of sulfide oxidation, and results suggest that none of the applied models can fully explain the isotopic composition of pit-lake waters. Most sulfur isotope data suggest a mixed source of aqueous sulfate including the oxidation of pyrite and dissolution of sulfate salts.