Molybdenum (Mo) stable isotopic variations as indicators of Mo attenuation in mine waste-rock drainage

Measurements of molybdenum (Mo) stable isotopes (δ98Mo) were applied along with conventional geochemical analyses of water and rock samples at a Mo mine in the USA to assess controls on Mo mobility in two unsaturated waste-rock storage facilities (WRSFs) that are differentiated by acidic and alkaline drainages. Unweathered waste rock, Mo ore, and mine tailings from the site were also analyzed to constrain the isotopic composition of the source of Mo entering mine drainage via weathering processes. The surfaces of weathered waste-rock samples were sequentially leached using a two-step chemical extraction to determine the distribution and isotopic composition of Mo among primary and secondary mineral assemblages. The observed variation in δ98Mo among all samples exceeded 5‰. The δ98Mo of Mo-rich ({\textgreater}340 μg/L) mine process water and pit wall runoff ranged from 0.6 to 0.7‰, which was equal within error to the average δ98Mo of waste rock, tailings, and ore samples and indicated negligible isotopic fractionation during molybdenite oxidative dissolution. In contrast, drainage from the base of both waste-rock storage facilities was isotopically heavier, ranging from 1.6 to 1.9‰. Coupled δ98Mo and Mo/SO42- ratios in mine drainage and sequential chemical extractions of weathered waste-rock surfaces indicated that Mo adsorption onto mineral surfaces was most likely driving the increase in δ98Mo in drainage from both WRSFs. Molybdenum stable isotope data provided a powerful tracer of processes controlling Mo mobility when used in conjunction with conventional geochemical analyses.