Abstract:
Aqueous geochemical techniques and analysis of pyrite surfaces have been used to study element partitioning between the aqueous and solid phase and to infer mechanisms that limit the concentrations of elements in porewater in a sulfide-rich mine tailings impoundment. Porewater samples and pyrite grains for surface analysis were collected from oxidized and unoxidized zones within the tailings. Surface analyses were conducted using a Time-of-Flight Laser-Ionization Mass Spectrometer (TOFLIMS). The porewater pH at the different sample locations varies from 3.85 to 6.98. High relative abundances of Na, K, Ca, Mg, Al, and Ni occur at the surfaces of the pyrite grains from all of the sample locations. The porewater concentrations of these elements in the low-pH zone may be controlled by precipitation or coprecipitation in secondary mineral coatings on the pyrite surface. Surface abundances of the metals Cu, Ag, Pb, Zn, and Cd are lowest, and porewater concentrations are highest, in the low-pH oxidized tailings. Surface abundances of As are greatest, and porewater concentrations are lowest, in the low-pH sulfide-oxidation zone. These trends vs. pH are consistent with an adsorption model for attenuation of Cu, Ag, Pb, Zn, Cd, and As from the porewater. The porewater Cu and Ag concentrations may be limited by replacement reactions that form secondary Cu and Ag sulfides at the pyrite surface. The highest abundance of C on the surface of the pyrite grains is in the shallow sulfide-oxidation zone; this interval coincides with large abundances of chemolithotrophic bacteria and may reflect populations of iron- and sulfur-oxidizing bacteria such as Thiobacilli.