AQUEOUS GEOCHEMISTRY AND ANALYSIS OF PYRITE SURFACES IN SULFIDE-RICH MINE TAILINGS

Aqueous geochemical techniques and analysis of pyrite surfaces have be en used to study element partitioning between the aqueous and solid ph ase and to infer mechanisms that limit the concentrations of elements in porewater in a sulfide-rich mine tailings impoundment. Porewater sa mples and pyrite grains for surface analysis were collected from oxidi zed and unoxidized zones within the tailings. Surface analyses were co nducted using a Time-of-Flight Laser-Ionization Mass Spectrometer (TOF LIMS). 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 oc cur at the surfaces of the pyrite grains from all of the sample locati ons. The porewater concentrations of these elements in the low-pH zone may be controlled by precipitation or coprecipitation in secondary mi neral coatings on the pyrite surface. Surface abundances of the metals Cu, Ag, Pb, Zn, and Cd are lowest, and porewater concentrations are h ighest, in the low-pH oxidized tailings. Surface abundances of As are greatest, and porewater concentrations are lowest, in the low-pH sulfi de-oxidation zone. These trends vs. pH are consistent with an adsorpti on model for attenuation of Cu, Ag, Pb, Zn, Cd, and As from the porewa ter. The porewater Cu and Ag concentrations may be limited by replacem ent reactions that form secondary Cu and Ag sulfides at the pyrite sur face. The highest abundance of C on the surface of the pyrite grains i s in the shallow sulfide-oxidation zone; this interval coincides with large abundances of chemolithotrophic bacteria and may reflect populat ions of iron- and sulfur-oxidizing bacteria such as Thiobacilli. Copyr ight (C) 1997 Elsevier Science Ltd.