MICROBIOLOGICAL, CHEMICAL, AND MINERALOGICAL CHARACTERIZATION OF THE KIDD-CREEK MINE TAILINGS IMPOUNDMENT, TIMMINS AREA, ONTARIO

Bacterial enumeration and geochemical characterization were undertaken at three sites an the sulfide-rich railings impoundment at the Kidd C reek metallurgical site, Timmins, Ontario, Canada. The three sites wer e selected to represent varying degrees of sulfide oxidation to assess the changes in water chemistry, in the mineralogical composition of t he railings, and in bacterial populations as the sulfide oxidation pro cess proceeds under natural field conditions. The first site was chara cterized as having negligible oxidation-derived alteration, the pH of the porewater varied from 6.5 to 7.5, and the concentrations of dissol ved constituents were similar to those observed in the deeper, unalter ed tailings. Mineralogical examination of the tailings grains indicate d that the sulfide surfaces were sharp and unreplaced. At this site, t he predominant sulfur-oxidizing bacteria were Thiobacillus thioparus a nd related species. The second site showed evidence of the onset of ac idification, the pH of the near-surface porewater had decreased to 5.5 , and the concentrations of some sulfide oxidation products, principal ly SO4 and Zn, had increased beyond those observed in the unaltered ra ilings. At the second site, T. thioparus and related species predomina ted. Al the third site the acidification process was well established. The pH of the shallow porewater had decreased to <4.0, and high conce ntrations of several sulfide oxidation products were observed. Mineral ogical examination of the tailings indicated the presence of alteratio n rims of ferric oxyhydroxide minerals that surrounded the oxidizing s ulfide grains. At this site the predominant bacterial groups,were T. f errooxidans and T. thiooxidans types. At the third site the bacterial abundance reached a maximum at 25-45 cm below the tailings surface. Th e occurrence of the peak abundances coincided with the interface betwe en the zones of the unaltered tailings and the overlaying altered sulf ides. This coincidence suggests that the iron oxyhydroxide coatings, w hich surround the partly oxidized sulfide grains, may shield the partl y altered grains, thereby inhibiting bacterial growth. The data are co nsistent with an initial chemical oxidation of the metal sulfides to p roduce elemental sulfur and polysulfides, which are subsequently oxidi zed by neutrophilic thiobacilli to produce sulfuric acid. After the pH has decreased to the 3.5-4.0 range, the acidophilic thiobacilli, such as T. ferrooxidans, can directly oxidize the metal sulfides.