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TitleHydrogeochemistry of mine tailings from a carbonatite-hosted Nb-REE deposit, Oka, Quebec, Canada
LicencePlease note the adoption of the Open Government Licence - Canada supersedes any previous licences.
AuthorDesbarats, A JORCID logo; Percival, J BORCID logo
SourceGeological Survey of Canada, Scientific Presentation 154, 2023, 35 pages, Open Access logo Open Access
PublisherNatural Resources Canada
Meeting29th International Applied Geochemistry Symposium; Vina del Mar; CL; October 23-28, 2022
Mediapaper; digital; on-line
File formatpdf
NTS31H/12; 31G/09
Lat/Long WENS -74.5000 -73.5000 45.7500 45.5000
Subjectsgeochemistry; hydrogeology; environmental geology; Science and Technology; Nature and Environment; mineralogy; hydrogeochemistry; tailings; tailings geochemistry; rare earths; rare earths geochemistry; rock analyses, rare earth elements; carbonatites; niobium
Illustrationsphotographs; location maps; diagrams; geochemical profiles; plots
ProgramEnvironmental Geoscience Cumulative effects in cobalt watersheds
Released2023 01 11
AbstractEnvironmental impacts associated with the mining of carbonatite deposits are an emerging concern due to the demand for critical metals. This study investigates the chemistry of tailings seepage at the former Saint Lawrence Columbium mine near Oka, Québec, Canada, which produced pyrochlore concentrate and ferroniobium from a carbonatite-hosted Nb-REE deposit. Its objectives are to characterize the mineralogy of the tailings and their pore water and effluent chemistries. Geochemical mass balance modeling, constrained by aqueous speciation modeling and mineralogy, is then used to identify reactions controlling the chemical evolution of pore water along its flow path through the tailings impoundment. The tailings are composed mainly of REE-enriched calcite (82 wt. %), biotite (12 wt. %) and fluorapatite (4 wt. %). Minor minerals include chlorite, pyrite, sphalerite, molybdenite and unrecovered pyrochlore. Secondary minerals include gypsum, barite and strontianite. Within the unsaturated zone, pore water chemistry is controlled by sulfide oxidation and calcite dissolution with acid neutralization. With increasing depth below the water table, pore water composition reflects gypsum dissolution followed by sulfate reduction and FeS precipitation driven by the oxidation of organic carbon in the tailings. Concomitantly, incongruent dissolution of biotite and chlorite releases K, Mg, Fe, Mn, Ba and F, forming kaolinite and Ca-smectite. Cation exchange reactions further remove Ca from solution, increasing concentrations of Na and K. Fluoride concentrations reach 23 mg/L and 8 mg/L in tailings pore water and effluent, respectively. At a pH of 8.3, Mo is highly mobile and reaches an average concentration of 83 µg/L in tailings effluent. Although U also forms mobile complexes, concentrations do not exceed 16 µg/L due to the low solubility of its pyrochlore host. Adsorption and the low solubility of pyrochlore limit concentrations of Nb to less than 49 µg/L. Cerium, from calcite dissolution, is strongly adsorbed although it reaches concentrations (unfiltered) in excess of 1 mg/L and 100 µg/L in pore water and effluent, respectively. Mine tailings from carbonatite deposits are enriched in a variety of incompatible elements with mineral hosts of varying reactivity. Some of these elements, such as F and Mo, may represent contaminants of concern because of their mobility in alkaline tailings waters.
Summary(Plain Language Summary, not published)
New technologies are driving increased demand for niobium (Nb) and rare earth elements (REE), which are critical metals found in carbonatite geological intrusions. However, potential environmental risks from mining critical metals are still largely unknown. To address knowledge gaps and NRCan objectives with respect to critical metals and sustainable resource development, the GSC Environmental Geoscience Program supported this study of drainage chemistry in tailings at the former Saint Lawrence Columbium mine near Oka, Québec. The mine produced Nb concentrate and ferroniobium alloy from a carbonatite-hosted Nb-REE deposit. Because the tailings are composed of over 60 % calcite, they do not generate acid mine drainage despite the presence of 0.5 % sulfide minerals. However, effluent from the tailings impoundment contains sulfate, fluoride and molybdenum at concentrations that may be harmful to aquatic life. Other trace metals including Nb, REE, uranium and zinc are present at low concentrations. The new knowledge generated through this study is transferable to similar carbonatite-hosted critical metal deposits that may undergo environmental impact assessments in the future.

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