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TitleTrace element mobility in mine waters from granitic pegmatite U-Th-REE deposits, Bancroft area, Ontario
AuthorDesbarats, A J; Percival, J B; Venance, K E
SourceApplied Geochemistry vol. 67, 2016 p. 153-167, https://doi.org/10.1016/j.apgeochem.2016.02.010
Year2016
Alt SeriesEarth Sciences Sector, Contribution Series 20150266
PublisherElsevier
Documentserial
Lang.English
Mediapaper; on-line; digital
File formatpdf
ProvinceOntario
NTS31C/13; 31D/16; 31E/01; 31F/04
AreaBancroft
Lat/Long WENS -78.3333 -77.8333 45.0833 44.8667
Subjectsgeochemistry; hydrogeology; pegmatites; mineralization; uranium; thorium; radionuclides; rare earths; trace element analyses; mine site development; water analyses; water quality
Illustrationslocation maps; ternary diagrams; tables; plots
ProgramTools for environmental impacts and adaptation for metal mining, Environmental Geoscience
AbstractSmall, low-grade, granitic pegmatite U-Th-REE deposits are found throughout the Proterozoic Grenville geological province of eastern Canada. Renewed interest in this class of deposit, prompted by recent spikes in the price of U and the search for REE, has raised public concerns about potential impacts to groundwater quality from exploration and mining activities. In order to inform public debate and government decision making, the Geological Survey of Canada has undertaken a study of groundwater quality at historical mining properties in the Bancroft area, focusing on the mobility of trace elements that may pose health risks. Groundwater samples were obtained from diamond drill holes, adits and flooded mine shafts. The mainly Ca-HCO3 type groundwaters have circum-neutral pH and low specific conductance values. Uranium, almost entirely in the dissolved (< 0.45µm) phase, is found at concentrations ranging between 0.6 and 2579 µg/L. The highest U concentrations are observed in oxic groundwaters where complexation with carbonate ions enhances U mobility. The lowest concentrations are found in more reducing groundwaters that are in equilibrium with uraninite and coffinite. Thorian uraninite and uranothorite are the main U-bearing minerals. Uranium concentrations in groundwater appear to be correlated with the uranothorite content of the deposits rather than with their U grade. Uranothorite may be the more soluble U mineral because of its non-ideal composition and metamict structure. Thorium is released concomitantly with U during the dissolution of uraninite or uranothorite. Total Th concentrations in groundwater range between 0.03 and 11.56 µg/L. Mass balance and stoichiometric considerations indicate that almost all This being immobilized very close to its source. The sums of total light REE (La-Gd) concentrations range between 0.12 and 117.43 µg/L. The sums of total heavy REE (Tb-Lu) concentrations range between 0.05 and 21.34 µg/L. Light REE are derived mainly from the dissolution of metamict allanite whereas the sources of heavy REE are widely dispersed among accessory minerals. Fractionation patterns of REE in the dissolved phase are flat or concave, with negative Ce anomalies associated with more oxic groundwaters. The data suggest preferential LREE and HREE complexation with organic and carbonate ligands, respectively. Fractionation patterns in the suspended particulate phase exhibit decreasing enrichment with atomic number from La to Gd and a flat profile from Tb to Lu. This is explained by preferential sorption of LREE and uniform sorption of HREE. The data suggest that a Mn precipitate is the most likely sorbent for REE.
Summary(Plain Language Summary, not published)
Small, low-grade, granite-hosted Uranium-Thorium-Rare Earth Element (REE) deposits are found throughout the Grenville geological province in Ontario, Québec and Labrador. Renewed interest in this class of deposit, prompted by spikes in the price of uranium and the search for REE, has raised concerns about potential impacts to groundwater quality from mining activities. In order to inform public debate and government decision making, NRCan scientists have investigated groundwater quality at historical mining sites in the Bancroft area, focusing on metals that may pose health risks. Results of this study show that elevated uranium concentrations are derived from more soluble uranothorite, rather than uraninite, in the presence of carbonate species from calcite dissolution. Thorium, released concurrently with uranium, is immobilized close to its source. REE, from the dissolution of allanite and other accessory minerals, occur at very low concentrations, mainly adsorbed on manganese precipitates.
GEOSCAN ID297053