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TitleGroundwater chemistry of uranium-thorium-rare earth element deposits, Bancroft area, Ontario
AuthorDesbarats, A J; Percival, J B
SourceGeological Survey of Canada, Open File 7992, 2016, 65 pages,
PublisherNatural Resources Canada
Documentopen file
Mediaon-line; digital
File formatreadme
File formatpdf; rtf; xlsx
NTS31D/16; 31E/01; 31F/04
Lat/Long WENS-78.4167 -77.8333 45.1667 44.8333
Subjectsgeochemistry; environmental geology; heavy metals contamination; metals; base metal deposits; environmental studies; environmental analysis; geochemical analyses; geochemical interpretations; water analyses; water geochemistry; arsenic; antimony; pollutants; groundwater pollution; groundwater geochemistry
Illustrationslocation maps; tables; photographs; plots; ternary diagrams; digital elevation models
Natural Resources Canada Library - Ottawa (Earth Sciences)
ProgramTools for environmental impacts and adaptation for metal mining, Environmental Geoscience
Released2016 04 20
Granitic pegmatite-hosted U-Th-REE deposits and related mineralized skarns and calcite dikes are widespread within the Grenville geological province of eastern Canada, although mining of these deposits has been limited to the Bancroft area of Ontario. Renewed exploration for this type of mineral deposit in Ontario and Québec has raised concerns about possible impacts to groundwater quality. In order to better inform public debate on uranium exploration, government policy, and regulatory decision-making, the Geological Survey of Canada (GSC) has undertaken a study of groundwater quality at ten historical mines and prospects in the Bancroft area focusing on trace elements of potential environmental concern. Groundwater samples were obtained from flooded shafts, flowing adits and diamond drill holes.
Uranium, from the oxidation and dissolution of uraninite and uranothorite, occurs at concentrations as high as 2580 ?g/L. The Health Canada (2014) Maximum Acceptable Concentration (MAC) of 20 ?g/L was exceeded in approximately 70% of samples. It was found that groundwaters exhibiting U concentrations less than the MAC are unlikely to exhibit radium- 226 and lead-210 radionuclide activities in excess of regulatory limits which are 0.5 and 0.2 Bq/L, respectively. The high mobility of U in shallow oxic groundwaters can be attributed to the availability of dissolved Ca and carbonate species for the formation of stable aqueous complexes known to inhibit adsorption. Because of its high mobility in shallow groundwater systems, U may pose an environmental risk where mine adits drain to nearby surface water receptors. This was found to be the case at the Amalgamated Rare Earth No.2 mine.
Thorium is released concurrently with U from the oxidation and dissolution of thorian uraninite and uranothorite. However, Th concentrations are very low with a maximum of only 11 ?g/L in unfiltered groundwater samples. Almost all Th is removed from solution close to source.
Allanite is the most likely source of Light Rare Earth Elements (La-Gd) whereas sources of Heavy Rare Earth Elements (Tb-Lu) appear to be dispersed among numerous minerals. A maximum total REE concentration of 126 ?g/L was observed in unfiltered groundwater samples.*** It was found that the ranges of Th, LREE and HREE concentrations observed in this study fall within the ranges observed in groundwaters from unmineralized Precambrian bedrock although mean concentrations are somewhat higher. Uranium concentrations in groundwater from U-Th- REE deposits exhibit a much wider range than background values and their mean is more than 50 times greater. The MAC for uranium (20 ?g/L) provides a reasonable threshold for distinguishing groundwaters in contact with U-Th-REE deposits from the natural background.
This retrospective investigation of environmental impacts to groundwater from historical mining activity in the Bancroft district provides geoscience data and knowledge useful to regulators and proponents of new mining projects in similar geological settings. This information can be used to anticipate and mitigate environmental risks associated with future resource development.
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 regulatory decision making, NRCan scientists have conducted a retrospective investigation of groundwater quality at historical mining sites in the Bancroft area, focusing on metals that may pose health risks. Results of this study show that uranium is highly mobile in shallow groundwater systems and may be present at high concentrations in mine drainage. Natural attenuation processes are not always effective in reducing uranium loading in discharge to downstream surface waters. Thorium and REE are not very mobile in the groundwater environment and occur at concentrations within the range of natural background values.