|Title||Groundwater chemistry of uranium-thorium-rare earth element deposits, Bancroft area, Ontario|
|Author||Desbarats, A J; Percival, J B|
|Source||Geological Survey of Canada, Open File 7992, 2016, 65 pages, https://doi.org/10.4095/297779 (Open Access)|
|Publisher||Natural Resources Canada|
|File format||pdf; rtf; xlsx|
|NTS||31D/16; 31E/01; 31F/04|
|Lat/Long WENS|| -78.4167 -77.8333 45.1667 44.8333|
|Subjects||geochemistry; 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|
|Illustrations||location maps; tables; photographs; plots; ternary diagrams; digital elevation models|
|Program||Tools for environmental impacts and adaptation for metal mining, Environmental Geoscience|
|Released||2016 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.