Geochemical responses of buried kimberlites in shallow groundwater in the Attawapiskat kimberlite field, James Bay Lowlands, Canada

Shallow groundwaters were examined in the Attawapiskat kimberlite region, James Bay Lowlands, Ontario, Canada in order to assess buried kimberlite impacts on shallow groundwater geochemistry. Kimberlite pathfinder metals consisting of light rare earth elements (LREEs), Ni, Cr, Ti, Ba, Rb, Cs, and the ratio of Mg/Ca are commonly elevated in peat groundwaters along sampling transects as far as 200 m outside of kimberlite margins. The principal mechanism in transporting these metals upwards from kimberlites, through up to 21 m of TSS and up to 4 m of peat, is upward movement of deep groundwater. This mechanism is inferred based on hydrogeology and geochemistry of groundwaters in the TSS and peat. Fractures along the boundaries between kimberlites and limestone are suggested to provide pathways for upward groundwater movement.;Groundwaters from TSS over limestone, from boreholes in limestone, and kimberlites have lower CH4 concentrations and variable Delta 13CDIC-CH4 values (55 to 70‰) consistent with biological methane oxidation. All but one CH4 sample in TSS groundwater over kimberlite indicates in situ formation. Less negative delta 2HH2O values (greater than the local meteoric water line) are typically those samples from TSS over kimberlites and suggest the production of CH4, as biological DIC reduction requires H+ ions from H2O to form CH4. The ratio of CH4 to the concentrations of Fe3+, SO42- and O2(aq) are greatest in the majority of TSS groundwater samples collected over kimberlites, and suggests kimberlites may indirectly influence the CO2-CH4 system by consuming oxidized ions in the overlying TSS.;The results of this study suggest that buried kimberlites can be identified through the detection of high Ni, Cr, Ti, LREEs, K, Rb, Ba concentrations, and the ratio of Mg/Ca in peat groundwaters over or near the kimberlite margin. However, geochemical processes such as metal -- DOM complexation, adsorption, and incorporation of metals into precipitated minerals are important considerations when identifying the spatial distribution of pathfinder metals along transects. Variations in CH4 emissions occur in soils and groundwaters overlying kimberlites could be due to the release of reduced ions during serpentinization reactions. Methane gas coupled with C isotopic variations in CH4 and DIC may represent a possible pathfinder geochemical tool to use in conjunction with pathfinder metals. (Abstract shortened by UMI.);Metal -- dissolved organic matter (DOM) speciation calculations using Visual MINTEQ 3.0 and the NICA-Donnan database were conducted for Nd, Ni, Ba, and K. These pathfinder metals were used, as they are analogs for trivalent and divalent transition metals, alkaline earth metals, and alkali metals, respectively. The model predicts almost 100% of soluble Nd, Ni, and Ba form complexes with DOM at sampling sites with little to no contribution from upwelling (minerotrophic) groundwater (i.e., dissolved organic carbon (DOC) concentrations = 40 to 132 mg/L, pH = 3.9 to 5.5, and log ionic strength = < -3). Comparatively, as little as 10% of Nd and Ni, and 0% K and Ba are predicted to complex with DOM under conditions with large contributions from upwelling groundwaters (i.e., DOC concentrations = < 40 mg/L, pH = 5.5 to 6.5, and log ionic strength = -3 to -2). Based on modeling calculations, competitive availability of DOM binding sites where ionic strength is elevated (upward migration of groundwater) is the dominant control on metal-DOM complexation. MINTEQ modeling calculations indicate soluble Ni and Nd are likely scavenged from solution under strong upwelling conditions (i.e. high ionic strength). Metal scavenging is likely due to increased adsorption onto precipitating ferrihydrite (based on logSIferr close to or greater than 0) coupled with decreases in metal fractions complexed with DOM. Total Ni and Nd concentrations are typically 5 times lower than waters with little to no upwelling and ferrihydrite saturation indices (logSIferr) strongly indicate precipitation (up to 5). Comparatively, the concentrations of dissolved Ni and Nd in peat groundwaters are elevated where they are predicted to dominantly complex with DOM, and ferrihydrite is highly under saturated (logSIferr -18 to 5). Metal complexation with DOM may effectively inhibits metal scavenging from solution.