Hydrogeological, Mineralogical, and Geochemical Characterization of a Gold Mine Tailings Containment Area at the Giant Mine, NT

Abstract

The Giant Mine is an underground and open-pit, greenstone-hosted gold mine in Yellowknife, Northwest Territories, Canada. Mining operations (1948-1999) resulted in waste products of flotation tailings (85 wt%) and roaster residues (14 wt%) and arsenic (As) trioxide roaster waste (ATRW; 1 wt%), which were deposited in tailings containment areas. A water treatment system has been operational since 1999, utilizing the Northwest Tailings Containment Area (NW-TCA) as intermediary storage. The hydrogeology, aqueous geochemistry, and mineralogy were examined to understand the loading and transport of As and antimony (Sb) within the NW-TCA.

From 597,000 to 1,050,000 m3 of mine dewatering effluent (As: 22.2 ± 4.7 mg L-1; Sb: 0.79 ± 0.11 mg L-1) transitioned through the NW-TCA during the study period (2017-2022). A water balance, constructed with components of precipitation, evaporation, change in surface storage, effluent in, and effluent out, found that on average, 384,000 m3 y-1 of water discharged out of the NW-TCA annually. Very large, persistent downward hydraulic gradients in the southern half of the containment area indicate the predominant direction of flow.

Stable water isotope and water chemistry measurements indicate porewater in areas of high-water table is influenced by mine dewatering effluent. Isotope mass balance indicates 60% of the mine dewatering effluent, which contains high concentrations of As, is sourced from water cycled through the NW-TCA. Mine dewatering effluent, NW-TCA surface-water, and the tailings porewater contain elevated concentrations of As, Sb, Zn, and other metal(loid)s. The impact of the mine dewatering effluent is minimal in areas with a deep vadose zone, containing lower concentrations of As. Hydrological simulations indicate groundwater flow rates into the deep groundwater flow system from the NW-TCA would be modest in the absence of the mine dewater pond because the system is near net evaporative in the absence of mine dewatering effluent.

Acid-base accounting indicates the tailings pose low or no acid-generation risk. Oxygen concentrations indicate sulfide oxidation extends to 4 m depth. Oxidative dissolution of sulfide minerals in the vadose zone releases As, Sb, Zn, Co, Ni, and Cu to the porewater. A portion of this As is subsequently incorporated into secondary Fe(III) (oxyhydr)oxides. Below the oxidation zone, reducing conditions drive As and Sb release by reductive dissolution of Fe(III)-bearing roaster residues. At the base of the TCA, highly reducing conditions result in partial As sequestration through sulfate (SO4) reduction. Infiltration of mine dewatering effluent has resulted in elevated groundwater As in areas influenced by the holding pond. Effective management of As and Sb requires consideration of spatial- and temporal-dependent mobilization and attenuation processes within the TCA.

Bulk As K-edge XANES linear combination fitting indicates that solid-phase hosts of As in the TCA are approximately equal proportions As(-I), associated with arsenopyrite and arsenical pyrite, and As(V)-O/As(III)-O associated with roaster residues and secondary Fe(III) (oxyhydr)oxides. Solid-phase hosts of Sb are primarily roaster residues based on results from SEM-EDS, Sb K-edge XANES, and chemical sequential extractions. Solid-phase characterization reveals that extensive sulfide oxidation is restricted to the upper 1 m of the profile, with the persistence of minimally altered arsenopyrite and pyrite. Alteration of Fe(III) oxide roaster residues was not detected in mineralogical or spectroscopic analysis. Aqueous As speciation contrasts with thermodynamic predictions, indicating redox disequilibrium and limits of microbial activity. Dissolved As(III) is present in the oxidizing vadose zone porewater, sourced from arsenopyrite oxidation. As(V) is present in reducing groundwater, attributed to the infiltration of oxygenated surface water and reductive dissolution of As(V)-bearing oxides. Methylated As compounds are present in the anoxic vadose zone and shallow groundwater zone, likely formed as a product of metabolic detoxification. In comparison to freshly deposited tailings analyzed in 1999, there is evidence of sulfide oxidation but low alteration of As-bearing minerals. Aqueous geochemical distributions of As and Sb in the neutral mine drainage sourced from sulfidic flotation tailings and roaster residues are not coupled.

This research provides new knowledge into the hydrogeochemical processes controlling contaminant metal(loid) release and attenuation in mixed mine wastes composed of sulfidic flotation tailings and roaster residues. Remediation strategies are planned to be implemented at the NW-TCA in the near future. Completion of a year-round water treatment plant will eliminate the use of the NW-TCA as a storage reservoir for mine dewatering effluent. The findings of this study can be used to inform estimates of the rate of contaminant release, attenuation, and transport in the NW-TCA under different management scenarios.