APPLICATION OF 1-D VERTICAL PROFILES OF δ2H AND CL TO EVALUATE VERTICAL LEAKAGE THROUGH A FRACTURED CRETACEOUS AQUITARD AT A PROPOSED MINE DEWATERING SITE, FORT Á LA CORNE, SASKATCHEWAN, CANADA

A proposed open pit diamond mine owned by Shore Gold Inc. is located in central Saskatchewan, Canada. Access to the kimberlites located in the late Cretaceous-aged Lower Colorado shale will require the development of a major dewatering program to depressurize the underlying Mannville Group aquifer. Secondary structures consisting of fractures, sand lenses, and kimberlite intrusions can compromise the ability of clay-rich bedrock aquitards to prevent migration of water through the system. As such, this study was conducted to assess the efficacy of the Lower Colorado shale aquitard to isolate the overlying intertill aquifers from the underlying Mannville Group aquifer during proposed dewatering. Water migration was characterized at two sites located approximately 5 km apart. One site consisted of a relatively simple shale aquitard system containing no secondary features (443 to 220 m asl) while the other shale aquitard was geologically complex (446 to 99 m asl). It consisted of fractures and the geotechnical and hydrogeological properties were influenced by kimberlite volcanism and subsequent hydrothermal alteration during deposition of the shale aquitard (99 to 112 Ma BP).

High-resolution, 1-D vertical profiles of conservative δ²H and Cl were determined from core samples collected every 1 m at both sites (to depths of 203 and 353 m below ground surface at the simple and complex site, respectively) to define the dominant vertical solute transport mechanisms. The 1-D tracer profiles from both the geologically simple and complex sites were consistent with diffusion as the dominant transport mechanism through the entire thickness of the Lower Colorado shale aquitard. Hydrothermal alteration during cooling of the kimberlite material may have reduced the effective porosity (n_e) at the complex site from 40% to 1-5%. Results also suggest that despite kimberlite emplacement in the study area, the shale has a relatively low hydraulic conductivity (2x10^-11 to 1x10^-10 m² s^-1); however, a high degree of fracturing within the kimberlite pipes may increase the hydraulic conductivity up to 3 orders of magnitude (9.1x10^-9 to 1.7x10^-7 m s^-1), thereby creating a potential conduit for water flow during dewatering.