HYDROCHEMICAL AND STABLE ISOTOPIC PROFILES OF FORMATION WATERS FROM THREE POTASH MINE-SHAFTS, SASKATCHEWAN, CANADA

The Prairie Evaporite Formation in the Elk Point Basin in Saskatchewan, Canada contains the largest deposits of potash in the world, supplying one third of the world’s demand. The problem of mine-level flooding has threatened the longevity of the potash mines in Saskatchewan since their construction. It is essential therefore to determine the origin of any inflows to remediate them or plan preventative maintenance. In the past, standard hydrochemical tracers (e.g. TDS, Cl) have met with limited success because the inflows pass through, and dissolve, the halites and sylvites of the Prairie Evaporite Formation. Non-standard hydrochemical tracers (e.g. stable isotopes, Br) have met with more success. Previous research has shown that stable isotopes (18O, D) are useful for fingerprinting formation waters in the basin and thus mine inflows. However, using Br as a tracer has not worked well in the past because of analytical difficulties.

Thus, the objective of this study was to update the existing stable isotopic profiles at the mines and to develop a Br profile using a promising new analytical technique (Epithermal Neutron Activation Analysis). Sixty-five inflow samples from access shafts were collected at three separate mines in order to construct three 1000 metre deep hydrochemical profiles for each mine. Measured TDS ranges from 3.2 g/L near the surface to over 514 g/L at the mine level (1025 m). The 18O composition ranges from –19.7 permil to 1.0 permil over the same depth range. Water types range from Na-SO4 (near the surface), to Na-Cl (mid-levels), to Ca-Cl2 (mine level). The Ca-Cl2 brines were observed at two different mines a distance of 55 km apart, indicating a significant distribution of this brine in the basin. Formation waters originated as mixtures of meteoric water, evaporated seawater, and dissolved evaporites. These hydrochemical and stable isotopic profiles are useful to study formation water origin, basin-scale fluid migration, and in potash mine planning.