The petrogenesis of brines in Devonian potash deposits of western Canada

Abstract

Recent mine-level floods in some potash mines in the Elk Point Basin of Saskatchewan, Canada, have demonstrated the need to identify accurately the source of water leaks so as to assist in remedial or preventative action. Most chemical constituents of the waters are inadequate tracers of the origin of these leaks because the concentrations of most elements change substantially during migration of the fluids to the mining level. In marked contrast to most of the chemical constituents dissolved in the water, oxygen and hydrogen isotopes are conservative elements of the water. Within the Elk Point Basin, δD- and δ 18O-values of basinal waters normally increase with depth because of mixing between trapped formation waters, and local surface waters such that the water in each aquifer within the basin generally has a unique isotopic composition. Floodwaters in the potash mines of Saskatchewan have chemical and stable isotopic compositions that indicate three different origins for mine-level fluids: (1) halite- and sylvite-saturated basinal brines from Devonian formations directly above the deposits, which are predominantly connate waters; (2) undersaturated waters of predominantly meteoric origin, from stratigraphically higher aquifers such as the Cretaceous Mannville Group; and (3) Ca-rich brines that may represent Devonian fluids associated with recrystallization of the evaporites. The salt-undersaturated formation waters most likely are directed to lower stratigraphic units through collapse structures or related features which may be reactivated through mining.