The potash-rich sylvinite zone mined at Esterhazy, Saskatchewan, contains irregular bodies of halite referred to as salt horses. Cuts through the salt horses range from less than 20 sq. ft to more than 200 sq. ft. Contacts between salt horses and the enclosing ore beds are normally irregular but well defined.There is significantly more bromide in both halite and sylvite in the salt horses (mean bromide 102 ppm and 1470 ppm respectively) than in adjacent ore beds (mean bromide 81 ppm and 1020 ppm respectively) although bromide contours within a horse bear little relationship to the geometric form of the horse.Rubidium in sylvite, unlike bromide, is not significantly different in the ore beds and salt horses (mean rubidium 37 ppm and 33 ppm respectively).Details of stratification, principally layers of clay–carbonate, can be traced from normal ore sections through the smaller salt horses. This suggests that the horses formed either contemporaneously with the ore beds or by later modification of the ore beds, without complete destruction of primary layering.It is suggested that the salt horses were formed by localized leaching of sylvite from the ore beds by brines saturated in sodium chloride. Some halite would be precipitated as sylvite dissolved. Prior to leaching, residual brines associated with the ore beds, capable of precipitating sylvite containing 1300 ppm bromide (near the upper range of bromide in sylvite of the ore beds), would be saturated in sodium chloride and potassium chloride. Up to 30% dilution of these residual brines with sodium chloride saturated brines (formed by the dissolution of halite) would produce brines which, in contact with the ore section, would dissolve sylvite and simultaneously precipitate halite. The resulting halite would contain about the same amount of bromide as the halite in the salt horses.No significant change in bromide or rubidium could be detected in the ore beds as a salt horse is approached. It is therefore unlikely that these trace elements can be used to predict the proximity of a salt horse during mine development. However, the higher bromide of the salt horses provides a possible means to differentiate, in drill core, between a barren hole due to a salt horse and a barren hole due to non-deposition, or erosion, of an ore zone.