Carnallite-Sylvite Relationships in the Middle Devonian Prairie Evaporite Formation, Saskatchewan
Abrupt lateral and vertical changes from red carnallitite (carnallite-halite rock) to red sylvinite (sylvite-halite rock) occur in the Prairie Evaporite Formation between Watrous and Kandahar, Saskatchewan. Such changes are of considerable economic importance, since carnallite has undesirable physical properties and a relatively low content of potassium. The following four types of relationship between sylvinite and carnallitite are considered: (1) the rocks are facies equivalents deposited in different areas from essentially contemporaneous brines; (2) carnallite formed by reaction of sylvite with magnesium chloride brines; (3) sylvite derived from carnallite by leaching of magnesium chloride; (4) sylvite, as presently found, not directly related to carnallite, but formed through solution of pre-existing sylvite with subsequent crystallization. Sylvinite overlies carnallitite, the reverse of a normal depositional sequence. The distribution of traces of bromide and rubidium in the chloride minerals indicates that red sylvinite was formed by leaching of magnesium chloride from carnallitite. This conclusion is substantiated by textural observations at carnallite-sylvite contacts, where red sylvite has replaced carnallite and inherited iron-oxide inclusions from carnallite. A comparison of the proportions of carnallite and sylvite, in a given potash zone, from a region of carnallitite to an adjacent region of sylvinite, reveals that the amount of sylvite present corresponds to the amount which could be derived from carnallite by leaching of magnesium chloride. The potash zones are cumulatively about 50 ft thicker where they are red carnallitite than where they are red sylvinite. Since the total thickness of salts can be determined by seismic techniques, this provides a valuable prospecting guide to the presence of local areas of carnallitite. Not all the sylvite present is derived from red carnallite. A clear variety of sylvite, termed “clear sylvite,” can be distinguished from red sylvite by the absence of iron-oxide inclusions and by a relatively small rubidium content (0.003 weight percent). Petrographic evidence indicates that red carnallite locally has replaced clear sylvite. Seventeen potassium-argon dates on red sylvite range from Permian to Mississippian; two samples gave younger ages. These are minimum ages and the alterations of carnallite to sylvite may have been essentially contemporaneous with deposition.