Summary:
Deposit Type
The geology of the potash-bearing beds of the Carlsbad area has been well documented. Overall, the potash-bearing beds may be described as bedded sedimentary rocks, deposited across the Delaware Basin and Northwest Shelf backreef from the Capitan Reef. The depositional sequences that developed in the Salado Formation consist of repetitive cycles that can be recognized by changes in mineralogy, sedimentary textures, and structures. Two types of cycles are differentiated as Type I and Type II. A complete Type I cycle ranges in thickness from 3 ft to 33 ft and consists of (in ascending order):
• A basal, mixed siliciclastic and carbonate mudstone
• Laminated to massive anhydrite-polyhalite
• Halite
• Halite with mud (argillaceous halite).
Type II is a thinner, less complete sequence and consists of halite that grades upward into argillaceous halite (Lowenstein 1988). The anhydrite-polyhalite beds are laterally continuous over large distances and are used as marker beds for correlation. Potash beds are not included in these sequences because potash is secondary and formed later than the basic depositional sequence.
Mineralization
In the Carlsbad Area, the potassium minerals, in order of decreasing abundance, are polyhalite, sylvite, carnallite, langbeinite, kainite, and leonite. Other potassium minerals occur only in minor amounts in association with the principal potassium minerals listed previously.
The minerals listed above can be described as follows (Schaller and Henderson 1932):
• Polyhalite is the most abundant potassium mineral in the Carlsbad Area. Beds of nearly pure polyhalite have thicknesses up to 8 ft and beds a foot or more thick are numerous.
• Sylvite often has a dark red or reddish-brown color due to hematite inclusions. Sylvite without the inclusions is a milky white color. Sylvite is typically mixed with halite and where clay is present in the mixture, it is in bands distinct from the sylvite.
• Carnallite is massive and compact showing no crystal faces. Crystals where seen are typically less than 1 millimeter (mm) in diameter. It occurs in small blebs with halite and sylvite.
• Langbeinite is found in distinct tetrahedral crystals that reach sizes up to ¾ inch. It is typically associated with halite and sylvite and often some kieserite. It has a distinct pink color in most samples and has a higher compressive strength than sylvite.
• Kainite is massive with poorly developed fibrous fracture surfaces and has a characteristic honey-yellow color. It is found in narrow bands between sylvite and langbeinite and is apparently a result of a reaction between the two.
• Leonite is typically found in small quantities in mixtures of other minerals, notably kainite and sylvite. Its color ranges from colorless to pale yellow. It is also found with polyhalite and anhydrite, but the relationship is unclear. It has also been found as a secondary replacement for kieserite.
Historically, the East Mine primarily mined sylvinite in the 10 ore zone. Current mining is predominantly taking place on the 5 mixed and 3 langbeinite ore zones. The 5 ore zone is a mixed ore consisting of variable amounts of K2O as langbeinite and sylvite. The 5 ore zone, predominant in langbeinite, is mined and blended with the 3 langbeinite ore. The common minerals found at the mine are halite, sylvite, clay (montmorillonite), sulfate minerals, and carnallite. The eastern sections of the mine have large deposits of carnallite and kieserite. The 10 ore zone is also characterized by isolated pods of barren clays. These clay pods range in size from a few square feet to several hundred thousand square feet. The location of these pods is random, and there is no known practical method of predicting their location.