Economic geology of the Carlsbad potash district, New Mexico

Abstract

—New Mexico produced 83% of domestic potash and 27% of domestic consumption in 1992. Fertilizer used 95% of U.S. production; 5% was used in chemicals. The potash industry of New Mexico produces sylvite (KCI), langbeinite (K 2 SO42Mg(SO4) 2 ) and artificial K 2 50 4 . Sylvite grade in New Mexico decreased from 20-25% K 2 0 in the 1950s to about 14% today. The average grade of langbeinite (first produced in 1940) remains 8-10% K 2 0. Reserves in the Known Potash Leasing Area (KPLA) will sustain production for 25 to 35 years and represent 57% of domestic reserves. Potash horizons extend far outside the KPLA, but none are economic. Commercial potash occurs in the middle or McNutt Member of the Salado Formation (Upper Permian Ochoan Stage). The potash distribution in the Salado is asymmetrical, suggesting a reflux model related to a bar-restricted marine embayment with dense brine underflow toward the bar. The dominantly seawater brine was magnesium-rich, producing primary carnallite or polyhalite, later recycled into the modern complex salt assemblage. The 400–ft-thick McNutt Member dips about I° east and contains 11 of 12 potential ore zones in the Salado. Potash zones are 3-10 ft thick and contain minable sylvite and/or langbeinite, together with halite and accessory minerals. The zones are consistent laterally, but are locally interrupted by barren halite (salt horses) formed later by undersaturated migrating fluids. Room-and-pillar mining, with pulled pillars, recovers >90% of the potash from depths ranging from 885 to 1400 ft by continuous-mining coal equipment or drilling and blasting. Beneficiation is by various separation, flotation, crystallization, leaching and heavy-media circuits optimized for each ore. The mineralogy and proportion of clay minerals, up to 7% of the ore, influence optimum milling procedures and ore grade cutoff. Agriculture, petroleum and nuclear waste disposal affect production of potash from southeastern New Mexico. Agricultural demand for fertilizer is a complex interaction between weather and climate, advances in crop genetics, soil science, farming practices, GNP of importing nations, farm income, population growth, efficient distribution systems, freight rates and backhauls, substitutes, taxes and tariffs. Petroleum companies compete for petroleum under potash and the Waste Isolation Pilot Project (WIPP) led to withdrawal of potash reserves.