Carbonatites in the Lemitar and Chupadera Mountains, Socorro County, New Mexico
— Virginia T. McLemore

Carbonatites are unique carbonate-rich rocks of apparent magmatic descent and are characterized by a distinct but variable mineralogy, composition, and associated alteration (Heinrich, 1966). Greater than 50 percent carbonate minerals and varying amounts of apatite, magnetite, pyroxenes, an other accessory minerals are characteristic of carbonatites. These unusual rocks are enriched in total iron, CaO, CO₂, 13205, and numerous minor and trace elements and depleted in SiO, relative to other igneous rock types. One of the remarkable features of carbonatites is the variation in texture, mineralogy, and chemical composition between carbonatites within the same complex. Carbonatites range from simple monomineralogic types to varieties containing complex mineral assemblages and variable compositions. Textures may also be diverse and complex. Late-stage hydrothermal mineralization and alteration products may occur. Economically, carbonatites are important because they may contain large quantities of numerous commodities, including "limestone," iron ore, vermiculite, barite, fluorite, phosphate, rare-earth elements, uranium, thorium, niobium, copper, titanium, strontium, and manganese (Heinrich, 1966).

Most carbonatites occur as one or more plugs or stocks surrounded by cone sheets and ring dikes of carbonatite and alkalic rocks. They generally are associated with alkalic or kimberlite provinces. However, many carbonatite complexes, including those in the Lemitar and Chupadera Mountains, occur as dike swarms with or without any associated alkalic rocks or kimberlites (Heinrich, 1966).

A halo of fenites, produced by fenitization, typically surrounds carbonatites and may be adjacent to carbonatite dikes. Fenitization is a distinct metasomatic alteration associated with carbonatites, kimber- lites, and alkalic rocks (Heinrich, 1966; McKie, 1966). Fenites are characterized by alkali feldspars, alkali hornblende, aegirine, apatite, calcite, and the depletion or absence of quartz. Fenitization is locally present adjacent to some Lemitar and Chupadera carbonatites.