New Mexico Geological Society Annual Spring Meeting — Abstracts

Cathodoluminescence (CL) is the characteristic visible radiation (color) produced in a mineral subjected to a bombardment of electrons. Many features of a sample are observed under CL that are not seen using standard optical petrography. Applications of CL include mineral identification, determining mineral distribution, observing textures and structures, among others. CL studies are especially useful in examining carbonatites and associated fenitization.

Carbonatites are unique carbonate-rich rocks of apparent magmatic descent and are characterized by a distrinct but variable mineralogy, composition, and associated alteration (fenitization). The Lemitar carbonatites exhibit bright red CL that is characteristic of carbonatites elsewhere in the world. The bright red luminescence is due to compositional variations in fine-grained carbonate minerals, predominantly calcite and dolomite. Apatite luminesces blue to green-gray to gray and is typically zoned. Magnetite occasionally exhibits CL zoning, not observed using optical microscopy. Examination with the electron microprobe reveals the zoning is caused by complex intergrowths of magnetite, ilmenite, rutile and/or leucoxene, calcite, and quartz.

Fenitization is the alkalic metasomatic alteration adjacent to carbonatites and alkalic rocks. In the Lemitar Mountains, unaltered county rocks exhibit less intense luminescence when exposed to CL than carbonatites and fenites. K-feldspars are typically blue to white or gray, or do not luminesce in unaltered rocks, whereas they luminesce brown or red in fenites, probably due to replacement by carbonate and/or activation of ferric iron, rare-earth elements, and manganese. Apatites luminesce yellow to green in unaltered and slightly fenitized rocks and luminesce blue to green-gray to gray in highly fenitized rocks. Slightly fenitized rocks exhibit veins of red luminescing carbonate along grain boundaries, within fractures, and replacing chlorite, feldspar, and hornblende. Highly fenitized rocks exhibit bright red luminecsent matrix with relict feldspars (replaced by carbonate) .