PETROLOGY, GEOCHEMISTRY, AND PARAGENESIS OF CRITICAL AND OTHER MINERALS IN THE CORNUDAS MOUNTAINS, NEW MEXICO

The Cornudas Mountains are a set of Eocene to Oligocene nepheline syenite, phonolite, syenite and volcanic breccia laccoliths, sills, plugs and dikes that form the northern extent of the Trans-Pecos alkaline magmatic province. The rocks in the Cornudas are alkaline, metaluminous to peraluminous A-type within plate granitoids. Wind Mountain, one of the largest intrusions in the Cornudas, has an approximate diameter of 2.5km and is mineralogically, texturally, and chemically zoned. Samples from Wind Mountain and Chess Draw are elevated in rare earth elements, or REE, (up to 3313 ppm in CORN123) and Nb (up to 1750 ppm in CORN-20-14). Pyrochlore is the predominant niobium mineral. REE are found in bastnasite, calcio-catapleiite, eudialyte, monazite, vitusite, zircon, and xenotime; all REE minerals are extremely fine (under 20 microns) disseminations, although eudialyte and monazite are coarser and better-crystallized. Wind Mountain nepheline syenites and phonolites exhibit a unique and complex mineral association between aegirine-augite, arfvedsonite, aenigmatite, hastingsite, and pyrochlore, which could be indicative of magma mixing, sodic metasomatism, or late-stage fractionation of magmatic fluids. Biotite forms the core of hornblende and aegirine crystals which are in turn replaced by arfvedsonite, aenigmatite, and hastingsite; a paragenesis opposite in order to what is seen in other alkaline complexes. Potential deposits of U, Be, REE, Nb, Au, Ag, and nepheline syenite exist in the Cornudas, but there has been no production with exception for nepheline syenite test shipments.