RARE EARTH ELEMENT (REE) POTENTIAL AND ORIGIN OF THE EPISYENITES IN THE CABALLO, BURRO, AND ZUNI MOUNTAINS, SOUTHERN NEW MEXICO

Episyenites found in the Caballo, Burro, and Zuni Mountains are brick-red, K-feldspar-rich rocks formed by desilicification and metasomatism of host rocks by alkali-rich fluids, possibly related to alkaline or carbonatite magmas. Based on mapping and geochemical analysis, the episyenites are metasomatic in origin and are found as pods, lenses (<300 m diameter) or dike-like bodies (<2 m wide, 400 m long). Some areas have numerous episyenites in a geographically restricted area, suggesting fracture control, possibly near the edges of the granitic magma bodies. The contacts between the episyenites and the host rocks vary from sharp to gradational. One example is the episyenite in the Jack Creek Rapakivi Granite at Ramsey Saddle (Burro Mountains), which exhibits a gradational contact, transitioning from the buff-colored host rapakivi granite to brick-red episyenite over a distance of 10s of meters. The large rapikivi feldspar crystals, diagnostic of the host granite, are overprinted by the brick-red episyenite, indicating that a fluid-driven metasomatic process was responsible. Similar replacement relationships are observed elsewhere, and are reinforced by microbeam observations of feldspar crystal texture and composition. The episyenites consist of K-feldspar with varying amounts of muscovite, hematite/goethite, chlorite, and plagioclase; quartz is locally absent. Accessory minerals include apatite, zircon, calcite, fluorite, magnetite and multiple REE-bearing minerals. They contain as much as 16% K₂O and have higher concentrations of REE (as much as 3167 ppm total REE), Th (as much as 9721 ppm) and U (as much as 2329 ppm) than most igneous rocks. Some episyenites contain high heavy REE (as much as 133 ppm Yb, 179 ppm Dy). The Nd isotope composition of an episyenite from the Burro Mountains (epsilon-Nd at 525 Ma of -5.8 and a depleted mantle model age of 1851 Ma) conforms to that of the surrounding granites and metamorphics and thus the metasomatism did not cause a change in the overall Sm/Nd ratio of the rock. Cambrian-Ordovician carbonatites are found in the Lemitar and Chupadera Mountains, Lobo Hill, Monte Largo and southern Colorado (Wet Mountains, Iron Mountain). It is possible that the episyenites are related to alkaline or carbonatite plutons at depth and are possibly related to this widespread magmatic event.