MINERALOGICAL AND GEOCHEMICAL CHARACTERIZATION OF THE VIRGIN MOUNTAIN PEGMATITE FIELD, NEVADA-ARIZONA

The world has seen a surge in interest in securing reliable supplies of metals and minerals critical to modern, energy efficient, and strategic technologies. Although essential to the functionality of these technologies, ‘critical metals’ are often plagued by supply risks and related restrictions, such as the global supply of a critical metal being dominantly controlled by a single country. This has driven a renewed interest in locating and developing domestic critical metal resources within the USA. Granitic pegmatites represent a significant source of critical metals with nearly half of these metals being previously or actively mined from these deposits; however, pegmatite petrogenetic and associated critical metal mineralizing processes are actively debated.

The Virgin Mountain Pegmatite Field on the border of Nevada and Arizona contains critical metal mineralized pegmatites (CM) that were previously mined for beryl, chrysoberyl, and sheet mica but remain relatively unstudied. The pegmatite field hosts both barren and CM pegmatites occurring in parallel, NE-trending, discontinuous sheets and lenses intruding Paleoproterozoic upper amphibolite to granulite facies metasedimentary rocks. The CM pegmatites are subdivided into chrysoberyl and beryl-columbite types, the former more deformed than the latter, whereas barren pegmatites host variable minor to accessory tourmaline, biotite, muscovite, garnet, and magnetite. A suite of metagranitoids, including tourmaline-muscovite and two-mica leucogranitic gneisses, biotite monzogranite gneiss, and granodiorite gneiss units, outcrop along the margin of the pegmatite field. Migmatite leucosomes are somewhat gradational with the barren pegmatites, potentially indicating an anatectic source for the pegmatite field. This study presents new findings from mineralogic field mapping, garnet electron microprobe analysis, and whole-rock geochemical data from pegmatites, migmatite leucosomes, and metagranitoids. These data indicate that the CM pegmatites are mineralogically and geochemically distinct from the co-spatial barren pegmatites, suggesting that a different petrogenetic origin such as a separate, more fractionated magmatic pulse or a lower degree of anatexis was required to generate the CM pegmatites in this region.