113-4 Reconnaissance Textural Observations of Aquamarine and the Beryllium-Rich, Gem-Hosting Pegmatite System of Mount Antero, Colorado (USA)

Session: Mineralogical Characterization of Economic Resources: From Critical Minerals to Gemstones (Posters)

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Authors:

Abstract:

Granitic pegmatites are derived from highly evolved silicic magmas and often contain many rare minerals, including beryl and in some cases, gem-quality beryl var. aquamarine. One world-class location where aquamarine, beryl, and other Be-rich minerals are found in association with granitic pegmatites, is Mount Antero, Colorado. At Antero, evolved granites (e.g., Mt. Antero granite and California leucogranite), granitic pegmatites, aplite dikes, and hydrothermal veins host colorless beryl, blue beryl, and aquamarine, as well as bertrandite, and phenakite. The granitoids are ~30 Ma and are coeval with local topaz rhyolite domes in the Arkansas Valley that are associated with the onset of local Rio Grande Rift extension. On Antero, the granitoids cut the ~36-35 Ma Mt. Princeton batholith, which was a locus of magmatism associated with the southern Rocky Mountain volcanic field. Prior studies synonymously use the terms pegmatite, dike, and veins for Antero mineral deposits, but it’s unclear whether any P-T differences exist between quartz and beryl in each deposit type, nor if there are any geochemical variations in beryl and aquamarine from each type. This research aims to characterize these potential differences. Here, we report reconnaissance hand sample and thin section petrography of the California leucogranite, and cross-cutting pegmatite and hydrothermal veins. The granite is composed of quartz with mosaic and feather textures, beryl (up to 3-mm), muscovite, bertrandite, spinel, and plagioclase. In the granite, blue beryl makes up less than 5% of the rock. The pegmatites are classified as RMG-Group 2 type [1] and some host unidirectional growths of quartz, beryl, and plagioclase up to ~26-mm-thick. In other pegmatites, beryl crystals (up to 6-mm-wide) grew adjacent to quartz, and transition into 4-mm-long bertrandite laths. The hydrothermal veins are predominantly aquamarine, which are fractured and filled by quartz, bertrandite intergrowths, and clay minerals. Plagioclase is nonexistent in the hydrothermal veins, while the aquamarine makes up ~80% of the veins. Ongoing work is focused on in-situ sampling of pegmatite and veins to fully document their field occurrences, and LA-ICP-MS and fluid inclusion studies of quartz and beryl from pegmatites, veins, and host granitoid. [1] Wise, M.A. et al., 2022, A proposed new mineralogical classification system for granitic pegmatites: The Canadian Mineralogist. DOI:10.3749/canmin.1800006

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6899

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