The geochemical formation and evolution of a beryllium-rich, beryl-hosting pegmatite system from Mount Antero, Colorado, USA

Abstract

Mount Antero, Colorado (USA) is a known locality for gem-quality beryl (var. aquamarine and goshenite), quartz (var. smoky), fluorite, phenakite, and bertrandite. These minerals primarily occur in and adjacent to pegmatites, hydrothermal veins, and related features within ~30 Ma A-type granites. The pegmatites investigated in this study come from one location on Mt. Antero and occur as spheroids which range from 5 to 75 cm. Two pegmatite compositions exist: beryl-rich and beryl-poor. The beryl-rich pegmatites may contain beryl, bertrandite, quartz, muscovite, biotite, alkali feldspar, and plagioclase feldspar. The beryl-poor pegmatites contain quartz, muscovite, biotite, alkali feldspar, and plagioclase feldspar. Based on this mineralogy, Mt. Antero pegmatites are classified as Residual Melts of Granite Magmatism Group 2 (RMG-2). LA-ICP-MS trace element analyses were performed on beryl and quartz from both pegmatite types. Lithium and Zn are enriched in the cores of euhedral beryls when compared to the rims, while Mg, Sc, V, Mn, Fe, Ga, Rb, and Cs are enriched in the rims when compared to the cores. Anhedral beryl in the pegmatites have a more similar chemical composition to the euhedral beryl rims than the euhedral beryl cores. When examining the Na/Li versus ppm Cs, euhedral beryl cores appear less fractionated than the euhedral rims and anhedral beryl. Thus, we argue that euhedral beryl cores formed first, followed by euhedral beryl rims and anhedral beryl, while crystallizing from a singular evolving melt. LA-ICP-MS quartz data shows that Mt. Antero RMG-2 pegmatites are chemically distinct from other worldwide pegmatite types on bivariate plots of Al versus Li, Be, B, Ti, Ge, Ga, and Rb. The most notable differences between Mt. Antero RMG-2 pegmatites and DPA-1, DPA-2, and RMG-1 pegmatites occur when plotting Al vs Be, B, Ga, and Rb. These plots show that Be, B, Al, Ga, and Rb all have elevated concentrations when compared to other pegmatites. Overall, these results indicate that the pegmatites on Mt. Antero are classified as RMG-2, crystallized from one evolving melt, and Mt. Antero RMG-2 pegmatites plot distinct from other documented pegmatite types based on quartz trace element geochemistry.