CRITICAL METAL GRANITIC PEGMATITES: PLUTON-DERIVED OR DIRECT ANATEXIS?

Granitic pegmatites represent an important current and future source of many of the metals classified as critical (e.g., Be, Nb, Li) by government and academic institutions for their importance in modern, green, and defense technologies. The prevailing model for petrogenesis of critical metal granitic pegmatite deposits is generation through extreme fractional crystallization of a granitic pluton. Recently, the petrogenetic processes responsible for the generation of these critical metal deposits, the understanding of which is essential for a successful exploration model, have been called into question. New research has identified lengthy temporal gaps (10⁷ Ma) between some pegmatites and their assumed parental plutons. This suggests that pegmatite genesis may not require a parental pluton but instead could be generated by other processes such as partial melting of a critical metal-enriched source or low-degree anatexis.

This study focuses on the Virgin Mountain pegmatite field on the border of Nevada and Arizona, which hosts Be- and Nb-mineralized pegmatites and a variety of barren pegmatites emplaced into a 1700 Ma upper amphibolite to granulite facies metamorphic complex. New LA-ICP-MS analysis of zircons from local pegmatites, migmatite leucosomes, and plutons have generated trace and rare earth element (TREE) data and U-Pb ages for this pegmatite field. The zircon U-Pb dates indicate that evolving plutons spatially linked with the granitic pegmatites (i.e., pegmatite dikes emanating from the plutons) intruded at ~1670-1800 Ma whereas barren and Be-Nb-mineralized pegmatites were emplaced at ~1630-1700 and ~1590-1660 Ma, respectively. The migmatite leucosomes overlap with the emplacement of both suites of pegmatites and formed at ~1640-1690 Ma. These data suggest that either (1) the pegmatites are indirectly genetically-related to the plutons (e.g., partial melting of the granites themselves), or (2) there is a cogenetic relationship between the migmatite leucosomes and the pegmatites but not the plutons, meaning the pegmatites were generated via direct anatexis. The new TREE data provides further constraints on these two models and is used to outline the most likely petrogenetic heritage of the barren and critical metal-mineralized pegmatites within the Virgin Mountain pegmatite field.