Geochemistry and structures of the exomorphic halo of the Volta Grande pegmatite, Brazil: Implications for net mass change and pegmatite emplacement

Abstract Traditional views regarding the essential role of water in forming pegmatite have been debated in favor of less water-intensive models, although the total volume of water present in pegmatite-forming liquids after emplacement is poorly constrained by field evidence. Non-isochemical interactions between country rocks and metasomatizing pegmatite-related fluids may offer crucial insights into their origin and properties. In our study of the Li-rich Volta Grande pegmatite, we draw on comprehensive geochemical exploration databases and structural volumetric strain constraints to compute representative mass-balance estimates. In the exomorphic halo of the main pegmatite, an initial stage of holmquistite-dominated metasomatism can be recognized, followed by biotitization, which produces a metasomatic rock with ca. 80% Li-Rb-Cs-biotite. Major and trace elements in biotitite indicate alteration characterized by a volume loss factor of ca. 0.94. Volume loss is compatible with structural profiles of a contact-parallel reverse shear zone that overprinted metasomatism, characterized by upper greenschist to lower amphibolite facies mylonitization microstructures. At these doubly constrained volume loss factors, estimates suggest that adding K and Li to the exomorphic halo accounts for approximately 3–5% and 2% by weight, respectively, of the original mass within the pegmatite. Conversely, about 10% of Ca in the pegmatite may have been drawn in from the country rocks during late-stage metasomatism. This finding is backed by textural evidence of Ca-rich phases in the pegmatite and supports the growing consensus regarding open-system interactions between pegmatite-forming liquids and surrounding country rocks, which are particularly effective in causing Ca incursions into pegmatite.