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 c. 80% Li-Rb-Cs-biotite. Major and trace elements in biotitite indicate alteration characterized by a volume loss factor of c. 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 growing consensus regarding open-system interactions between pegmatite-forming liquids and surrounding country rocks, which are particularly effective in causing calcium incursions into pegmatite.