Fluorinated Granitic Magmas: A Comprehensive Geochemical Study

Fluorine, a highly reactive halogen, is understood to be responsible for the unusual behavior of elements in silicic magmas. Rare metals, including economically critical elements like Nb, Ta, and rare-earth elements (REEs), in particular deviate from their expected geochemistry in fluorinated systems, leading to a need to understand fluorine in granitic melts. The thesis presents many avenues of exploration into the interaction of F and the major, minor, and trace element geochemistry and mineralogy of granites. The first part of this work details a method to measure trace elements, including F, by pressing finely-ground powders into a cellulose-bound pellet, then analyzing by x-ray fluorescence (for F) and laser- ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS). This method overcomes previous in-house shortcomings by obviating the need to dissolve samples in strong acids, decreasing the dilution effects of fluxing, and removing the possibility of F volitization during high-temperature fusion. The second part presents studies of real granites, including the results of a global literature review of reported halogens in granitoids around the world, and a focused discussion of two specific rare-metal plutons from the Eastern Desert of Egypt. One of these, Abu Dabbab, is a world-class deposit of Ta, and is interpreted to be an isolated magmatic system that self-metasomatized. The other, Abu Rusheid, is highly enriched in U and REEs, and appears that the mineralization of this pluton occurred over an extended period of time as an open magmatic system. The final part of the thesis similarly presents a compilation and novel study of computational, molecular dynamics simulations of fluorinated magmas. The observed preferred F speciations (Al and Si) are not thermodynamically ideal, as the species are not charge neutral, but may be made into a neutral species as AlOF. The F-high field strength species are longer lived, but less common, which F-alkali species are abundant and short- lived.