From Granite to Pegmatite : Petrographic, Structural, and Geochemical Insights into Rare‐Element Enrichment in the Varuträsk System, Sweden

Sammanfattning

This thesis investigates the geological and geochemical processes responsible for rareelement (Li-Cs-Ta) enrichment in the Varuträsk granite-pegmatite system, located in the Paleoproterozoic Svecofennian domain of northern Sweden. The primary research question addresses how the magmatic evolution, structural controls, and fluid–rock interactions are recorded in mica minerals. The study employs an integrated methodology combining petrographic analysis, electron microprobe (EPMA) geochemistry of biotite, muscovite, and lepidolite, and structural characterization of both granitic and pegmatitic units. Geochemical and multivariate statistical analyses are used to discriminate between mica types and to track magmatic and hydrothermal processes. Key findings indicate rare-element enrichment in the Varuträsk pegmatite system results from a twostage crustal melting history, with pegmatiteforming melts derived from residual metasedimentary sources and emplaced along reactivated brittle faults/fracture planes, some of which show evidence of plastic deformation especially within the Varuträsk deposit. Structural controls facilitated emplacement of the shallowly dipping, lensoid Varuträsk pegmatite (in contrast to the steeply dipping regional pegmatites) and promoted fluid exsolution during late-stage magmatic evolution. Mica chemistry reveals systematic trends in halogen and alkali metal enrichment, supporting a model where fluid-driven processes, rather than simple fractional crystallization, are the main drivers of ore formation. The conclusions highlight the potential influence of structural pathways, inferred from regional context, and magmatichydrothermal transitions in concentrating rare elements. However, new structural mapping was not undertaken as part of this work; conclusions are based on petrology and mineral chemistry. The research demonstrates that mica geochemistry, especially F, Rb, Cs, and AlVI contents, provides robust tracers for distinguishing magmatic versus hydrothermal signatures and for identifying ore-forming processes in LCT pegmatite systems. The findings have broader implications for the exploration and genetic modeling of rare-element pegmatites in ancient orogenic terranes.