LATE MIOCENE VAPOR-PHASE ENRICHMENT OF REE IN BASALTIC ENCLAVES

As REE ore becomes more socially important we studied a mixed magma dome in central Idaho to identify the style and process of mineralization. Porphyritic mafic enclaves of trachybasalt to trachyandesite contain highly altered plagioclase and anorthoclase as well as sanidine and fayalite xenocrysts from the peralkaline rhyolite. Arfvedsonite surrounds enclaves and vesicles and are disseminated through the devitrified rhyolite. Vesicles locally contain kaolinite and La-Nd monazite with sparse fluorite. A K-Ar age of feldspar of about 7 Ma has been reported, but feldspars have been found to be commonly altered. U-Pb ages of zircons from the rhyolite using LA-ICP-MS are generally 8 Ma with one 2.8 Ga core, but few zircons from the rhyolite were large enough for analyses. Vapor phase microcrystalization of low-compatible elements was observed in clay filled vesicles of the alkali-rich mafic enclaves. This occurrence is not economical due to the relatively low HREE concentrations, small volume, and post emplacement silicification; however, this deposit displays evidence of multiple magma mixing processes in an extrusive setting along a normal fault.