The major and trace element geochemistry of silicate melt inclusions was investigated within late Paleozoic felsic rhyolites from the Piskahegan and Harvey Formations of southern New Brunswick, Canada, in order to provide further insight into the genetic history of the volcanic- and caldera-related U mineralization that occurs in the region. Glassy melt inclusions analyzed by laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS) and electron microprobe show enrichment in most incompatible trace elements, but a marked depletion in Ba, Sr, and Eu compared to whole rock. At Harvey, melt trapped in early quartz phenocrysts (“preeruptive” inclusions) and in late quartz aggregates (“syneruptive” inclusions) within the groundmass of the rhyolites was significantly more fractionated than melt trapped in quartz phenocrysts at Piskahegan. Fractionation was associated with the crystallization of feldspar and resulted in progressive enrichment of the melt in U, Th, B, LILE, LREE, and other metals, as well as an increase in the U/Th ratio of the melt. A higher degree of melt fractionation, combined with postmagmatic leaching, may have been prerequisites for mineralization at Harvey. Because felsic volcanic rocks are highly susceptible to alteration, melt inclusion analysis may be the only method capable of providing constraints on melt chemistry and evolution in such ancient volcanic terrains. This may enable the evaluation of the economic potential of such terrains if the initial U and Th concentration, as well as the U/Th ratio of the volcanic products, affect the ultimate mineralizing potential of the system.

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