MAGMATIC ENRICHMENT OF URANIUM, THORIUM AND RARE EARTH ELEMENTS IN LATE PALEOZOIC RHYOLITES OF SOUTHERN NEW BRUNSWICK, CANADA: EVIDENCE FROM SILICATE MELT INCLUSIONS

The major and trace–element geochemistry of silicate melt inclusions were investigated within Late Paleozoic felsic rhyolites from the Piskahegan and Harvey Groups 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 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 (“pre-eruptive” inclusions) and in late quartz aggregates (“syn-eruptive” 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, and an increase in the U/Th ratio of the melt, despite low F concentrations. A higher degree of melt fractionation combined with post-magmatic leaching may have been prerequisites for mineralization at Harvey, exhibiting anomalously high U (20 ± 2.0 ppm, 1σ, n=42) and Th (45 ± 6.4 ppm, 1σ, n=42) concentrations, especially in the “syn-eruptive” inclusions (U, 76 ± 12 ppm; Th, 118 ± 21 ppm, 1σ, n=7) when compared to non-mineralized whole rock (U, 9.0 ppm; Th, 45 ppm, 1σ, n=2). Evolving magmas at Piskahegan were relatively less affected by fractionation, with the U and Th concentrations remaining quite constant from the melt phase (U, 11 ± 3.6 ppm; Th, 37 ± 8.0 ppm, 1σ, n=35) to non-mineralized rocks (U, 37 ppm; Th, 37 ppm, n=1). Since whole–rocks are highly susceptible to alteration, melt inclusion analysis may be the only method capable of providing constraints on melt chemistry/evolution in such ancient volcanic terrains. Melt inclusion analysis may also enable the evaluation of the economic potential of such terrains if the initial U and Th concentration, and U/Th ratio of the volcanic products impact the ultimate mineralizing potential of the system.