Petrogenetic evolution of Late Paleozoic rhyolites of the Harvey Group, southwestern New Brunswick (Canada) hosting uranium mineralization

Abstract

The 360 Ma subaerial felsic volcanic and volcaniclastic rocks of the Harvey Group form a belt about 15 km long and 3 km wide in southwestern New Brunswick (Canada) that has been correlated with parts of the Mount Pleasant caldera complex, the site of a significant polymetallic (tin, tungsten, molybdenum, indium and bismuth) deposit. The Harvey volcanic rocks are highly fractionated peraluminous within-plate F-rich rhyolites, which host uranium mineralization. The rocks were modified by late-magmatic and post-magmatic processes. A comparison of the composition of whole rocks and melt inclusions in the quartz phenocrysts shows that some trace elements, including U, were affected by the post-magmatic processes. Their flat REE patterns accompanied by distinct negative Eu anomalies are typical of highly evolved F-rich leucogranites and rhyolites. Nd isotopic ratios (ɛNd(360) = +0.6 to −1.0) are similar to those of the felsic rocks of the Mount Pleasant complex. The Harvey rhyolites were generated by extensive fractional crystallization of andesites of the Mount Pleasant caldera. The melt evolved at the apex of the magma chamber where volatile elements become concentrated. The Harvey rhyolite (with melt inclusions containing ~20 ppm U) had the potential to develop a significant U mineralization. The erupted glassy rhyolite is a favorable U source rock amendable to leaching by post-magmatic hydrothermal and meteoric water. The high Th/U ratios in the Harvey volcanic rocks compared to the low ratios in the U-rich melt inclusions is indicative of such a process.