Neodymium isotopic study of rare earth element sources and mobility in hydrothermal Fe oxide (Fe-P-REE) systems

摘要

Dare earth element (REE)-enriched, igneous-related hydrothermal Fe-oxide hosted (Fe-P-REE) systems from four areas in North America have been analyzed for their neodymium isotopic composition to constrain REE sources and mobility in these systems. The Nd isotopic results evidence a common pattern of REE concentration from igneous sources despite large differences in age (Proterozoic to Tertiary), tectonic setting (subduction vs, intraplate), and magmatic style (mafic vs. felsic). In the Middle Proterozoic St. Francois Mountains terrane of southeastern Missouri, epsilon(Nd) for Fe-P-REE (apatite, monazite, xenotime) deposits ranges from +3.5 to +-5.1, similar to associated felsic to intermediate igneous rocks of the same age (epsilon(Nd) = +2.6 to +6.2) At the mid-Jurassic Humboldt mafic complex in western Nevada, epsilon(Nd) for Fe-P-REE (apatite) mineralization varies between + 1.1 and +2.4, similar to associated mafic igneous rocks (-1.0 to +3.5). In the nearby Cortez Mountains in central Nevada, mid-jurassic felsic volcanic and plutonic rocks (epsilon(Nd) = -2.0 to -4.4) are associated with Fe-P-REE (apatite-monazite) mineralization having similar epsilon(Nd) (-1.7 to -2.4). At Cerro de Mercado, Durango, Mexico,all assemblages analyzed in this Tertiary rhyolite-hosted Fe oxide deposit have identical isotopic compositions with epsilon(Nd) = -2.5. These data are consistent with coeval igneous host rocks being the primary source of REE in all four regions, and are inconsistent with a significant contribution of REE from other sources. Interpretations of the origin of these hydrothermal systems and their concomitant REE mobility must account for nonspecialized igneous sources and varied tectonic settings. Copyright (C) 2000 Elsevier Science Lt. [References: 46]