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

Dare earth element (REE)-enriched, igneous-related hydrothermal Fe-oxide ho sted (Fe-P-REE) systems from four areas in North America have been analyzed for their neodymium isotopic composition to constrain REE sources and mobi lity in these systems. The Nd isotopic results evidence a common pattern of REE concentration from igneous sources despite large differences in age (P roterozoic to Tertiary), tectonic setting (subduction vs, intraplate), and magmatic style (mafic vs. felsic). In the Middle Proterozoic St. Francois Mountains terrane of southeastern Mi ssouri, epsilon(Nd) for Fe-P-REE (apatite, monazite, xenotime) deposits ran ges from +3.5 to +-5.1, similar to associated felsic to intermediate igneou s rocks of the same age (epsilon(Nd) = +2.6 to +6.2) At the mid-Jurassic Hu mboldt mafic complex in western Nevada, epsilon(Nd) for Fe-P-REE (apatite) mineralization varies between + 1.1 and +2.4, similar to associated mafic i gneous rocks (-1.0 to +3.5). In the nearby Cortez Mountains in central Neva da, 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,a ll assemblages analyzed in this Tertiary rhyolite-hosted Fe oxide deposit h ave 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 o f REE from other sources. Interpretations of the origin of these hydrotherm al systems and their concomitant REE mobility must account for nonspecializ ed igneous sources and varied tectonic settings. Copyright (C) 2000 Elsevie r Science Lt.