U-Th-Pb dating, trace elements, and Sr-Nd isotopes of monazite and allanite as recorders for multi-stage rare earth element mineralization and remobilization in carbonatite dike systems

Abstract

Carbonatite hosts the most important rare earth resources in the world, but the precise timing, ore-forming history, and mechanism of rare earth mineralization in carbonatite systems are still in debate. Here, we report a rare corona texture of monazite-allanite-fluorapatite from the Huangjiagou carbonatite in the Lesser Qinling of central China, and demonstrate that the U-Th-Pb dating, trace elements, and Sr-Nd isotopes of these minerals in the corona are useful tools to unravel multiple-stage events for rare earth element (REE) mineralization and mobilization. The first mineralization event took place at ca. 219 Ma as revealed by the monazite U-Pb age, the same as regional carbonatite forming ages, but the Th-Pb age has been disturbed, which shows a negative correlation with Th contents. The second mineralization event occurred at ca. 128 Ma, as revealed by in situ U-Pb dating of allanite, coeval with the intrusions of neighboring I-type granite. The initial Sr-Nd isotope ratios of allanite show a downtrend from the center to the rim of monazite-allanite-apatite coronas to approach the ratios of neighboring granite, indicating an increasing effect by the metasomatism of magmatic-hydrothermal fluids during the growth of these REE-mineral coronas. Therefore, a two-episode REE mineralization was recognized with the replacement of ca. 219 Ma monazite by ca. 128 Ma allanite-apatite coronas on the function of magmatic-hydrothermal fluid metasomatism, and this process accompanies the disturbance of Th/Pb geochronology in monazite. Allanite as the product of monazite dissolution can represent the later-stage REE mineralization tracing the REE reworking processes under the hydrothermal conditions in carbonatite systems. Our study highlights the implication of monazite-allanite-fluorapatite coronas on the REE remobilization and mineralization in carbonatite systems.