Reaction aureoles around uraninites within biotite and plagioclase: evidence of low-temperature sequential fluid alteration and LREE-mobilization from monazite

Abstract

AbstractOptical microscopy, Raman spectroscopy and electron probe microanalyses were conducted on a migmatitic metapelite to investigate the effects of alpha radiation and subsequent alteration at the interface between uraninite inclusions and the host minerals biotite, chlorite and albitic plagioclase. The study reveals (1) anomalous colouration under the polarizing microscope; (2) pertinent changes in the characteristic Raman spectra of host phases; (3) reaction aureoles, composed of secondary phases (viz. chlorite and K-feldspar) of various sizes (∼15–45 μm) within biotite and plagioclase, respectively; (iv) K-feldspatization and sericitization of plagioclase at the grain boundaries; (5) agreement between the observed α-penetration depths and the Monte-Carlo simulation results; and (6) dissolution textures within the matrix monazites. Analysis of the compositions of the reaction aureoles in albitic plagioclase reveals a systematic distribution of K-feldspar-, LREE- and clay-rich zones; while the same in biotite is composed of secondary chlorite. The growth sequence of the secondary phases indicates an influx of a K-rich fluid, following intense radiation damage, efficiently superimposed by LREE-metasomatism and later acidic alteration. These changes took place under low-temperature (≤150°C) conditions, wherein radial cracks (within plagioclase) and cleavages/fractures (within biotite) favoured fluid infiltration-circulation into the reaction aureoles. Depletion of the LREEs from the dissolved matrix monazites and their enrichment as a discrete LREE phase within the damaged aureoles in plagioclase demonstrate micrometre-scale LREE mobility.