Contrasting alteration textures and geochemistry of allanite from uranium-fertile and barren granites: Insights into granite-related U and ion-adsorption REE mineralization

Abstract

Allanite is an important rare earth element (REE)-U-bearing mineral in granites, and it can act as a metal source for the formation of some hydrothermal uranium deposits and ion-adsorption REE deposits. To investigate the potential of allanite as a mineral probe of granite-related uranium mineralization processes and the formation of ion-adsorption REE deposits, we present textures, geochemistry, and in situ U-Pb isotope data for allanite from the fertile Changjiang granite associated with the Changjiang uranium ore field and barren Jiufeng granite in the Zhuguangshan batholith, South China. Alteration of allanite in the Changjiang granite is characterized by the altered domains with lower backscattered electron (BSE) intensities than the unaltered domains and replacement by other secondary minerals such as REE-fluorocarbonates, calcite, fluorite, thorite, clay minerals, quartz, chlorite, and epidote. Crystals from the Jiufeng granite were partly replaced by the altered domains appearing darker in BSE images and minor REE-fluorocarbonates. The darker domains of the Changjiang and Jiufeng allanite grains have higher Fe 3+ /(Fe 3+ + Fe 2+ ) ratios and U concentrations than those of the brighter domains, indicating that the alteration of allanite was probably related to more oxidized fluids. This study suggests that the Changjiang granite might have been subjected to the influx of F-and CO 2 -bearing fluids. The brighter domains of the Changjiang and Jiufeng allanite grains have weighted mean 207 Pb-corrected 206 Pb/ 238 U ages of 156.7 ± 4.3 Ma and 161.6 ± 5.3 Ma, respectively, consistent with the corresponding zircon 206 Pb/ 238 U ages of 156.1 ± 1.4This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America.The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.